Hydrogels for phosphate binding and release

Abstract

Hydrogels are provided formed from a first polymer comprising a plurality of amine moieties and a second polymer comprising a plurality of cyclic acid anhydride moieties. Water filtration systems comprising the hydrogels and methods for removing phosphate from a water source using the hydrogels are also provided.

Description

[0001] HYDROGELS FOR PHOSPHATE BINDING AND RELEASE CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority to United States Provisional Patent Application No. 63 / 714,516, filed October 31, 2024, the disclosure of which is incorporated herein by reference in its entirety.

[0002] STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0003] This invention was made with government support under CBET2019435 awarded by the National Science Foundation. The government has certain rights in the invention.

[0004] BACKGROUND

[0005] Phosphorus is ubiquitous in vital biological processes, including the physiology of living organisms, plant growth and health, and photosynthesis. Over 95% of mined phosphate rock is used as fertilizers and animal feed supplements to support the food supplies in the increasing global population (see United States Geological Survey, "Mineral commodity summaries 2023" (Reston, VA), pp. 134). Unfortunately, the excessive application of fertilizers has resulted in runoff into waterways, leading to eutrophication and freshwater contamination, posing threats to drinking water sources and aquatic biodiversity (see: D. L. Correll, The Role of Phosphorus in the Eutrophication of Receiving Waters: A Review. J. Environ. Qual 27, 261-266 (1998); and Y. Zhang et al., Cause and effect of N / P ratio decline with eutrophication aggravation in shallow lakes. Sci. Total Environ. 627, 1294-1302 (2018)). Nearly 90% of the world's population faces risks associated with phosphorus-related pollution, where anthropogenic phosphorus inputs exceed basin waste assimilation capacity (see M. M. Mekonnen, A. Y. Hoekstra, Global Anthropogenic Phosphorus Loads to Freshwater and Associated Grey Water Footprints and Water Pollution Levels: A High- Resolution Global Study. Water Resour. Res.54, 345-358 (2018)). A viable economic and environmental solution is to recycle phosphate from P-excessive sources, such as eutrophic water bodies, to P-depleted agricultural fields, thereby reducing greenhouse emissions and lowering energy consumption (see S. R. Golroudbary, M. El Wali, A. Kraslawski, Environmental sustainability of phosphorus recycling from wastewater, manure and solid wastes. Sci. Total Environ. 672, 515-524 (2019)).

[0006] Over half of the excessive phosphate in the soil eventually ends up in water bodies (see C. Alewell et al., Global phosphorus shortage will be aggravated by soil erosion. Nat. Common. 11, 4546 (2020)). Among various P sources in the water, soluble inorganic phosphate is directly available to capture, while organic phosphorus compound requires decomposition to inorganic phosphate before removal. Soluble inorganic phosphate accounts for >50% of phosphorus in wastewater phosphorus (see K. Venkiteshwaran, P. J. McNamara, B. K. Mayer, Meta-analysis of non-reactive phosphorus in water, wastewater, and sludge, and strategies to convert it for enhanced phosphorus removal and recovery. Sci. Total Environ. 644, 661-674 (2018)). Numerous approaches have been developed to capture inorganic phosphates and meet the quality standards. Chemical precipitation and enhanced biological phosphorus removal (EBPR) are two technologies applied in wastewater treatment plants. Nevertheless, the environmental instability of biological treatment and viable regeneration of phosphate precipitates requires a more environmentally friendly, cost-effective, and robust recycling process (see M. K. Perera, J. D. Englehardt, A. C. Dvorak, Technologies for Recovering Nutrients from Wastewater: A Critical Review. Environ. Eng. Sci. 36, 511-529 (2019)).

[0007] The current strategies to recycle P are multifaceted and include a broad range of inorganic and organic chemical systems. Specifically, the inorganic sorbents include metal oxide-based materials, i.e., zirconium and lanthanum-based materials, showing high selectivity and capacity towards phosphate (see: Q. He et al., Phosphate removal and recovery by lanthanum-based adsorbents: A review for current advances. Chemosphere 303, 134987 (2022); R. Liu et al., Review of metal (hydr)oxide and other adsorptive materials for phosphate removal from water. J. Environ. Chem. Eng. 6, 5269-5286 (2018); and S. M. Ribet, B. Shindel, R. dos Reis, V. Nandwana, V. P. Dravid, Phosphate Elimination and Recovery Lightweight (PEARL) membrane: A sustainable environmental remediation approach. Proc. Natl. Acad. Sci. U.S.A. 118, e2102583118 (2021)). For these metal-based systems, phosphate adsorption relies on the high binding affinity between sorbent and phosphate, the same as chemical precipitation, relying on the insolubility of metal phosphate salts. However, the desorption of captured phosphate is complex. High affinity and insolubility guarantee high P removal capacity but restrict bonded and precipitated phosphate from being recovered. The release of bonded phosphate on metal oxide typically requires high concentrations (>1 M) of alkaline or acidic conditions at high processing temperatures (see: J. Li et al., Lanthanum-based adsorbents for phosphate reutilization: Interference factors, adsorbent regeneration, and research gaps. Sustainable Horizons 1, 100011 (2022); and P. Zhang, M. He, S. Huo, F. Li, K. Li, Recent progress in metal-based composites toward adsorptive removal of phosphate: Mechanisms, behaviors, and prospects. Chem. Eng. J. 446, 137081 (2022)), posing challenges to environmental sustainability.

[0008] New materials and systems are needed to remove and recycle phosphate from wastewater streams efficiently. This disclosure addresses this as well as other needs.

[0009] SUMMARY

[0010] The present disclosure provides compositions, devices comprising said compositions, and methods of making and using said compositions. More particularly, the present disclosure provides hydrogels capable of selectively adsorbing phosphate from a water source. Water filtration devices containing said hydrogels and methods of using said hydrogels are also provided.

[0011] In one aspect, a hydrogel is provided. In some aspects, the hydrogel can be formed from a first polymer including a plurality of amine moieties. In some aspects, the hydrogel can be formed from a second polymer including a plurality of cyclic acid anhydride moieties. In some aspects, the second polymer can be crosslinked by the first polymer.

[0012] In some aspects, the first polymer includes a polyalkylamine. In some aspects, the first polymer includes polyethyleneimine (PEI). In some aspects, the first polymer includes branched polyethyleneimine (PEI). In some aspects, the first polymer includes linear polyethyleneimine (PEI).

[0013] In some aspects, the second polymer includes a copolymer of at least one cyclic anhydride and one or more additional monomers. In some aspects, the copolymer can be an alternating copolymer. In some aspects, the copolymer can be a statistical or random copolymer. In some aspects, the at least one cyclic anhydride includes succinic anhydride. In some aspects, the one or more additional monomers include a monomer having one or more alkenyl moieties. In some aspects, the one or more additional monomers include a vinyl ether or a vinyl ester. In some aspects, the one or more additional monomers include an alkyl vinyl ether. In some aspects, the one or more additional monomers include methyl vinyl ether. In some aspects, the one or more additional monomers include ethyl vinyl ether. In some aspects, the one or more additional monomers include vinyl acetate or vinyl propionate.

[0014] In some aspects, the second polymer includes a constitutional repeating unit having the structure:

[0015]

[0016] wherein all variables are as defined herein.

[0017] In some aspects, the one or more additional monomers include at least one hydrophilic monomer.

[0018] In some aspects, the second polymer includes poly(methyl vinyl ether-alt-maleic anhydride) (PMVEMA).

[0019] In some aspects, a ratio of a first repeating unit of the first polymer to a second repeating unit of the second polymer can be from about 6:1 to about 1:6. In some aspects, a ratio of a first repeating unit of the first polymer to a second repeating unit of the second polymer can be about 3:1.

[0020] In some aspects, the hydrogel can be capable of selective adsorption of phosphate from water in contact with the hydrogel at a pH from about 2.0 to about 7.0. In some aspects, the hydrogel can be capable of selective adsorption of phosphate over nitrate from water when a mass ratio of nitrate to phosphate in the water in contact with the hydrogel is less than about 2.5.

[0021] In some aspects, the hydrogel can be capable of desorption of phosphate from the hydrogel at a pH of about 9 to about 11. In some aspects, the hydrogel can be capable of desorption of phosphate from the hydrogel when contacted with a hydroxide solution (such as a sodium hydroxide or potassium hydroxide solution).

[0022] In another aspect, a water filtration system is provided. In some aspects, the water filtration system includes a hydrogel described herein. In some aspects, the water filtration system includes a filter column or a flow bed.

[0023] In another aspect, a method for removing phosphate from a water source is provided. In some aspects, the method includes contacting the water source with a hydrogel described herein. In some aspects, the water source includes a waterway (such as a river, lake, or stream) or agricultural runoff. In some aspects, phosphate can be selectively removed over nitrate from the water source. In some aspects, contacting the water source with the hydrogel includes filtering the water source through a water filtration system comprising the hydrogel. In some aspects, the method further includes contacting the hydrogel with a hydroxide solution. In some aspects, the phosphate can be desorbed from the hydrogel.

[0024] The details of one or more aspects of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description, the drawings, and the claims.

[0025] DESCRIPTION OF DRAWINGS

[0026] FIG. 1 provides a schematic of the chemical structures of polyethyleneimine (PEI), poly (methyl vinyl ether-a / t-maleic anhydride) (PMVEMA), and PEI: PMVEMA hydrogel.

[0027] FIGs. 2A-2C depict the adsorption kinetics of phosphate at different initial concentrations. (FIG. 2A) absorbed P per mass of sorbent at time t (qt) vs. time;

[0028] (FIG. 2B) solution pH vs. time; (FIG. 2C) pH versus distribution of phosphate species (see S. Kim, Y. H. Park, J. B. Lee, H. S. Kim, Y.-E. Choi, Phosphorus adsorption behavior of industrial waste biomass-based adsorbent, esterified polyethylenimine-coated polysulfone-Escherichia coli biomass composite fibers in aqueous solution. J. Hazard. Mater.400, 123217 (2020)). FIGs. 3A-3D depict the equilibrium absorption capacity and phosphate removal by PEI / PMVEMA (3:1) (FIGs. 3A and 3C), and (1:3) (FIGs. 3B and 3D) hydrogels as a function of P concentration, sorbent concentration, and pH.

[0029] FIGs. 4A-4F depict the effects of pH on the desorption of P preloaded hydrogels. The hydrogels were initially loaded with phosphate in solutions of (FIGs. 4A and 4D) 2.0 mg / L, (FIGs. 4B and 4E) 8.0 mg / L, and (FIGs. 4C and 4F) 20.0 mg / L of P, all at varying pH levels: 3.0, 4.5, and 6.0. On the x-axis, the pH values denote the conditions during preloading. The data points on the graph are represented as follows: the red square for desorption at pH 9.0, the green circle at pH 10.0, and the blue up-triangle at pH 11.0.

[0030] FIG. 5 provides a schematic illustration of electrostatic interactions between phosphates, carboxyls, and amines under different pH conditions (top). Brown double-sided arrows represent electrostatic repulsion forces, and red double-sided arrows are for attraction forces. The distribution of phosphate species as a function of pH is shown at the bottom. Fraction means the ratio of the corresponding form and total phosphate. The pKa of PEI is 9.5, and PEI turns globally neutral at pH above 11. The pKa of succinic acid are 4.2 and 5.6, respectively.

[0031] FIGs. 6A-6C depict the selectivity of phosphate adsorption in binary phosphatenitrate systems. pH effects on the absorbed P (FIG. 6A), absorbed N (FIG. 6B), and separation factor (FIG. 6C) in the phosphate-nitrate solution. pH effects on the absorbed P.

[0032] FIG. 7 provides Fourier transform infrared (FTIR) spectra of PEI, PMVEMA, PEI / PMVEMA dried gel (PEI / PMVEMA), PEI / PMVMEA upon loading with P (PEI / PMVMEA+P), and monosodium phosphate.

[0033] FIG. 8 depicts the stability of the PEI / PMVEMA hydrogel as a function of pH. Initial pH represents the hydrogel systems' pH after storage for one week.

[0034] FIG. 9 depicts the pseudo-first-order (left) and pseudo-second-order (right) fitting of sorption kinetics, t is time, qt is the sorption capacity at time t.

[0035] FIG. 10 provides titration curves of PEI, succinic acid, PEI: PMVEMA=3:1 hydrogel, and sodium chloride. The pKa of PEI is ~9.5, and the succinic acid is 4.2 and 5.6, in agreement with the literature reported (see: G. D. Pinching, R. G. Bates, Second dissociation constant of succinic acid from 0° to 50° C. J Res Natl Bureau Stand 45 (1950); and G. D. Pinching, R. G. Bates, First dissociation constant of succinic acid from 0 to 50 c and related thermodynamic quantities. J Research NBS 45, 444-449 (1950)).

[0036] FIG. 11 provides breakthrough curves of PEI / PMVEMA hydrogel with influent at pH 3.0, 4.5, and 6.0.

[0037] FIGs. 12A-12B depict (FIG. 12A) breakthrough curves of P absorption of PEI / PMVMEA hydrogel at pH 4.5 and 6 BV / h, equivalently 0.05 mL / min. (FIG.

[0038] 12B) Cycles of P desorption of phosphate-loaded PEI / PMVEMA hydrogel at pH 12 and 6 BV / h.

[0039] FIG. 13 depicts the competitive absorption of phosphate and sulfate on PEI / PMVEMA hydrogel at a flow rate of 6 BV / h and at pH 4.5.

[0040] FIGs. 14A-14B depict (FIG. 14A) Desorption of phosphate and sulfate with 0.01 M NaOH at a flow rate of 6 BV / h. (FIG. 14B) Molar ratio of phosphate and sulfate in the effluent.

[0041] Like reference symbols in the various drawings indicate like elements.

[0042] DETAILED DESCRIPTION

[0043] The following description of the disclosure is provided as an enabling teaching of the disclosure in its best, currently known aspects. Many modifications and other aspects disclosed herein will come to mind to one skilled in the art to which the disclosed compositions and methods pertain, benefiting from the teachings presented in the descriptions herein and the associated drawings. Therefore, it is understood that the disclosures are not limited to the specific aspects disclosed and that modifications and other aspects are intended to be included within the scope of the appended claims. The skilled artisan will recognize many variants and adaptations of the aspects described herein. These variants and adaptations are intended to be included in the teachings of this disclosure and to be encompassed by the claims herein. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

[0044] As is apparent to those of skill in the art upon reading this disclosure, each of the individual aspects described and illustrated herein has discrete components and features that may be readily separated from or combined with the features of any of the other several aspects without departing from the scope or spirit of the present disclosure.

[0045] Any recited method can be carried out in the order of events recited or any other order that is logically possible. Unless otherwise expressly stated, it is in no way intended that any method or aspect set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not explicitly state in the claims or descriptions that the steps are to be limited to a particular order, it is in no way intended that an order be inferred in any respect. This holds for any possible non-express basis for interpretation, including logic concerning the arrangement of steps or operational flow, meaning derived from grammatical organization or punctuation, or the number or type of aspects described in the specification.

[0046] All publications mentioned herein are incorporated by reference to disclose and describe the methods or materials in connection with which the publications are cited. The publications discussed herein are provided solely for their disclosure before the filing date of the present application. Further, the dates of publication provided herein can be different from the actual publication dates, which can require independent confirmation.

[0047] It is also to be understood that the terminology herein describes particular aspects only and is not intended to be limiting. Unless defined otherwise, all technical and scientific terms herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosed compositions and methods belong. It can be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and relevant art and should not be interpreted in an idealized or overly formal sense unless expressly defined herein. Before describing the various aspects of the present disclosure, the following definitions are provided and should be used unless otherwise indicated. Additional terms may be defined elsewhere in the present disclosure.

[0048] As used herein, “comprising” is interpreted as specifying the presence of the stated features, integers, steps, or components, but does not preclude the presence or addition of one or more features, integers, steps, components, or groups thereof. Moreover, each of the terms “by,” “comprising,” “comprises,” “comprised of,” “including,” “includes,” “included,” “involving,” “involves,” “involved,” and “such as” is used in its open, non-limiting sense and maybe used interchangeably. Further, the term “comprising” is intended to include examples and aspects encompassed by the terms “consisting essentially of ’ and “consisting of.” Similarly, “consisting essentially of’ is intended to include examples encompassed by the term “consisting of.” As used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context dictates otherwise.

[0049] Ratios, concentrations, amounts, and other numerical data can be expressed herein in a range format. Further, the endpoints of each range are significant both concerning the other endpoint and independently of the other endpoint. There are many values disclosed herein, and each value is also disclosed as “about” that particular value in addition to the value itself. For example, if the value “10” is disclosed, then “about 10” is also disclosed. Ranges can be expressed herein as from “about” one particular value and to “about” another particular value. Similarly, when values are expressed as approximations, using the antecedent “about,” the particular value forms a further aspect. For example, if the value “about 10” is disclosed, then “10” is also disclosed.

[0050] When a range is expressed, a further aspect includes from the one particular value and to the other particular value. For example, where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the disclosure, e.g., the phrase “x to y” includes the range from ‘x’ to ‘y’ as well as the range greater than ‘x’ and less than ‘y’. The range can also be expressed as an upper limit, e.g., ‘about x, y, z, or less’ and should be interpreted to include the specific ranges of ‘about x,’ ‘about y,’ and ‘about z’ as well as the ranges of ‘less than x,’ ‘less than y.’ and ‘less than z.’ Likewise, the phrase ‘about x, y, z, or greater’ should be interpreted to include the specific ranges of ‘about x,’ ‘about y,’ and ‘about z’ as well as the ranges of ‘greater than x,’ greater than y,’ and ‘greater than z.’ In addition, the phrase “about ‘x’ to ‘y’,” where ‘x’ and ‘y’ are numerical values, includes “about ‘x’ to about y.”

[0051] Such a range format is used for convenience and brevity and, thus, should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range but also to include all the individual numerical values or sub- ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. To illustrate, a numerical range of “about 0.1% to 5%” should be interpreted to include not only the explicitly recited values of about 0.1% to about 5% but also include individual values (e.g., about 1%, about 2%, about 3%, and about 4%) and the sub-ranges (e.g., about 0.5% to about 1.1%; about 5% to about 2.4%; about 0.5% to about 3.2%, and about 0.5% to about 4.4%, and other possible sub-ranges) within the indicated range.

[0052] As used herein, the terms “about,” “approximate,” “at or about,” and “substantially” mean that the amount or value in question can be the exact value or a value that provides equivalent results or effects as recited in the claims or taught herein. That is, amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact but may be approximate, larger or smaller, as desired, reflecting tolerances, conversion factors, rounding, measurement error, and the like, and other factors known to those of skill in the art such that equivalent results or effects are obtained. In some circumstances, the value that provides equivalent results or effects cannot be reasonably determined. In such cases, as used herein, “about” and “at or about” mean the nominal value indicated ±10% variation unless otherwise indicated or inferred. In general, an amount, size, formulation, parameter, or other quantity or characteristic is “about,” “approximate,” or “at or about,” whether or not expressly stated to be such. Where “about,” “approximate,” or “at or about” is used before a quantitative value, the parameter also includes the specific quantitative value itself unless expressly stated otherwise.

[0053] Compounds are described using standard nomenclature. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this disclosure belongs.

[0054] The compounds described herein include enantiomers, mixtures of enantiomers, diastereomers, tautomers, racemates, and other isomers, such as rotamers, as if each is specifically described, unless otherwise indicated or otherwise excluded by context. It is to be understood that the compounds provided herein may contain chiral centers. Such chiral centers may be of either the R) or (S) configuration. The compounds provided herein may either be enantiomerically pure or be diastereomeric or enantiomeric mixtures. Unless stated to the contrary, a formula with chemical bonds shown only as solid lines and not as wedges or dashed lines contemplates each possible isomer, e.g., each enantiomer, diastereomer, and meso compound, and a mixture of isomers, such as a racemic or scalemic mixture.

[0055] Compounds described herein may contain one or more double bonds and, thus, potentially give rise to cis / trans (E / Z) isomers and other conformational isomers. Unless stated to the contrary, all such possible isomers and mixtures of such isomers are contemplated.

[0056] Compounds described herein may also present as an equilibrium of tautomers. For example, ketones with an a-hydrogen can exist in an equilibrium of the keto and enol forms. Likewise, amides with an N-hydrogen can exist in an equilibrium of the amide form and the imidic acid form. Unless stated to the contrary, all possible tautomers of the compounds described herein are contemplated.

[0057] A dash (“-”) that is not between two letters or symbols indicates a point of attachment for a substituent. For example, -(C=0)NH2is attached through the carbon of the keto (C=O) group.

[0058] The term “substituted,” as used herein, means that any one or more hydrogens on the designated atom or group is replaced with a moiety selected from the indicated group, provided that the designated atom’s normal valence is not exceeded and the resulting compound is stable. For example, when the substituent is oxo (i.e., =0), two hydrogens on the atom are replaced. For example, a pyridyl group substituted by oxo is a pyridine. Combinations of substituents and / or variables are permissible only if such combinations result in stable compounds or useful synthetic intermediates. A stable active compound is a compound that can be isolated and / or formulated into a form with a shelf life of at least one month. A stable manufacturing intermediate or precursor to an active compound is stable if it does not degrade within the period needed for reaction or other use. A stable moiety or substituent group is one that does not degrade, react, or fall apart within the period necessary for use. Non-limiting examples of unstable moieties are those that combine heteroatoms in an unstable arrangement, as typically known and identifiable to those of skill in the art.

[0059] Any suitable group may be present on a “substituted” or “optionally substituted” position that forms a stable molecule and meets the desired purpose of the disclosure and includes, but is not limited to: halo, nitro, cyano, azido, oxo, Ci-Ce alkyl, Ci-Ce haloalkyl, C2-C6 alkenyl, C2-C6 alkynyl, (C3-C6 cycloalkyl) (C0-C3 alkyl)-, (3- to 8- membered monocyclic or bicyclic heterocycle)-(Co-C6 alkyl)-, (6- to 10-membered monocyclic or bicyclic aryl)-(Co-C6 alkyl)-, (5- to 10-membered monocyclic or bicyclic heteroaryl)-(Co-C6 alkyl)-, A O-(Co-C6 alkyl)-, A S-(Co-C6 alkyl)-, (A AyN)-(Co-C6 alkyl)-, AZC(O)-(CO-C6 alkyl)-, AZC(N)-(CO-C6 alkyl)-, and AzS(O)-(Co-C6 alkyl)-, and AZS(O)2-(CO-C6 alkyl)-, wherein Axand Av are independently selected at each occurrence from Aa, AZC(O)-, AZC(N)-, AZS(O)-, and AzS(0)2-, each of which maybe optionally substituted with one or more B groups as allowed by valency; wherein A is independently selected at each occurrence from hydrogen, halo, Ci-Cealkyl, Ci-Cohaloalkyl, C2-C6alkenyl, C2-C6alkynyl, (C3-C7cycloalkyl)-(Co-C3alkyl)-, (4- to 6-membered heterocycle)-(Co-C3alkyl)-, (5- to 10-membered monocyclic or bicyclic aryl)-(Co-C3alkyl)-, (5- to 10-membered monocyclic or bicyclic heteroaryl)-(Co-C3alkyl)-, - Aa, -SA“, and -NAaAb, each of which may be optionally substituted with one or more B groups as allowed by valency; wherein Aaand Abare independently selected at each occurrence from hydrogen, Ci-Cealkyl, Ci-Cehaloalkyl, C2-C6alkenyl, C2-C6alkynyl, (C3-C7cycloalkyl)-(CO-C3alkyl)-, (4- to 6-membered heterocycle)-(Co-C3alkyl)-, (5- to 10-membered monocyclic or bicyclic aryl)-(Co-C3alkyl)-, (5- to 10-membered monocyclic or bicyclic heteroaryl)-(Co-C3alkyl)-, each of which may be optionally substituted by one or more B groups as allowed by valency; and wherein B is independently selected at each occurrence from hydrogen, halo, nitro, cyano, azido, oxo, Ci-Ce alkyl, Ci-Ce haloalkyl, C2-Ce alkenyl, C2-Ce alkynyl, (C3-Ce cycloalkyl) (Co-C3alkyl)-, (3- to 8-membered monocyclic or bicyclic heterocycle)-(Co-Co alkyl)-, (6- to 10-membered monocyclic or bicyclic aryl)-(Co-C6 alkyl)-, (5- to 10-membered monocyclic or bicyclic heteroaryl)-(Co-C6 alkyl)-, AP -, APS-, A('N-, A°C(O)-, A°C(O)-O-, A°C(O)-NA^-, A°S(0)2-, A°S(0)2-0-, and A°S(0)2-NA<*-, wherein A° is independently selected at each occurrence from AP, halo, APO-, and APAIN-, and wherein AP and A^ are independently selected at each occurrence from hydrogen, Ci-Ce alkyl, Ci-Ce haloalkyl, C2-Ce alkenyl, C2-Ce alkynyl, (C3-Ce cycloalkyl) (Co-C3alkyl)-, (3- to 8-membered monocyclic or bicyclic heterocycle)-(Co-Co alkyl)-, (6- to 10-membered monocyclic or bicyclic aryl)-(Co-C6 alkyl)-, and (5- to 10-membered monocyclic or bicyclic heteroaryl)-(Co-C6 alkyl)-.

[0060] The terms for various functional groups as used herein are not intended to be limited to monovalent radicals and may include polyvalent radical groups as appropriate, such as divalent, trivalent, tetravalent, pentavalent, and hexavalent groups, and the like, based on the position and location of such groups in the compounds described herein as would be readily understood by the skilled person in the context in which said functional groups are recited.

[0061] As used herein, the term “allowed by valency” in the context of a substituent means that any substituent maybe present at a position on a moiety, provided that the total number, type, and arrangement of bonds formed by that substituent on the moiety do not exceed its established valency according to recognized principles of chemical bonding. Specifically, substituents may be incorporated at a site only insofar as the resulting structure adheres to the maximum number of bonds and electronic requirements compatible with the valency of the constituent atoms. In a representative aspect, a substituent on a moiety is allowed by valency when the substituent replaces a hydrogen atom on the atom or moiety.

[0062] As used herein, the symbol “

[0063]

[0064] ” (which hereinafter can be referred to as “a point of attachment bond”) denotes a bond that is a point of attachment between two chemical entities, one of which is depicted as being attached to the point of attachment bond and the other of which is not depicted as being attached to the point XY - i

[0065] of attachment bond. For example, “? ” indicates that the chemical entity “XY” is bonded to another chemical entity via the point of attachment bond. Furthermore, the specific attachment point to the non-depicted chemical entity can be specified by XY - i inference. For example, the compound CH3-R3, wherein R3 is H or “ « ” infers that when R3 is “XY”, the point of attachment bond is the same bond as the bond by which R3 is depicted as being bonded to CH3.

[0066] As used herein, the designation of a polyvalent moiety without specifying the specific order of attachment is intended to cover all possible arrangements. By way of example, a compound represented by the formula:

[0067] A-X-B,

[0068] wherein X is NHC(=O) embraces both:

[0069] O O JL B A JI

[0070] and

[0071] “Halo” or “halogen” independently indicates any of fluoro, chloro, bromo, or iodo. The term “nitro” as used herein, is represented by the formula — N02.

[0072] The term “cyano” as used herein, is represented by the formula — CN

[0073] The term “azido” as used herein, is represented by the formula -N3.

[0074] The term “oxo” as used herein, is represented by the formula =0.

[0075] “Alkyl” is a straight chain or branched saturated aliphatic hydrocarbon group. In certain aspects, the alkyl is C1-C2, C1-C3, or Ci-Ce (i.e., the alkyl chain can be 1, 2, 3, 4, 5, or 6 carbons in length). The specified ranges used herein indicate an alkyl group with a length of each range member described as an independent species. For example, Ci-Cealkyl as used herein, indicates an alkyl group having 1, 2, 3, 4, 5, or 6 carbon atoms and is intended to mean that each of these is described as an independent species, and Ci-C4alkyl as used herein, indicates an alkyl group having from 1, 2, 3, or 4 carbon atoms and is intended to mean that each of these is described as an independent species. When Co-Cnalkyl is used herein in conjunction with another group, for example (C3-C7cycloalkyl)Co-C4alkyl, or -Co-C4(C3-C7cycloalkyl), the indicated group, in this case cycloalkyl, is either directly bound by a single covalent bond (Coalkyl), or attached by an alkyl chain, in this case 1, 2, 3, or 4 carbon atoms. Alkyls can also be attached via other groups such as heteroatoms, as in -O-Co-C4alkyl(C3-C7cycloalkyl). Examples of alkyl include but are not limited to, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, t-butyl, n-pentyl, isopentyl, tert-pentyl, neopentyl, n-hexyl, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane. In some aspects, the alkyl group is optionally substituted as described herein.

[0076] “Haloalkyl” refers to an alkyl group that is substituted with one or more halo groups, e.g., fluoro, chloro, bromo, iodo, or combinations thereof.

[0077] “Cycloalkyl” is a saturated or partially unsaturated mono- or multi-cyclic hydrocarbon ring system. When composed of two or more rings, the rings may be joined together in a fused or bridged fashion. Non-limiting examples of typical cycloalkyl groups include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, and cycloheptyl. In some aspects, the cycloalkyl group is optionally substituted as described herein.

[0078] “Alkenyl” is a straight or branched chain aliphatic hydrocarbon group having one or more carbon-carbon double bonds, each of which is independently either cis or trans, that may occur at a stable point along the chain. Non-limiting examples include C2-C4alkenyl and C2-C6alkenyl (i.e., having 2, 3, 4, 5, or 6 carbons). The specified ranges used herein indicate an alkenyl group having each member of the range described as an independent species, as described above for the alkyl moiety. Examples of alkenyl include but are not limited to, ethenyl and propenyl. In one aspect, the alkenyl group is optionally substituted as described herein.

[0079] “Alkynyl” is a straight or branched chain aliphatic hydrocarbon group having one or more carbon-carbon triple bonds that may occur at any stable point along the chain, for example, C2-C4alkynyl or Ch-Cealkynyl (i.e., having 2, 3, 4, 5, or 6 carbons). The specified ranges used herein indicate an alkynyl group, with each member of the range described as an independent species, as described above for the alkyl moiety. Examples of alkynyl include, but are not limited to, ethynyl, propynyl, 1-butynyl, 2-butynyl, 3-butynyl, 1-pentynyl, 2-pentynyl, 3-pentynyl, 4-pentynyl, 1-hexynyl, 2-hexynyl, 3-hexynyl, 4-hexynyl, and 5-hexynyl. In one aspect, the alkynyl group is optionally substituted as described herein.

[0080] “Aryl” indicates an aromatic group containing only carbon in the aromatic ring or rings. In one aspect, the aryl group contains 1 to 3 separate or fused rings and is 6 to 14 or 18 ring atoms, without heteroatoms as ring members. Such aryl groups may be further substituted with carbon or non-carbon atoms or groups when indicated. Such substitution may include the fusion to a 4- to 7- or 5- to 7-membered saturated or partially unsaturated cyclic group that optionally contains 1, 2, or 3 heteroatoms independently selected from N, O, B, P, Si, and S, to form, for example, a 3,4-methylenedioxyphenyl group. Aryl groups include, for example, phenyl and naphthyl, including 1-naphthyl and 2-naphthyl. In one aspect, aryl groups are pendant. An example of a pendant ring is a phenyl group substituted with a phenyl group. In one aspect, the aryl group is optionally substituted as described herein. The term “heterocycle” refers to saturated and partially saturated heteroatomcontaining ring radicals, where the heteroatoms may be selected from N, O, and S. The term heterocycle includes monocyclic 3-12-membered rings, as well as bicyclic 5-16 membered ring systems (which can include fused, bridged, or spiro bicyclic ring systems). It does not include rings containing -O-O-, -O-S-, and -S-S- portions. Examples of saturated heterocycle groups include saturated 4- to 7-membered monocyclic groups containing 1 to 4 nitrogen atoms [e.g., pyrrolidinyl, imidazolidinyl, piperidinyl, pyrrolinyl, azetidinyl, piperazinyl, and pyrazolidinyl]; saturated 4- to 6-membered monocyclic groups containing 1 to 2 oxygen atoms and 1 to 3 nitrogen atoms [e.g., morpholinyl]; and saturated 3- to 6-membered heteromonocyclic groups containing 1 to 2 sulfur atoms and 1 to 3 nitrogen atoms [e.g., thiazolidinyl]. Examples of partially saturated heterocycle radicals include, but are not limited, dihydrothienyl, dihydropyranyl, dihydrofuryl, and dihydrothiazolyl. Examples of partially saturated and saturated heterocycle groups include, but are not limited to, pyrrolidinyl, imidazolidinyl, piperidinyl, pyrrolinyl, pyrazolidinyl, piperazinyl, morpholinyl, tetrahydropyranyl, thiazolidinyl, dihydrothienyl, 2,3-dihydro-benzo[i,4]dioxanyl, indolinyl, isoindolinyl, dihydrobenzothienyl, dihydrobenzofuryl, isochromanyl, chromanyl, 1,2-dihydroquinolyl, 1, 2,3,4-tetrahydro-isoquinolyl, 1,2,3,4-tetrahydro-quinolyl, 2,3,4,4a,9,9a-hexahydro-iH-3-aza-fluorenyl, 5,6,7-trihydro-i,2,4-triazolo[3,4-a]isoquinolyl, 3,4-dihydro-2H-benzo[i,4]oxazinyl, benzo[i,4]dioxanyl, 2,3,-dihydro-iH-benzo[d]isothazol-6-yl, dihydropyranyl, dihydrofuryl, and dihydrothiazolyl. Bicyclic heterocycle includes groups wherein the heterocyclic radical is fused with an aryl radical, and the attachment point is the heterocycle ring. Bicyclic heterocycle also includes heterocyclic radicals that are fused with a carbocyclic radical. Representative examples include but are not limited to, partially unsaturated condensed heterocyclic groups containing 1 to 5 nitrogen atoms, for example, indoline and isoindoline, partially unsaturated condensed heterocyclic groups containing 1 to 2 oxygen atoms and 1 to 3 nitrogen atoms, partially unsaturated condensed heterocyclic groups containing 1 to 2 sulfur atoms and 1 to 3 nitrogen atoms, and saturated condensed heterocyclic groups containing 1 to 2 oxygen or sulfur atoms. In one aspect, the heterocycle group is optionally substituted as described herein.

[0081] “Heteroaryl” refers to a stable monocyclic, bicyclic, or multicyclic aromatic ring that contains from 1 to 4, or in some aspects 1, 2, or 3 heteroatoms selected from N, O, S, B, and P (and typically selected from N, O, and S) with remaining ring atoms being carbon, or a stable bicyclic or tricyclic system containing at least one 5, 6, or 7 membered aromatic ring which contains from 1 to 4, or in some aspects from 1 to 3 or from 1 to 2, heteroatoms selected from N, O, S, B, or P, with remaining ring atoms being carbon. In one aspect, the only heteroatom is nitrogen. In one aspect, the only heteroatom is oxygen. In one aspect, the only heteroatom is sulfur. Monocyclic heteroaryl groups typically have from 5 to 6 ring atoms. In some aspects, bicyclic heteroaryl groups are 8- to 10-membered heteroaryl groups, that is groups containing 8 or 10 ring atoms in which one 5-, 6-, or 7-membered aromatic ring which contains from 1 to 4 heteroatoms selected from N, O, S, B, or P is fused to a second aromatic or non-aromatic ring, wherein the point of attachment is an aromatic ring. When the total number of S and O atoms in the heteroaryl ring exceeds 1, these heteroatoms are not adjacent to one another within the ring. In one aspect, the total number of S and O atoms in the heteroaryl ring is not more than 2. In another aspect, the total number of S and O atoms in the heteroaryl ring is not more than 1. Examples of heteroaryl groups include, but are not limited to, pyridinyl, imidazolyl, imidazopyridinyl, pyrimidinyl, pyrazolyl, triazolyl, pyrazinyl, furyl, thienyl, isoxazolyl, thiazolyl, oxadiazolyl, oxazolyl, isothiazolyl, pyrrolyl, quinolinyl, isoquinolinyl, tetrahydroisoquinolinyl, indolyl, benzimidazolyl, benzofuranyl, cinnolinyl, indazolyl, indolizinyl, phthalazinyl, pyridazinyl, triazinyl, isoindolyl, pteridinyl, purinyl, triazolyl, thiadiazolyl, furazanyl, benzofurazanyl, benzothiophenyl, benzothiazolyl, benzoxazolyl, quinazolinyl, quinoxalinyl, naphthyridinyl, and furopyridinyl. In one aspect, the heteroaryl group is optionally substituted as described herein.

[0082] Certain materials, compounds, compositions, and components disclosed herein can be obtained commercially or readily synthesized using techniques generally known to those of skill in the art. For example, the starting materials and reagents used in preparing the disclosed compounds and compositions are either available from commercial suppliers, such as Sigma-Aldrich (formally MilliporeSigma, Burlington, MA) or Thermo Fisher Scientific Inc. (Waltham, MA), or are prepared by methods known to those skilled in the art following procedures set forth in references such as Fieser and Fieser's Reagents for Organic Synthesis (John Wiley and Sons, 2007); Organic Reactions (John Wiley and Sons, 2004); March's Advanced Organic Chemistry, (John Wiley and Sons, 8thEdition); and Larock's Comprehensive Organic Transformations (John Wiley and Sons, 3rdedition, 2017).

[0083] The present disclosure provides hydrogels capable of selectively adsorbing phosphate from a water source upon contact with the hydrogel. The disclosed hydrogels are capable of desorbing the phosphate when exposed to alkaline conditions, allowing for the potential recycling of the phosphate. In some aspects, the hydrogel can be formed from a first polymer and a second polymer. In some aspects, the first polymer can include a plurality of amine moieties. In some aspects, the second polymer can include a plurality of cyclic acid anhydride moieties.

[0084] In some aspects, the hydrogel is crosslinked. In some aspects, the second polymer is crosslinked by the first polymer. In other aspects, the first polymer is crosslinked by the second polymer. While not wishing to be bound by any one theory, crosslinking between the first polymer and second polymer may occur by reaction of a portion of the amine moieties of the first polymer with a portion of the cyclic acid anhydride moieties of the second monomer. For example, crosslinking may occur via the formation of amide and / or cyclic imide linkages from the amine and cyclic acid anhydride moieties.

[0085] In some aspects, the first polymer includes a polyalkylamine. In some aspects, the first polymer includes polyethyleneimine (PEI). In some aspects, the first polymer includes branched PEI. In other aspects, the first polymer includes linear PEI.

[0086] In some aspects, the second polymer includes a copolymer of at least one cyclic anhydride and one or more additional monomers. In some aspects, the copolymer is an alternating copolymer. In other aspects, the copolymer is a statistical or random copolymer.

[0087] As used herein, a “cyclic acid anhydride” or “cyclic anhydride” refers to an acid anhydride derived by the loss of water between two oxoacid functional groups (e.g., carboxylic, sulfonic, etc.) in the same molecule so as to close a ring. An “acid anhydride” as used herein refers to a moiety and / or compound having two acyl groups, either the same or different, bonded to the same oxygen atom.

[0088] As used herein, the cyclic acid anhydride includes at least one polymerizable moiety that differs from the acid anhydride moiety. In some aspects, the at least one polymerizable moiety includes an alkenyl moiety. In some aspects, the cyclic acid anhydride includes an acid anhydride moiety and an alkenyl moiety. A representative example of a cyclic anhydride as may be used herein includes, but is not limited to, succinic anhydride.

[0089] In aspects where the second polymer is a copolymer, the one or more additional monomers may include any suitable monomer. In some aspects, the one or more additional monomers may include a monomer having one or more alkenyl moieties. In some aspects, the one or more additional monomers include at least one hydrophilic monomer.

[0090] In some aspects, the one or more additional monomers include a vinyl ether or a vinyl ester. In some aspects, the one or more additional monomers include an alkyl vinyl ether. Suitable examples of such monomers include, but are not limited to, methyl vinyl ether and ethyl vinyl ether. Further examples include vinyl acetate or vinyl propionate.

[0091] In some aspects, the second polymer includes a constitutional repeating unit having the structure:

[0092]

[0093] wherein RAis selected from hydrogen, RXO-, and RZC(O)-;

[0094] Rxis independently selected at each occurrence from Ra, RZC(O)-, RZC(N)-, RZS(O)-, and RZS(0)2-, each of which maybe optionally substituted with one or more Y groups as allowed by valency;

[0095] Rzis independently selected at each occurrence from hydrogen, halo, Ci-Cealkyl, Ci-Cohaloalkyl, C2-C6alkenyl, C2-C6alkynyl, (C3-C7cycloalkyl)-(Co-C3alkyl)-, (4- to 6-membered heterocycle)-(Co-C3alkyl)-, (5- to 10-membered monocyclic or bicyclic aryl)-(Co-C3alkyl)-, (5- to 10-membered monocyclic or bicyclic heteroaryl)-(Co-C3alkyl)-, -ORa, -SRa, and -NRaRb, each of which maybe optionally substituted with one or more Y groups as allowed by valency; and

[0096] Raand Rbare independently selected at each occurrence from hydrogen, Ci-Cealkyl, Ci-Cehaloalkyl, C2-C6alkenyl, Cb-Cealkynyl, (C3-C7cycloalkyl)-(CO-C3alkyl)-, (4- to 6-membered heterocycle)-(Co-C3alkyl)-, (5- to 10-membered monocyclic or bicyclic aryl)-(Co-C3alkyl)-, (5- to 10-membered monocyclic or bicyclic heteroaryl)-(Co-C3alkyl)-, each of which may be optionally substituted by one or more Y groups as allowed by valency; and Y is independently selected at each occurrence from hydrogen, halo, nitro, cyano, azido, oxo, Ci-Ce alkyl, Ci-Ce haloalkyl, C2-C6 alkenyl, C2-C6 alkynyl, (C3-C6 cycloalkyl) (C0-C3 alkyl)-, (3- to 8-membered monocyclic or bicyclic heterocycle)-(Co-Co alkyl)-, (6- to 10-membered monocyclic or bicyclic aryl)-(Co-C6 alkyl)-, (5- to 10-membered monocyclic or bicyclic heteroaryl)-(Co-C6 alkyl)-, RPO-, RPS-, RPR^N-, R°C(O)-, R°C(O)-O-, R°C(O)-NRq-, R°S(0)2-, R°S(0)2-0-, and R°S(0)2-NRq-, wherein R° is independently selected at each occurrence from RP, halo, RPO-, and RpRqN-, and wherein RP and Rq are independently selected at each occurrence from hydrogen, Ci-Ce alkyl, Ci-Ce haloalkyl, C2-C6 alkenyl, C2-C6 alkynyl, (C3-C6 cycloalkyl) (C0-C3 alkyl)-, (3- to 8-membered monocyclic or bicyclic heterocycle)-(Co-Co alkyl)-, (6- to 10-membered monocyclic or bicyclic aryl)-(Co-C6 alkyl)-, and (5- to 10-membered monocyclic or bicyclic heteroaryl)-(Co-C6 alkyl)-.

[0097] In some aspects, RAis hydrogen.

[0098] In some aspects, RAis Rx0-.

[0099] In some aspects, RAis Rx0-, wherein Rxis selected from hydrogen, methyl, ethyl, isopropyl, trifluoromethyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, and phenyl, each of which may be optionally substituted by one or more (such as 1, 2, 3, 4, or 5) Y groups as allowed by valency.

[0100] In some aspects, RAis Rx0-, wherein Rxis RZC(O)-, wherein Rzis selected from Rzis selected from hydrogen, chloro, bromo, -OH, -NH2, methyl, ethyl, isopropyl, trifluoromethyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, and phenyl, each of which maybe optionally substituted by one or more (such as 1, 2, 3, 4, or 5) Y groups as allowed by valency.

[0101] In some aspects, RAis RZC(O)-. In some aspects, RAis RZC(O)-, wherein Rzis selected from Rzis selected from hydrogen, chloro, bromo, -OH, -NH2, methyl, ethyl, isopropyl, trifluoromethyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, and phenyl, each of which may be optionally substituted by one or more (such as 1, 2, 3, 4, or 5) Y groups as allowed by valency.

[0102] In some aspects, the second polymer can include poly(methyl vinyl ether-alt -maleic anhydride) (PMVEMA).

[0103] In some aspects, a ratio of a first repeating unit of the first polymer to a second repeating unit of the second polymer is from about 6:1 to about 1:6, including exemplary values of about 6:1, about 5.5:1, about 5:1, about 4.5:1, about 4:1, about 3.5:1, about 3:1, about 2.5:1, about 2:1, about 1.5:1, about 1:1, about 1:1.5, about 1:2, about 1:2.5, about 1:3, about 1:3.5, about 1:4, about 1:4.5, about 1:5, about 1:5.5, about 1:6, or any subrange formed from the above exemplary values. In particular aspects, a ratio of a first repeating unit of the first polymer to a second repeating unit of the second polymer is about 3:1.

[0104] The hydrogels described are capable of selective adsorption of phosphate from water under neutral to acidic conditions. As used herein, “adsorption” refers to the transfer of a material from a liquid phase onto to a solid surface by formation of a physical and / or chemical bond with said surface. As used herein, “selective adsorption” refers to preferential adsorption of phosphate by the hydrogel as compared to one or more other anions present in the water, e.g., having a selectivity factor of greater than one for phosphate over the one or more other anions. In some aspects, the hydrogel is capable of selective adsorption of phosphate from water in contact with the hydrogel at a pH from about 2.0 to about 7.0. In some aspects, the hydrogels described herein are capable of selective adsorption of phosphate over nitrate from water. In some aspects, the hydrogel is capable of selective adsorption of phosphate over nitrate from water when the mass ratio of nitrate to phosphate in the water in contact with the hydrogel is less than about 2.5.

[0105] The hydrogels described herein are further capable of desorption of any bound phosphate upon exposure of the hydrogel to alkaline conditions. As used herein, “desorption” refers to the release of an adsorbed material from a solid surface into a liquid phase. In some aspects, the hydrogel is capable of desorption of phosphate from the hydrogel at a pH of about 9 to about 11. Desorption of phosphate from hydrogel can occur when the hydrogel is contacted with a hydroxide solution, for example, such as a sodium hydroxide or potassium hydroxide solution. In some particular aspects, the hydroxide solution is a potassium hydroxide solution, upon which potassium phosphate may be isolated. Isolating phosphate from the hydrogel as potassium salts may be particularly advantageous with respect to recycling of the phosphate, as this is the preferred salt form for applications such as agriculture. In another aspect, a water filtration system is provided. Such a water filtration system may include a hydrogel as described herein. Based on the properties of the hydrogels found therein, such water filtration systems would be capable of removing phosphate from a water source via adsorption. Further, phosphate adsorbed to the hydrogel in the water filtration system would be optionally desorbed and recycled.

[0106] Any implementation of water filtration system capable of housing a sorbent, such as the hydrogels described herein, is suitable according to the present disclosure. One representative example of a suitable implementation of the water filtration system includes a filter column. Another representative example of a suitable implementation includes a flow bed.

[0107] In a further aspect, methods are provided for removing phosphate from a water source. In some aspects, the method can include contacting the water source with a hydrogel as described herein.

[0108] Any water source containing (e.g., contaminated by) phosphate may be used in the described methods. Representative examples may include waterways, such as but not limited to rivers, lakes, or streams, springs, reservoirs, aquifers, sewage, or groundwater supplies or impoundments. In particular examples, the water source may include agricultural runoff, e.g., runoff contaminated with commonly used fertilizers, or a water source as described above that has been combined with or contaminated by agricultural runoff.

[0109] In some aspects, phosphate can be selectively removed over nitrate from the water source using the methods described herein.

[0110] In some aspects, contacting the water source with the hydrogel includes filtering the water source through a water filtration system including the hydrogel. This may be performed using any suitable water filtration system as described herein.

[0111] In some aspects, the methods described herein may further include contacting the hydrogel with a hydroxide solution, whereupon the phosphate is desorbed from the hydrogel. Thus, phosphate that has been removed by a water source using the described methods may be subsequently isolated and recycled for further use, such as but not limited to fertilizer.

[0112] In view of the described compounds, compositions, devices, and methods, certain more particular aspects of the disclosure are described below. These particularly recited aspects should not, however, be interpreted to have any limiting effect on any different claims containing different or more general teachings described herein, or that the “particular” aspects are somehow limited in some way other than the inherent meanings of the language and formulae literally used therein.

[0113] Aspects A

[0114] Aspect Al. A hydrogel formed from:

[0115] a first polymer including a plurality of amine moieties; and

[0116] a second polymer including a plurality of cyclic acid anhydride moieties.

[0117] Aspect A2. The hydrogel of any aspect herein, such as aspect Al,

[0118] wherein the second polymer is crosslinked by the first polymer.

[0119] Aspect A3. The hydrogel of any aspect herein, such as aspect Al or aspect A2, wherein the first polymer includes polyethyleneimine (PEI).

[0120] Aspect A4. The hydrogel of any aspect herein, such as any one of aspects A1-A3, wherein the first polymer includes branched polyethyleneimine (PEI).

[0121] Aspect A5. The hydrogel of any aspect herein, such as any one of aspects A1-A4, wherein the second polymer includes a copolymer of at least one cyclic anhydride and one or more additional monomers.

[0122] Aspect A6. The hydrogel of any aspect herein, such as aspect A5, wherein the copolymer is an alternating copolymer.

[0123] Aspect A7. The hydrogel of any aspect herein, such as aspect A5 or aspect A6, wherein the cyclic anhydride includes succinic anhydride.

[0124] Aspect A8. The hydrogel of any aspect herein, such as any one of aspects A5-A7, wherein the one or more additional monomers include at least one hydrophilic monomer.

[0125] Aspect A9. The hydrogel of any aspect herein, such as any one of aspects A5-A8, wherein the one or more additional monomers include an alkyl vinyl ether.

[0126] Aspect A10. The hydrogel of any aspect herein, such as aspect A9, wherein the alkyl vinyl ether includes methyl vinyl ether or ethyl vinyl ether.

[0127] Aspect All. The hydrogel of any aspect herein, such as any one of aspects A1-A10, wherein the second polymer includes poly(methyl vinyl ether-alt-maleic anhydride) (PMVEMA). Aspect A12. The hydrogel of any aspect herein, such as any one of aspects A1-A11, wherein a ratio of a first repeating unit of the first polymer to a second repeating unit of the second polymer is from about 6:1 to about 1:6.

[0128] Aspect A13. The hydrogel of any aspect herein, such as any one of aspects A1-A12, wherein a ratio of a first repeating unit of the first polymer to a second repeating unit of the second polymer is about 3:1.

[0129] Aspect A14. The hydrogel of any aspect herein, such as any one of aspects A1-A13, wherein the hydrogel is capable of selective adsorption of phosphate from water in contact with the hydrogel at a pH from about 2.0 to about 7.0.

[0130] Aspect A15. The hydrogel of any aspect herein, such as any one of aspects A1-A14, wherein the hydrogel is capable of selective adsorption of phosphate over nitrate from water when the mass ratio of nitrate to phosphate in the water in contact with the hydrogel is less than about 2.5.

[0131] Aspect A16. The hydrogel of any aspect herein, such as any one of aspects A1-A15, wherein the hydrogel is capable of desorption of phosphate from the hydrogel at a pH of about 9 to about 11.

[0132] Aspect A17. The hydrogel of any aspect herein, such as aspect A16, wherein the hydrogel is capable of desorption of phosphate from the hydrogel when contacted with a hydroxide solution (such as a sodium hydroxide or potassium hydroxide solution).

[0133] Aspect A17. A water filtration system including the hydrogel of any aspect herein, such as any one of aspects A1-A16.

[0134] Aspect A18. The water filtration system of any aspect herein, such as aspect A17, wherein the water filtration system includes a filter column or a flow bed.

[0135] Aspect A19. A method for removing phosphate from a water source, the method including contacting the water source with the hydrogel of any aspect herein, such as any one of aspects A1-A16.

[0136] Aspect A20. The method of any aspect herein, such as aspect A19, wherein the water source includes a waterway (such as a river, lake, or stream) or agricultural runoff. Aspect A21. The method of any aspect herein, such as aspect A19 or aspect A20, wherein phosphate is selectively removed over nitrate from the water source. Aspect A22. The method of any one of aspects A19-A21, wherein contacting the water source with the hydrogel includes filtering the water source through a water filtration system including the hydrogel.

[0137] Aspect A23. The method of any aspect herein, such as any one of aspects A19-A22, further including contacting the hydrogel with a hydroxide solution, whereupon the phosphate is desorbed from the hydrogel.

[0138] Additional Aspects B

[0139] Aspect Bi. A hydrogel formed from:

[0140] a first polymer including a plurality of amine moieties; and

[0141] a second polymer including a plurality of cyclic acid anhydride moieties. Aspect B2. The hydrogel of any aspect herein, such as aspect Bi,

[0142] wherein the second polymer is crosslinked by the first polymer.

[0143] Aspect B3. The hydrogel of any aspect herein, such as aspect 1 or aspect B2, wherein the first polymer includes a polyalkylamine.

[0144] Aspect B4. The hydrogel of any aspect herein, such as any one of aspects B1-B3, wherein the first polymer includes polyethyleneimine (PEI).

[0145] Aspect B5. The hydrogel of any aspect herein, such as any one of aspects B1-B4, wherein the first polymer includes branched polyethyleneimine (PEI).

[0146] Aspect B6. The hydrogel of any aspect herein, such as any one of aspects B1-B4, wherein the first polymer includes linear polyethyleneimine (PEI).

[0147] Aspect B7. The hydrogel of any aspect herein, such as any one of aspects B1-B6, wherein the second polymer includes a copolymer of at least one cyclic anhydride and one or more additional monomers.

[0148] Aspect B8. The hydrogel of any aspect herein, such as aspect B7, wherein the copolymer is an alternating copolymer.

[0149] Aspect B9. The hydrogel of any aspect herein, such as aspect B7, wherein the copolymer is a statistical or random copolymer.

[0150] Aspect Bio. The hydrogel of any aspect herein, such as any one of aspects B7-B9, wherein the at least one cyclic anhydride includes succinic anhydride. Aspect Bn. The hydrogel of any aspect herein, such as any one of aspects B7-B10, wherein the one or more additional monomers include a monomer having one or more alkenyl moieties.

[0151] Aspect B12. The hydrogel of any aspect herein, such as any one of aspects B7-B11, wherein the one or more additional monomers include a vinyl ether or a vinyl ester. Aspect B13. The hydrogel of any aspect herein, such as any one of aspects B7-B12, wherein the one or more additional monomers include an alkyl vinyl ether.

[0152] Aspect B14. The hydrogel of any aspect herein, such as any one of aspects B7-B13, wherein the one or more additional monomers include methyl vinyl ether.

[0153] Aspect B15. The hydrogel of any aspect herein, such as any one of aspects B7-B14, wherein the one or more additional monomers include ethyl vinyl ether.

[0154] Aspect B16. The hydrogel of any aspect herein, such as any one of aspects B7-B15, wherein the one or more additional monomers include vinyl acetate or vinyl propionate.

[0155] Aspect B17. The hydrogel of any aspect herein, such as any one of aspects B7-B16, wherein the second polymer includes a constitutional repeating unit having the structure:

[0156]

[0157] wherein RAis selected from hydrogen, RXO-, and RZC(O)-;

[0158] Rxis independently selected at each occurrence from Ra, RZC(O)-, RZC(N)-, RZS(O)-, and RZS(0)2-, each of which maybe optionally substituted with one or more Y groups as allowed by valency;

[0159] Rzis independently selected at each occurrence from hydrogen, halo, Ci-Cealkyl, Ci-Cohaloalkyl, C2-C6alkenyl, C2-C6alkynyl, (C3-C7cycloalkyl)-(Co-C3alkyl)-, (4- to 6-membered heterocycle)-(Co-C3alkyl)-, (5- to 10-membered monocyclic or bicyclic aryl)-(Co-C3alkyl)-, (5- to 10-membered monocyclic or bicyclic heteroaryl)-(Co-C3 alkyl)-, -0Ra, -SRa, and -NRaRb, each of which maybe optionally substituted with one or more Y groups as allowed by valency; and

[0160] Raand Rbare independently selected at each occurrence from hydrogen, Ci-Cealkyl, Ci-Cehaloalkyl, C2-C6alkenyl, C2-C6alkynyl, (C3-C7cycloalkyl)-(Co-C3alkyl)-, (4- to 6-membered heterocycle)-(Co-C3alkyl)-, (5- to 10-membered monocyclic or bicyclic aryl)-(Co-C3alkyl)-, (5- to 10-membered monocyclic or bicyclic heteroaryl)-(Co-C3alkyl)-, each of which may be optionally substituted by one or more Y groups as allowed by valency; and

[0161] Y is independently selected at each occurrence from hydrogen, halo, nitro, cyano, azido, oxo, Ci-Ce alkyl, Ci-Ce haloalkyl, C2-C6 alkenyl, C2-C6 alkynyl, (C3-Ce cycloalkyl) (Co-C3alkyl)-, (3- to 8-membered monocyclic or bicyclic heterocycle)-(Co-Co alkyl)-, (6- to 10-membered monocyclic or bicyclic aryl)-(Co-C6 alkyl)-, (5- to 10-membered monocyclic or bicyclic heteroaryl)-(Co-C6 alkyl)-, RPO-, RPS-, RPR^N-, R°C(O)-, R°C(O)-O-, R°C(O)-NRq-, R°S(0)2-, R°S(0)2-0-, and R°S(0)2-NRq-, wherein R° is independently selected at each occurrence from RP, halo, RPO-, and RpRqN-, and wherein RP and Rc' are independently selected at each occurrence from hydrogen, Ci-Ce alkyl, Ci-Ce haloalkyl, C2-C6 alkenyl, C2-C6 alkynyl, (C3-Ce cycloalkyl) (Co-C3alkyl)-, (3- to 8-membered monocyclic or bicyclic heterocycle)-(Co-Co alkyl)-, (6- to 10-membered monocyclic or bicyclic aryl)-(Co-C6 alkyl)-, and (5- to 10-membered monocyclic or bicyclic heteroaryl)-(Co-C6 alkyl)-.

[0162] Aspect B18. The hydrogel of any aspect herein, such as aspect B17, wherein RAis RXO-.

[0163] Aspect B19. The hydrogel of any aspect herein, such as aspect B18, wherein Rxis Ci-Cc> alkyl.

[0164] Aspect B20. The hydrogel of any aspect herein, such as aspect B18, wherein Rxis RZC(O)-.

[0165] Aspect B21. The hydrogel of any aspect herein, such as aspect B20, wherein Rzis Ci-Cc> alkyl.

[0166] Aspect B22. The hydrogel of any aspect herein, such as any one of aspects B7-B21, wherein the one or more additional monomers include at least one hydrophilic monomer. Aspect B23. The hydrogel of any aspect herein, such as any one of aspects B1-B22, wherein the second polymer includes poly(methyl vinyl ether-alt-maleic anhydride) (PMVEMA).

[0167] Aspect B24. The hydrogel of any aspect herein, such as any one of aspects B1-B23, wherein a ratio of a first repeating unit of the first polymer to a second repeating unit of the second polymer is from about 6:1 to about 1:6.

[0168] Aspect B25. The hydrogel of any aspect herein, such as any one of aspects B1-B24, wherein a ratio of a first repeating unit of the first polymer to a second repeating unit of the second polymer is about 3:1.

[0169] Aspect B26. The hydrogel of any aspect herein, such as any one of aspects B1-B25, wherein the hydrogel is capable of selective adsorption of phosphate from water in contact with the hydrogel at a pH from about 2.0 to about 7.0.

[0170] Aspect B27. The hydrogel of any aspect herein, such as any one of aspects B1-B26, wherein the hydrogel is capable of selective adsorption of phosphate over nitrate from water when a mass ratio of nitrate to phosphate in the water in contact with the hydrogel is less than about 2.5.

[0171] Aspect B28. The hydrogel of any aspect herein, such as any one of aspects B1-B27, wherein the hydrogel is capable of desorption of phosphate from the hydrogel at a pH of about 9 to about 11.

[0172] Aspect B29. The hydrogel of any aspect herein, such as aspect B28, wherein the hydrogel is capable of desorption of phosphate from the hydrogel when contacted with a hydroxide solution (such as a sodium hydroxide or potassium hydroxide solution).

[0173] Aspect B30. A water filtration system including the hydrogel of any aspect herein, such as any one of aspects B1-B29.

[0174] Aspect B31. The water filtration system of any aspect herein, such as aspect B30, wherein the water filtration system includes a filter column or a flow bed.

[0175] Aspect B32. A method for removing phosphate from a water source, the method including contacting the water source with the hydrogel of any aspect herein, such as any one of aspects B1-B29. Aspect B33. The method of any aspect herein, such as aspect B32, wherein the water source includes a waterway (such as a river, lake, or stream) or agricultural runoff. Aspect B34. The method of any aspect herein, such as aspect B32 or aspect B33, wherein phosphate is selectively removed over nitrate from the water source.

[0176] Aspect B35. The method of any aspect herein, such as any one of aspects B32-B34, wherein contacting the water source with the hydrogel includes filtering the water source through a water filtration system including the hydrogel.

[0177] Aspect B36. The method of any aspect herein, such as any one of aspects B32-B35, further including contacting the hydrogel with a hydroxide solution, whereupon the phosphate is desorbed from the hydrogel.

[0178] A number of aspects of the disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other aspects are within the scope of the following claims.

[0179] By way of non-limiting illustration, examples of certain aspects of the present disclosure are given below.

[0180] EXAMPLES

[0181] The following examples are set forth below to illustrate the compounds, compositions, articles, devices, and methods claimed herein, along with associated methods and results according to the disclosed subject matter. These examples are not intended to be inclusive of all aspects of the subject matter disclosed herein, but rather to illustrate representative methods and results. These examples are not intended to exclude equivalents and variations of the present disclosure, which are apparent to one skilled in the art.

[0182] Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in °C or is at ambient temperature, and pressure is at or near atmospheric. There are numerous variations and combinations of reaction conditions, e.g., component concentrations, temperatures, pressures, and other reaction ranges and conditions that can be used to optimize the product purity and yield obtained from the described process. Only reasonable and routine experimentation will be required to optimize such process conditions.

[0183] Example 1. Functional Hydrogels for Selective Phosphate Removal from Water and Release on Demand

[0184] Commercial overuse and soaring prices of phosphate fertilizers are causing a negative economic and environmental impact, threatening human health, clean water, and food security. To solve the phosphorus (P) cycling challenge, we developed a polyethyleneimine (PEI) / poly(methyl vinyl ether-co-maleic anhydride) (PMVEMA) hydrogel system capable of efficient capture and release of inorganic phosphates with high selectivity in the presence of nitrate. This example investigated the synergetic effect of the molar composition of PEI and PMVEMA within the hydrogel for a broad range of eluent pH conditions and P loadings to establish the capturing capacity and selectivity of the system toward nitrate. The PEI-enriched hydrogel system showed a high P capturing capacity between pH 2.0 and 7.0 with a maximum P capture of 65 mg of P / g of sorbent at an equilibrium pH of 4.5, comparable to high-performance metal oxide systems. Desorption studies showed that the system could efficiently release captured inorganic phosphate with an efficiency of 96% at pH 11.0 under mild conditions (<0.001 M NaOH), independent of the preloaded amount of phosphates and system history. The selectivity factor (a) was dependent on the pH of the eluent and was equal to approximately 50 when in the presence of nitrates.

[0185] The past few decades have witnessed the development of phosphate capture featuring metal oxides / hydroxides, commercial resins, and other compositions that feature high phosphate adsorption. However, phosphate desorption from these systems only occurs under harsh conditions (>0.1 M NaOH or more and often elevated temperature). Protein-based capturing agents provide both selectivity and a high degree of desorption. However, these protein-based materials are costly and have low adsorption capacities. In this example, we introduce a gel synthesized by crosslinking commercially available materials. This gel exhibits a high capacity for phosphate capture and releases the cargo under mild conditions (<0.001 M NaOH and at ambient temperature).

[0186] Introduction Table 1 summarizes the capacity, optimal adsorption condition, and desorption of diverse types of materials. Compared to various sorbents, including organic and inorganic, the PELPMVEMA hydrogel exhibits high selectivity and high capturing capacity toward inorganic phosphate, enables complete regeneration, and demonstrates stability over time in a wide range of pH conditions.

[0187] Table 1. Sorption and desorption capacity (including conditions) for P using various sorbents. Selectivity is included in known systems.

[0188] Metal Oxide Hydride

[0189]

[0190] Commercial Resin

[0191]

[0192] Other Types of Organic Sorbents

[0193]

[0194] PEI-based Sorbents

[0195]

[0196] Note: Poor selectivity indicates that P sorption reduces to <50% capacity when competing anions are present compared to the absence of competing anions.

[0197] Moderate selectivity indicates that P sorption ranges from 50 to <85% capacity when competing anions are present compared to the absence of competing anions.

[0198] High selectivity indicates that P sorption is >85% capacity when competing anions are present compared to the absence of competing anions.

[0199] Since wastewater and natural water often have pH ranging from 6.0 to 9.0, at which inorganic phosphates mainly manifest in H2PO4- and HP042-. Consequently, cationic materials offer promising phosphate removal capabilities (32). Sorbents based on quaternary ammonium compounds are permanently charged and exhibit high adsorption capacity over an extensive pH range (26). The commercial ion exchange organic resins have a relatively low capacity (~io mg P / g). The regeneration of permanently positively charged sorbents requires a high concentration (>0.1 M) of salts or alkaline conditions (26, 33). Polyethylenimine (PEI) emerged as a favorable alternative due to its cost-effectiveness and high density of amino groups. Previously, PEI has been incorporated into other materials, such as silica and lanthanum hydroxide, to enhance the P capturing capacity (30, 34). However, pure amine-based materials show limited abilities for regeneration and selectivity for P capture in the presence of other competing anions, such as nitrate (NO3-) (26, 33). Phosphate binding peptides and proteins (PBP) overcome this limitation by having extraordinary selectivity towards phosphate and removing P to ultralow concentrations (35). The adsorption and desorption of phosphate on PBP can be achieved under mild conditions. However, the prohibitive cost of synthesizing biosorbents for P capture and their low capacity (<1 mg P / g) needs improvement. Hydrogel materials are three-dimensional networks that allow unrestricted ion diffusion access to the phosphate binding compared to traditional non-porous sorbents (36). This example integrates poly (methyl vinyl ether-co-maleic anhydride) (PMVEMA) as part of the hydrogel formulation to accomplish two objectives. First, maleic anhydride groups along the polymer backbone allow facile functionalization with primary amines under ambient conditions. This modification enables the direct employment of PMVEMA chains as crosslinker agents in combination with primary amines present in the PEI to form hydrogels (FIG. 1). It is important to note that PMVEMA has ultra-low toxicity (37, 38), an essential material attribute in water purification applications. The second aspect of PMVEMA is the ability to convert anhydride groups into carboxylates in the presence of water. The studies on the PBP structure show that carboxylates play a pivotal role in recognizing phosphate (39, 40), which implies that the combination of amines and carboxylate groups may improve the material selectivity toward the phosphates in the presence of nitrate. Here, we describe a strategy to fabricate a hydrogel system that utilizes the synergy between amino and carboxyl groups to mimic the chemical structure of PBP for enhanced capturing capacity and selectivity toward phosphate. This example develops a P-recycling technology that balances adsorption capacity, selectivity, and ease of recovery. Developing such a system allows for the reuse of captured phosphate, enabling P sustainability and providing economic gains.

[0200] In the present example, we (1) test the pH stability of hydrogel with a wide pH range, (2) study the kinetics of P adsorption by PELPMVEMA hydrogel, (3) explore the pH dependence and the effects of PELPMVEMA composition on equilibrium P adsorption, (4) investigate the pH dependence of desorption, and (5) evaluate the P capturing selectivity by varying pH and concentrations of nitrate. Typically, the phosphate-P concentration in the sewage is lower than 10 mg / L (41). Our experimental concentrations are therefore chosen comparably to the P concentration in sewage.

[0201] Results and Discussion

[0202] Fourier-transform infrared (FTIR) spectroscopy confirmed the crosslinking between PMVEMA and PEI and the hydrolysis of residual anhydride groups (see FIG. 7 and Table 2). The IR data also confirm that monosodium phosphate gets captured by the PELPMVEMAgel.

[0203] While pH adjustment is commonly employed for adsorption and desorption cycles, pH changes can also trigger chemical bond cleavage in many commonly studied hydrogel systems. For instance, the amide bond in polyacrylamide hydrolyzes in basic conditions (42), and the glycosidic bond in chitosan hydrolyzes in acidic conditions (43), decreasing the system's long-term stability. We measured the swelling ratios of the crosslinked systems (SR) by exposing the hydrogels to acidic and basic conditions for 7 days and titrating them to pH 7.0. Given that the P capturing capacity was suppressed at pH 2.0 and the P desorption efficiency was close to 100% above pH 11.0 (vide infra), the experimental pH range was set from 2.0 to 12.0. If the crosslinking density of the hydrogel decreases due to bond cleavages, the swelling ratio should increase accordingly (44). As seen in FIG. 8, the swelling ratio of PEI: PMVEMA hydrogel exposed to pH 2.0 to 12.0 remains within the SR =25 to 27 range, indicating the PELPMVEMA network is stable across a broad pH range.

[0204] FIGs. 2A-2C display the results of adsorption experiments for the PELPMVEMA (3:1) hydrogel exposed to various phosphate concentrations (2.0, 4.0, 6.0, 8.0, and 20.0 mg / L). Adsorption was modeled using pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetics. See FIG. 9 for details. Our analysis indicates that the PSO model is a better predictor of experimental kinetics than PFO models based on the fitting of correlation coefficients. This finding suggests that the adsorption rate depends on the adsorption capacity rather than on the concentration of the sorbent (46). The equilibrium adsorption capacity in different initial P concentrations was similar for both models. The equilibrium adsorption capacity notably increases with the initial concentration from 2.0 to 6.0 mg / L, with a slower increase between 6.0 and 8.0 mg / L, suggesting that maximum adsorption capacity is attained at an initial concentration above 6.0 mg / L (FIG. 2A). The hydrogel phosphate sorption rate constants (ki for PFO and k2for PSO) reached a minimum at a phosphate concentration of 4.0 mg / L. While not wishing to be bound by any one theory, we hypothesize that adsorption kinetics for phosphate concentration between 2.0 to 4.0 mg / L can be represented as solute-limited adsorption, where the sorbent provides abundant adsorption sites, while the solute concentration is insufficient to saturate it. The situation changes in the phosphate solution concentration between 6.0 and 20.0 mg / L, where a sorbent site-limited scenario better describes the kinetics. The adsorption kinetics of the 20.0 mg / L phosphate system does not possess a clear trend, and the experimental data are close to the equilibrium adsorption capacity (~ 55.0 mg / g), implying that the system has reached equilibrium in less than 10 min. We monitored the pH change of the solution versus time after adding the PELPMVEMA hydrogel to the phosphate solution. The observed drop in the solution pH is attributed to the fact that gels initially pre-conditioned with aqueous solution at pH=3.o contain an abundance of free protons (FIG. 2B). Interestingly, adding the 27 pL of 1 mM HC1 (pH=3), the same volume as the swollen gel, into 10 mL of water only resulted in a final pH of 5.7, significantly different from the pH value upon adding PELPMVEA hydrogel. The addition of phosphate solution at a higher concentration caused the overall pH of the solution to eventually increase to higher values (4.0 to 7.0), which is indicative that H2PO4- ions regulate the system pH (FIG.

[0205] 2C). When added to the deionized water, the gel released protons to the solution and reached an equilibrium pH in less than 10 min. However, after adding the gel to phosphate solutions, the system reached equilibrium pH in ~ 5 hours. The interactions between the gel and phosphates delayed the proton-releasing process, where HPO42’ is converted to H2PO4’. Higher residual phosphate concentrations resulted in higher equilibrium pHs, which agrees with the ionization of HPO42’ to H2PO43-. The equilibrium pH of the hydrogel at the 8.0 mg / L P solution only reached a pH value of 7.2, which is close to the pKa2of monohydrogen phosphate. The rate of pH change follows this order: 4.0 mg / L > 8.0 mg / L > 6.0 mg / L > 2.0 mg / L. The corresponding kinetic rate coefficients (ki) follow the order: 4.0 mg / L < 8.0 mg / L < 6.0 mg / L < 2.0 mg / L (c. Table 5). The rate of pH decrease exhibits an inverse correlation with the kinetic coefficients. Given the pH changes and adsorption kinetics, the adsorption of phosphates may be related to the conversion of HPO42’ to H2PO4-. The abundance of protons within the hydrogel accelerates phosphate adsorption. While not wishing to be bound by any one theory, the hypothesis also explains the fast equilibrium process of hydrogel in the 20.0 mg / L phosphate solution, in which the pH change is insufficient to transform HPO42’ to H2PO4’. The equilibrium adsorption capacity was measured for PELPMVEMA (3:1) and PEI: PMVEMA (1:3) hydrogel systems. The initial goal for this example was to fabricate a hydrogel system capable of high capturing capabilities toward phosphate anions while maintaining the selective adsorption behavior toward other commonly encountered anions, such as nitrate. This high capacity and selectivity need the presence of both protonated amines (PEI) and carboxylate anions (PMVEMA) within the capturing matrix. The equilibrium solution pH will affect the concentration and the charge states of amines, carboxyls, and phosphate groups. Therefore, it is necessary to understand the influence of pH on the charged states of amine and carboxyl groups during phosphate adsorption. PELPMVEMA (3:1) (PEI-rich) and (1:3) (PMVEMA-rich) hydrogels were tested for phosphate adsorption under different initial phosphate concentrations and pH. The feed ratio of PEI and PMVEMA determines the final hydrogel chemical composition. As illustrated in Table 3, the increase in PELPMVEMA ratio causes elevated nitrogen content in the hydrogel network. Given that the PEI is the sole nitrogen source, the increase in N content indicates that more PEI is incorporated into the hydrogel. However, the PEI hydrogel incorporation efficiency was not proportional to the rise in feed ratio. Moreover, the chloride content follows the trend of the nitrogen content, implying that the chloride is attached to protonated PEI in the hydrogel.

[0206] The phosphate adsorption capacity of PELPMVEMA (3:1) hydrogel increases notably with the increase in phosphate concentrations from 2.0 to 8.0 mg / L, with only a marginal increase for concentrations above 8.0 mg / L (FIGs. 3A, 3C). To test the system's scalability, we measured the phosphate capturing capacity of 0.5 g / L sorbent exposed to 40.0 mg / L P with different pHs. The phosphate adsorption behavior of 0.1 g / L sorbent at 8 mg / L P is identical to that of 0.5 g / L sorbent at.0 mg / L, indicating the potential of expandability to large flow bed configurations. Comparing the adsorption capacity and P removal of PELPMVEMA (3:1) (PEI-rich) and PELPMVEMA (1:3) (PMVEMA-rich) hydrogels in Figs. 3A-3D, the PEI-rich hydrogel exhibited significantly greater P adsorption capacity than the PMVEMA-rich hydrogel under identical conditions. For example, 0.1 g / L of PEI-rich hydrogel removed ~9O% of the phosphate in the 2.0 mg / L P solution at pH 4.5, while the PMVEMA-rich hydrogel could only remove ~3O% of the phosphate. The maximum P adsorption capacity of PEI-rich hydrogel was ~65.o mg / g, while that of the PMVEMA-rich hydrogel was only ~6.o mg / g. The difference in capturing capacity results from the different compositions of the two gels. The PEI-rich hydrogel contains more amines capable of protonating and attracting negatively charged phosphates (H2PO4-, HPO42-, PO43-). As stated previously, the high content of nitrogen in PEI: PMVEMA (3:1) from elemental analysis (cf. Table 3) confirmed that PEI: PMVEMA (3:1) hydrogel contained more PEI and fewer PMVEMA units than PELPMVEMA (1:3) hydrogel.

[0207] The phosphate adsorption capacity of 0.1 g / L PEI: PMVEMA=i:3 hydrogel exposed to the 2.0 and 4.0 mg / L phosphate solutions exhibit similar trends, indicating that the system reached a maximum adsorption capacity (FIGs. 3B, 3D). Increasing the sorbent concentration of PELPMVEMA (1:3) from 0.1 to 0.5 g / L in the 2.0 mg / L phosphate solution removed 5 times more phosphate. Additionally, the equilibrium adsorption capacity remained the same. It also demonstrated the scalability of PELPMVEMA hydrogel in dealing with various concentrations of P solutions.

[0208] Experimental data in FIGs. 3A-3D demonstrate an increase in equilibrium phosphate adsorption capacity from pH 2.0 to 4.0, followed by a slight decline for higher pH values. This trend was independent of initial phosphate concentration and gel composition. The maximum adsorption capacity of hydrogels shifts from pH 4.0 to 4.5 as the content of PEI in the system increases. At 2.i<pH<pKa the carboxyl groups will deprotonate to form carboxylate (-COO’) and repel H2PO4-electrostatically if pH increases. Since the pKai of the carboxylic acid from PMVEMA is around 4.2, lowering the pH from 4.5 to 4 will decrease the carboxylate portion in the carboxyl groups, leading to a decrease in the electrostatic repulsion between carboxylates and phosphates. Therefore, a hydrogel with a high content of carboxyl groups exhibits maximum capacity toward phosphate sorption at a lower pH.

[0209] The protonation and deprotonation of active moieties in the hydrogel could explain the adsorption and desorption of phosphate. FIG.5 illustrates the dependence of the charged state of the hydrogel functional groups with pH and phosphate. In the pH range between 2.0 and 3.0, protonated amines stay undisturbed, and carboxylic acid stays neutral, as shown in FIG.5. Increasing the pH from 2.0 to 4.0 will deprotonate the uncharged H3PO4molecules into a charged anionic state of H2PO4’. The increasing number of anionic phosphate ions leads to increased absorbed phosphates in the hydrogel due to the attractive electrostatic interactions with the protonated amine. At pH above 4.0, the decrease in absorbed P could be attributed to two reasons. Firstly, the carboxylic acid (-COOH) starts to deprotonate, resulting in the electrostatic repulsion of dihydrogen phosphates (H2PO4’), thus diminishing the P adsorption capacity. Secondly, more carboxylates (-COO) are formed to attract protonated amines and compete with absorbed phosphate as the pH increases. The titration curve in FIG. 10 shows the deprotonation process of succinic acid with two pKaS as 4.2 and 5.6.

[0210] As the pH increases above 9.0, the protonated amines from PEI undergo deprotonation. The carboxyl groups are converted into negatively charged carboxylates, as depicted in FIG. 5. At pH above 11.0, the protonated amines (-NR2H+) get completely deprotonated into the neutral amines (-NR2), and only the repulsive forces between carboxylate and phosphate dominate the system when phosphate leaves the sorbent into the bulk solution. The experimental desorption data agrees with the PEI protonation process, where the pKa of PEI is around 9.5, and PEI turns completely uncharged at pH above 11.0 (c. FIG. 10). The pH dependence on desorption also agrees with the observation Mira et al. found for chitosan. However, the absolute efficiency varies (47). The synergy of interactions between carboxylates, amines, and phosphates determines the adsorption capacity of hydrogels at different pHs. From elemental analysis, the chloride content drops from 17.99% to o after the hydrogel is soaked with phosphate, as shown in Table 4.

[0211] Phosphate replaces the chloride in the pristine hydrogel, supporting the ion exchange mechanism.

[0212] Natural water bodies contain various anions, including nitrate, potentially competing with phosphate for adsorption sites (48). The United States Environment Protection Agency set a safe limit for nitrate-N in drinking water to 10 mg / L. Understanding the potential competition of nitrate for adsorption sites is crucial to capture phosphates selectively. The distribution coefficient and separation factor are key parameters to evaluate phosphate adsorption selectivity, which can be calculated as follows:

[0213] Distribution coefficient:

[0214] Kd=q(1) qeis the adsorption capacity at equilibrium, and Ce is the concentration at equilibrium.

[0215] Separation factor:

[0216]

[0217] In calculating the separation factor, we use the distribution coefficient Kd2 of phosphate as the numerator and Kdi of nitrate as the denominator.

[0218] PEI: PMVEMA (3:1) hydrogel was evaluated in a nitrate-phosphate binary anionic system. As shown in FIGs. 6A-6C, the phosphate adsorption capacity remains unaffected as the nitrate-N concentration is no more than 4.0 mg / L but starts declining as the nitrate-N concentration is > 10.0 mg / L. The separation factors of the phosphate-nitrate binary system are all greater than 1 (FIG. 6A), demonstrating the PEEPMVEMA hydrogel has a high selectivity of phosphate capturing in the coexistence of nitrate. The separation factor evaluates the selectivity, depicting the pH effects on selective P adsorption. The separation factor for the nitrate-phosphate binary systems rises with increasing pH, indicating that elevating pH could improve selectivity P adsorption in the binary mixtures with nitrates. The mechanism driving pH changes and the separation factor remains unclear. Two assumptions are considered to explain the influence of pH on the change of separation factor: 1) Phosphates are converted from H2PO4- to HPO42’, and the Columbic attraction forces between the protonated PEI and the anionic phosphate increase with increasing pH.

[0219] 2) carboxylates serve as hydrogen bond acceptors with monohydrogen phosphate and dihydrogen phosphate (COO- — H-OP) at high pH. The pKa of nitric acid is -1.3 (49). Nitrate is fully deprotonated above pH 3 and, therefore, cannot form hydrogen bonds with carboxylate. The hydrogen bond interaction between COO- and HPO42' / H2PO4- may enhance the selection of phosphate capturing (50).

[0220] Conclusions

[0221] A hydrogel, PEEPMVEMA, was prepared and applied for phosphate adsorption and desorption. The PEEPMVEMA hydrogel exhibited stability at all tested pH values from 2.0 to 12.0. Results of P adsorption kinetics and pH indicated that the adsorption process was accompanied by a solution pH drop, and the interaction between the hydrogel and phosphate delayed pH change. We modeled the kinetics of P sorption using the pseudo-first order and pseudo-second order adsorption models. The adsorption process depended primarily on the sorbent capacity. As the phosphate concentration increased, the adsorption process transitioned from a solute-insufficient state to a sorbent -insufficient state, as evidenced by equilibrium adsorption capacity and kinetic rate coefficients. We probed the effects of hydrogel composition, pH, initial phosphate concentrations, and competing anions on equilibrium phosphate adsorption. The maximum P capturing capacity of PELPMVEMA (3:1) was 65.0 mg of P / g at equilibrium pH 4.5, and PEI: PMVEMA (1:3) was 6.0 mg of P / g at pH 4.0. Phosphate gets released from the gel at pH 9-11 and <0.001 M NaOH at ambient conditions. The presence of nitrate had a minor negative impact on phosphate adsorption when the N: P mass ratio was below 2.5. Increasing equilibrium pH in binary ionic solutions improved the selectivity of P capturing. We studied the effects of pH and preloading conditions on desorption. The desorption efficiencies of pH were ~72% at pH 9.0, ~83% at pH 10.0, and ~96% at pH 11.0, independent of the preloaded phosphates and pre-conditioned pHs. A mechanism of synergistic effects of carboxyl and amino groups explains the adsorption and desorption of phosphate.

[0222] Materials and Methods

[0223] Chemicals. Disodium hydrogen phosphate, sodium dihydrogen phosphate, branched polyethyleneimine (PEI) (Mn=10 kDa), Poly(methyl vinyl ether-a / t-maleic anhydride) (PMVEMA) (Mn=80 kDa), hydrochloric acid, and sodium nitrate were purchased from Sigma-Aldrich. Sodium hydroxide, potassium bromide (FTIR grade), and dimethylsulfoxide (DMSO) were purchased from Fisher Scientific. All materials were used without further purification.

[0224] Hydrogel preparation. All tested hydrogels were prepared in DMSO at different molar ratios feeds of PMVEMA and PEI. The repeat unit of branched PEI is (-C11H66N11-), and PMVEMA is (-C7H8O4-) based on the chemical structure provided by Sigma-Aldrich. For example, the PELPMVEMA (3:1) ratio means that a 3:1 ratio of the repeat units of PEI and PMVEMA was used during the gel formation. In the typical experiment, 9.0 w / v% (repeat unit concentration 0.192 M) PEI solution in 10.0 ml of DMSO was added dropwise into 10.0 mL of 1.0 w / v% (repeat unit concentration 0.064 M) PMVEMA in DMSO under continuous stirring at 300 rpm to prepare the PEI: PMVEMA (3:1) gel. The solution instantaneously becomes visibly viscous, indicating the gel formation. The extent of the reaction (c. FIG. 7) can be visually monitored by the disappearance of the solution's red color due to the solvatochromism of PMVEMA in DMSO (51). After 30 minutes, the mixture was transferred to another flask containing a large quantity of deionized water at pH 3.0 and conditioned for 24 hours to remove DMSO, uncrosslinked PEI, and PMVEMA. The washing process was repeated at least five times to ensure the complete removal of unreacted materials.

[0225] Characterization. The extent of the crosslinking reaction was monitored using FTIR spectroscopy (Nicolet iNio MX). The spectra were collected between 600 and 4000 cm-1with 4 cm-1resolution using a nitrogen-cooled MCT-A detector (256 scans in total for each sample). Tested samples were evaluated in ATR mode (Ge crystal) and transmission mode using KBr pellets. All samples were dried for 72 hours before characterization.

[0226] Stability experiments. The stability of hydrogels was evaluated by exposing them to deionized water at the pHs of 2.0, 4.5, 7.0, 9.5, and 12.0. The solution pH was controlled by adding appropriate amounts of NaOH / HCl and measured by Mettler Toledo Seven Excellence pH meter S400. The samples were kept in the solution for a week and then titrated to pH 7.0. Hydrogels were filtered and dried. The swelling ratios of gels before and after exposure to water were estimated using the following equation:

[0227]

[0228] where mSwoiien is the mass of the swollen gel, and mdry is the mass of the dry gel. Phosphate adsorption studies. A stock of 100 mg / L of phosphate-P solution was prepared by dissolving Na2HPO4in deionized water. Solutions with various P concentrations were prepared by dilution of the stock solution. In the typical experiment, the hydrogel of a given mass was added to the P solution at a specific concentration. The solutions were filtered using P8 paper to remove the hydrogel particulate and assess the amount of phosphorus captured by the hydrogels. The residual phosphate concentration was determined using the standardized molybdenum blue method (52). The ultraviolet-visible (UV-Vis) spectra were collected by Thermo Scientific Evolution 300 spectrophotometer, and phosphate concentrations were calibrated at 880 nm. The phosphorus adsorption calculation is reported based on the dry hydrogel mass (m).

[0229] Phosphate adsorption kinetics were studied for phosphate concentration between 2.0 and 20.0 mg / L with 0.1 mg / mL of PELPMVEMA (3:1) hydrogel. Adsorption capacity (qt) at time t was calculated as follows:

[0230]

[0231] Ci is the initial P concentration before adsorption, Ct is the residual P concentration of the solution at time t, Vis the solution volume, m is the sorbent mass, and Cs is the sorbent concentration.

[0232] Equilibrium adsorption was conducted at different P concentrations from 2.0 mg / L to 20.0 mg / L with 0.1 mg / L of PELPMVMEA (3:1) hydrogel. Additionally, 40.0 mg / L P solution with 0.5 mg / L of PELPMVMEA (3:1) hydrogel were used to explore the performance scalability of the system. Similar studies were performed with 0.1 mg / L of PELPMVMEA (1:3) hydrogel and 4.0 mg / L P concentration with 0.5 mg / mL of PELPMVMEA (1:3) hydrogel. Equilibrium pH was controlled by adding NaOH / HCl and measured by pH meter after 24 h. Equilibrium adsorption capacity (qe) and removal (R%) were calculated as follows:

[0233]

[0234] where Ct is the residual P concentration of the solution at time t, Ce is the residual P concentration at equilibrium, Cs is the sorbent concentration.

[0235] Desorption experiments: 0.1 g / L PELPMVEMA (3:1) hydrogel was immersed in three different concentrations of phosphate solutions (2.0, 8.0, 20.0 mg / L) at three different pHs (3.0, 4.5, 6.0). The hydrogel preloaded with phosphates was filtered and transferred to the same volume of deionized water and conditioned to pH 9, 10, and 11 by adding NaOH. All experiments were performed in triplicate. The total desorbed P and desorption efficiency were calculated as follows:

[0236] Desorbed P =Cd

[0237] mXV(7)

[0238]

[0239] where m is the mass of sorbent. Cd is the concentration of P at equilibrium desorption, and V is the solution volume. Absorbed P was calculated in the adsorption experiments.

[0240] Selectivity experiments: A stock of 40.0 mg / L of nitrate-N solutions was prepared by NaNO3in deionized water. Binary nitrate and phosphate solutions were prepared by stoichiometrically mixing NaNO3and Na2HPO4in different concentrations. The hydrogel PELPMVEMA (3:1) with a mass of 0.1 g / L was added to the binary anionic solution. The equilibrium pH was conditioned by NaOH / HCl and measured after mixing for 24 h. A Dionex ICS-5000+ Ion Chromatography system measured the phosphate and nitrate concentrations.

[0241] References for Example 1

[0242] 1. United States Geological Survey, "Mineral commodity summaries 2O23"(Reston, VA), pp. 134.

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[0244] 3. Y. Zhang et al., Cause and effect of N / P ratio decline with eutrophication aggravation in shallow lakes. Sci. Total Environ. 627, 1294-1302 (2018).

[0245] 4. M. M. Mekonnen, A. Y. Hoekstra, Global Anthropogenic Phosphorus Loads to Freshwater and Associated Grey Water Footprints and Water Pollution Levels: A High- Resolution Global Study. Water Resour. Res.54, 345-358 (2018).

[0246] 5. S. R. Golroudbary, M. El Wali, A. Kraslawski, Environmental sustainability of phosphorus recycling from wastewater, manure and solid wastes. Sci. Total Environ.

[0247] 672, 515-524 (2019).

[0248] 6. C. Alewell et al., Global phosphorus shortage will be aggravated by soil erosion. Nat. Commun. 11, 4546 (2020).

[0249] 7. K. Venkiteshwaran, P. J. McNamara, B. K. Mayer, Meta-analysis of non-reactive phosphorus in water, wastewater, and sludge, and strategies to convert it for enhanced phosphorus removal and recovery. Sci. Total Environ. 644, 661-674 (2018). 8. M. K. Perera, J. D. Englehardt, A. C. Dvorak, Technologies for Recovering Nutrients from Wastewater: A Critical Review. Environ. Eng. Sci. 36, 511-529 (2019).

[0250] 9. Q. He et al., Phosphate removal and recovery by lanthanum-based adsorbents: A review for current advances. Chemosphere 303, 134987 (2022). 10. R. Liu et al., Review of metal (hydr)oxide and other adsorptive materials for phosphate removal from water. J. Environ. Chem. Eng. 6, 5269-5286 (2018).

[0251] 11. S. M. Ribet, B. Shindel, R. dos Reis, V. Nandwana, V. P. Dravid, Phosphate Elimination and Recovery Lightweight (PEARL) membrane: A sustainable environmental remediation approach. Proc. Natl. Acad. Sci. U. SA. 118, 02102583118 (2021).

[0252] 12. J. Li et al., Lanthanum-based adsorbents for phosphate reutilization: Interference factors, adsorbent regeneration, and research gaps. Sustainable Horizons 1, 100011 (2022).

[0253] 13. P. Zhang, M. He, S. Huo, F. Li, K. Li, Recent progress in metal-based composites toward adsorptive removal of phosphate: Mechanisms, behaviors, and prospects. Chem. Eng. J.446, 137081 (2022).

[0254] 14. K. A. Tee, S. Ahmed, M. A. H. Badsha, K. C. J. Wong, I. M. C. Lo, Comprehensive review and future research directions on using various lanthanum-based adsorbents for selective phosphate removal. Clean Technol. Environ. Policy 25, 1783-1805 (2023).

[0255] 15. J. Xie, Y. Lin, C. Li, D. Wu, H. Kong, Removal and recovery of phosphate from water by activated aluminum oxide and lanthanum oxide. Powder Technol. 269, 351-357 (2015).

[0256] 16. X. Mi et al., Lanthanum activated palygorskite for selective phosphate separation from aqueous media: Comprehensive understanding of adsorptive behavior and mechanism affected by interfering substances. Chem. Eng. J. 443, 136423 (2022).

[0257] 17. Y. Mu et al., Synthesis of Fe3O4@Phoslock® composites and the application in adsorption of phosphate from aqueous solution. Environmental Science and Pollution Research 29, 60674-60686 (2022). 18. S. Tanada et al., Removal of phosphate by aluminum oxide hydroxide. J. Colloid Interface Sei. 257, 135-140 (2003).

[0258] 19. X. Huang, G. D. Foster, R. V. Honeychuck, J. A. Schreifels, The Maximum of Phosphate Adsorption at pH 4.0: Why It Appears on Aluminum Oxides but Not on Iron Oxides. Langmuir 25, 4450-4461 (2009).

[0259] 20. D. Guaya, C. Valderrama, A. Farran, C. Armijos, J. L. Cortina, Simultaneous phosphate and ammonium removal from aqueous solution by a hydrated aluminum oxide modified natural zeolite. Chem. Eng. J. 271, 204-213 (2015).

[0260] 21. N. Kawasaki, F. Ogata, H. Tominaga, Selective adsorption behavior of phosphate onto aluminum hydroxide gel. J. Hazard. Mater. 181, 574-579 (2010).

[0261] 22. Z. Ajmal et al., Phosphate removal from aqueous solution using iron oxides: Adsorption, desorption and regeneration characteristics. J. Colloid Interface Sei.

[0262] 528, 145-155 (2018).

[0263] 23. R. Chitrakar et al., Phosphate adsorption on synthetic goethite and akaganeite. J. Colloid Interface Sei. 298, 602-608 (2006).

[0264] 24. B. D. Martin, S. A. Parsons, B. Jefferson, Removal and recovery of phosphate from municipal wastewaters using a polymeric anion exchanger bound with hydrated ferric oxide nanoparticles. Water Sei. Technol. 60, 2637-2645 (2009).

[0265] 25. T. NUR. "Nitrate, phosphate and fluoride removal from water using adsorption process", University of Technology Sydney, New South Wales, Australia (2014).

[0266] 26. M. R. Awual, A. Jyo, Assessing of phosphorus removal by polymeric anion exchangers. Desalination 281, 111-117 (2011).

[0267] 27. Y. Shang, K. Guo, P. Jiang, X. Xu, B. Gao, Adsorption of phosphate by the cellulose-based biomaterial and its sustained release of laden phosphate in aqueous solution and soil. Int. J. Biol. Macromol. 109, 524-534 (2018).

[0268] 28. S. Pap et al., Low-cost chitosan-calcite adsorbent development for potential phosphate removal and recovery from wastewater effluent. Water Res. 173, 115573 (2020). 29. J. Wei, X. Meng, X. Wen, Y. Song, Adsorption and recovery of phosphate from water by amine fiber, effects of co-existing ions and column filtration. J. Environ. Sci.

[0269] 87, 123-132 (2020).

[0270] 30. M. Xanthopoulou et al., Phosphate Removal Using Polyethylenimine Functionalized Silica-Based Materials. Sustainability 13, 1502 (2021).

[0271] 31. E. Zong et al., Reusable Hyperbranched Polyethylenimine-Functionalized Ethyl Cellulose Film for the Removal of Phosphate with Easy Separation. ACS Omega 6, 505-515 (2021).

[0272] 32. E. E. O. Odjadjare, A. I. Okoh, Physicochemical quality of an urban municipal wastewater effluent and its impact on the receiving environment. Environ. Monit. Assess. 170, 383-394 (2010).

[0273] 33. G. Xu et al., Enhanced phosphate removal from wastewater by recyclable fiber supported quaternary ammonium salts: Highlighting the role of surface polarity. Chem. Eng. J.416, 127889 (2021).

[0274] 34. S. Li et al., PVA / PEI crosslinked electrospun nanofibers with embedded La(OH)3nanorod for selective adsorption of high flux low concentration phosphorus. J. Hazard. Mater.384, 121457 (2020).

[0275] 35. J. M. Hutchison, F. B. Hussein, B. K. Mayer, Evaluating Sustainable Development Pathways for Protein- and Peptide-Based Bioadsorbents for Phosphorus Recovery from Wastewater. Environ. Sci. Technol. 57, 16317-16326 (2023).

[0276] 36. H. Hu, Y. W. Tong, Y. He, Current insight into enhanced strategies and interaction mechanisms of hydrogel materials for phosphate removal and recovery from wastewater. Sci. Total Environ.892, 164514 (2023).

[0277] 37. T. Iglesias et al., In vitro evaluation of the genotoxicity of poly(anhydride) nanoparticles designed for oral drug delivery. Int. J. Pharm.523, 418-426 (2017).

[0278] 38. A. Mira, C. R. Mateo, R. Mallavia, A. Falco, Poly(methyl vinyl ether-alt-maleic acid) and ethyl monoester as building polymers for drug-loadable electrospun nanofibers. Sci. Rep. 7, 17205 (2017). 39- A. E. Hargrove, S. Nieto, T. Zhang, J. L. Sessler, E. V. Anslyn, Artificial Receptors for the Recognition of Phosphorylated Molecules. Chem. Rev. m, 6603-6782 (2011).

[0279] 40. A. K. H. Hirsch, F. R. Fischer, F. Diederich, Phosphate Recognition in Structural Biology. Angew. Chem. Int. Ed. 46, 338-352 (2007).

[0280] 41. Y. Ye et al., Insight into biological phosphate recovery from sewage. Bioresour. Technol. 218, 874-881 (2016).

[0281] 42. T. Tanaka et al., Phase Transitions in Ionic Gels. Phys. Rev. Lett. 45, 1636-1639 (1980).

[0282] 43. K. M. Varum, M. H. Ottoy, O. Smidsrod, Acid hydrolysis of chitosans. Carbohydr. Polym. 46, 89-98 (2001).

[0283] 44. Y. Zhou, L. Jin, Hydrolysis-induced large swelling of polyacrylamide hydrogels. Soft Matter 16, 5740-5749 (2020).

[0284] 45. S. Kim, Y. H. Park, J. B. Lee, H. S. Kim, Y.-E. Choi, Phosphorus adsorption behavior of industrial waste biomass-based adsorbent, esterified polyethylenimine-coated polysulfone-Escherichia coli biomass composite fibers in aqueous solution. J. Hazard. Mater. 400, 123217 (2020).

[0285] 46. T. A. Saleh, "Chapter 4 - Isotherm models of adsorption processes on adsorbents and nanoadsorbents" in Interface Science and Technology, T. A. Saleh, Ed. (Elsevier, 2022), vol. 34, pp. 99-126.

[0286] 47. T. Jozwiak et al., Recovery of phosphorus as soluble phosphates from aqueous solutions using chitosan hydrogel sorbents. Sei. Rep. 11, 16766 (2021).

[0287] 48. N. Gros, B. Gorenc, Performance of ion chromatography in the determination of anions and cations in various natural waters with elevated mineralization. J. Chromatogr. A 770, 119-124 (1997).

[0288] 49. D. R. Lide, CRC handbook of chemistry and physics (CRC press, 2004), vol.

[0289] 85- 50. B. Wu, J. Wan, Y. Zhang, B. Pan, I. M. C. Lo, Selective Phosphate Removal from Water and Wastewater using Sorption: Process Fundamentals and Removal Mechanisms. Environ. Sei. Technol. 54, 50-66 (2020). 51. E. Zhao et al., Poly[(maleic anhydride)-alt-(vinyl acetate)]: A Pure Oxygenic Nonconjugated Macromolecule with Strong Light Emission and Solvatochromic Effect. Macromolecules 48, 64-71 (2015).

[0290] 52. US EPA, Method 365.3: Phosphorus, all forms (colorimetric, ascorbic acid, two reagent). United States Environmental Protection Agency (USEPA), Washington, DC (1978).

[0291] Example 2. Supplementary Materials

[0292] FIG. 7 shows the IR spectra of PEI, PMVEMA, PEI / PMVEMA gel, and PEI / PMVEMA exposed to phosphate solution (add concentration) (PEI / PMVEMA+P) and neat monosodium phosphate. Table 2 summarizes the characteristic absorbance peaks of studied systems. The PEI spectrum shows the peaks at 1654 cm-1(primary amine), 1562 cm-1(secondary amine), and between 1120-1010 cm-1(C-N stretching) (26). In the PMVEMA spectrum, the C=O stretching in maleic anhydride exhibits two peaks at 1856 cm-1(asymmetric) and 1785 cm-1(symmetric), and the C-O-C stretching in anhydride has a peak at 1234 cm-1(27). PMVEMA is moisture-sensitive and slowly undergoes hydrolysis, as evidenced by the appearance of C=O (carboxylate) peaks at 1724 cm-1and 928 cm-1. The extent of the crosslinking reaction is monitored by the disappearance of peaks at 1856 cm-1, 1785 cm-1, and 1234 cm-1(maleic anhydride) and the appearance of peaks at 1550 cm-1and 1637 cm-1from amide, 1697 cm-1and 1770 cm-1from imides (26, 28). The shoulder peak at 1724 cm-1carboxyl (C=O) stretching in PEI / PMVEMA hydrogel indicates residual carboxyl groups left intact upon exposure to water.

[0293] The FTIR spectrum of monosodium phosphate is included for comparison as H2PO4-represents the primary form of phosphates within the pH range from 2.1 to 7.0, wherein the gels are conditioned and tested for adsorption. The analysis of PEI / PMVEMA hydrogel exposed to the P solution shows that C-N adsorption at 1087 cm-1(stretching) for pure hydrogel shifts to 1042 cm-1in the presence of captured phosphate anions (29). Moreover, the P-0 deformation peak shifts from 990 cm-1in monosodium phosphate to 980 cm-1in PEI / PMVEMA+P (30).

[0294] Table 2. Characteristic peaks of IR spectra

[0295]

[0296] Table 3. Elemental analysis of PEI / PMVEMA with different PEI / PMVEMA molar feed ratios.

[0297]

[0298] Table 3 depicts the elemental composition of gels as a function of molar feed ratios. Experimental data agrees with the theoretical values of pure PEI and PMVEMA (shown in parentheses). Nitrogen content originates from PEI and decreases with increasing PMVEMA feed ratio. While this provides evidence that the PEI / PMVEMA hydrogel has more PMVEMA units when fed with more PMVEMA, the decrease in nitrogen content is not proportional to the increase in PMVEMA content. The chloride content originates from the addition of hydrochloric acid. Chloride content decreases with decreasing PEI units due to decreased available amine groups.

[0299] Table 4. Elemental analysis of PEI / PMVEMA hydrogels before and after absorption and desorption.

[0300]

[0301] Stage 1 represents pristine PEI: PMVEMA=3:1 hydrogel conditioned at pH 3 and dried without absorption; Stage 2 represents the hydrogel that was dried after soaking in 20 mg / L of P solution; Stage 3 represents hydrogel that was preloaded with P and dried after desorption at pH 11.

[0302] Kinetic analysis of P sorption

[0303] Pseudo-first order model:

[0304]

[0305] Pseudo-second order model:

[0306]

[0307] Table 5 summarizes the fitting parameters. FIG. 9 plots the corresponding fitted curves.

[0308] Table 5. Kinetic parameters of pseudo-first and pseudo-second-order reactions fitted for phosphate absorption at different initial concentrations.

[0309] Pseudo-first order

[0310]

[0311] Pseudo-second order

[0312]

[0313]

[0314] References for Example 2

[0315] 1. Q. He et al., Phosphate removal and recovery by lanthanum-based adsorbents: A review for current advances. Chemosphere 303, 134987 (2022).

[0316] 2. K. A. Tee, S. Ahmed, M. A. H. Badsha, K. C. J. Wong, I. M. C. Lo, Comprehensive review and future research directions on using various lanthanum-based adsorbents for selective phosphate removal. Clean Technol. Environ. Policy 25, 1783-1805 (2023).

[0317] 3. J. Xie, Y. Lin, C. Li, D. Wu, H. Kong, Removal and recovery of phosphate from water by activated aluminum oxide and lanthanum oxide. Powder Technol. 269, 351-357 (2015).

[0318] 4. X. Mi et al., Lanthanum activated palygorskite for selective phosphate separation from aqueous media: Comprehensive understanding of adsorptive behavior and mechanism affected by interfering substances. Chem. Eng. J. 443, 136423 (2022).

[0319] 5. Y. Mu et al., Synthesis of Fe3O4@Phoslock® composites and the application in adsorption of phosphate from aqueous solution. Environmental Science and Pollution Research 29, 60674-60686 (2022).

[0320] 6. S. Tanada et al., Removal of phosphate by aluminum oxide hydroxide. J. Colloid Interface Sei.257, 135-140 (2003).

[0321] 7. X. Huang, G. D. Foster, R. V. Honeychuck, J. A. Schreifels, The Maximum of Phosphate Adsorption at pH 4.0: Why It Appears on Aluminum Oxides but Not on Iron Oxides. Langmuir 25, 4450-4461 (2009).

[0322] 8. D. Guaya, C. Valderrama, A. Farran, C. Armijos, J. L. Cortina, Simultaneous phosphate and ammonium removal from aqueous solution by a hydrated aluminum oxide modified natural zeolite. Chem. Eng. J.271, 204-213 (2015).

[0323] 9. N. Kawasaki, F. Ogata, H. Tominaga, Selective adsorption behavior of phosphate onto aluminum hydroxide gel. J. Hazard. Mater.181, 574-579 (2010). 10. Z. Ajmal et al., Phosphate removal from aqueous solution using iron oxides: Adsorption, desorption and regeneration characteristics. J. Colloid Interface Sei.

[0324] 528, 145-155 (2018).

[0325] 11. R. Chitrakar et al., Phosphate adsorption on synthetic goethite and akaganeite. J. Colloid Interface Sei. 298, 602-608 (2006).

[0326] 12. B. D. Martin, S. A. Parsons, B. Jefferson, Removal and recovery of phosphate from municipal wastewaters using a polymeric anion exchanger bound with hydrated ferric oxide nanoparticles. Water Sei. Technol. 60, 2637-2645 (2009).

[0327] 13. T. NUR (2014) Nitrate, phosphate and fluoride removal from water using adsorption process. (University of Technology Sydney, New South Wales, Australia).

[0328] 14. M. R. Awual, A. Jyo, Assessing of phosphorus removal by polymeric anion exchangers. Desalination 281, 111-117 (2011).

[0329] 15. Y. Shang, K. Guo, P. Jiang, X. Xu, B. Gao, Adsorption of phosphate by the cellulose-based biomaterial and its sustained release of laden phosphate in aqueous solution and soil. Int. J. Biol. Macromol. 109, 524-534 (2018).

[0330] 16. S. Pap et al., Low-cost chitosan-calcite adsorbent development for potential phosphate removal and recovery from wastewater effluent. Water Res. 173, 115573 (2020).

[0331] 17. J. Wei, X. Meng, X. Wen, Y. Song, Adsorption and recovery of phosphate from water by amine fiber, effects of co-existing ions and column filtration. J. Environ. Sei.

[0332] 87, 123-132 (2020).

[0333] 18. M. Xanthopoulou et al., Phosphate Removal Using Polyethylenimine Functionalized Silica-Based Materials. Sustainability 13, 1502 (2021).

[0334] 19. E. Zong et al., Reusable Hyperbranched Polyethylenimine-Functionalized Ethyl Cellulose Film for the Removal of Phosphate with Easy Separation. ACS Omega 6, 505-515 (2021).

[0335] 20. G. D. Pinching, R. G. Bates, Second dissociation constant of succinic acid from 0° to 50° C. J Res Natl Bureau Stand 45 (1950).

[0336] 21. G. D. Pinching, R. G. Bates, First dissociation constant of succinic acid from 0 to 50 c and related thermodynamic quantities. J Research NBS 45, 444449 (1950). Example 3. Use in Column of PEI / PMVEMA (3:1) Hydrogel

[0337] Materials and Methods

[0338] The hydrogel was prepared as described above.

[0339] The volume of the column is approximately 0.5 cm3. The flow rate was controlled by the pump. Flow rate 0.05 mL / min is equivalent to 6 BV / h.

[0340] Number of bed volumes (BV) is calculated as follows:

[0341]

[0342] The absorbed P and desorbed P are calculated as follows, where c represents the phosphate concentration:

[0343]

[0344] Effects of influent pH. The pH of influent 8 mg / L phosphate-P solution is controlled by adding HC1 to 3.0, 4.5, and 6.0. The effluent concentration of phosphate was measured by molybdenum blue method.

[0345] Regeneration cycles. 8 mg / L of phosphate solution at pH 4.5 was chosen as the influent solution at a flow rate of 6 BV / h. Desorption was performed with 0.01 mol / L NaOH solution at 6 BV / h using the same gel from the absorption test. After the desorption, the gel will be flushed with deionized water for 144 BV to remove residual NaOH. Three cycles of absorption and desorption were conducted on the same gel.

[0346] Competing absorption with sulfate and selective desorption: 8 mg / L of phosphate and 8 mg / L of sulfate solution was conditioned to pH 4.5 and flowed at 6 BV / h.

[0347] Before the desorption test, the PEI / PMVEMA hydrogel first absorbed 8 mg / L of phosphate and sulfate at pH 4.5 for 480 BV and at 6 BV / h. Desorption of the presoaked hydrogel was performed with 0.01 mol / L NaOH solution at 6 BV / h.

[0348] Effects of influent pH From the above batch studies, we identified a pH of approximately 4.5 for the sorption of phosphate using PEI / PMVEMA (3:1) hydrogel. Phosphoric acid, whose pKai is 2.2 and pKa2is 7.7, undergoes species change in response to pH variations. PEI / PMVEMA hydrogel has two major functional groups, carboxylic acids and amine groups. The pKa of amine groups is between 9 and 11, while the ones of acids are 4.2 and 5.6. The two functional groups can undergo protonation and deprotonation to change forms as well, therefore, affecting the P sorption process.

[0349] We conducted the PEI / PMVEMA hydrogel in fixed-bed column with influent at pH 3, 4.5, and 6. As shown in FIG. 11, an effluent P concentration below 8 mg / L means the phosphate in the influent was absorbed by the hydrogel packed in the column. When the effluent phosphate concentration equaled the influent concentration, it signified that the adsorption sites on the hydrogel had become saturated. The highest phosphate sorption capacity was observed at pH 4.5, followed by pH 6.0 and pH 3.0. This variation in phosphate sorption capacity with pH is attributed to the protonation states of both the hydrogel functional groups and the phosphate species. The synergistic effects of amine, carboxyl, and phosphate determine the absorption capacity. Below pH 9, the amine groups are protonated and charged positively. At pH 3, phosphate primarily exists as H3PO4, which is not engaged in strong electrostatic interactions. At pH 6, phosphate is anionic to be attracted by protonated amines. However, the carboxyl groups are deprotonated in the hydrogel and repel anions electrostatically. At pH 4.5, the carboxyl groups are slightly deprotonated, and anionic phosphate allows for favorable electrostatic interactions with cationic protonated amines, resulting in the highest overall phosphate sorption capacity in this region.

[0350] Table 6. Total absorption of phosphate varying with influent pH

[0351]

[0352] Regeneration Cycles

[0353] PEI / PMVEMA hydrogel was loaded with phosphate in the 8 mg / L phosphate solution at pH 4.5. The desorption analysis of phosphate-loaded PEI / PMVEMA hydrogel was conducted with 0.01 M NaOH solution (pH 12) at 6 BV / h. Three absorption-desorption cycles were performed, as shown in FIGs. 12A-12B. The trends of the three desorption processes in FIG. 12B were similar, where the phosphate concentration of the initial effluent is o before 10 BV and follows a Gaussian distribution as a function of time from 10 to 100 BV. The highest concentration of phosphate-P in the desorption effluent is over 100 mg / L. Table 7 summarizes the total absorbed and desorbed phosphate, as well as the regeneration efficiency across the three cycles. The PEI / PMVEMA hydrogel demonstrated excellent reusability, with over 85% regeneration capacity in each cycle and a desorption efficiency exceeding 97%. The eluted phosphate can be further utilized in various applications.

[0354] The desorption probably results from the deprotonation of PEI / PMVEMA hydrogel. In the beginning of desorption, the preloaded PEI / PMVEMA was in an acidic condition, and the functional groups were protonated, and the hydrogel still had strong affinity to phosphates. As the basic effluent starts to pass through the column, PEI / PMVEMA hydrogel bonded with phosphate started to deprotonate and release phosphate. Desorption was a competitive process of chemical deprotonation and physical transport, including convection and diffusion.

[0355] Table 7. Cycles of absorption and desorption of phosphate

[0356]

[0357] Competing absorption with sulfate and selective desorption Sulfate is commonly present in most water sources and competes with phosphate for the active sites of the hydrogel. To investigate this competitive adsorption, we conducted experiments using an equimolar solution of sulfate and phosphate at concentrations of 8 mg / L. As shown in FIG. 13, the effluent first comes with phosphate at around 10 BV and then follows with sulfate at around 150 BV. After 350 BV, the effluent phosphate concentration exceeded the influent concentration of 8 mg / L, which can be attributed to sulfate displacing the adsorbed phosphate ions. Phosphate concentration peaked at 450 BV and gradually decreased over time until it equaled the influent concentration of 8 mg / L. Simultaneously, the effluent sulfate concentration increased steadily to 8 mg / L, indicating continuous sulfate adsorption by the hydrogel. Ultimately, the hydrogel becomes fully saturated with sulfate.

[0358] To investigate the desorption behavior of phosphate and sulfate, the PEI / PMVEMA hydrogel was preloaded with both ions and desorbed using a 0.01 M NaOH solution.

[0359] FIGs. 14A-14B show the elution of phosphate and sulfate by species and bed volumes. Phosphate was desorbed first, followed by sulfate, suggesting a difference in the anionic affinities of the two species to the hydrogel. As is more apparent in FIG. 14B, the ratio of phosphate and sulfate is greater than 1 before 20 BV, demonstrating that the phosphate was preferentially washed out in the early state. The P / S ratio dropped dramatically after 15 BV, showing that the sulfate was the dominant desorbed component. This time discrepancy in the elution of phosphate and sulfate presents an opportunity for selective desorption, where controlling time and other parameters could allow for preferential release of the target ion.

[0360] The fixed-bed column is a widely used technology in industrial applications, valued for its ability to process large volumes of liquid with precise control. This example demonstrates that the PEI / PMVEMA hydrogel exhibits a high phosphate sorption capacity, reaching up to 96 mg / g, and can be reused over multiple cycles in column experiments. Moreover, given that sulfate exhibits a higher affinity for hydrogel compared to phosphate, it is possible to finely tune the desorption sequence to achieve selective separation of different anions.

[0361] The compositions and methods of the appended claims are not limited in scope by the specific compositions and methods described herein, which are intended as illustrations of a few aspects of the claims, and any compositions and methods that are functionally equivalent are intended to fall within the scope of the claims. Various modifications of the compositions and methods, in addition to those shown and described herein, are intended to fall within the scope of the appended claims. Further, while only certain representative compositions and method steps disclosed herein are specifically described, other combinations of the compositions and method steps also are intended to fall within the scope of the appended claims, even if not specifically recited. Thus, a combination of steps, elements, components, or constituents may be explicitly mentioned herein; however, other combinations of steps, elements, components, and constituents are included, even though not explicitly stated.

Claims

WHAT IS CLAIMED IS:

1. A hydrogel formed from:a first polymer comprising a plurality of amine moieties; anda second polymer comprising a plurality of cyclic acid anhydride moieties.

2. The hydrogel of claim 1,wherein the second polymer is crosslinked by the first polymer.

3. The hydrogel of claim 1 or claim 2, wherein the first polymer comprises a polyalkylamine.

4. The hydrogel of any one of claims 1-3, wherein the first polymer comprises polyethyleneimine (PEI).

5. The hydrogel of any one of claims 1-4, wherein the first polymer comprises branched polyethyleneimine (PEI).

6. The hydrogel of any one of claims 1-4, wherein the first polymer comprises linear polyethyleneimine (PEI).

7. The hydrogel of any one of claims 1-6, wherein the second polymer comprises a copolymer of at least one cyclic anhydride and one or more additional monomers.

8. The hydrogel of claim 7, wherein the copolymer is an alternating copolymer.

9. The hydrogel of claim 7, wherein the copolymer is a statistical or random copolymer.

10. The hydrogel of any one of claims 7-9, wherein the at least one cyclic anhydride comprises succinic anhydride.

11. The hydrogel of any one of claims 7-10, wherein the one or more additional monomers comprise a monomer having one or more alkenyl moieties.

12. The hydrogel of any one of claims 7-11, wherein the one or more additional monomers comprise a vinyl ether or a vinyl ester.

13. The hydrogel of any one of claims 7-12, wherein the one or more additional monomers comprise an alkyl vinyl ether.14- The hydrogel of any one of claims 7-13, wherein the one or more additional monomers comprise methyl vinyl ether.

15. The hydrogel of any one of claims 7-14, wherein the one or more additional monomers comprise ethyl vinyl ether.

16. The hydrogel of any one of claims 7-15, wherein the one or more additional monomers comprise vinyl acetate or vinyl propionate.

17. The hydrogel of any one of claims 7-16, wherein the second polymer comprises a constitutional repeating unit having the structure:wherein RAis selected from hydrogen, RXO-, and RZC(O)-;Rxis independently selected at each occurrence from Ra, RZC(O)-, RZC(N)-, RZS(O)-, and RZS(0)2-, each of which maybe optionally substituted with one or more Y groups as allowed by valency;Rzis independently selected at each occurrence from hydrogen, halo, Ci-Cealkyl, Ci-Cohaloalkyl, C2-C6alkenyl, C2-C6alkynyl, (C3-C7cycloalkyl)-(Co-C3alkyl)-, (4- to 6-membered heterocycle)-(Co-C3alkyl)-, (5- to 10-membered monocyclic or bicyclic aryl)-(Co-C3alkyl)-, (5- to 10-membered monocyclic or bicyclic heteroaryl)-(Co-C3alkyl)-, -ORa, -SRa, and -NRaRb, each of which maybe optionally substituted with one or more Y groups as allowed by valency; andRaand Rbare independently selected at each occurrence from hydrogen, Ci-Cealkyl, Ci-Cehaloalkyl, C2-C6alkenyl, C2-C6alkynyl, (C3-C7cycloalkyl)-(CO-C3alkyl)-, (4- to 6-membered heterocycle)-(Co-C3alkyl)-, (5- to 10-membered monocyclic or bicyclic aryl)-(Co-C3alkyl)-, (5- to 10-membered monocyclic or bicyclic heteroaryl)-(Co-C3alkyl)-, each of which may be optionally substituted by one or more Y groups as allowed by valency; andY is independently selected at each occurrence from hydrogen, halo, nitro, cyano, azido, oxo, Ci-Ce alkyl, Ci-Ce haloalkyl, C2-C6 alkenyl, C2-C6 alkynyl, (C3-C6 cycloalkyl) (C0-C3 alkyl)-, (3- to 8-membered monocyclic or bicyclic heterocycle)-(Co-Co alkyl)-, (6- to 10-membered monocyclic or bicyclic aryl)-(Co-C6 alkyl)-, (5- to 10-membered monocyclic or bicyclic heteroaryl)-(Co-C6 alkyl)-, RPO-, RPS-, RPRC'N-, R°C(O)-, R°C(O)-O-, R°C(O)-NRq-, R°S(0)2-, R°S(0)2-0-, and R°S(0)2-NRq-, wherein R° is independently selected at each occurrence from RP, halo, RPO-, and RpRqN-, and wherein RP and Rc' are independently selected at each occurrence from hydrogen, Ci-Ce alkyl, Ci-Ce haloalkyl, C2-C6 alkenyl, C2-C6 alkynyl, (C3-C6 cycloalkyl) (C0-C3 alkyl)-, (3- to 8-membered monocyclic or bicyclic heterocycle)-(Co-Co alkyl)-, (6- to 10-membered monocyclic or bicyclic aryl)-(Co-C6 alkyl)-, and (5- to 10-membered monocyclic or bicyclic heteroaryl)-(Co-C6 alkyl)-.

18. The hydrogel of claim 17, wherein RAis Rx0-.

19. The hydrogel of claim 18, wherein Rxis Ci-Ce alkyl.

20. The hydrogel of claim 18, wherein Rxis RZC(O)-.

21. The hydrogel of claim 20, wherein Rzis Ci-Ce alkyl.

22. The hydrogel of any one of claims 7-21, wherein the one or more additional monomers comprise at least one hydrophilic monomer.

23. The hydrogel of any one of claims 1-22, wherein the second polymer comprises poly(methyl vinyl ether-alt-maleic anhydride) (PMVEMA).

24. The hydrogel of any one of claims 1-23, wherein a ratio of a first repeating unit of the first polymer to a second repeating unit of the second polymer is from about 6:1 to about 1:6.

25. The hydrogel of any one of claims 1-24, wherein a ratio of a first repeating unit of the first polymer to a second repeating unit of the second polymer is about 3:

1.

26. The hydrogel of any one of claims 1-25, wherein the hydrogel is capable of selective adsorption of phosphate from water in contact with the hydrogel at a pH from about 2.0 to about 7.0.

27. The hydrogel of any one of claims 1-26, wherein the hydrogel is capable of selective adsorption of phosphate over nitrate from water when a mass ratio of nitrate to phosphate in the water in contact with the hydrogel is less than about 2.5.

28. The hydrogel of any one of claims 1-27, wherein the hydrogel is capable of desorption of phosphate from the hydrogel at a pH of about 9 to about 11.

29. The hydrogel of claim 28, wherein the hydrogel is capable of desorption of phosphate from the hydrogel when contacted with a hydroxide solution (such as a sodium hydroxide or potassium hydroxide solution).

30. A water filtration system comprising the hydrogel of any one of claims 1-29.

31. The water filtration system of claim 30, wherein the water filtration system comprises a filter column or a flow bed.

32. A method for removing phosphate from a water source, the method comprising contacting the water source with the hydrogel of any one of claims 1-29.

33. The method of claim 32, wherein the water source comprises a waterway (such as a river, lake, or stream) or agricultural runoff.

34. The method of claim 32 or claim 33, wherein phosphate is selectively removed over nitrate from the water source.

35. The method of any one of claims 32-34, wherein contacting the water source with the hydrogel comprises filtering the water source through a water filtration system comprising the hydrogel.

36. The method of any one of claims 32-35, further comprising contacting the hydrogel with a hydroxide solution, whereupon the phosphate is desorbed from the hydrogel.

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