Composition of matter for selective separation of palladium and methods of use and manufacture thereof

Abstract

A composition of matter for selective separation of a Pd species from a mixture is provided, wherein the composition includes an a having a support and at least one ligand attached to the support, wherein the at least one ligand includes a S donor with least one S atom and a N donor with at least one N atom, and the at least one ligand selectively adsorbs the Pd species over one or more competing ions in the mixture that contains a Pt species. Also provided are methods of preparing and using the composition or adsorbent, an article of manufacture including the composition or adsorbent, and a use of the composition or adsorbent for selective separation of a Pd species.

Description

COMPOSITION OF MATTER FOR SELECTIVE SEPARATION OF PALLADIUM AND METHODS OF USE AND MANUFACTURE THEREOFCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U.S. Provisional Application Serial No. 63 / 638,959, filed on April 26, 2024, and entitled “THIOL-FUNCTIONALIZED CELLULOSE ADSORBENTS FOR THE SELECTIVE SEPARATION OF PALLADIUM OVER PLATINUM IN AQUEOUS SOLUTION”, the entirety of which is incorporated herein by reference.FIELD OF THE INVENTION

[0002] The present invention generally relates to the field of adsorbent materials. In particular, the present invention is directed to compositions of matter for selective separation of palladium (Pd) and methods of use and manufacture thereof.BACKGROUND

[0003] Palladium (Pd, Atomic No. 46) and platinum (Pt, Atomic No. 78) are platinum group metals (PGMs) with outstanding catalytic activity and corrosion resistance. Pd and Pt can be used in automotive catalytic converters, petroleum catalysts, and fuel cell vehicles, among others. The rapidly developing hydrogen fuel cell industry and the use of catalytic converters in automobiles have increased the demand of PGMs. However, the supply of PGMs is limited, as their abundance is relatively rare in the Earth’s crust (0.0063 ppm), which poses a potential risk of shortage. Some industrial wastes, such as catalytic converters and electronic wastes, contain large amounts of Pd and Pt species (27-120 mg / kg), and recovery and recycling of PGMs from such secondary sources is therefore desirable. Conventional recovery methods of PGMs include chemical precipitation, solvent extraction, and ion exchange, but these methods often generate secondary pollution or exhibit low efficiency or selectivity. In addition, the selective separation of palladium (Pd) over platinum (Pt) remains challenging, as Pd and Pt exhibit similar electronic configurations and chemical properties. SUMMARY OF THE DISCLOSURE

[0004] An aspect is a composition of matter as disclosed herein for selective separation of a Pd species from a mixture. The composition includes an adsorbent, wherein the adsorbent includes a support and at least one ligand attached to the support. The at least one ligand includes a S donor having at least one S atom and a N donor having at least one N atom. The at least one ligand selectively adsorbs the Pd species over one or more competing ions, including a Pt species, in the mixture.

[0005] Another aspect is a method of preparing the adsorbent described in this disclosure. The method includes providing a support having at least one surface group. The method further includes providing a bifunctional linker, wherein the bifunctional linker includes a first reactive group and a second reactive group. The method further includes connecting the bifunctional linker to the at least one surface moiety of the support using the first reactive group. The method further includes providing the at least one ligand. The method further includes attaching the at least one ligand to the second reactive group of the bifunctional linker to produce the adsorbent.

[0006] Another aspect is a method of using the adsorbent described in this disclosure. The method includes contacting the adsorbent with a mixture of the Pd species and one or more competing ions. The method further includes selectively adsorbing the Pd species for a period of time.

[0007] Another aspect is an adsorbent for selective separation of a Pd species from a mixture, wherein the adsorbent is prepared using the method described in this disclosure.

[0008] Another aspect is an article of manufacture for selective separation of a Pd species from a mixture, wherein the article of manufacture includes the composition or adsorbent described in this disclosure.

[0009] Another aspect is the use of the composition or adsorbent described in this disclosure for selective separation of a Pd species from a mixture.

[0010] These and other aspects and features of nonlimiting embodiments of the present invention will become apparent to those skilled in the art upon review of the following description of specific nonlimiting embodiments of the invention in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] For the purpose of illustrating the invention, the drawings show aspects of one or more embodiments of the invention. However, it should be understood that the present invention is not limited to the precise arrangements and instrumentalities shown in the drawings.

[0012] FIG. 1 depicts synthetic schemes of three cellulose adsorbents functionalized with 2- aminothiophenol (Cell-AP), 2-mercaptopyridine (Cell-MP), and 2-mercaptobenzothiazole (Cell- MBT), respectively.

[0013] FIG. 2A depicts Fourier-transform infrared spectroscopy (FTIR) data of Cell-AP, Cell- MP, and Cell-MBT adsorbents before adsorption of Pd(II).

[0014] FIG. 2B depicts the FTIR spectra of Cell-AP, Cell-MP, and Cell-MBT adsorbents after adsorption of Pd(II), i.e., Cell-AP -Pd, Cell-MP -Pd, and Cell-MBT-Pd.

[0015] FIG. 2C depicts a plot of Brunauer-Emmett-Teller (BET) adsorption and desorption isotherm of unfunctionalized cellulose.

[0016] FIG. 2D depicts a plot of BET pore size distribution of unfunctionalized cellulose.

[0017] FIG. 2E depicts a plot of BET adsorption and desorption isotherm of Cell-AP adsorbent.

[0018] FIG. 2F depicts a plot of BET pore size distribution of Cell-AP adsorbent.

[0019] FIG. 3A depicts data showing the effect of pH on the adsorption of Pd(ll) and Pt (IV) byCell-AP adsorbent. Adsorption experiments were performed using 10 mb Pd / Pt mix solution, at an initial concentration of 200 mg / L each, and 10 mg adsorbent, over a contact time of 48 h.

[0020] FIG. 3B depicts data showing the effect of pH on the adsorption of Pd(II) and Pt (IV) by Cell-MP adsorbent. Adsorption experiments were performed using 10 mb Pd / Pt mix solution, at an initial concentration of 200 mg / L each, and 10 mg adsorbent, over a contact time of 48 h.

[0021] FIG. 3C depicts data showing the effect of pH on the adsorption of Pd(II) and Pt (IV) by Cell-MPT adsorbent. Adsorption experiments were performed using 10 mb Pd / Pt mix solution, at an initial concentration of 200 mg / L each, and 10 mg adsorbent, over a contact time of 48 h.

[0022] FIG. 3D depicts a plot comparing the adsorption capacity of Pd (mg / g) and the qe-pd / qe-pt ratio of Cell-AP, Cell-MP, and Cell-MBT adsorbents with previous studies, showing superior performance of the functionalized cellulose adsorbents.

[0023] FIG. 4A depicts a plot showing the effect of initial concentration on the adsorption of Pd(II) by Cell-AP, Cell-MP, and Cell-MBT adsorbents. The lines represent fits using the Langmuir isotherm model and the Freundlich isotherm model, respectively. Adsorption experiments were performed using 10 mL Pd solution, at an initial concentration from 20-220 mg / L, and 10 mg adsorbent, over a contact time of 48 h. Adsorption experiments for Cell-AP and Cell-MP adsorbents were performed at pH = 1.0, and adsorption experiments for Cell-MBT adsorbents were performed at pH = 2.5.

[0024] FIG. 4B depicts a plot showing linear fits for the adsorption of Pd(ll) by Cell-AP, Cell- MP, and Cell-MBT adsorbents based on the linearized Langmuir adsorption model.

[0025] FIG. 5 A depicts a plot showing the adsorption of Pd(II) by Cell-AP, Cell-MP, and Cell- MBT adsorbents as a function of contact time. Adsorption experiments were performed using 10 mL Pd solution, at an initial concentration of 100 mg / L, and 10 mg adsorbent. Adsorption experiments for Cell-AP and Cell-MP were performed at pH = 1.0, and adsorption experiments for Cell-MBT were performed at pH = 2.5. The lines represent fits using the pseudo-second-order (PSO) model, the pseudo-first-order (PFO) model, and the Elovich model, respectively.

[0026] FIG. 5B depicts a plot showing linear fits for the adsorption of Pd(II) by Cell-AP, Cell- MP, and Cell-MBT adsorbents based on the linearized PSO model.

[0027] FIG. 5C depicts a plot showing linear fits for the adsorption of Pd(II) by Cell-AP, Cell- MP, and Cell-MBT adsorbents based on the linearized PFO model.

[0028] FIG. 5D depicts a plot showing linear fits for the adsorption of Pd(II) by Cell-AP, Cell- MP, and Cell-MBT adsorbents based on the linearized Elovich model.

[0029] FIG. 6A depicts a plot showing the adsorption efficiency (%) for Cell-AP, Cell-MP, and Cell-MBT adsorbents over 5 adsorption / desorption cycles. The adsorption test was performed using 10 mb Pd-Pt mix solution, at an initial concentration 100 mg / L, and 10 mg adsorbent, at pH = 2.5, in an HC1 solution, over a contact time 24 h, using a batch test at 25 °C.

[0030] FIG. 6B depicts a plot showing the desorption efficiency (%) for Cell-AP, Cell-MP, and Cell-MBT adsorbents over 5 adsorption / desorption cycles. The desorption test was performed via regeneration using 3 M HC1 over a contact time of 24 h.

[0031] FIG. 7 depicts a plot showing the selectivity of Cell-AP, Cell-MP, and Cell-MBT adsorbents in a 19-element mix solution. Adsorption experiments were performed using 10 m , 100 mg / L Pd-Pt solution containing 18 competing ions each at 20 mg / L, 5 mg adsorbent, at pH = 1, pH = 2, and pH = 3, respectively, over a contact time of 48 h.

[0032] FIG. 8A depicts X-ray photoelectron spectroscopy (XPS) data of cellulose, Cell-MP, and Cell-MP adsorbents after Pd(II) adsorption (i.e., Cell-MP -Pd).

[0033] FIG. 8B depicts Pd 3d spectra of Cell-MP and Cell-MP-Pd, respectively, measured using XPS.

[0034] FIG. 8C depicts Cl 2p and S 2p spectra of Cell-MP and Cell-MP-Pd, respectively, measured using XPS.

[0035] FIG. 8D depicts CHNS elemental compositions of various chemical compositions and materials described herein.

[0036] FIG. 9 depicts a proposed mechanism of adsorption between Cell-AP, Cell-MP and Cell- MBT adsorbents and PdCL2' ions.

[0037] FIG. 10A depicts electrostatic potential (ESP) maps of a portion of Cell-AP, Cell-MP, and Cell-MBT adsorbents based on density functional theory (DFT) calculations. Heteroatoms (i.e., atoms other than C and H) and / or their respective computed charges are marked accordingly.

[0038] FIG. 10B depicts optimized structures of Cell-AP, Cell-MP, and Cell-MBT adsorbents coordinated with PdCL2’ based on DFT calculations. Heteroatoms (i.e., atoms other than C and H), computed bond lengths, and computed atomic charges are labeled accordingly.

[0039] FIG. 10C depicts optimized structures of Cell-AP, Cell-MP, and Cell-MBT adsorbents coordinated with PtCk2’ based on DFT calculations. Heteroatoms (i.e., atoms other than C and H), computed bond lengths, and computed atomic charges are labeled accordingly.

[0040] FIG. 11. depicts the HOMO and LUMO energy of multiple metal ions and ligands (including Pd2+, Pt4+, and R-S-R ligand) calculated using a Gaussian molecular orbital (MO) function (Basis set: LANL2DZ, RB3LYP, solvent = water). The gap between HOMO and LUMO is proportional to the hardness level, r|, of a Lewis acid or Lewis base.

[0041] FIG. 12 depicts exemplary scanning electron microscopy (SEM) images of Cell-AP (top) and Cell-MP (bottom) adsorbents. The scale bars represent 100 pm (left) and 5pm (right), respectively.

[0042] The drawings are not necessarily to scale and may be illustrated by phantom lines, diagrammatic representations, and fragmentary views. In certain instances, details that are not necessary for an understanding of the embodiments or that render other details difficult to perceive may have been omitted.DETAILED DESCRIPTION

[0043] To facilitate the understanding of this invention, a number of terms are defined below and throughout the disclosure. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict, the present specification, including definitions, will control. The terminology herein is used to describe specific embodiments of the invention, but their usage does not limit the invention, except as outlined in the claims. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety.

[0044] It is to be understood that any aspect and / or element of any embodiment of the method(s) described herein or otherwise may be combined in any way to form additional embodiments of the method(s), all of which are within the scope of the method(s).

[0045] As used herein, including the claims, the phrase “at least some” means “one or more” and includes the case of only one. Thus, e.g., the phrase “at least some ABCs” means “one or more ABCs” and includes the case of only one ABC.

[0046] As used herein, including the claims, the term “at least one” should be understood as meaning “one or more” and therefore includes both embodiments that include one or multiple components. Furthermore, dependent claims that refer to independent claims that describe features with “at least one” have the same meaning, both when the feature is referred to as “the” and “the at least one”.

[0047] As used herein, the term “portion” means some or all. Therefore, for example, “a portion of X” may include some of “X” or all of “X”. In the context of a conversation, the term “portion” means some or all of the conversation.

[0048] As used herein, including the claims, the phrase “using” means “using at least” and is not exclusive. Thus, e.g., the phrase “using X” means “using at least X”. Unless specifically stated by use of the word “only”, the phrase “using X” does not mean “using only X”.

[0049] As used herein, including the claims, the phrase “based on” means “based in part on” or “based, at least in part, on” and is not exclusive. Thus, e.g., the phrase “based on factor X” means “based in part on factor X” or “based, at least in part, on factor X”. Unless specifically stated by use of the word “only”, the phrase “based on X” does not mean “based only on X”.

[0050] In general, as used herein, including the claims, unless the word “only” is specifically used in a phrase, it should not be read into that phrase.

[0051] As used herein, including the claims, the phrase “distinct” means “at least partially distinct”. Unless specifically stated, distinct does not mean fully distinct. Thus, e.g., the phrase “X is distinct from Y” means that “X is at least partially distinct from Y” and does not mean that “X is fully distinct from Y”. Thus, as used herein, including the claims, the phrase “X is distinct from Y” means that X differs from Y in at least some way.

[0052] It should be appreciated that the words “first”, “second”, and so on, in the description and claims, are used to distinguish or identify, and not to show a serial or numerical limitation.

[0053] Similarly, letter labels (e.g., “(A)”, “(B)”, “(C)”, and so on, or “(a)”, “(b)”, and so on) and / or numbers (e.g., “(i)”, “(ii)”, and so on) are used to assist in readability and to help distinguish or identify, and are not intended to be otherwise limiting or to impose or imply any serial or numerical limitations or orderings. Similarly, words such as “particular”, “specific”, “certain”, and “given”, in the description and claims, if used, are to distinguish or identify, and are not intended to be otherwise limiting.

[0054] As used herein, including the claims, the terms “multiple” and “plurality” mean “two or more,” and include the case of “two”. Thus, e.g., the phrase “multiple ABCs” means “two or moreABCs” and includes “two ABCs”. Similarly, e.g., the phrase “multiple PQRs” means “two or more PQRs” and includes “two PQRs”.

[0055] The present invention also covers the exact terms, features, values, and ranges, etc., in case these terms, features, values, and ranges, etc., are used in conjunction with terms such as “about”, “around”, “generally”, “substantially”, “essentially”, “at least”, etc. Thus, e.g., “about 3” or “approximately 3” shall also cover exactly 3, and “substantially constant” shall also cover exactly constant.

[0056] As used herein, unless stated otherwise, the terms “about” or “approximately” refer to a value that is within 10% above or below the value being described.

[0057] As used herein, including the claims, singular forms of terms are to be construed as also including the plural form and vice versa, unless the context indicates otherwise. Thus, it should be noted that as used herein, the singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. In other words, terms such as “a”, “an”, and “the” are not intended to refer to only a singular entity but include the general class of which a specific example may be used for illustration.

[0058] Throughout the description and claims, the terms “comprise”, “including”, “having”, “contain”, and their variations should be understood as meaning “including but not limited to” and are not intended to exclude other components unless specifically so stated.

[0059] It will be appreciated that variations to the embodiments of the invention can be made while still falling within the scope of the invention. Alternative features serving the same, equivalent, or similar purpose can replace features disclosed in the specification, unless stated otherwise. Thus, unless stated otherwise, each feature disclosed represents one example of a generic series of equivalent or similar features.

[0060] Use of exemplary language, such as “for instance”, “such as”, “for example” (“e.g.,”), and the like, is merely intended to better illustrate the invention and does not indicate a limitation on the scope of the invention unless specifically so claimed.

[0061] While the invention has been described in connection with what is presently considered to be the most practical and embodiments thereof are further described in the examples below, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

[0062] The following description sets forth various examples along with specific details to provide a thorough understanding of claimed subject matter. It will be understood by those skilled in the art, however, that claimed subject matter may be practiced without one or more of the specific details disclosed herein. Further, in some circumstances, well-known methods, procedures, systems, and / or components have not been described in detail in order to avoid unnecessarily obscuring claimed subject matter. The illustrative embodiments described in the detailed description and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and make part of this disclosure.Composition of Matter / Adsorbent

[0063] An objective of the present application is to provide a composition of matter for selective separation of a Pd species from a mixture. In some embodiments, the composition includes an adsorbent. As used herein, an “adsorbent” is a type of substance or material that adsorbs one or more chemical species on its surface. Accordingly, as used herein, “adsorption” is a surface-based process where one or more chemical species adhere to the surface of an adsorbent in order to be isolated, concentrated, purified, or eliminated. A chemical species to be captured by an adsorbent is accordingly termed an adsorbate, and may include one or more atoms, ions, molecules / biomolecules, and / or the like, depending on the exact use case. An adsorbent typically has a porous structure with a large specific surface area, i.e., the total surface area per unit mass, in order to increase its contact with an adsorbate. An adsorption process is typically driven by the high surface energy of undercoordinated atoms or ions in an adsorbent, and by binding to an adsorbate, an adsorbent reduces its surface energy to reach a more stable thermodynamic state; this process is governed by the Second Law of Thermodynamics. Depending on the nature and / or strength of binding interaction, an adsorption process may be further classified as physisorption or chemisorption. Physisorption is typically driven by weaker intermolecular interactions such as van der Waals forces, with an adsorption enthalpy often in the range of 20 to 40 kJ / mol. In contrast, chemisorption is typically driven by stronger chemical bonds (e.g., covalent bonds, ionic bonds, and / or coordinate bonds) instead, with an adsorption enthalpy often in the range of 80 to 240 kJ / mol. It is worth noting that such distinction is not absolute and permits intermediate cases.

[0064] An adsorption process is typically engineered to be selective, and an adsorbent is often designed for a particular type of adsorbate(s) to which it preferentially binds with a high affinity. More specifically, in metal recovery or recycling processes, adsorption is a promising technology that typically uses functionalized solid substrates as a stationary phase to recover critical elements from a mobile liquid phase, usually acidic aqueous solutions frequently encountered in surface (i.e., above-ground) or traditional (i.e., underground) mining. Adsorbents are highly versatile and can be chemically tailored to improve their selectivity for target ions. Adsorbents can also be easily regenerated for repeated use. These features make adsorption an efficient, cost-effective, and environmental -friendly technique, especially for isolation of precious metals from low-concentration feeds.

[0065] In some embodiments, an adsorbent can include a support and at least a ligand attached to the support. In some embodiments, an adsorbent can include a support, at least a ligand, and one or more linkers covalently connected to and separating the support and the at least a ligand.

[0066] In some embodiments, an adsorbent can be prepared using any method(s) of preparation, or combination of thereof, as described herein in this disclosure. In some embodiments, an adsorbent can be used for selective separation of a Pd species according to method(s) of use, or combination thereof, as described herein in this disclosure.Support

[0067] In some embodiments, an adsorbent described herein includes a support. As used herein, a “support” is a substance, often a nonreactive solid with a high surface area, to which one or more chemical species are attached to impart and / or amplify specific chemical functions. In some embodiments, a support can include a flexible support. As used herein, a “flexible” support is a type of support with an ability to be bent, twisted, reshaped, or deformed without breaking.

[0068] In some embodiments, a support can include unsubstituted or substituted cellulose. As used herein, “cellulose” is an organic compound with a chemical formula of (CeHio05)n and a polysaccharide that includes hundreds to thousands of 0(1— >4) linked D-glucose units. Cellulose is the most abundant biopolymer on Earth, found in trees, waste from agricultural crops and other biomass, and a promising green adsorbent due to its biodegradability and non-toxicity.

[0069] In some embodiments, a support can include silica. As used herein, “silica” or “silicon dioxide” is a compound of silicon and oxygen at a 1 :2 molar ratio. As nonlimiting examples, silica can include mesoporous silica. As used herein, “mesoporous” silica” is a type of silica with an ordered arrangement of mesopores having a pore size from 2 to 50 nm. Mesoporous silica can have aspecific surface area above 300 m2 / g and be used in applications such as biomedicine, catalysis, adsorption, energy storage, and / or the like.

[0070] In some embodiments, a support can include activated alumina. As used herein, “activated alumina” is a porous form of aluminum oxide. Activated lumina typically has a large specific surface area, often in the range of 200-300 m2 / g, and can be used in various types of application such as heterogenous catalysts, desiccants, and adsorbents. As nonlimiting examples, activated alumina can include phases such as a-alumina, P-alumina, y-alumina, 6-alumina, 0- alumina, r|-alumina, and / or the like.

[0071] In some embodiments, a support can include a zeolite. As used herein, a “zeolite” is a type of a microporous, crystalline aluminosilicate mineral with a general formula ofMn+i / n(A102) (SiO2)x-yH2O, where M11+is either a metal ion or H+. Similar to microporous silica and activated alumina, zeolites are also characterized by a large specific surface area and frequently used as adsorbents or supports for heterogeneous catalysts. Zeolites have microporous structures with a typical pore diameter of 0.3-0.8 nm.

[0072] In some embodiments, a support can include a polymer resin. As used herein, a “polymer” is a large molecule with repeating structural units connected to one another by covalent chemical bonds. As used herein, a “resin” or “polymer resin” is a type of polymer that can harden permanently. As a nonlimiting example, a polymer resin can include a polystyrene resin. As used herein a “polystyrene resin” is a type of resin polymerized, at least in part, using styrene as a monomer. As another nonlimiting example, a polymer or polymer resin can include an ion-imprinted polymer. As used herein, an “ion-imprinted” polymer is a type of polymer material with functional monomer units forming binding sites, pores, or cavities that specifically bind to one or more target ions or molecules.

[0073] In some embodiments, a support can include activated carbon. As used herein, “activated carbon”, also known as activated charcoal, is a form of carbonaceous material processed or activated to have a porous structure and a large specific surface area. Activated carbon can have a porous amorphous structure with cylindrical, rectangular, or irregularly shaped pores having diameters in the range of 0.8-10 nm (micropores), 10-50 nm (mesopores), and / or 50-2000 nm (macropores). Activated carbon can have high surface areas ranging from 700 to 1800 m2 / g. Activated carbon can be used for adsorption or chemical reactions, such as without limitation for filtering contaminants from water and air.

[0074] In some embodiments, a support can include graphene. As used herein, “graphene” is a single layer of graphite as a two-dimensional sheet of fused benzene rings. In some embodiments, a support can include graphene oxide. As used herein, “graphene oxide” is a modified, at least partially oxidized form of graphene with one or more oxygen-containing functional groups attached to the graphene backbone. Nonlimiting examples of such functional groups include a carboxylic acid, epoxide group, hydroxyl group, and / or the like. These functional groups can impart additional chemical reactivity and specificity to graphene.Surface Group

[0075] In some embodiments, a support can include one or more surface groups. As used herein, a “surface group” or “surface moiety” is a chemical structure or functional group that is exposed at the exterior of a support and capable of / available for chemical functionalization or modifications. In some embodiments, a support such as cellulose, silica, activated alumina, etc., can include at least one hydroxyl group on its surface.

[0076] As a nonlimiting example, when a support includes unsubstituted cellulose, each glucose monomer includes three hydroxyl groups, as shown in FIG. 1. As a result, cellulose has a high concentration of surface hydroxyl groups (18.78 mmol -OH / g). However, cellulose suffers from a low resistance to acidic environment. There have been ongoing efforts in modifying the surface chemistry of cellulose, such as without limitation by functionalizing the native hydroxyl groups, to enhance the stability of cellulose in acidic environments and its adsorption performance towards target ions. This modification may include without limitation esterification, etherification, and grafting of N-, O-, or S-donor ligands to the surface of cellulose. Grafting a strategically engineered ligand can improve the adsorption capacity and enhance the selectivity of Pd(II) over competing ions such as Pt(IV).Ligand

[0077] In some embodiments, the adsorbent includes at least one ligand attached to the support. As used herein, a “ligand” is a chemical species capable of binding with and / or stabilizing another chemical species, typically through coordinate covalent bond. A ligand can include a neutral molecule or an ion, and usually contains relatively polarizable elements such as O, N, P, S, or the like that can function as a Lewis base by donating its lone-pair electron(s) to an electron-deficient Lewis acid species.

[0078] In some embodiments, a ligand selectively adsorbs a Pd species. In some embodiments, a ligand selectively adsorbs a Pd species over a Pt species. In some embodiments, a Pd species canbe adsorbed to and desorbed from a ligand or adsorbent in a reversible or substantially reversible manner. In some embodiments, an adsorbent described herein can be configured as a reusable adsorbent. Additional details will be provided below in this disclosure.S Donor

[0079] In some embodiments, the ligand includes a S donor. As used herein, a “sulfur donor” or “S donor” is a chemical species, or a fragment or functional group therein, that contains one or more S atoms. The S atom(s) within a S donor can contain one or more lone pairs available for forming a coordinate bond with a metal / metal cation. In some embodiments, a S donor can include at least one sp3-hybridized S atom. Additionally, and / or alternatively, in some embodiments, a S donor can include at least one sp2-hybridized S atom.

[0080] In some embodiments, a S donor can include one or more functional groups selected from a list consisting of a mercaptan, an alkyl thiol, an aryl thiol, a monothiol, a dithiol, a trithiol, a sulfide, a disulfide, a polysulfide, a thioester, a thiocarbonyl, a substituted or unsubstituted thiophenol, a substituted or unsubstituted naphthanethiol, a substituted or unsubstituted thiophene, a substituted or unsubstituted thiolane, a substituted or unsubstituted thiazole, a substituted or unsubstituted benzothiazole, a substituted or unsubstituted thiazolidine, a substituted or unsubstituted isothiazole, a substituted or unsubstituted benzisothi azole, a substituted or unsubstituted isothiazolidine, a substituted or unsubstituted oxathiole, a substituted or unsubstituted benzoxathiole, a substituted or unsubstituted oxathiolane, a substituted or unsubstituted isoxathiole, a substituted or unsubstituted benzisoxathiole, a substituted or unsubstituted isoxathiolane, a substituted or unsubstituted dithiole, a substituted or unsubstituted benzodithiole, a substituted or unsubstituted dithiolane, a substituted or unsubstituted thiadiazole, a substituted or unsubstituted benzothiadiazole, a substituted or unsubstituted dithiazole, a substituted or unsubstituted benzodithiazole, a substituted or unsubstituted thiatriazole, a substituted or unsubstituted thiatetrazole, a substituted or unsubstituted thiopyran, a substituted or unsubstituted thiane, a substituted or unsubstituted thiazine, a substituted or unsubstituted benzothiazine, a substituted or unsubstituted phenothiazine, a substituted or unsubstituted thiomorpholine, a substituted or unsubstituted oxathiin, a substituted or unsubstituted oxathiane, a substituted or unsubstituted dithiin, a substituted or unsubstituted dithiane, a substituted or unsubstituted trithiin, a substituted or unsubstituted trithiane, a substituted or unsubstituted thiepine, a substituted or unsubstituted thiepane, a substituted or unsubstituted thiazepine, a substituted or unsubstituted thiazepane, a substituted or unsubstituted thiocane, a substituted or unsubstituted thionine, and a substituted or unsubstituted thionane.

[0081] In some embodiments, a S donor can include a mercaptan, an alkyl thiol, an aryl thiol, a monothiol, a dithiol, a trithiol, a sulfide, a disulfide, a polysulfide, a thioester, or a thiocarbonyl group.

[0082] In some embodiments, a S donor can include a substituted or unsubstituted thiophenol, a substituted or unsubstituted naphthanethiol, a substituted or unsubstituted thiophene, or a substituted or unsubstituted thiolane.

[0083] In some embodiments, a S donor can include a substituted or unsubstituted thiazole, a substituted or unsubstituted benzothiazole, a substituted or unsubstituted thiazolidine, a substituted or un substituted isothiazole, a substituted or un substituted benzisothiazole, or a substituted or unsubstituted isothiazolidine.

[0084] In some embodiments, a S donor can include a substituted or unsubstituted oxathiole, a substituted or un substituted benzoxathiole, a substituted or unsubstituted oxathiolane, a substituted or unsubstituted isoxathiole, a substituted or unsubstituted benzisoxathiole, a substituted or unsubstituted isoxathiolane, a substituted or unsubstituted dithiole, a substituted or unsubstituted benzodithiole, or a substituted or unsubstituted dithiolane.

[0085] In some embodiments, a S donor can include a substituted or unsubstituted thiadiazole, a substituted or un substituted benzothiadi azole, a substituted or unsubstituted dithiazole, a substituted or unsubstituted benzodi thiazole, a substituted or unsubstituted thiatriazole, or a substituted or unsubstituted thiatetrazole.

[0086] In some embodiments, a S donor can include a substituted or unsubstituted thiopyran, a substituted or unsubstituted thiane, a substituted or unsubstituted thiazine, a substituted or unsubstituted benzothiazine, a substituted or unsubstituted phenothiazine, a substituted or unsubstituted thiomorpholine, a substituted or unsubstituted oxathiin, a substituted or unsubstituted oxathiane, a substituted or unsubstituted dithiin, or a substituted or unsubstituted dithiane.

[0087] In some embodiments, a S donor can include a substituted or unsubstituted trithiin, a substituted or unsubstituted trithiane, a substituted or unsubstituted thiepine, a substituted or unsubstituted thiepane, a substituted or unsubstituted thiazepine, a substituted or unsubstituted thiazepane, a substituted or unsubstituted thiocane, a substituted or unsubstituted thionine, and a substituted or unsubstituted thionane.

[0088] According to hard and soft acid and base (HSAB) theory, soft (i.e., more polarizable) Lewis acids, such as metal cations of Pd, have an higher affinity for soft Lewis bases, following an order of S > N > O. Therefore, an S-donor can separate Pd from coexisting metals in a solution. S-containing ligands can be used to create surface-functionalized materials such as carbon disulfide- modified cellulose, dithiocarbamate-modified cellulose, trithiocyanuric acid-modified ZrCh, and 2- aminophenyl disulfide-modified silica, among others. These S-functionalized adsorbents have a relatively high affinity for palladium, with an adsorption capacity (qc) ranging from 44.6-455.47 mg Pd / g of functionalized adsorbent, and selectivity separation factors (SFpa / M, wherein M = a metal other than Pd) ranging from 289 to 10,729.N Donor

[0089] The at least one ligand includes a N donor. As used herein, a “nitrogen donor” or “N donor” is a chemical species, or a fragment or functional group therein, that contains one or more N atoms. The N atom(s) within a N donor can contain one or more lone pairs available for accepting a proton (H+), thereby functioning as a Bronsted-Lowry base. In other words, the N atom(s) within a N donor can be basic. In some embodiments, a N donor can include at least one sp3-hybridized N atom. Additionally, and / or alternatively, in some embodiments, a N donor can include at least one sp2-hybridized N atom. In other words, the N atom(s) within a N donor can have a pKaof at least 6.

[0090] In some embodiments, a N donor can include one or more functional groups selected from a list consisting of a substituted or unsubstituted primary amine, a substituted or unsubstituted secondary amine, a substituted or unsubstituted tertiary amine, a substituted or unsubstituted alkylamine, a substituted or unsubstituted arylamine, a substituted or unsubstituted imine, a substituted or unsubstituted pyrrole, a substituted or unsubstituted benzopyrrole, a substituted or unsubstituted pyrrolidine, a substituted or unsubstituted imidazole, a substituted or unsubstituted benzimidazole, a substituted or unsubstituted imidazolidine, a substituted or unsubstituted pyrazole, a substituted or unsubstituted indazole, a substituted or unsubstituted pyrazolidine, a substituted or unsubstituted oxazole, a substituted or unsubstituted benzoxazole, a substituted or unsubstituted oxazolidine, a substituted or unsubstituted isoxazole, a substituted or unsubstituted benzisoxazole, a substituted or unsubstituted isoxazolidine, a substituted or unsubstituted thiazole, a substituted or unsubstituted benzothiazole, a substituted or unsubstituted thiazolidine, a substituted or unsubstituted isothiazole, a substituted or unsubstituted benzisothi azole, a substituted or unsubstituted isothiazolidine, a substituted or unsubstituted triazole, a substituted or unsubstituted benzotri azole, a substituted or unsubstituted oxodiazole, a substituted or unsubstituted benzoxadiazole, a substituted or unsubstituted thiadiazole, a substituted or unsubstituted benzothiadi azole, a substituted or unsubstituted dioxazole, a substituted or unsubstituted benzodi oxole, a substituted or unsubstituted dithiazole, a substituted or unsubstituted benzodithiazole, a substituted or unsubstituted tetrazole, asubstituted or unsubstituted oxatriazole, a substituted or unsubstituted thiatriazole, a substituted or unsubstituted pentazole, a substituted or unsubstituted oxatetrazole, a substituted or unsubstituted thiatetrazole, a substituted or unsubstituted pyridine, a substituted or unsubstituted quinoline, a substituted or un substituted piperidine, a substituted or unsubstituted pyrazine, a substituted or unsubstituted quinoxaline, a substituted or unsubstituted naphthyridine, a substituted or unsubstituted benzodiazine, and a substituted or unsubstituted diazanaphthalene, a substituted or unsubstituted pyrimidine, a substituted or unsubstituted quinazoline, a substituted or unsubstituted pyridazine, a substituted or unsubstituted phthalazine, a substituted or unsubstituted cinnoline, a substituted or unsubstituted piperazine, a substituted or unsubstituted oxazine, a substituted or unsubstituted benzoxazine, a substituted or unsubstituted morpholine, a substituted or unsubstituted thiazine, a substituted or unsubstituted benzothiazine, a substituted or unsubstituted phenothiazine, a substituted or unsubstituted thiomorpholine, a substituted or unsubstituted triazine, a substituted or unsubstituted benzotriazine, a substituted or unsubstituted triazinane, a substituted or unsubstituted tetrazine, a substituted or unsubstituted purine, a substituted or unsubstituted azepine, a substituted or unsubstituted azepane, a substituted or unsubstituted azocine, a substituted or unsubstituted azocane, a substituted or unsubstituted azonine, a substituted or unsubstituted azonane.

[0091] In some embodiments, a N donor can include a substituted or unsubstituted primary amine, a substituted or unsubstituted secondary amine, a substituted or unsubstituted tertiary amine, a substituted or unsubstituted alkylamine, a substituted or unsubstituted arylamine, or a substituted or unsubstituted imine.

[0092] In some embodiments, a N donor can include a substituted or unsubstituted pyrrole, a substituted or unsubstituted benzopyrrole, or a substituted or unsubstituted pyrrolidine.

[0093] In some embodiments, a N donor can include a substituted or unsubstituted imidazole, a substituted or unsubstituted benzimidazole, or a substituted or unsubstituted imidazolidine.

[0094] In some embodiments, a N donor can include a substituted or unsubstituted pyrazole, a substituted or unsubstituted indazole, a substituted or unsubstituted pyrazolidine, a substituted or unsubstituted oxazole, a substituted or unsubstituted benzoxazole, a substituted or unsubstituted oxazolidine, a substituted or unsubstituted isoxazole, a substituted or unsubstituted benzisoxazole, or a substituted or unsubstituted isoxazolidine.

[0095] In some embodiments, a N donor can include a substituted or unsubstituted thiazole, a substituted or un substituted benzothiazole, a substituted or unsubstituted thiazolidine, a substitutedor unsubstituted isothiazole, a substituted or unsubstituted benzisothi azole, or a substituted or unsubstituted isothiazolidine.

[0096] In some embodiments, a N donor can include a substituted or unsubstituted triazole, a substituted or unsubstituted benzotriazole, a substituted or unsubstituted oxodiazole, a substituted or unsubstituted benzoxadiazole, a substituted or unsubstituted thiadiazole, a substituted or unsubstituted benzothiadiazole, a substituted or unsubstituted dioxazole, a substituted or unsubstituted benzodi oxole, a substituted or unsubstituted dithiazole, or a substituted or unsubstituted benzodithiazole.

[0097] In some embodiments, a N donor can include a substituted or unsubstituted tetrazole, a substituted or unsubstituted oxatriazole, a substituted or unsubstituted thiatriazole, a substituted or unsubstituted pentazole, a substituted or unsubstituted oxatetrazole, or a substituted or unsubstituted thiatetrazole.

[0098] In some embodiments, a N donor can include a substituted or unsubstituted pyridine, a substituted or unsubstituted quinoline, or a substituted or unsubstituted piperidine.

[0099] In some embodiments, a N donor can include a substituted or unsubstituted pyrazine, a substituted or unsubstituted quinoxaline, a substituted or unsubstituted naphthyridine, a substituted or unsubstituted benzodiazine, and a substituted or unsubstituted di azanaphthalene, a substituted or unsubstituted pyrimidine, a substituted or unsubstituted quinazoline, a substituted or unsubstituted pyridazine, a substituted or unsubstituted phthalazine, a substituted or unsubstituted cinnoline, or a substituted or unsubstituted piperazine.

[0100] In some embodiments, a N donor can include a substituted or unsubstituted oxazine, a substituted or unsubstituted benzoxazine, a substituted or unsubstituted morpholine, a substituted or unsubstituted thiazine, a substituted or unsubstituted benzothiazine, a substituted or unsubstituted phenothiazine, or a substituted or unsubstituted thiomorpholine.

[0101] In some embodiments, a N donor can include a substituted or unsubstituted triazine, a substituted or unsubstituted benzotriazine, a substituted or unsubstituted triazinane, a substituted or unsubstituted tetrazine, a substituted or unsubstituted purine, a substituted or unsubstituted azepine, a substituted or un substituted azepane, a substituted or unsubstituted azocine, a substituted or unsubstituted azocane, a substituted or unsubstituted azonine, or a substituted or unsubstituted azonane.

[0102] In some embodiments, the S atom(s) in a S donor and the N atom(s) in a N donor can be separated by 1, 2, 3, 4, 4, 5, 6, 7, 8, 9, or 10 chemical bonds.

[0103] It is worth noting that the designation of S donors and N donors can be arbitrary, and a fraction, moiety, or functional group of the ligand can include both a S donor and a N donor at the same time. In some embodiments, a ligand can include an aminothiophenol group such as without limitation a 2-aminothiophenol (AP) group, a 3 -aminothiophenol group, a 4-aminothiophenol group, or a mono- or poly-substituted variation thereof. In some embodiments, a ligand can include a mercaptopyridine group, such as without limitation a 2-mercaptopyridine (MP) group, a 3- mercaptopyridine, or a 4-mercaptopyridine, or a mono- or poly-substituted variation thereof. In some embodiments, a ligand can include a mercaptobenzothiazole group, such as without limitation 2- mercaptobenzothiazole (MBT) group, or a mono- or poly-substituted variation thereof.

[0104] Surprisingly, by incorporating a ligand that contains both a S donor and a N donor, an adsorbent can adsorb Pd species both at a high capacity and with a good selectivity, due to a synergic effect of i) the electrostatic attraction between a protonated N atom and PdCh2' and ii) the highly specific coordination between a S atom and Pd2+. This disclosure therefore provides a practical approach for the selective separation of Pd (e.g., Pd(II) over competing ions such as Pt(IV), with a superior efficiency for overall metal recovery or recycling processes.Pd Species

[0105] The composition, adsorbent, and / or the at least one ligand, as described above, selectively adsorbs a Pd species from a mixture. As used herein, a “Pd species” is a chemical species that contains the Pd element. A Pd species can include any ion, salt, coordination complex, etc., of Pd deemed relevant by a person of ordinary skill in the art, upon reviewing the entirety of this disclosure. In some embodiments, the Pd species can include Pd(II) or Pd2+. In some embodiments, the Pd species can include Pd(IV) or Pd4+. In some embodiments, the Pd species can include PdCU2'. Mixture and pH

[0106] In some embodiments, a mixture described herein can include a homogeneous mixture such as a solution. In some embodiments, a mixture described herein can include an aqueous or nonaqueous solution of one or more Pd species alongside one or more competing ions. In some embodiments, a mixture described herein can include or be sourced from an industrial waste stream. In some embodiments, a mixture described herein can have a pH of at least 0.5 and no greater than 3.5. As nonlimiting examples, a mixture described herein can have a pH of about 1.0, about 1.5, about 2.0, about 2.5, about 3.0, and / or the like. As further nonlimiting examples, a mixture described herein can have a pH of at least 0.5 and no greater than 1.0, at least 1.0 and no greater than 1.5, at least 1.5 and no greater than 2.0, at least 2.0 and no greater than 2.5, at least 2.5 and no greater than3.0, at least 3.0 and no greater than 3.5, and / or the like. In some embodiments, the pH of a mixture described herein may be adjusted using an acid such as without imitation HC1 or a base such as without limitation NaOH.Competing Ions

[0107] The composition, adsorbent, and / or the at least one ligand, as described above, selectively adsorbs a Pd species over one or more competing ions. As used herein, a “competing ion” is an ionic species that competes with a chemical species of interest in binding to an adsorbent or ligand. In some embodiments, a competing ion can include an ion of a metal or metalloid. In some embodiments, the one or more competing ions can include an ion of Mg (e.g., Mg2+), Al (e.g., Al3+), Cu (e.g., Cu2+), Fe (e.g., Fe2+or Fe3+), Mn (e.g., Mn2+), Zn (e.g., Zn2+), Sc (e.g., Sc3+), As (e.g., As3+), Cd (e.g., Cd2+), Sb (e.g., Sb5+), Hg (e.g., Hg2+or Hg22+), Pb (e.g., Pb2+or Pb4+), Ga (e.g., Ga3+), Ge (e.g., Ge4+), Pt (e.g., Pt2+, Pt4+, or PtCl62'), La (e.g., La3+), Yb (e.g., Yb3+), Th (e.g., Th4+), or U (e.g., U4+). In some embodiments, the one or more competing ions can include one or more ions of elements selected from a list consisting of Mg (e.g., Mg2+), Al (e.g., Al3+), Cu (e.g., Cu2+), Fe (e.g., Fe2+or Fe3+), Mn (e.g., Mn2+), Zn (e.g., Zn2+), Sc (e.g., Sc3+), As (e.g., As3+), Cd (e.g., Cd2+), Sb (e.g., Sb5+), Hg (e.g., Hg2+or Hg22+), Pb (e.g., Pb2+or Pb4+), Ga (e.g., Ga3+), Ge (e.g., Ge4+), Pt (e.g., Pt2+, Pt4+, or PtCle2'), La (e.g., La3+), Yb (e.g., Yb3+), Th (e.g., Th4+), or U (e.g., U4+). In some embodiments, the one or more competing ions can include an ion a light metal (e.g., Mg2+, Al3+). In some embodiments, the one or more competing ions can include an ion of a transitional metal (e.g., Cu2+, Fe3+, Mn2+, Zn2+, Sc2+). In some embodiments, the one or more competing ions can include an ion of a heavy metal / metalloid (e.g., As3+, Cd2+, Sb5+, Hg2+, Pb2+, Ga3+, Ge4+). In some embodiments, the one or more competing ions can include an ion of a precious metal (e.g., PtCle2'). In some embodiments, the one or more competing ions can include an ion of a rare earth elements (e.g., La3+, Yb3+, Th4+, U4+).

[0108] In some embodiments, the one or more competing ions includes a Pt species. As used herein, a “Pt species” is a chemical species that contains the Pt element. A Pt species can include any ion, salt, coordination complex, etc., of Pt deemed relevant by a person of ordinary skill in the art, upon reviewing the entirety of this disclosure. In some embodiments, the Pt species can include Pt(II) or Pt2+. In some embodiments, the Pt species can include Pt(IV) or Pt4+. In some embodiments, the Pt species can include PdCL2'.

[0109] It is worth noting that most Pd-related selectivity studies have focused on isolation of Pd from its mixtures with other base metals (e.g., Cu, Fe, Zn) or precious metals (Au, Ag), whereas efforts in achieving Pd selectivity over Pt are very limited.

[0110] Pd often coexists with Pt in waste catalytic converters, oil-refining catalysts, and fuel cell catalysts, making it necessary to separate these two elements from one another. However, separation of palladium over platinum is a challenge for metal recovery processes due to their highly similar physical / chemical properties, such as without limitation atomic radii (1.79 A and 1.83 A, respectively), electronic configuration (4d10for and 5d96s1, respectively), and common oxidation states (+2 and +4). Among the adsorbents that have been synthesized for separating Pd from Pt, the separation factor of Pd over Pt (SFpd / pt) is in the range of 10-18, and the ratio of adsorption capacity for Pd over Pt (qe-pd / qe-pt) is less than 2.68 (Table 2), which is significantly lower compared to separation of Pd over other competing ions.[0U1] It is important to understand the mechanism of Pd / Pt selectivity. PdCL2’ (5.5 A) is slightly smaller in size than PtCk2’ (8.3 A). Tailoring adsorbent pore size to such specificity can be challenging. A Metal organic frameworks (MOFs) adsorbent, U1O-66-NH2, have been synthesized, and it was determined based on DFT calculations that the binding energy between PdCh2' and the adsorbent was lower than that between PtCL2’ and the adsorbent. However, this study does not provide insight into the interactions between Pd and a ligand at an atomic or molecular level, thereby lacking insight regarding the binding energy between Pd / Pt and a ligand.Linkage

[0112] In some embodiments, the at least one ligand can be attached to the support though a S donor. Such attachment can be either direct or indirect (i.e., through a spacer or linker). In some cases, such attachment can be achieved using a coupling reaction to create a covalent linkage between the S donor (or a precursor thereof) and the support (or a precursor thereof), consistent with details described elsewhere in this disclosure.

[0113] In some embodiments, the at least one ligand can be covalently attached to the support through a linker. As used herein, a “linker” is a chemical structure connecting two or more chemical species together while keeping them separated by a certain distance. A linker usually contains non- reactive, often repetitive chemical structures and is typically anchored to a chemical species via one or more chemical bonds, such as without limitation covalent bonds or coordinate bonds. The linker described herein can be flexible, semiflexible, semi-rigid, or rigid, and may include, or be constructed from, any suitable linker or linking chemistry as recognized by a person of ordinary skillin the art, upon reviewing the entirety of this disclosure. In some embodiments, a linker can include one or more aliphatic chains (e.g., methylene groups), aromatic chains (e.g., phenylene groups), polar chains (e.g., peptide groups or ethylene oxide groups), and / or the like. In some embodiments, a linker can be constructed using a bifunctional linker and one or more coupling reactions.Nonlimiting examples of suitable coupling reactions include without limitation click chemistry such as copper(l)-catalyzed azide-alkyne cycloaddition and carbodiimide coupling reactions such as DCC-NHS coupling, DCC / DMAPP coupling, and EDC-NHS coupling, among others.

[0114] In some embodiments, a linker can include at least one and no greater than 20 C atoms (i.e., a C1-C20 linker). As nonlimiting examples, a linker can include a Ci linker, a C2 linker, a C3 linker, a C4 linker, a C5 linker, a Ce linker, a C7 linker, a Cs linker, a C9 linker, a C10 linker, a Cn linker, a C12 linker, a C13 linker, a C14 linker, a C15 linker, a Ci6 linker, a C17 linker, a Cis linker, a C19 linker, or a C20 linker. In some embodiments, a linker can include one or more methylene units. As a nonlimiting example, a linker can include a structure of (CH2)m, where m is 1-20. In some embodiments, a linker can include one or more phenylene units. As a nonlimiting example, a linker can include a structure of (CeHOn, where n is 1-10. In some embodiments, a linker can include one or more polyethylene glycol units. As a nonlimiting example, a linker can include a structure of (CH2CH2O)O, where o is 1-10. In some embodiments, a linker can include a -C=C- bond, a -C=C- bond, a phenylene (-CeHr-), a carbonyl (-C(O)-), an ester (-C(O)OC-), an ether ( COC ), and / or an amide (-C(O)NH-). In some embodiments, a linker can include one or more branched or cyclic moieties, such as without limitation / -propyl, / -butyl, cyclopropyl, cyclopentyl, cyclohexyl, etc. In some embodiments, a linker can include one or more substituents such as without limitation one or more halogen atoms. As nonlimiting examples, a linker can include or be prepared using chloroacetyl chloride or thionyl chloride.

[0115] In some embodiments, a support can include a plurality of surface groups (e.g., hydroxyls), consistent with details described elsewhere in this disclosure. Accordingly, in some embodiments, a portion of the surface groups can serve as anchors and be functionalized by a ligand and / or a linker. In some embodiments, at least 20% and no greater than 80% of the plurality of the surface groups (e.g., hydroxyl groups) can be functionalized. As nonlimiting examples, about 20%, about 30%, about 40%, about 50%, about 60%, about 70%, about 80%, etc., of the surface groups (e.g., hydroxyls) can be functionalized. As further nonlimiting examples, at least 20% and no greater than 30%, at least 30% and no greater than 40%, at least 40% and no greater than 50%, at least 50% and no greater than 60%, at least 60% and no greater than 70%, at least 70% and no greater than80%, etc., of the surface groups (e.g., hydroxyls) can be functionalized. As a further nonlimiting example, when the support includes unfunctionalized cellulose, the density of total hydroxyl groups its surface is 18.78 mmol / g, whereas the density of active hydroxyl groups is estimated to be 6.26 mmol / g, since only 2 out of 10 oxygen atoms (and only 1 out of 3 hydroxyl groups) can be functionalized due to the steric hindrance imposed by large ligands.

[0116] In some embodiments, a linker can be attached to the support by reacting with a surface group (e.g., hydroxyl) of the support, consistent with details described elsewhere in this disclosure. Accordingly, in some embodiments, a linker can be connected to the support through an ester group (-C(O)OC-), an ether group (-COC-), and / or the like.Structural Features of Adsorbent

[0117] In some embodiments, the adsorbent can have a specific surface area of at least 2 m2 / g and no greater than 10 m2 / g. As nonlimiting examples, the adsorbent can have a specific surface area of approximately 2 m2 / g, approximately 3 m2 / g, approximately 4 m2 / g, approximately 5 m2 / g, approximately 6 m2 / g, approximately 7 m2 / g, approximately 8 m2 / g, approximately 9 m2 / g, approximately 10 m2 / g, and / or the like. As further nonlimiting examples, the adsorbent can have a specific surface area of at least 2 m2 / g and no greater than 3 m2 / g, at least 3 m2 / g and no greater than 4 m2 / g, at least 4 m2 / g and no greater than 5 m2 / g, at least 5 m2 / g and no greater than 6 m2 / g, at least 6 m2 / g and no greater than 7 m2 / g, at least 7 m2 / g and no greater than 8 m2 / g, at least 8 m2 / g and no greater than 9 m2 / g, or at least 2 m2 / g and no greater than 10 m2 / g, among others.

[0118] In some embodiments, the adsorbent can have a pore volume of at least 20 cm3 / kg and no greater than 60 cm3 / kg. As nonlimiting examples, the adsorbent can have a pore volume of at least 20 cm3 / kg and no greater than 30 cm3 / kg, at least 30 cm3 / kg and no greater than 40 cm3 / kg, at least 40 cm3 / kg and no greater than 50 cm3 / kg, at least 50 cm3 / kg and no greater than 60 cm3 / kg, and / or the like. As further nonlimiting examples, the adsorbent can have a pore volume of approximately 20 cm3 / kg, approximately 25 cm3 / kg, approximately 30 cm3 / kg, approximately 35 cm3 / kg, approximately 40 cm3 / kg, approximately 45 cm3 / kg, approximately 50 cm3 / kg, approximately 55 cm3 / kg, approximately 60 cm3 / kg, and / or the like.

[0119] In some embodiments, the adsorbent can have an average pore diameter of at least 25 nm and no greater than 32 nm. As nonlimiting examples, the adsorbent can have an average pore size of at least 25 nm and no greater than 26 nm, at least 26 nm and no greater than 27 nm, at least 27 nm and no greater than 28 nm, at least 28 nm and no greater than 29 nm, at least 29 nm and nogreater than 30 nm, at least 30 nm and no greater than 31 nm, at least 31 nm and no greater than 32 nm, and / or the like.Method of Preparation

[0120] Another objective of the present application is to provide a method of preparing a composition of matter or adsorbent for selective separation of a Pd species from a mixture.

[0121] In some embodiments, the method can include i) providing a support having at least one surface group; ii) providing a bifunctional linker having a first reactive group and a second reactive group; iii) connecting the bifunctional linker to the at least one surface group of the support using the first reactive group; iv) providing at least one ligand; and v) attaching the at least one ligand to the second reactive group of the bifunctional linker to produce the adsorbent.

[0122] In some embodiments, step iii) or v) can be performed using triethylamine (EtsN) or 4- dimethylaminopyridine (DMAP) as a catalyst.

[0123] In some embodiments, steps iii) or v) can be performed for 4-20 hours. As nonlimiting examples, step iii) or v) can be performed under room temperature for 4-6 h, 6-8 h, 8-10 h, 10-12 h, 12-14 h, 14-18 h, 18-20 h, and / or the like. As further nonlimiting examples, step iii) or v) can be performed under room temperature for approximately 2 h, approximately 4 h, approximately 6h, approximately 8 h, approximately 10 h, approximately 12 h, approximately 14 h, approximately 16 h, approximately 18 h, or approximately 20 h.

[0124] In some embodiments, steps iii) or v) can be performed under room temperature. In some embodiments, step iii) or v) can be performed under room temperature for 4-20 hours. In some embodiments, step iii) or v) can be performed under a temperature of at least 5 °C and no greater than 50 °C. As nonlimiting examples, step iii) or v) can be performed under a temperature of at least 5 °C and no greater than 10 °C, at least 10 °C and no greater than 15 °C, at least 15 °C and no greater than 20 °C, at least 20 °C and no greater than 30 °C, at least 30 °C and no greater than 40 °C, at least 40 °C and no greater than 50 °C, and / or the like. As further nonlimiting examples, step iii) or v) can be performed under a temperature of approximately 5 °C, approximately 10 °C, approximately 15 °C, approximately 20 °C, approximately 25 °C, approximately 30 °C, approximately 35 °C, approximately 40 °C, approximately 45 °C, approximately 50 °C, and / or the like.

[0125] In some embodiments, the method can include A) providing a support having at least one surface group; B) connecting a bifunctional linker to the at least one surface group of the supportusing a first reactive group of the bifunctional linker; and C) attaching at least one ligand to a second reactive group of the bifunctional linker to produce the adsorbent.

[0126] In some embodiments, step B) or C) can be performed using triethylamine (EtsN) or 4- dimethylaminopyridine (DMAP) as a catalyst.

[0127] In some embodiments, step B) or C) can be performed for 4-20 hours. As nonlimiting examples, step B) or C) can be performed under room temperature for 4-6 h, 6-8 h, 8-10 h, 10-12 h, 12-14 h, 14-18 h, 18-20 h, and / or the like. As further nonlimiting examples, step B) or C) can be performed under room temperature for approximately 2 h, approximately 4 h, approximately 6h, approximately 8 h, approximately 10 h, approximately 12 h, approximately 14 h, approximately 16 h, approximately 18 h, or approximately 20 h.

[0128] In some embodiments, step B) or C) can be performed under room temperature. In some embodiments, step B) or C) can be performed under room temperature for 4-20 hours. In some embodiments, step B) or C) can be performed under a temperature of at least 5 °C and no greater than 50 °C. As nonlimiting examples, step B) or C) can be performed under a temperature of at least 5 °C and no greater than 10 °C, at least 10 °C and no greater than 15 °C, at least 15 °C and no greater than 20 °C, at least 20 °C and no greater than 30 °C, at least 30 °C and no greater than 40 °C, at least 40 °C and no greater than 50 °C, and / or the like. As further nonlimiting examples, step B) or V) can be performed under a temperature of approximately 5 °C, approximately 10 °C, approximately 15 °C, approximately 20 °C, approximately 25 °C, approximately 30 °C, approximately 35 °C, approximately 40 °C, approximately 45 °C, approximately 50 °C, and / or the like.

[0129] In some embodiments, connecting the bifunctional linker can include connecting the bifunctional linker to at least 20% and no greater than 80% of a plurality of surface groups. As nonlimiting examples, connecting the bifunctional linker can include connecting the bifunctional linker to about 20%, about 30%, about 40%, about 50%, about 60%, about 70%, about 80%, etc., of the surface groups (e.g., hydroxyls). As further nonlimiting examples, connecting the bifunctional linker can include connecting the bifunctional linker to at least 20% and no greater than 30%, at least 30% and no greater than 40%, at least 40% and no greater than 50%, at least 50% and no greater than 60%, at least 60% and no greater than 70%, at least 70% and no greater than 80%, etc., of the surface groups (e.g., hydroxyls).

[0130] As used herein, a “bifunctional linker” is a chemical species or precursor, with at least two reactive functional groups, that is used in one or more synthetic route to create a linker. In someembodiments, a bifunctional linker can include at least one and no greater than 20 C atoms (i.e., a C1-C20 bifunctional linker). As nonlimiting examples, a bifunctional linker can include a Ci bifunctional linker, a C2 bifunctional linker, a C3 bifunctional linker, a C4 bifunctional linker, a C5 bifunctional linker, a Ce bifunctional linker, a C7 bifunctional linker, a C& bifunctional linker, a C9 bifunctional linker, a C10 bifunctional linker, a C11 bifunctional linker, a C12 bifunctional linker, a C13 bifunctional linker, a C14 bifunctional linker, a C15 bifunctional linker, a Ci6 bifunctional linker, a C17 bifunctional linker, a Cis bifunctional linker, a C19 bifunctional linker, and a C20 bifunctional linker. In some embodiments, a bifunctional linker can include one or more methylene units. As a nonlimiting example, a bifunctional linker can include a structure of (CH2)m, where m is 1-20. In some embodiments, a bifunctional linker can include one or more phenylene units. As a nonlimiting example, a bifunctional linker can include a structure of (CeH^n, where n is 1-10. In some embodiments, a bifunctional linker can include one or more polyethylene glycol units. As a nonlimiting example, a bifunctional linker can include a structure of (CH2CH2O)o, where o is 1-10. In some embodiments, a bifunctional linker can include a -C=C- bond, a -C=C- bond, a phenylene (-C6H4-), a carbonyl (-C(O)-), an ester (-C(O)OC-), an ether (-COC-), and / or an amide (- C(O)NH-). In some embodiments, a bifunctional linker can include one or more branched or cyclic moieties, such as without limitation / -propyl, / -butyl, cyclopropyl, cyclopentyl, cyclohexyl, etc. In some embodiments, a bifunctional linker can include one or more substituents such as without limitation one or more halogen atoms.

[0131] In some embodiments, the first reactive group or the second reactive group can include a chloro group. In some embodiments, the first reactive group or the second reactive group can include an acyl chloride group (-C(O)Cl). In some embodiments, the bifunctional linker can include 2- chloroacetyl chloride (FIG. 1), 3 -chloropropionyl chloride, 4-chlorobutyryl chloride, 5- chloropentanoyl chloride, 6-chlorohexanoyl chloride, or an analogue thereof with a chloro group, an acyl group, and 8 to 20 carbon atoms separating in between.

[0132] In some embodiments, the composition described herein can be prepared using a solvent selected from a group consisting of dimethylformamide (DMF), dimethyl sulfoxide (DMSO), dichloromethane (DCM), tetrahydrofuran (THF), methanol, ethanol, and acetone. In some embodiments, the composition described herein can be prepared using a solvent such as without limitation DMF, DMSO, DCM, THF, methanol, ethanol, acetone, and / or the like.Method of Use

[0133] Another objective of the present application is to provide a method of preparing a composition of matter or adsorbent for selective separation of a Pd species from a mixture.

[0134] In some embodiments, the method includes i) adding an adsorbent to a mixture of a Pd species and one or more competing ions; and ii) selectively adsorbing the Pd species for a period of at least 2 hours and no greater than 100 hours.

[0135] As nonlimiting examples, the method can include selectively adsorbing the Pd species for a period of at least 2 hours and no greater than 10 hours, at least 10 hours and no greater than 20 hours, at least 20 hours and no greater than 30 hours, at least 30 hours and no greater than 40 hours, at least 40 hours and no greater than 50 hours, at least 50 hours and no greater than 60 hours, at least 60 hours and no greater than 70 hours, at least 70 hours and no greater than 80 hours, at least 80 hours and no greater than 90 hours, at least 90 hours and no greater than 100 hours, and / or the like. As further nonlimiting examples, the method can include selectively adsorbing the Pd species for a period of approximately 2 h, approximately 4 h, approximately 6 h, approximately 8 h, approximately 10 h, approximately 20 h, approximately 30 h, approximately 40 h, approximately 50 h, approximately 60 h, approximately 70 h, approximately 80 h, approximately 90 h, approximately 100 h, and / or the like. In some embodiments, the method can include selectively adsorbing the Pd species for a period of approximately 12 h, approximately 24 h, approximately 36 h, approximately 48 h, approximately 72 h, approximately 96 h, and / or the like.

[0136] In some embodiments, the method can include, before step i), acidifying the mixture or adjusting the acidity of the mixture to reach a pH of at least 0.5 and no greater than 3.5. As nonlimiting examples, the method can include acidifying the mixture or adjusting the acidity of the mixture to reach a pH of about 1.0, about 1.5, about 2.0, about 2.5, about 3.0, and / or the like. As further nonlimiting examples, the method can include acidifying the mixture or adjusting the acidity of the mixture to reach a pH of at least 0.5 and no greater than 1.0, at least 1.0 and no greater than 1.5, at least 1.5 and no greater than 2.0, at least 2.0 and no greater than 2.5, at least 2.5 and no greater than 3.0, at least 3.0 and no greater than 3.5, and / or the like. In some embodiments, the method can include acidifying the mixture or adjusting the acidity of the mixture using an acid such as without imitation HC1 or a base such as without limitation NaOH.

[0137] In some embodiments, the method can further include desorbing the adsorbed Pd species from the adsorbent. In some embodiments, the adsorbed Pd species can be desorbed from the adsorbent using an HC1 solution or an acidic solution similar thereto.

[0138] In some embodiments, the method can further include recycling the adsorbent using a centrifuge.Article of Manufacture

[0139] Another objective of the present application is to provide a method of preparing an article of manufacture that includes a composition of matter or adsorbent described elsewhere in this disclosure. In some embodiments, the article of manufacture can include a cartridge. In some embodiments, the article of manufacture can include a fdter unit. In some embodiments, the article of manufacture can include an adsorbent bed. In some embodiments, the article of manufacture can include a column.EXAMPLE

[0140] This example describes the design, synthesis, and characterization of three S- functionalized adsorbents and the use thereof for selective separation of Pd over Pt. These S- functionalized adsorbents were prepared by functionalizing cellulose substrates with 2- aminothiophenol (Cell-AP), 2-mercaptopyridine (Cell-MP), and 2-mercaptobenzothiazole (Cell- MBT), respectively. The adsorption capacities of these S-functionalized adsorbents were determined at different pH, initial Pd concentrations, contact time, and competing ion conditions, which were correlated with isotherm, kinetic, and selectivity studies. The separation factor of Pd over Pt was calculated, and the reusability of adsorbents was evaluated through multiple cycles of adsorption and desorption. These S-functionalized adsorbents exhibited outstanding adsorption capacity for Pd(II), reaching 163.25 ± 2.5 mg / g (at pH = 1), 92.51 ± 1.2 mg / g (at pH = 1), and 27.48 ± 0.5 mg / g (at pH = 2.5), respectively. Isotherm and kinetic studies revealed that the adsorption of Pd(II) followed the Langmuir isotherm and a pseudo-second-order model, which indicates that i) the adsorption of Pd(II) occurred on a monolayer and ii) chemisorption is the rate-limiting step. These S-functionalized adsorbents showed a selective adsorption of Pd(II) over Pd(IV), with a maximum separation factor (SFpd / pt) of 64.50, which is among the highest separation factors ever reported. The mechanism of such selectivity was investigated by XPS, FTIR, and DFT calculations, which showed that PdCL2' has i) a smaller size, ii) a smaller H0M0-LUM0 gap, and iii) a higher binding energy to ligands compared to PtCL2'. These evidences collectively help explain the higher affinity for Pd(II) over Pt(IV) of these S-functionalized adsorbents.Methods

[0141] Materials

[0142] The materials used for the functionalization reactions, including cellulose powder, triethylamine (TEA, > 99.5%), chloroacetyl chloride (98%), 2-aminothiophenol (AP, 99%), 2- mercaptopyridine (MP, 99%), and 2-mercaptobenzothiazole (MBT, 97%) were all purchased from Sigma-Aldrich (USA). The organic solvents used for synthesis and washing, such as N,N- dimethylformamide (DMF, 99.8%, extra dry) and methanol (> 99%, extra pure), were purchased from Thermo Fisher Scientific (USA). Metal compounds such as palladium (II) chloride (99%) and potassium hexachloroplatinate (IV) (> 99.9%), which were used for preparing Pb and Pt solutions and standards, were purchased from Sigma-Aldrich. For competing metal ions in the experiments, in addition to Pt, 17 different ions were chosen. These chemicals were purchased as single-element standards, at a concentration of 1,000 ug / mL in dilute HC1, from Texas Scientific Products (USA). HC1 (36.5-38.0%), HN03(67-70%), and NaOH (> 97%) (VWR chemicals) were used for pH adjustment and sample dilution before Inductively Coupled Plasma Mass Spectrometry (ICP-MS) measurements.

[0143] Functionalization

[0144] Preparation of chloroacetyl chloride-functionalized cellulose (Cell-Cl): adsorbent precursor (Cell-Cl) was synthesized by pretreating cellulose powder. The cellulose was dried at 45 °C for 24 hours. 1.6 mL (20 mmol / L) chloroacetyl chloride and 1.4 mL (10 mmol / L) triethylamine were dissolved in 30 mL DMF, and 1.0 g dried cellulose was added to the solution and stirred at room temperature for 12 hours. The reaction produces a light-yellow solid (Cell-Cl), which was filtrated, washed with 30 mL methanol (6 times) and 30 mL deionized (DI) water (3 times), and dried at 45 °C for 12 hours.

[0145] Preparation of 2-aminothiophenol-functionalized cellulose (Cell-AP), 2- mercaptopyridine-functionalized cellulose (Cell-MP), and 2-mercaptobenzothiazole-functionalized cellulose (Cell-MBT): 0.3 g dried Cell-Cl and 0.4 mL triethylamine were firstly dissolved in 30 mL DMF, and an S-donor precursor, i.e., AP (1.126 g, 9 mmol), MP (1.00 g, 9 mmol), or MBT (1.505 g, 9 mmol), was slowly added to the solution. The solutions were sonicated for 5 minutes and stirred for 12 hours at room temperature. The products, which are bright yellow solid (Cell-AP), dark yellow solid (Cell-MP), and yellow solid (Cell-MBT), respectively, were washed with 30 mL methanol (6 times), 30 mL DI water (3 times), and finally dried at 45 °C for 12 hours. The reaction schemes are represented in FIG. 1.

[0146] Spectroscopic Characterization

[0147] The S donor-functionalized adsorbents were characterized by different spectroscopic techniques. Fourier-transform infrared spectroscopy (FTIR) was performed on a Bruker vertex-70 equipped with a diamond detector, where samples were scanned from 500 to 4000 cm'1with 400 scans per sample. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a Physical Electronics VersaProbe III equipped with a monochromatic Al ka X-ray source, with dual charge neutralization by electrons and argon ions. CHNS analysis was performed using a UNICUBE Elemental analyzer equipped with a thermal detector (TCD) to determine the mass percentages of elements C, H, N, S of the samples. The Brunauer-Emmett-Teller (BET) analysis was conducted by Micromeritics TriStar II Plus analyzer at 77.35 K using nitrogen gas as the adsorbate. The pH of adsorbate solutions was tested by Mettler Toledo SevenCompact pH meter S220. The concentration of metal ions in the solution was analyzed by Agilent 7800 ICP-MS.

[0148] Adsorption Experiments

[0149] Batch adsorption experiments were conducted in a 15-mL round-bottom polystyrene tube, with 10 mL Pd-Pt binary solution and 10 mg as-synthesized adsorbents. The tubes were mixed with a rotatory agitator in end-over-end fashion at a speed of 30 rpm for 48 hours at room temperature. After 48 hours of mixing, the solution was centrifuged at 4000 rpm for 5 mins. After adsorption, the supernatant liquid was removed for the measurement of metal concentration by ICP- MS. The solid adsorbents were dried at 45 °C for 12 hours for spectroscopic characterization. To test the effect of pH, Pd-Pt solutions were prepared at pH = 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0, with the same initial concentration of 100 mg / L Pd. pH adjustments were performed using 1 M HC1 and 1 M NaOH. Adsorption was conducted at each pH, and the adsorption capacity was calculated at different conditions as follows:

[0150] where qeis the equilibrium adsorption capacity (mg / g), Coand Ceare the initial and final concentrations of Pd-Pt solution (mg / L), respectively, V is the volume of the Pd-Pt solution (L), and m is the mass of adsorbent (g). The distribution coefficient, Kd(L / g), was calculated as follows:

[0151] where qeand Ceare the equilibrium adsorption capacity (mg / g) and equilibrium concentration (mg / L), respectively. The isotherm study was carried out at different initial concentrations ranging from 80 mg / L to 240 mg / L, at exemplary pH = 1.0 for Cell-AP and Cell-MP adsorbents and at exemplary pH = 2.5 for Cell-MBT adsorbents, based on the results described in further detail below in this disclosure. The kinetic studies were performed under exemplary pHconditions for each adsorbent by sampling at specific intervals between 0-48 h. The selectivity of Pd against competing ions was determined at pH = 1.0, 2.0, and 3.0. Competing ions included without limitation magnesium (Mg), aluminum (Al), scandium (Sc), manganese (Mn), iron (Fe), copper (Cu), zinc (Zn), gallium (Ga), germanium (Ge), arsenic (As), cadmium (Cd), antimony (Sb), lanthanum (La), ytterbium (Yb), lead (Pb), thorium (Th), and uranium (U). The initial concentration of each ion was 20 mg / L. The separation factor (SF) was calculated as follows:

[0152] where Kd Pdand Kd Mn+ are the distribution coefficients of Pd and a competing ion, M, respectively. For the desorption step, the adsorbents were immersed in 10 mb 3 M HC1 solution and equilibrated for 24 hours. The solution was then centrifuged at 4000 rpm for 5 min, and the supernatant was removed and sampled to determine the desorption efficiency. The adsorbent was then washed with DI water 3 times and underwent 5 additional adsorption cycles. The efficiency of desorption and re-adsorption of each cycle was determined by ICP-MS.

[0153] DFT Calculations

[0154] Density functional theory (DFT) analysis was performed on a Gaussian 16 software to investigate the molecular properties and binding energies of ligand-metal complexes. The basis set #opt B3LYP / LANL2DZ / solvent = water was used for the optimization analysis for metals, ligands, and complexes. After optimization, the atomic charge of each atom and bond length of each bond were calculated and labeled. Based on the electron density calculations, the electrostatic potential (ESP) map was drawn to show the charge distribution of ligand molecules. The singlepoint energy results were used for calculating the binding energy (Ebind) between metal ions and ligands as follows:Ebincl EComplex Eme[aiEngand(4)

[0155] where Ecomplex, Eme taand EUgandrefer to the total energy (kcal / mol) of the ligandmetal complex, metal, and ligand, respectively. Molecular orbital (MO) analysis was used to calculate the HOMO and LUMO energy of each metal and ligand, which contributed to the calculation of their hardness index (q) and softness index (o) as follows:

[0156] where I and A refer to the ionization energy and electron affinity, respectively, which are approximately equal to —EHOMOand —ELUMO, respectively. The hardness index, T], is approximatelyhalf of HOMO-LUMO gap, and the softness index, <7, is reversely proportional to the hardness index, rj.Results and Discussion

[0157] Characterization

[0158] The synthetic scheme for S-functionalized Cell-AP, Cell-MP, and Cell-MBT adsorbents are shown in FIG. 1. The S-functionalized adsorbents were characterized with FTIR, XPS, and CHNS techniques to evaluate their chemical properties.Table 1. BET Analysis Results of Unfunctionalized Cellulose and Cell-AP, Cell-MP, and Cell-MBT Adsorbents.Adsorbent BET surface Pore volume Avg pore size (nm) Pore width with area (m2 / g) (cm3 / kg) max. volume (nm)Cellulose 0.85 4.35 20.45 5.43Cell-AP 5.62 38.62 27.45 6.34Cell-MP 3.26 24.41 29.98 5.87Cell-MBT 7.73 48.05 24.88 5.88

[0159] The FTIR spectra of cellulose, Cell-Cl, and three S-functionalized adsorbents are shown in FIG. 2A. The broad peak at 3280 cm'1represents the stretching vibration of abundant O-H bonds in cellulose, while the peaks at 2850 cm'1and 1027 cm'1correspond to the stretching vibration of C H and C-0 bonds, all of which form the backbone of cellulose structure. The peak at 1726 cm'1represents the C=O stretching vibration, which was absent in pure cellulose but was present in Cell- Cl and the three S-functionalized cellulose, which suggests that chloroacetyl chloride was successfully linked to the cellulose backbone and remained stable during subsequent preparation of Cell-AP, Cell-MP, and Cell-MBT. There was also a new peak at 736 cm'1in the spectrum for Cell- Cl, which represents the C-Cl vibration due to the addition of chloroacetyl chloride. After functionalization with S-donor ligands, Cell-AP, Cell-MP, and Cell-MBT all showed a new strong peak at 750 cm'1, which corresponds to the C-S stretching vibration. This peak indicates that S- donor ligands were successfully attached. There was no apparent peak at 2550 cm'1, which suggests that the S atoms were not in the form of -SH, and that it was -SH, not -NH2, that substituted the Cl atom in Cell-Cl to produce Cell-AP. For Cell-MP and Cell-MBT, there was a new peak at 1420 cm'1corresponding to the C=N bond, which originated from ligands MP and MBT. In contrast, Cell-APshowed a strong peak at 1608 cm'1, which represents the bending vibration of N-H bonds. The new peaks of C-Cl, C-S, C=N, and N-H together indicate that the functionalization process shown in FIG. 1 proceeded successfully.

[0160] The X-ray photoelectron spectroscopy (XPS) analysis was also performed for cellulose, Cell-MP, and Cell-MP after adsorption (Cell-MP-Pd) (FIG. 8A). Pure cellulose shows distinct peaks of O Is and C Is, indicating that cellulose contains only carbon and oxygen elements. After functionalization, Cell-MP shows new peaks of S 2s and S 2p at 220.0 eV and 162.0 eV, respectively, which illustrates that S-donor ligand MP has been successfully attached to cellulose. A new N 1 s peak was observed at 381 .0 eV, which originates from the pyridine structure in MP. A detailed spectrum for C Is revealed a higher content of O-C=O and C-H for Cell-MP compared to unfunctionalized cellulose, as a result of the functionalization of the S-donor ligand. The BET analysis showed that the surface area increased from 0.85 m2 / g for unfunctionalized cellulose to 3.26-7.73 m2 / g for functionalized cellulose adsorbents (Table 1). The pore volume increased from 4.35 m2 / g for unfunctionalized cellulose to 24.41-48.05 cm3 / kg after functionalization. The pore width with maximum volume of S-functionalized adsorbents ranged from 5.87 nm to 6.34 nm (FIG. 2C and 2D), which were larger than the size of PdCh2' (5.5 A) and PtCk2' (8.3 A).

[0161] The CHNS analysis was performed for cellulose, Cell-Cl, and the three S donor- functionalized adsorbents (FIG. 8D). The quantitative results (by weight %) showed that cellulose is mainly composed of carbon (42.4%) and oxygen (49.7%), as expected. The density of hydroxyl groups on cellulose surface was 18.78 mmol / g, and the density of active hydroxyl groups was estimated to be 6.26 mmol / g, since every 2 out of 10 oxygen atoms (and every 2 out of 6 hydroxyl groups) could be functionalized due to the steric hindrance by large ligands. After functionalization using the S-donor ligands, the oxygen content decreased, and the percentages of nitrogen and sulfur increased accordingly as new ligands were attached. The percentage of sulfur atoms in Cell-AP, Cell-MP, and Cell-MBT were 5.87%, 5.66%, and 9.75%, respectively, which indicated that the densities of the tethered ligands, i.e., AP, MP, and MBT molecules, were 1.84, 1.77, and 1.52 mmol / g, respectively. The grafting yields of the three adsorbents were calculated accordingly as 29.3%, 28.3%, and 24.3%, for Cell-AP, Cell-MP, and Cell-MBT respectively, based on the density of pristine, active hydroxyl groups. The grafting yield of Cell-MBT was slightly lower than that of Cell-AP and Cell-MP, due to the larger size of the MBT ligand, which makes it harder to be grafted at a higher density due to steric hindrance. The grafting efficiency may be further improved by adding catalysts such as 4-dimethylaminopyridine (DMAP) to the reaction system.

[0162] Effect of pH

[0163] pH is an important parameter to consider for adsorption experiments, as it can alter the metal species and / or chemical properties of adsorbents. The pH values investigated in this example were in the range of 0.0-3.0 (FIGS. 3A-D) to simulate a true industrial leaching solution. The results show that the highest adsorption occurred at pH = 0.5-1.5 for Cell-AP and Cell-MP, and at pH = 2.0- 3.0 for Cell-MBT. The observed effect of pH on adsorption may be due to the speciation of palladium and platinum, which is a function of pH. The dominant species for Pd at high Cl' concentration (>10‘2 5M, pH < 2.5 in HC1 solution) is [PdCh]2', while species [PdCh]’ and [PdCh]0become more abundant at lower Cl' concentrations (10'5-10'2 :,M, 2.5 < pH < 5 in HC1 solution). As for Pt, the dominant Pt species are [PtCh]2’ and [PtCh]2’ at high CT concentration and [PtCh]’ and [PtCh]0at low CF concentrations.

[0164] As pH decreases, the dominant Pd species changes from [PdCh]0and [PdCh]’ to [PdCh]2’, with an increasingly negative charge. At the same time, the unreacted amine and pyridine on adsorbents may undergo protonation in an acidic environment to become ammonium and pyridinium ions, respectively. The cation NH3 have a higher affinity for dianion PdCh2’ than monoanion [PdCh]’, which is likely the reason why lower pH was more favorable for Cell-AP and Cell-MP in adsorbing Pd or Pt species. These observations suggest that in addition to chelation, electrostatic attraction may also play a role in the adsorption mechanism, which could reduce the selectivity of Pd / Pt, since NH3 may attract both PdCh2’ and PtCh2’. The selectivity ratio of qefor Pd over Pt was 6.36 and 9.14 for Cell-AP and Cell-MP (Table 2), respectively. The reason why Cell-AP has a lower selectivity for Pd than Cell-MP is likely that the amine group (pKa= 9.2) is more basic and typically easier to protonate compared to pyridine-N (pKa= 5.5) and accordingly has a stronger electrostatic attraction towards metal ions besides the chelation reaction. As a result, [PdCh]2' and [PtCh]2' can both be adsorbed, since they both have two negative charges. The main mechanism of adsorption, however, remains the preferential chelation to Pd(ll) by sulfur atoms (more details in section “Adsorption Mechanism” below), which contributed to the relatively high selectivity of Pd over Pt compared with prior studies. A more acidic environment increases the electrostatic attraction between the adsorbent and the Pd / Pt species, thereby increasing the adsorption capacity for both but at the expense of selectivity. This is an important trade-off to consider when designing an adsorption system.

[0165] It is also shown that the optimal adsorption capacity of Pd (qe,pa) for Cell-AP and Cell- MP were 126.0 mg / g and 83.73 mg / g, respectively, at pH = 1.0, but the optimal capacity for Cell-MBT was 27.48 mg / g at pH = 2.5. The qe,pd of Cell-AP was approximately 4.6 times that of Cell- MBT, but the ligand density of Cell-AP (29.3%) did not differ significantly from that of Cell-MBT (24.3%). The poor adsorption of Cell-MBT was unexpected but could be explained by the steric hindrance of MBT. The larger size of MBT makes it harder for [PdCh]2' ions to diffuse through the cellulose structure and bind to a ligand, the mechanism of which will be further explained by DFT calculation discussed later in section “Adsorption Mechanism” below. The decreasing adsorption capacity under lower pH indicates that electrostatic attraction did not play a main role for Cell-MBT, likely because the N atom in the MBT ligand did not undergo protonation and remained electrically neutral under acidic conditions.Table.2. A Comparison Between S-Functionalized Cellulose Adsorbents Described Herein andOther Types of Absorbents.Adsorbents Target Adsorption qe-pd / qe- Competing Condition ReferenceIons Capacity (mg / g) ptratio Ions"" Pd PtSulfur heteroatoms- Pd, Pt 177.72 66.32 2.68 Mg(II), 6 M HC1 Torrejos, R E.C. et al., modified crown ether Al(III), “Multidentate thia-crown Cr(III), ethers as hyper-crosslinked macroporous adsorbent Mn(II), resins for the efficientFe(III), Pd / Pt recovery and Ni(II) separation from highly acidic spent automotive catalyst leachate”, Chem. Eng. J., 2021; 424: 130379.Dialdehyde Pd, Pt 89.38 80.83 1.11 Zn(II), pH 2.0 Asere, T.G. et al., carboxymethyl Fe(III), “Dialdehyde cellulose Co(II), carboxymethyl cellulose cross-linked chitosan for Cu(II) the recovery of palladium and platinum from aqueous solution”, React. Fund.Polym., 2019; 141:145- 154.Leaf biomass T. Pd, Pt 41.86 22.50 1.86 N.A. pH 2.0 Ramakul. P. et al., catappa L. “Biosorption of palladium(II) and platinum(IV) from aqueoussolution using tannin from Indian almond (Terminalia catappa L.) leaf biomass: Kinetic and equilibrium studies”. Chem. Eng. J.. 2012; 193-194: 102-111.Ammonium- Pd, Pt 1 19.3 242.6 0.49 Fe(II), pH 7.5 Chen, M. et al., “Removal functionalized Zr4+Co(III) of metal-cyanideMOF complexes and recovery of Pt(II) and Pd(II) from wastewater using an alkali- tolerant metal-organic resin”, J Hazard Mater , 2021; 406: 124315.polyacrylonitrile-based sorbent”, Int. J. Miner. Process , 2015; 137:52-58Mesoporous carbon, Pd, 63.85 78.03 0.82 Sn(IV) pH 3.0 Zalupski. P R et al., “TheCMK-3 Pt, Au Adsorption of Gold, Palladium, and Platinum from Acidic Chloride Solutions on Mesoporous Carbon”. Solvent Extr. IonTannin with Pd, 187.29 288.71 0.65 Cu(II) 0. I M Gurung, M et al , tetraethylenepentamine Pt, Au JJQ “Persimmon tannin-based new sorption material for resource recycling and recovery of precious metals”, Chem. Eng. J.. 2013; 228:405-414.Thiourea-modified Pd, Pt 129.90 1 12.40 1.15 Cu(II), pH 2.0 Zhou, L. et al., “Adsorption chitosan Pb(II), of platinum(IV) andCd(II), palladium(II) from aqueous solution by thiourea- Zn(II), modified chitosan Ca(II) microspheres”, J. Hazard.Mater. , 2009', 172(1):439- 46.Dimethylamine- Pd, 65.98 120.94 0.55 Cu(II), 0.5 M Parajuli, D. et al., “Total modified lignophenol Pt, Au Zn(II),recovery of gold.Ni(II), palladium, and platinum using lignophenolFe(III) derivative’’, Miner. Eng. , 2009; 22(13): 1173-1178.Ethylenediamine- Pd, Pt 65 00 79 00 0 82pH 2 0 Zhou, L. et al.. “Adsorption modified chitosan of platinum(IV) and palladium(II) from aqueous solution by magnetic crosslinking chitosan nanoparticles modified with ethylenediamine”, J. Hazard. Mater. , 2010;182(l-3):518-24.Polyamme-modified Pd, Pt 31 25 50 00 0 63 Ir(III), 1 M HC1 Fayemi. O.E. et al.. beads / nanofibers Rh(TTI) “Adsorption and separation of platinum and palladium by polyamine functionalized polystyrene- based beads andAP-modified cellulose Pd, Pt 163.25 19.83 6.36 17 ions pH = 1.0 This studyMP-modified cellulose Pd, Pt 92.51 9.16 9.14 17 ions pH = 1.0 This studyMB T -modified Pd, Pt 27.48 4.68 5.87 17 ions pH = 2.5 This study cellulose*N.A.: Not available.

[0166] Adsorption Isotherm

[0167] The study of adsorption isotherms provides additional insights regarding the dominant mechanism governing the adsorption. Adsorption isotherms were collected for Cell-AP, Cell-MP, and Cell-MBT adsorbents at different initial concentrations, ranging from 80-240 mg / L of Pd (FIGS.4A-B). The maximum adsorption capacities of Cell-AP, Cell-MP, Cell-MBT were 163.25, 92.51, and 22.30 mg / g, respectively, which are among the highest of adsorbents summarized in Table 2.Table 3. Isotherm Parameters for Pd Adsorption by Cell-AP, Cell-MP, And Cell-MBT Adsorbents.Exp. Langmuir isotherm Freundlich isothermQm(mg / g) (Lu mg / g) KLR2Kf(L / mg)1 / nn R2CelLAP 163.25 166.64 0.27 0.9934 6 98 5.80 0.9745Cell-MP 92.51 100.00 0.29 0.9942 5 62 6.68 0.8031Cell-MBT 22 30 23 99 0 084 0 9354 2 06 3 05 0 8854

[0168] The adsorption isotherm was studied by applying Langmuir (Eq.7 and Eq.8) orFreundlich models (Eq.9 and Eq. 10) to the experimental data using the following fitting equations:

[0169] Langmuir:

[0171] where Ceis the equilibrium concentration (mg / L), qeis the adsorption capacity (mg / g), Qmis the Langmuir maximum adsorption capacity (mg / g), KLrepresents the Langmuir isotherm constant, and Kf is the Freundlich isotherm constant (L / mg)l nThe equation 8 and 10 are the linearized versions of equation 7 and 9, respectively. The fitting parameters of both isotherms are shown in FIG. 4B and Table 3. The Qmvalues calculated by Langmuir isotherms of the three adsorbents are 166.64, 100.00, and 23.99 mg / g for CelLAP, Cell-MP, and Cell-MBT, respectively, which are close to the experimental maximum qevalue. The Langmuir isotherm assumes a monolayer adsorption where each reactive site is occupied by a molecule and no interaction occurs between adsorbed molecules; the Freundlich isotherm refers to processes that allow multi-layer adsorption and interactions between adsorbed molecules. It can be seen from FIGS. 4A-B and Table 3 that the Langmuir isotherm results in a better fit with the experimental data. The R2value of the Langmuir model is >0.935 for all three S-functionalized adsorbents, while the R2value of the Freundlich model is only 0.803-0.974. The better fit for Langmuir isotherm suggests that adsorption of three sulfur-modified adsorbents is likely a monolayer adsorption that takes place on a single layer, or on a homogeneous surface, that has a uniform distribution of identical or substantially identical adsorption sites.

[0172] Adsorption Kinetics

[0173] The kinetics study was conducted at different contact times up to 48 hours for Cell-AP, Cell-MP, and Cell-MBT to determine the reaction rate of the adsorption. The adsorption of the three adsorbents increased as the contact time increases and reaches equilibrium at about 12 hours, where it levels off for the remaining 36 hours of the test (FIGS. 5A-D). The adsorption reached 119.21, 78.89, and 22.30 mg / g for Cell-AP, Cell-MP, and Cell-MBT adsorbents, respectively. The adsorbed mass reached more than half of the maximum adsorption values at 4 hours, which is relatively efficient among Pd adsorption materials. There are several kinetic models that can be used for describing the adsorption processes, including the pseudo-first-order (PFO) model, the pseudo- second-order (PSO) model, and the Elovich model. The respective fitting equations are listed as below:

[0177] where qtand qeare the adsorption capacities at t time and at equilibrium (mg / g), while and k2refer to the pseudo-first-order constant (-1) and pseudo-second-order constant(g / (mg h)), respectively. Constant a and ? in equation (13) represent the initial adsorption rate (mg / (g h)) and desorption constant (g / mg), respectively. These equations represent three different scenarios. The pseudo-first-order model assumes that the rate is proportional to unoccupied sites and the rate-limiting step is the physisorption. The pseudo-second-order model suggests that the rate is proportional to the square of sites and the rate-limiting step involves chemisorption. The Elovich model assumes that the active sites are heterogenous and associated with different activation energies. It can be seen from FIGS. 5A-D and Table 4 that the R2for the pseudo-second-order fitting were 0.9957, 0.9991, 0.9951 for Cell-AP, Cell-MP, and Cell-MBT adsorbents, respectively, which were significantly higher than that for the pseudo-first-order and the Elovich model. This result indicates that the pseudo-second-order model can better explain the mechanism of adsorption. Thisresult also suggests that chemisorption is likely the main rate-limiting process for Pd(II) adsorption, and there was likely strong interaction between ligands and metals such as the formation of covalent bonds.Table 4. Kinetic Parameters of Adsorption by Cell-AP, Cell-MP, and Cell-MBT Adsorbents.Exp. Pseudo-first-order (PFO) Pseudo-second-order (PSO) Elovich modelQe(mg / g) qeK, R2qek2■ 103R2a fi R2(mg / g) (h-1) (mg / g) (g / (mg h)) (mg / (g h)) (g / mg)Cell-AP 1 19.21 52.56 0 049 0.9725 1 19.76 4.14 0.9957 1647.8 0.074 0.9729Cell-MP 78 89 23 52 0 045 0 8964 79 24 12 34 0 9991 1 1902 1 0 142 0 9752Cell-MBT 22 30 13 24 0 048 0 9672 23 37 14 28 0 9951 38 46 0 276 0 9906

[0178] Desorption Studies

[0179] The desorption of Pd was conducted under an acidic environment. 1 M, 2 M, and 3 M HC1 solution were used as eluting solution, of which 3 M HC1 was found to be the most effective for the S-functionalized cellulose adsorbents described herein. The high concentration of H+competes with Pd2+for active adsorption sites on the adsorbents (e.g., S donor and N donor), such that H+displaces Pd2+, leading to the desorption of Pd2+. Five cycles of desorption and re-adsorption were performed, and it can be seen from FIGS. 6A-B that the re-adsorption capacities of Cell-AP, Cell- MP, and Cell-MBT stayed stable after a few cycles, which supported the reusability of the S- functionalized adsorbents. The re-adsorption capacity after five cycles reached around 60%. The loss of adsorption capacity was probably caused by the unwashed adsorption sites and the mass loss of materials during the desorption-adsorption cycles. The desorption efficiency was calculated based on the adsorption extent of the previous cycle. The results showed that the elution efficiency of 3 M HC1 was consistent throughout the 5 cycles, at around 90%. The desorption efficiency of Cell-AP adsorbents appeared to be slightly lower than that of Cell-MP and Cell-MBT adsorbents, which suggests that Cell-AP adsorbent had a stronger binding affinity to Pd2+that makes Pd2+harder to desorb, consistent with the superior adsorption performance of Cell-AP adsorbent.

[0180] Selectivity Studies

[0181] The selectivity studies of Cell-AP, Cell-MP, and Cell-MBT adsorbents were performed in solution, by mixing Pd2+with 17 other competing ions at a pH ranging from 1-3, with an initial concentration of 20 mg / L (FIG. 7). The competing metals include light metals (Mg2+, Al3+), transitional metals (Cu2+, Fe3+, Mn2+, Zn2+, Sc2+), heavy metal s / metalloids (As3+, Cd2+, Sb?+, Hg2+,Pb2+, Ga3+, Ge4+), precious metals (PtCk2-), and rare earth elements (La3+, Yb3+, Th4+, U4+), which take into consideration the possible types of metal that may be present in industrial waste streams. All three adsorbents show great selectivity for Pd(II) over other competing ions. The separation factors (SFpd / M, M = a metal or metalloid) for the three S-functionalized adsorbents are shown in Table 5 and range from 49.44-13373.97, which is comparable to previous studies. Generally, the selectivity for Pd2+follows the order Cell-AP > Cell-MP > Cell-MBT, which is reflective of the higher adsorption capacities of Cell-AP and Cell-MP for Pd(II). Some ions such as Fe3+, Sb5+, Yb3+, and Th4+show a higher adsorption capacity, which indicates that these ions may have a higher affinity to the S-functionalized adsorbents used in this study.Table 5. Separation factors (SF) of Cell-AP, Cell-MP, and Cell-MBT for Pd(II) against 17 competing ions. Adsorption condition: 10 mb mixture of 19 elements. Initial concentration: 100 mg / L for Pd / Pt and 20 mg / L for other ions. 10 mg adsorbent, contact time: 48 h, batch test at 25 °C.Cell-AP Cell-MP Cell-MBT pH 1 pH 2 pH 3 pH 1 pH 2 pH 3 pH 1 pH 2 pH 3Mg2+483.77 663.13 2161.74 1961.89 1077.24 1020.65 768.01 661.24 944.56Al3+5324 65 4354.35 5597.83 416 09 149.51 5805.90 182.59 148.60 100.70Sc2+3770 85 3106.53 3488.33 583 32 794.54 310.83 740.35 592.08 878.68Mn2+4578 14 3705.10 4608.56 3843.88 363.66 278.40 830.25 1873.69 964.35Fe3+65.79 121.83 160.23 126 83 60.84 41.71 609.62 455.67 672.34Cu2+3551 05 1 1042.13 503.96 2251.83 409.48 233.50 331.52 291.51 743.08Zn2+10276.15 8295.02 13373.97 5273.1 1 2865.62 3185.68 1007.42 872.13 1492.25Ga3+4525 30 1842.72 3200.48 1512.48 362.57 822.28 753.01 775.41 1048.64Ge4+2121 38 1620.14 2000.18 801 67 2260.62 529.05 744.45 596.88 1294.96As3+392.56 344.48 428.15 415 69 1814.44 1440.30 498.10 398.96 674.57PdCl42’ 1.00 1.00 1 00 1.00 1.00 1.00 1.00 1.0 1.0PtCl62‘ 64.50 23.95 16.58 47.60 21.23 18.14 4.41 7.48 12.50Cd2+2749 61 643.56 824.35 395 20 492.96 386.50 769.24 616.93 162662.79Sb5+7376 76 7332.83 11817.25 145 19 132.05 57.47 164.63 77.34 363.75Pb2+2304 57 580.53 414.36 422 51 537.89 1702.29 1053.09 703.45 3145.78La3+1709 98 783.38 831.29 365 28 225.79 157.08 376.80 305.51 520.68Yb3+548.37 335.22 412.77 414 59 482.23 351.94 95.40 49.44 157.15Th4+123.09 75.41 89.10 228 87 273.02 1012.99 82.14 75.73 1048.73U4+920.80 235.80 435.00 425 40 221.63 1434.02 112.10 49.73 175.45

[0182] The high selectivity of Pd(II) over other competing ions can be explained by the HSAB theory, since a soft-base S donor and soft metal ions such as Pd2+have more diffuse electron clouds and more efficient orbital overlap, which allows a more stable complex to form therebetween (see section “Adsorption Mechanism” below for details on selective mechanism). The chelating adsorbents grafted by N-donor ligands also have a similar effect, but via a different mechanism, i.e., electrostatic attraction. The protonated NHs+in acidic media can attract anions, e.g., [PdCh]2' and [PtCk]2' ions, but repels cations such as Mg2+, Fe3+, Al3+, Cu2+, Cd2, As3+, La3+, and Pb2+. However, N-donor ligands have limited selectivity between PdCU2' and PtCk2', as both are divalent anions.

[0183] Conversely, the selectivity of Pd(II) over Pt(IV) was high for this study compared to prior studies, which is attributed to the S-donor ligands functionalized to the surface of cellulose adsorbents. The ratios of adsorption capacity of Pd(II) over Pt(IV) were 6.36, 9.14, and 5.87 for Cell-AP, Cell-MP, and Cell-MBT adsorbents, respectively, while prior studies tend to have relatively low ratios in the range of 0.49-2.68 (Table 2). The separation factor SFpa / pt ranges from 7.484 to 64.504 in this study, which is significantly higher compared to 10.9-18.0 in prior studies. The superior selectivity of Pd over Pt in this study is likely due to the incorporation of S-donor ligands into the cellulose backbone and the resulting chelation mechanism (see section “Adsorption Mechanism” below).

[0184] Adsorption Mechanism

[0185] The mechanism of metal adsorption on the adsorbents described herein were investigated by different techniques including XPS, FTIR, CHNS, and DFT calculations to provide further insight regarding the mechanism of interaction between ligands and metals.

[0186] The XPS spectra of Cell-MP before and after adsorption are shown in FIG. 8. It can be seen from FIG. 8A that new peaks showed up at 340 eV and 200 eV, which represent the Pd 3d and Cl 2p, respectively. These peaks indicate that [PdCU]2’ has been successfully adsorbed onto the surface of the adsorbents. The peak of Pd 3d can be further resolved into Pd 3d 3 / 2 (342 eV) and Pd 3d 5 / 2 (337 eV), which correspond to the characteristic spectra of divalent Pd cations. The peak of S 2s and S 2p shifted from 211.0 eV and 164.0 eV to a higher binding energy by 0.5-1.0 eV after the adsorption process, which indicates that sulfur atoms have participated in the formation of a coordinate bond with Pd. The same result was observed for the nitrogen atoms, as the N Is peak shifted from 399.0 eV to a higher binding energy by 1.0 eV, which indicates that the nitrogen atoms were also involved in the coordinate bond between ligands and Pd. In contrast, the O ls peak and C Is peak remained at 533.0 eV and 286.0 eV, respectively, which indicates that oxygen atom andcarbon atoms were not substantially involved in the coordinate bonds. Collectively, the XPS results showed that it was primarily S and N atoms that contributed to the coordination reaction between Pd(II) and ligands (FIG. 9). CHNS results (FIG. 8D) show the elemental composition of S- functionalized adsorbents before and after adsorption. The total percentage of C, H, N, S decreased after adsorption, which indicates that more Pd and Cl atoms were attached to the surface of cellulose.

[0187] The DFT calculations of binding energy, bond lengths, and atomic charge of adsorbentmetal complexes were performed to provide further insight into the mechanism of the adsorption process (FIGS. 10A-C). FIG. 10A shows the electrostatic potential (ESP) map of the three S- functionalized adsorbents. Regions around N and S atoms have relatively higher electron density than O or C atoms in the ligands. The charge of S atoms in Cell-AP, Cell-MP, and Cell-MBT are 0.145, 0.196, and 0.256, respectively, which indicates that the S atom in Cell-AP has a higher electron density and a higher tendency to donate its electron(s) to Pd(II). These results are consistent with the energy calculation results. Energy calculation results showed that the binding energies between Pd(II) and Cell-AP, Cell-MP, and Cell-MBT were -76.25, -73.53, and -63.80 kcal / mol, respectively. The lower the binding energy, the more spontaneous the binding reaction. The order of binding energy is consistent with the trend of adsorption capacity among the three types of S- functionalized adsorbents, which follows the order of Cell-AP > Cell-MP > Cell-MBT (FIGS. 3A- D). In addition, the S-Pd bond lengths also follow the same trend. The bond length between S atom and Pd(II) ion in Cell-AP, Cell-MP, and Cell-MBT adsorbents were 2.432 A, 2.568 A, and 2.603 A, respectively. Since a shorter bond length indicates a stronger bond, these results are also consistent with the trend of adsorption capacity. After adsorption, the charge of S atoms increases to 0.360- 0.394 and the charge of Pd decreases to 0.007-0.121, which is consistent with the coordination mechanism proposed above, where sulfur atoms donated electron density to Pd(II) and created a coordinate bond therebetween. Additionally, Pd(II) adopted a stable square planar geometry with two Cl atoms, one S atom, and one N atom from the ligands.

[0188] Selective Mechanism of Pd / Pt Complex Formation

[0189] The DFT energy calculation provided further insights regarding why adsorbents described herein have higher selectivity for Pd over Pt. Pt(IV) forms a stable octahedral structure with four Cl atoms, one S atom, and one N atom from the ligands (FIG. 10C). The binding energy of Pt(IV) with Cell-AP, Cell-MP, and Cell-MBT were -53.43, -48.07, and -35.22 kcal / mol, respectively, which are more positive than those of Pd(II) with the respective ligands. This trend is consistent with the experimental results that three S-functionalized adsorbents have a higher affinityto Pd(II) compared with Pt(IV). This trend is also consistent with prior study in which MOF material BDC-NH2 showed higher binding energy for PtCL2' than PdCL2'. Based on the DFT calculations, the volume of PdCL2' and PtCL2’ are 101.70 and 138.58 cm3 / mol, respectively. The larger size of PtCL2' makes it harder to go through a porous medium, which can provide an additional basis of selectivity for PdCL2' over PtCL2'.Table 6. The HOMO and LUMO energy of several Lewis acid and base (in Hartree).Lewis LUMO HOMO Hardness Softness Lewis LUMO HOMO Hardness Softness acid (- ) (<T) base (q) (cr)Li+0.0089 -1.9631 0.9860 1.0141 F‘ 0 9555 -0.1699 0 5627 1.7770K+-0.0117 -0.7341 0.361 1 2.7685 CL 0 7708 -0.1979 0 4844 2.0642Ca2+-0.0257 -1.1077 0.5410 1.8484 L 0 4267 -0.2048 0 3158 3.1664Cu2+-0.2118 -0.6462 0.2172 4.6040 H2O 0 0736 -0.3051 0 1893 5.2802Fe2+-0.2124 -0.3954 0.0914 10.9325 OH- 0 1789 -0.1294 0 1541 6.4865Ag+-0.1020 -0.3308 0.1143 8.7427 NH30 0987 -0.2454 0 1721 5.8100Au3+-0.6406 -0.6819 0.0206 48.3909 RNH2-0.0326 -0.2002 0 0837 11.9367Pd2+-0.3412 -0.3871 0.0229 43.5350 R-S-H -0.1128 -0.2429 0 0650 15.3680Pt4+-0.5534 -0.6674 0.0569 17.5515 R-S-R -0.1760 -0.2274 0 0257 38.9029

[0190] The molecular orbital (MO) analysis also provides insights regarding the preference for Pd(II) over Pt(IV). DFT analysis was used to calculate molecular orbital information for a number of Lewis acids (metals / metal cations) and Lewis bases (ligands) (FIG. 11, Table 6). The magnitude of the H0M0-LUM0 gap is directly proportional to the hardness index, r], and inversely proportional to the softness index, a. The traditional hard acids (e.g., Li+, K+, and Ca2+) and hard bases (e.g., F’, C1‘, and I') tend to have a large H0M0-LUM0 gap and, accordingly, a strong tendency to lose or gain electrons. Hard acids prefer hard bases and form bonds that have a larger ionic character. In contrast, soft acids (e.g., Pd2+, Pt4+, and Au3+) and soft bases (e.g., R-S-R, R-S-H) have a small H0M0-LUM0 gap and, accordingly, a smaller tendency to lose or gain electrons. Instead, they tend to have more polarizable and diffuse electron clouds. Soft acids preferentially interact with soft bases by sharing electrons and form bonds with a larger covalent character. It is shown in FIG. 11 that R-S-R ligand has a smaller H0M0-LUM0 gap than N-donor or O-donor ligands, which supports the observation that S-donor ligands are more suitable for the separation of soft metals, such as Pd2+and / or Pt4+in this example. Additionally, Pd2+has a smaller H0M0-LUM0 gap than Pt4+, which indicates that Pd2+does not have a strong tendency to lose or gain electrons, asevidenced by its higher polarizability and higher softness index. The electron cloud of Pd2+is more diffuse and easily distorted than that of Pt4+, and thus Pd2+is more likely to coordinate with S donors.

[0191] From another perspective, a stronger coordination interaction can occur when two orbitals have similar energy. The closer the energy between the LUMO of a Lewis acid and the HOMO of a Lewis base, the stronger the coordinate bond will be (such as Li+and Cl’). The HOMO energy of ligand R-S-R is very close to the LUMO energy of Pd2+, which means that it is easier for electrons from the HOMO of the R-S-R to transfer to the LUMO of Pd2+, and the electrons may be almost equally shared between the metal and ligand, thus forming a stable covalent complex (FIG. 11). However, the difference in energy between the HOMO of R-S-R and the LUMO of Pt4+is slightly larger, which indicates that the complex therebetween is less stable, and the coordination reaction is not as favorable as that between R-S-R and Pd2. In summary, the selectivity of Pd over Pt may be a result of the smaller size (as PdCL2'), the higher softness index, and / or the smaller H0M0-LUM0 gap of Pd2+when compared with Pt4+.Conclusion

[0192] To achieve a selective separation of Pd over Pt in an aqueous solution, three S- functionalized adsorbents were synthesized by covalently attaching 2-aminothiophenol (AP), 2- mercaptopyridine (MP), and 2-mercaptobenzothiazole (MBT), respectively, onto a cellulose matrix. The adsorption capacities of Cell-AP, Cell-MP, and Cell-MBT were 163.25, 92.51, and 27.48 mg / g for Pd(II), and 20.70, 9.16, and 3.60 mg / g for Pt(IV), respectively. The S-functionalized adsorbents showed a good selectivity for Pd(II) over Pt(IV). The S-functionalized adsorbents also showed a higher selectivity for Pd(II) over other competing ions including Fe(III), Cu(II), Hg(II), Sc(III) and U(IV), with a separation factor ranging from 49.44 to 13373.97. The isotherm study showed that the adsorption process follows the Langmuir isotherm representing a single-layer adsorption, and the kinetic study showed that the process follows a pseudo-second-order reaction. The adsorbents were characterized with FTIR, XPS, and CHNS analysis, which showed that AP, MP, and MBT ligands were successfully attached to the surface of cellulose and Pd(II) was coordinated and adsorbed to the ligands. The adsorption mechanism was investigated by DFT calculations, which showed that PdCL2’ have a smaller size, a higher softness index, and a lower binding energy with ligands compared to PtCL2’.

[0193] FIG. 12 depicts exemplary scanning electron microscopy (SEM) images of Cell-AP andCell-MP adsorbents.

[0194] The foregoing has been a detailed description of illustrative embodiments of the invention. Various modifications and additions can be made without departing from the spirit and scope of this invention. Features of each of the various embodiments described above may be combined with features of other described embodiments as appropriate in order to provide a multiplicity of feature combinations in associated new embodiments. Furthermore, while the foregoing describes a number of separate embodiments, what has been described herein is merely illustrative of the application of the principles of the present invention. Additionally, although particular methods herein may be illustrated and / or described as being performed in a specific order, the ordering is highly variable within ordinary skill to achieve methods, systems, and software according to the present disclosure. Accordingly, this description is meant to be taken only by way of example, and not to otherwise limit the scope of this invention.

[0195] Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the spirit and scope of the present invention.

Claims

CLAIMS1. A composition of matter for selective separation of a Pd species from a mixture, wherein the composition comprises: an adsorbent, wherein the adsorbent comprises: a support; and at least one ligand attached to the support, wherein: the at least one ligand comprises: a S donor comprising at least one S atom; and a N donor comprising at least one N atom; and the at least one ligand selectively adsorbs the Pd species over one or more competing ions in the mixture, wherein the one or more competing ions comprises a Pt species.

2. The composition according to claim 1, wherein the support comprises a flexible support.

3. The composition according to claim 1 or 2, wherein the support comprises unsubstituted or substituted cellulose.

4. The composition according to any one of claims 1-3, wherein the support comprises silica, activated alumina, or zeolite.

5. The composition according to any one of claims 1-4, wherein the support comprises a polymer resin or ion-imprinted polymer.

6. The composition according to claim 5, wherein the polymer resin comprises a polystyrene resin.

7. The composition according to any one of claims 1-6, wherein the support comprises activated carbon, graphene, or graphene oxide.

8. The composition according to any one of claims 1-7, wherein the N donor includes one or more functional groups selected from a list consisting of a substituted or unsubstitutedprimary amine, a substituted or unsubstituted secondary amine, a substituted or unsubstituted tertiary amine, a substituted or unsubstituted alkylamine, a substituted or unsubstituted arylamine, a substituted or unsubstituted imine, a substituted or unsubstituted pyrrole, a substituted or unsubstituted benzopyrrole, a substituted or unsubstituted pyrrolidine, a substituted or unsubstituted imidazole, a substituted or unsubstituted benzimidazole, a substituted or unsubstituted imidazolidine, a substituted or unsubstituted pyrazole, a substituted or unsubstituted indazole, a substituted or unsubstituted pyrazolidine, a substituted or unsubstituted oxazole, a substituted or unsubstituted benzoxazole, a substituted or unsubstituted oxazolidine, a substituted or unsubstituted isoxazole, a substituted or unsubstituted benzisoxazole, a substituted or unsubstituted isoxazolidine, a substituted or unsubstituted thiazole, a substituted or unsubstituted benzothiazole, a substituted or unsubstituted thiazolidine, a substituted or unsubstituted isothiazole, a substituted or unsubstituted benzisothi azole, a substituted or unsubstituted isothiazolidine, a substituted or unsubstituted triazole, a substituted or unsubstituted benzotri azole, a substituted or unsubstituted oxodiazole, a substituted or unsubstituted benzoxadi azole, a substituted or unsubstituted thiadiazole, a substituted or unsubstituted benzothiadiazole, a substituted or unsubstituted dioxazole, a substituted or unsubstituted benzodi oxole, a substituted or unsubstituted dithiazole, a substituted or unsubstituted benzodithiazole, a substituted or unsubstituted tetrazole, a substituted or unsubstituted oxatriazole, a substituted or unsubstituted thiatriazole, a substituted or unsubstituted pentazole, a substituted or unsubstituted oxatetrazole, a substituted or unsubstituted thiatetrazole, a substituted or unsubstituted pyridine, a substituted or unsubstituted quinoline, a substituted or unsubstituted piperidine, a substituted or unsubstituted pyrazine, a substituted or unsubstituted quinoxaline, a substituted or unsubstituted naphthyridine, a substituted or unsubstituted benzodiazine, and a substituted or unsubstituted diazanaphthalene, a substituted or unsubstituted pyrimidine, a substituted or unsubstituted quinazoline, a substituted or unsubstituted pyridazine, a substituted or unsubstituted phthalazine, a substituted or unsubstituted cinnoline, a substituted or unsubstituted piperazine, a substituted or unsubstituted oxazine, a substituted or unsubstituted benzoxazine, a substituted or unsubstituted morpholine, a substituted or unsubstituted thiazine, a substituted or unsubstituted benzothiazine, a substituted or unsubstituted phenothiazine, a substituted or unsubstituted thiomorpholine, a substituted or unsubstituted triazine, a substituted or unsubstituted benzotriazine, a substituted orunsubstituted triazinane, a substituted or unsubstituted tetrazine, a substituted or unsubstituted purine, a substituted or unsubstituted azepine, a substituted or unsubstituted azepane, a substituted or unsubstituted azocine, a substituted or unsubstituted azocane, a substituted or unsubstituted azonine, a substituted or unsubstituted azonane.

9. The composition according to any one of claims 1-8, wherein the S donor includes one or more functional groups selected from a list consisting of a mercaptan, an alkyl thiol, an aryl thiol, a monothiol, a dithiol, a trithiol, a sulfide, a disulfide, a polysulfide, a thioester, a thiocarbonyl, a substituted or un substituted thiophenol, a substituted or un substituted naphthanethiol, a substituted or unsubstituted thiophene, a substituted or unsubstituted thiolane, a substituted or unsubstituted thiazole, a substituted or unsubstituted benzothiazole, a substituted or unsubstituted thiazolidine, a substituted or unsubstituted isothiazole, a substituted or unsubstituted benzisothiazole, a substituted or unsubstituted isothiazolidine, a substituted or unsubstituted oxathiole, a substituted or unsubstituted benzoxathiole, a substituted or unsubstituted oxathiolane, a substituted or unsubstituted isoxathiole, a substituted or unsubstituted benzisoxathiole, a substituted or unsubstituted isoxathiolane, a substituted or unsubstituted dithiole, a substituted or unsubstituted benzodithiole, a substituted or unsubstituted dithiolane, a substituted or unsubstituted thiadiazole, a substituted or unsubstituted benzothiadiazole, a substituted or unsubstituted dithiazole, a substituted or unsubstituted benzodithiazole, a substituted or unsubstituted thiatriazole, a substituted or unsubstituted thiatetrazole, a substituted or unsubstituted thiopyran, a substituted or unsubstituted thiane, a substituted or unsubstituted thiazine, a substituted or unsubstituted benzothiazine, a substituted or unsubstituted phenothiazine, a substituted or unsubstituted thiomorpholine, a substituted or unsubstituted oxathiin, a substituted or unsubstituted oxathiane, a substituted or unsubstituted dithiin, a substituted or unsubstituted dithiane, a substituted or unsubstituted trithiin, a substituted or unsubstituted trithiane, a substituted or unsubstituted thiepine, a substituted or unsubstituted thiepane, a substituted or unsubstituted thiazepine, a substituted or unsubstituted thiazepane, a substituted or unsubstituted thiocane, a substituted or unsubstituted thionine, and a substituted or unsubstituted thionane.

10. The composition according to any one of claims 1-9, wherein the ligand comprises an aminothiophenol group, a mercaptopyridine group, or a mercaptobenzothiazole group.

11. The composition according to claim 10, wherein the aminothiophenol group is a 2- aminothiophenol (AP) group.

12. The composition according to any one of claims 10-11, wherein the mercaptopyridine group is a 2-mercaptopyridine (MP) group.

13. The composition according to any one of claims 10-12, wherein the mercaptobenzothiazole group is a 2-mercaptobenzothiazole (MBT) group.

14. The composition according to any one of claims 1-13, wherein the at least one ligand is attached to the support though the S donor.

15. The composition according to any one of claims 1-14, wherein the one or more competing ions include an ion of one or more elements selected from a group consisting of Mg, Al, Cu, Fe, Mn, Zn, Sc, As, Cd, Sb, Hg, Pb, Ga, Ge, Pt, La, Yb, Th, and U.

16. The composition according to any one of claims 1-15, wherein the Pd species comprises PdCl42'.

17. The composition according to any one of claims 1-16, wherein the mixture comprises an industrial waste stream.

18. The composition according to any one of claims 1-17, wherein the at least one ligand is covalently attached to the support through a linker.

19. The composition according to any one of claims 1-18, wherein the linker is chloroacetyl chloride or thionyl chloride.

20. The composition according to any one of claims 1-19, wherein the composition is prepared using a solvent selected from a group consisting of dimethylformamide (DMF), dimethylsulfoxide (DMSO), dichloromethane (DCM), tetrahydrofuran (THF), methanol, ethanol, and acetone.

21. The composition according to any one of claims 1-20, wherein the support comprises at least one hydroxyl group on its surface.

22. The composition according to any one of claims 18-21, wherein the linker is attached to the support by reacting with the at least one hydroxyl group.

23. The composition according to any one of claims 18-22, wherein: the support comprises a plurality of hydroxyl groups on its surface; and at least 20% and no greater than 80% of the plurality of hydroxyl groups is functionalized by the at least a ligand.

24. The composition according to any one of claims 18-23, wherein the linker is attached to the support through an ester group (-C(O)OC-) or an ether group (-COC-).

25. The composition according to any one of claims 18-24, wherein the linker comprises at least one and no greater than 20 C atoms.

26. The composition according to any one of claims 18-25, wherein the linker further comprises a -C=C- bond, a -C=C- bond, a phenylene (-C6H4-), a carbonyl (-C(O)-), an ester (- C(O)OC-), an ether (-COC-), or an amide (-C(O)NH-).

27. The composition according to any one of claims 18-26, wherein the linker further comprises one or more branched or cyclic moieties.

28. The composition according to any one of claims 18-27, wherein the linker comprises one or more halogen atoms.

29. The composition according to any one of claims 1-28, wherein the mixture has a pH of at least 0.5 and no greater than 3.5.

30. The composition according to any one of claims 1-29, wherein the adsorbent has a specific surface area of at least 2 m2 / g and no greater than 10 m2 / g.

31. The composition according to any one of claims 1-30, wherein the adsorbent has a pore volume of at least 20 cm3 / kg and no greater than 60 cm3 / kg.

32. The composition according to any one of claims 1-31, wherein the adsorbent has an average pore diameter of at least 25 nm and no greater than 32 nm.

33. The composition according to any one of claims 1-32, wherein the adsorbent is a reusable adsorbent.

34. The method of preparing the adsorbent according to any one of claims 1-33, wherein the method comprises: i) providing a support having at least one surface group; ii) providing a bifunctional linker having a first reactive group and a second reactive group; iii) connecting the bifunctional linker to the at least one surface group of the support using the first reactive group; iv) providing at least one ligand; and v) attaching the at least one ligand to the second reactive group of the bifunctional linker to produce the adsorbent.

35. The method of preparing the adsorbent according to claim 34, wherein the at least one surface group includes a hydroxyl.

36. The method of preparing the adsorbent according to claim 34 or 35, wherein: the support comprises a plurality of surface groups; and connecting the bifunctional linker comprises connecting the bifunctional linker to at least 20% and no greater than 80% of the plurality of surface groups.

37. The method of preparing the adsorbent according to any one of claims 34-36, wherein the bifunctional linker comprises at least one and no greater than 20 C atoms.

38. The method of preparing the adsorbent according to any one of claims 34-37, wherein the first reactive group or the second reactive group comprises a chloro group.

39. The method of preparing the adsorbent according to any one of claims 34-38, wherein the first reactive group or the second reactive group comprises an acyl chloride group (-C(O)Cl).

40. The method of preparing the adsorbent according to any one of claims 34-39, wherein the bifunctional linker comprises 2-chloroacetyl chloride, 3 -chloropropionyl chloride, 4- chlorobutyryl chloride, 5-chloropentanoyl chloride, or 6-chlorohexanoyl chloride.

41. The method of preparing the adsorbent according to any one of claims 34-40, wherein step iii) or v) is performed using triethylamine (EtaN) or 4-dimethylaminopyridine (DMAP) as a catalyst.

42. The method of preparing the adsorbent according to any one of claims 34-41, wherein steps iii) or v) is performed under room temperature for 4-20 hours.

43. A method of using the adsorbent according to any one of claims 1-42 for selective separation of the Pd species, the method comprising: i) contacting the adsorbent with a mixture of the Pd species and one or more competing ions; and ii) selectively adsorbing the Pd species for a period of at least 2 hours and no greater than 100 hours.

44. The method of using the adsorbent according to claim 43, further comprising, before step i), acidifying the mixture to reach a pH of at least 0.5 and no greater than 3.5.

45. The method of using the adsorbent according to claim 43 or 44, wherein the method further comprises desorbing the adsorbed Pd species from the adsorbent using an HC1 solution or a mixture containing Thiourea and HC1.

46. The method of using the adsorbent according to any one of claims 43-45, wherein the method further comprises recycling the adsorbent using a centrifuge.

47. An adsorbent for selective separation of a Pd species from a mixture, wherein the adsorbent is prepared using the method according to any one of claims 34-42.

48. An article of manufacture for selective separation of a Pd species from a mixture, wherein the article of manufacture comprises the adsorbent according to any one of claims 1-47.

49. The article of manufacture according to claim 48, wherein the article of manufacture is a cartridge, fdter unit, adsorbent bed, or column.

50. Use of the adsorbent according to claims 1-49 for selective separation of a Pd species from a mixture.

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