Preparation method and application of calcium ion double-crosslinked three-dimensional network hydrogel

Abstract

The application provides a preparation method and application of a calcium ion double-crosslinked three-dimensional network hydrogel, and belongs to the field of seawater desalination. The method comprises the following steps: mixing a lithium source, a titanium source and an iron source according to a molar ratio of Li:Ti:Fe=2(1+x):1:x, stirring and ultrasonic treating after adding anhydrous ethanol, calcining to obtain a precursor, and immersing the precursor in an acid solution, washing and drying to obtain an iron-doped titanium-based lithium ion sieve; mixing an aqueous solution of polyvinyl alcohol, gellan gum and sodium alginate, adding a multi-walled carbon nanotube dispersion liquid and the iron-doped titanium-based lithium ion sieve to obtain a composite precursor solution; adding a calcium chloride solution to the composite precursor solution to perform a double-crosslinking reaction; performing freeze-thaw cycle treatment on the crosslinked system, and then freeze-drying to obtain a calcium ion double-crosslinked three-dimensional network hydrogel. The preparation method and application provided by the application can simultaneously, efficiently and stably realize fresh water production and selective lithium resource recovery from a complex seawater environment.

Description

Technical Field

[0001] This application relates to the field of seawater desalination technology, and in particular to a method for preparing and applying a calcium ion double crosslinked three-dimensional network hydrogel. Background Technology

[0002] Lithium is a core strategic resource for new energy vehicles, energy storage batteries, and high-end electronic products. With the accelerated global energy transition, lithium demand is surging, and extracting lithium from the vast reserves of seawater (approximately 260 billion tons) has become an important direction for ensuring resource security. However, the extremely low concentration of lithium ions in seawater, coupled with high concentrations of sodium, magnesium, and potassium ions, makes efficient, economical, and selective lithium extraction a significant challenge.

[0003] Currently, the main technologies for extracting lithium from seawater or brine include solvent extraction, membrane separation, electrochemical methods, and adsorption methods. Among these, adsorption methods have attracted attention due to their simplicity and low energy consumption. Of the many adsorbents, titanium-based lithium-ion sieves (HTO) are particularly noteworthy for their ability to extract lithium from seawater or brine. + Due to its high selectivity, good chemical stability, and cycling performance, it is considered one of the most promising adsorbent materials. In recent years, in order to improve extraction efficiency, researchers have attempted to introduce solar energy to drive the process and have developed a variety of photothermal adsorption systems, utilizing the local thermal effect generated by photothermal conversion to accelerate ion migration and adsorption.

[0004] However, existing technical solutions still have limitations. On the one hand, traditional powdered HTO faces problems such as difficulty in recovery, easy agglomeration and loss, and high mass transfer resistance in actual water treatment, which limits its engineering applications. On the other hand, most reported photothermal adsorption systems are often single-function, focusing only on lithium extraction or freshwater production, making it difficult to achieve synergistic harvesting of resources and water; at the same time, the balance between the dispersion stability of adsorbed components, photothermal conversion efficiency, and long-term cycling performance in the system has not yet been effectively resolved. Summary of the Invention

[0005] In view of this, this application provides a method for preparing a calcium ion double crosslinked three-dimensional network hydrogel and its application, in order to solve the problems of powder adsorbents being difficult to recover and prone to agglomeration, photothermal adsorption systems having limited functionality, and insufficient long-term operational stability in the prior art.

[0006] Specifically, this application is implemented through the following technical solution:

[0007] The first aspect of this application provides a method for preparing a calcium ion double crosslinked three-dimensional network hydrogel, the method comprising:

[0008] A lithium source, a titanium source, and an iron source are mixed in a molar ratio of Li:Ti:Fe=2(1+x):1:x. Anhydrous ethanol is added, the mixture is stirred, and ultrasonically treated. The mixture is then calcined at 800~900℃ to obtain a precursor. The precursor is then leached with an acid solution, washed, and dried to obtain an iron-doped titanium-based lithium-ion sieve. Wherein, x=0.05~0.15.

[0009] An aqueous solution of polyvinyl alcohol, gellan gum, and sodium alginate was mixed, and then a multi-walled carbon nanotube dispersion and the iron-doped titanium-based lithium-ion sieve were added and mixed to obtain a composite precursor solution.

[0010] A calcium chloride solution was added to the composite precursor solution to carry out a double crosslinking reaction;

[0011] The cross-linked system was subjected to freeze-thaw cycles and then freeze-dried to obtain a calcium ion double cross-linked three-dimensional network hydrogel.

[0012] A second aspect of this application provides a calcium ion dual crosslinked three-dimensional network hydrogel, the calcium ion dual crosslinked three-dimensional network hydrogel comprising:

[0013] A three-dimensional network matrix composed of calcium ion-crosslinked polyvinyl alcohol, gellan gum, and sodium alginate;

[0014] Multi-walled carbon nanotubes uniformly dispersed in the three-dimensional network matrix;

[0015] And an iron-doped titanium-based lithium-ion sieve that is uniformly dispersed in the form of nanoparticles and anchored in the three-dimensional network matrix.

[0016] The third aspect of this application provides an application of a calcium ion dual cross-linked three-dimensional network hydrogel, which is used for simultaneous seawater desalination and lithium ion extraction.

[0017] This application presents a method for preparing and applying a calcium-ion dual-crosslinked three-dimensional network hydrogel. Through the synergistic design of steps involving iron doping optimization, calcium-ion dual crosslinking, and freeze-thaw cycle reinforcement, a structurally stable and functionally integrated calcium-ion dual-crosslinked three-dimensional network hydrogel is successfully constructed. First, lithium-ion sieves are prepared through iron doping and a specific process, providing the compositional and structural basis for its subsequent functions. Second, through solution mixing of a specific polymer and a calcium-ion dual-crosslinking reaction, a stable three-dimensional network framework with both chemical crosslinking and ion anchoring effects is constructed in one step while achieving uniform dispersion of functional components (carbon nanotubes and lithium-ion sieves). Finally, freeze-thaw cycles and freeze-drying further physically strengthen the network and solidify its hierarchical porous structure. The synergistic effect of each step in this method results in a hydrogel with a stable macroscopic morphology, uniform component distribution, and internal channels conducive to mass transfer and photothermal interaction, laying a structural foundation for the material's subsequent efficient adsorption, photothermal, and other multifunctional integrated applications. Attached Figure Description

[0018] Figure 1 The flowchart is for Example 1 of the preparation method of the calcium ion double crosslinked three-dimensional network hydrogel provided in this application;

[0019] Figure 2 The SEM morphology, EDS elemental distribution diagram and structural schematic diagram of the PGS, PGST and PGST@HTO-Fe composite hydrogels shown in this application are as follows:

[0020] Figure 3 XPS full and fine spectra of the PGS, PGST and PGST@HTO-Fe composite hydrogels shown in this application;

[0021] Figure 4 Characterization of the hydrophilicity, antifouling properties and mechanical properties of the PGST@HTO-Fe composite hydrogel shown in this application;

[0022] Figure 5 The photothermal properties and optical absorption characterization of the composite hydrogel shown in this application;

[0023] Figure 6 The diagram shows the solar evaporation and seawater desalination performance of the composite hydrogel presented in this application.

[0024] Figure 7 This is a graph showing the lithium-ion adsorption performance of the composite hydrogel illustrated in this application;

[0025] Figure 8 The diagram shows the photothermal-assisted lithium adsorption, performance optimization, and comprehensive application performance of the composite hydrogel shown in this application.

[0026] Figure 9 The diagram shows the photocatalytic degradation performance of the composite hydrogel illustrated in this application.

[0027] Figure 10 The diagram shows the DFT calculation and mechanism analysis of HTO and HTO-Fe / CNT as presented in this application. Detailed Implementation

[0028] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this application.

[0029] The terminology used in this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The singular forms “a,” “the,” and “the” used herein are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used herein refers to and includes any and all possible combinations of one or more of the associated listed items.

[0030] It should be understood that although the terms first, second, third, etc., may be used in this application to describe various information, such information should not be limited to these terms. These terms are only used to distinguish information of the same type from one another. For example, without departing from the scope of this application, first information may also be referred to as second information, and similarly, second information may also be referred to as first information. Depending on the context, the word "if" as used herein may be interpreted as "when," "when," or "in response to determination."

[0031] The following specific embodiments are given to illustrate the technical solution of this application in detail.

[0032] Example 1

[0033] Figure 1 This is a flowchart of Example 1 of the preparation method for the calcium ion double crosslinked three-dimensional network hydrogel provided in this application. Please refer to... Figure 1 The method provided in this embodiment may include:

[0034] S101. A lithium source, a titanium source, and an iron source are mixed in a molar ratio of Li:Ti:Fe=2(1+x):1:x. Anhydrous ethanol is added, the mixture is stirred, and ultrasonically treated. The mixture is then calcined at 800~900℃ to obtain a precursor. The precursor is then leached with an acid solution, washed, and dried to obtain an iron-doped titanium-based lithium-ion sieve. Wherein, x=0.05~0.15.

[0035] It should be noted that the experimental materials were prepared first. Specifically, the following materials were purchased from China National Pharmaceutical Group Chemical Reagent Co., Ltd.: gellan gum (GG), sodium alginate (SA), polyvinyl alcohol (PVA), anhydrous ethanol (C2H5OH), hydrochloric acid (HCl), anhydrous lithium chloride (LiCl), and chlorides of sodium, magnesium, potassium, and calcium. Multi-walled carbon nanotubes (CNTs) were purchased from Chengdu Times Nanotechnology Co., Ltd. Titanium dioxide (TiO2), lithium carbonate (Li2CO3), and iron(III) oxide (Fe3O4) were purchased from Shanghai Aladdin Reagent Co., Ltd. All chemical reagents used in the experiment were of analytical grade and required no further purification.

[0036] It should be noted that the raw materials used in this step are lithium-containing compounds, titanium-containing compounds, and iron-containing compounds. Specifically, the lithium source can be lithium carbonate, lithium hydroxide, or lithium nitrate, etc.; the titanium source can be titanium dioxide, tetrabutyl titanate, etc.; and the iron source can be ferric oxide, iron(II,III) oxide, or ferric nitrate, etc. These raw materials have high purity and reactivity. Preferably, in the embodiments provided in this application, lithium carbonate (Li2CO3) can be used as the lithium source, titanium dioxide (TiO2) as the titanium source, and iron(II,III) oxide (Fe3O4) as the iron source. This combination of raw materials is readily available, the stoichiometry is easy to control, the reaction is complete at high temperatures, and it is environmentally friendly.

[0037] It should be noted that the raw materials are mixed in a molar ratio of Li:Ti:Fe = 2(1+x):1:x. The excess Li is used to compensate for potential lithium volatilization losses during high-temperature calcination, ensuring the final precursor has a complete stoichiometric structure. The parameter x represents the Fe-to-Ti doping molar ratio, with a value ranging from 0.05 to 0.15. This range is set based on two considerations: firstly, an appropriate amount of Fe... 3+ Doping can replace Ti 4+ Sites, introducing oxygen vacancies into the crystal lattice and modulating the electronic structure, thereby reducing the subsequent lithium ion (Li) concentration. + On the one hand, excessive doping can reduce the energy barrier of diffusion and increase the adsorption active sites; on the other hand, excessive doping (x>0.15) may lead to excessive lattice distortion, destroy the stability of the main structure, or even form impurity phases, which is not conducive to adsorption performance. Preferably, x=0.10.

[0038] Furthermore, the solid powder raw materials weighed according to the above proportions are placed together in an organic dispersion medium such as anhydrous ethanol. Vigorous mechanical stirring is performed first, followed by ultrasonic treatment, in order to utilize mechanical shear force and the cavitation effect of ultrasound to completely break up the agglomeration of the raw material powder and achieve a highly uniform mixture of lithium source, titanium source and iron source at the microscale.

[0039] The uniformly dispersed slurry is dried at 323 K to obtain a uniformly mixed, dried precursor powder. This powder is then placed in a high-temperature furnace (such as a tube furnace) and calcined under air or an inert atmosphere (such as nitrogen) at a temperature ranging from 800°C to 900°C for 4 to 10 hours. Specifically, this high-temperature process is the stage where a solid-state reaction occurs, with the goal of generating iron-doped lithium titanate (Li₂TiO₃:Fe, denoted as LTO-Fe). Taking the reaction of lithium carbonate, titanium dioxide, and iron(III) oxide in air as an example, the main chemical reaction can be represented as: (1+x)Li₂CO₃ + TiO₂ + x / 3Fe₃O₄ + (1 / 2-x / 2)O₂ → Li₂TiO₃:Fe (LTO-Fe) + CO₂↑. The essence of this reaction is the formation of lithium titanate crystals with a doped structure. When using other raw materials or under different atmospheres, the reactants and products may be adjusted, but ultimately, an LTO-Fe precursor is formed.

[0040] The temperature range of 800-900℃ was chosen because this range is sufficient to decompose lithium carbonate and allow it to undergo a sufficient solid-state reaction with titanium dioxide and iron tetroxide, generating well-crystallized iron-doped lithium titanate (LTO-Fe) spinel or rock salt structure phase. Too low a temperature will result in incomplete reaction and the presence of unreacted raw materials; too high a temperature may cause excessive grain growth, severe lithium volatilization, or the formation of undesirable phases. In one specific embodiment, calcination at 850℃ for 8 hours is preferred to ensure complete reaction and obtain a precursor with a suitable grain size, laying the structural foundation for subsequent efficient ion exchange.

[0041] The LTO-Fe precursor obtained by calcination was immersed in an acid solution for proton (H) reaction. + ) and lithium ions (Li + The exchange reaction involves inorganic acids such as hydrochloric acid (HCl) or sulfuric acid (H₂SO₄), with concentrations ranging from 0.5 to 1.0 mol / L. A 0.75 mol / L hydrochloric acid solution is preferred, as this concentration provides a suitable amount of H₂. + Concentration, in order to effectively remove Li + At the same time, the corrosion of the titanium-based framework is relatively mild, which is conducive to maintaining the structural stability of the ion sieve.

[0042] The leaching process is carried out under heating conditions, which can increase the kinetic rate of ion exchange. The temperature range is typically 300K to 350K, and the leaching time is typically 10 to 15 hours. The preferred leaching conditions are continuous stirring at 333K for 12 hours, under which the exchange reaction is more complete. The ion exchange reaction in this process can be represented as: Li₂TiO₃:Fe(solid) + 2H⁺ + (solution) H₂TiO₃:Fe(solid) + 2Li + (In solution), through this reaction, lithium ions are released from the crystal lattice and enter the solution, while protons enter the crystal lattice, forming a solution rich in H+. + Ion sieve structure with vacancy.

[0043] After the reaction is complete, the solid is separated by centrifugation or filtration, and repeatedly washed with deionized water until the wash solution is neutral (pH≈7) to remove residual acid and exchanged Li. + Finally, the solid product is dried to obtain the target product—iron-doped titanium-based lithium-ion sieve (HTO-Fe). At this stage, the HTO-Fe crystal framework is rich in H+. + Occupied, available for exchange Li + The vacancy is filled, and the modification effect introduced by Fe doping is retained, thus combining high adsorption capacity with good kinetic performance.

[0044] It should be noted that the core of this step is to prepare a precursor through a high-temperature solid-phase reaction, followed by ion exchange through acid leaching, and finally obtain the target adsorbent material.

[0045] S102. Mix the aqueous solutions of polyvinyl alcohol, gellan gum and sodium alginate, add the multi-walled carbon nanotube dispersion and the iron-doped titanium-based lithium ion sieve, and mix to obtain a composite precursor solution.

[0046] It should be noted that polyvinyl alcohol (PVA) is a synthetic polymer with excellent film-forming properties, biocompatibility, and high mechanical strength. Its molecular chain is rich in hydroxyl groups (-OH), which can form numerous hydrogen bonds with gellan gum (GG), sodium alginate (SA), and subsequently added nanomaterials. These hydrogen bonds serve as physical cross-linking points, enhancing the toughness, elasticity, and structural stability of the hydrogel. Both gellan gum (GG) and sodium alginate (SA) are natural anionic polysaccharides. GG can form a thermally reversible gel upon heating and cooling, and its molecular chain contains abundant carboxyl groups (-COOH) and hydroxyl groups (-OH). The molecular chain of SA is composed of guluronic acid (G) and mannuronic acid (M) units. The carboxyl groups on its G units are associated with divalent cations (such as Ca²⁺, Ca²⁺, and Ca²⁺). 2+ It possesses highly specific binding ability. Choosing the combination of GG and SA allows for the utilization of their synergistic crosslinking properties, particularly in the subsequent introduction of Ca... 2+ It can preferentially combine with the G unit of SA to form an egg-box structure, and can also coordinate with the carboxyl group of GG to achieve double cross-linking of calcium ions, thus constructing a three-dimensional network prototype with both rigidity and toughness.

[0047] PVA, GG, and SA were prepared into aqueous solutions with mass percentage concentrations of 10 wt%, 2 wt%, and 3 wt%, respectively. This concentration range ensures that the polymers have suitable chain entanglement and solution viscosity, facilitating processing and mixing while providing sufficient functional group density for subsequent crosslinking. Too low a concentration results in insufficient skeletal strength; too high a concentration results in an overly viscous solution, which is detrimental to the uniform dispersion of nanomaterials and subsequent operations.

[0048] The three polymer solutions were mixed at a mass ratio of PVA:GG:SA = 2:1:1. This ratio was determined after a systematic evaluation of the mechanical properties, hydrophilicity, and moldability of hydrogels with different ratios (e.g., 1:1:1, 1:2:1, 1:1:2). The 2:1:1 ratio ensures that PVA provides sufficient mechanical reinforcement while maintaining a moderate total amount of GG and SA, which can be effectively combined with sufficient Ca... 2+ Effective cross-linking forms a stable network, while avoiding excessive gel brittleness or excessively rapid ion exchange caused by an excessive amount of anionic polysaccharides. At this ratio, the long chains of PVA and the short chains of GG / SA interweave and entangle with each other, and are pre-assembled through hydrogen bonds, providing an ideal polymer template for calcium ion cross-linking.

[0049] Multi-walled carbon nanotubes (CNTs) are introduced in this step due to their unique tubular structure, extremely high specific surface area, excellent broadband light absorption, and superior mechanical properties. Specifically, CNTs play three main roles in this system: firstly, as a highly efficient photothermal conversion agent, they absorb sunlight and convert it into heat energy, driving interfacial water evaporation; secondly, as a nano-reinforcing material, they physically adsorb and entangle with polymer chains through their large specific surface area, transferring stress and improving the overall mechanical strength of the hydrogel; and thirdly, as an electron conduction channel, they form interfacial coupling with HTO-Fe, promoting the separation and transfer of photogenerated charges and enhancing potential photocatalytic activity.

[0050] Multi-walled carbon nanotubes (CNTs) are added in the form of a pre-dispersion with a mass percentage concentration of 2 wt%. This concentration is set to balance dispersion stability and the CNT content in the final composite material. If the concentration is too low, the CNT solid content per unit volume will be low, resulting in limited photothermal and reinforcing effects; if the concentration is too high, the viscosity of the dispersion will increase dramatically, and the CNTs will easily re-agglomerate, making it difficult to disperse uniformly in the polymer solution. The 2 wt% dispersion can better maintain the individual dispersion state of CNTs under ultrasonic or shear force, facilitating their uniform embedding into the polymer network. In the specific embodiment provided in this application, 10 mL of the above-mentioned 2 wt% CNT dispersion can be added. This amount allows the CNT content in the final hydrogel to reach a balance point, achieving efficient photothermal conversion without affecting the hydrophilicity or pore structure of the gel due to excessive CNTs.

[0051] It should be noted that the HTO-Fe prepared in step S101 is responsible for specifically recognizing and capturing lithium ions from water. However, the direct application of nano-sized HTO-Fe powder presents challenges such as easy agglomeration and difficulty in recycling. Therefore, it is necessary to uniformly and stably immobilize it in the hydrogel network to be formed. The amount of HTO-Fe added directly affects the balance between the adsorption capacity and mechanical / transport properties of the final composite material. By studying the effects of different addition amounts, such as 0.1g, 0.2g, and 0.3g, on the lithium adsorption capacity and solar evaporation rate of the hydrogel, it was found that, preferably, when the HTO-Fe addition amount is 0.2g, the composite material can achieve excellent lithium balance adsorption capacity while maintaining a high evaporation rate. If the addition amount is too low, there will be insufficient adsorption sites; if the addition amount is too high, excessive particles will partially block the network pores of the gel, hindering water transport, leading to a decrease in evaporation efficiency, and may also affect the integrity of the gel due to local stress concentration.

[0052] Furthermore, the aforementioned polymer mixture, CNT dispersion, and HTO-Fe powder were subjected to prolonged mechanical stirring at a moderate speed under heating conditions. Heating helps reduce the viscosity of the PVA / GG / SA mixture, enhancing the thermal mobility of the molecular chains and facilitating the diffusion and dispersion of CNT and HTO-Fe nanoparticles in the solution. Continuous stirring provides the necessary shear force, further breaking down the soft aggregates of CNTs and the hard aggregates of HTO-Fe, dispersing them as nanoscale individuals or small aggregates. Simultaneously, it promotes the coating or adsorption of polymer molecular chains (especially the -OH groups of PVA and the -COOH / -OH groups of GG / SA) onto the surfaces of CNT and HTO-Fe nanoparticles. This adsorption is primarily achieved through van der Waals forces, hydrogen bonds, and possible coordination interactions (weak interactions between polymer carboxyl / hydroxyl groups and metal ions on the HTO-Fe surface). This initial interfacial bonding prevents severe phase separation or precipitation of the nanomaterials during subsequent crosslinking and drying processes. Through this mixing, a viscous and stable composite precursor solution containing a uniformly dispersed CNT network and HTO-Fe nanoparticles is finally obtained. The components in this composite precursor solution are pre-assembled through weak interactions, allowing for subsequent Ca2+ processing. 2+ Under the guidance of [them], a robust chemical-physical integrated network is formed.

[0053] S103. Add calcium chloride solution to the composite precursor solution to carry out a double crosslinking reaction.

[0054] It should be noted that calcium chloride (CaCl2) is a source of calcium. 2+ One of the most commonly used and effective reagents, it has the advantages of excellent water solubility, complete ion dissociation, low price, and good biocompatibility. 2+As a divalent cation, its ionic radius and charge density enable it to undergo strong and specific complexation with certain anionic polymers. The double crosslinking in this step refers to Ca... 2+ Simultaneously, it coordinates with two key sites in the complex precursor solution to form a three-dimensional network.

[0055] Specifically, the main cross-linking with the anionic polysaccharide (GG / SA) is the primary driving force for hydrogel formation. As mentioned above, the G units of sodium alginate (SA) interact with Ca... 2+ They have a very high affinity, and the two are combined through the "egg-box model," that is, a Ca 2+ It coordinates with the four carboxyl oxygen atoms on the two G units to form rigid, locally ordered crosslinking points. Simultaneously, the carboxyl groups on the gellan gum (GG) chain can also interact with Ca... 2+ Similar ion coordination occurs, although its binding constant may be slightly lower than that of SA, in this Ca 2+ The complexation with the carboxyl groups of GG / SA forms the first and most important chemical cross-linking framework of the entire hydrogel network. Secondly, the assisted cross-linking with iron-doped titanium-based lithium-ion sieves (HTO-Fe) is crucial for achieving stable adsorbent loading and preventing loss. In step S102, the polymer chains were initially coated onto the surface of the HTO-Fe particles through mixing. When Ca... 2+ After addition, it binds not only to free polymer carboxyl groups, but also to carboxyl groups on the polymer chain, and to hydroxyl groups (-OH) on the surface of HTO-Fe particles or to partially exposed metal ion sites (such as Fe). 3+ Ti 4+ Coordination occurs. This Ca... 2+ The indirect bridging of polymer-HTO-Fe forms a second crosslinking point, chemically anchoring the inorganic adsorbent nanoparticles to the organic polymer network, thereby solving the problem of powder loss.

[0056] It should be noted that a certain volume (e.g., 5 ml) of calcium chloride (CaCl2) solution is added to the composite precursor solution obtained in step S102. The concentration and amount of the CaCl2 solution should be sufficient to provide the calcium ions required for sufficient cross-linking with the anionic polysaccharide (GG / SA) in the system and to promote anchoring with HTO-Fe.

[0057] After adding the CaCl2 solution, the mixture was stirred at 75°C for 2 hours. This temperature helps maintain the low viscosity of the system and promotes the growth of CaCl2. 2+ Uniform diffusion throughout the sol prevents uneven gelation caused by excessively rapid local cross-linking; and continuous stirring helps Ca... 2+The dispersion ensures that the crosslinking reaction proceeds relatively uniformly throughout the entire process, while the 2-hour stirring time preserves the Ca... 2+ Sufficient time is allowed for the GG and SA carboxyl groups and HTO-Fe surface sites to fully interact, forming a sufficiently dense and stable three-dimensional cross-linked network. After the cross-linking reaction is complete, the system is kept at 75°C for about 30 minutes. The purpose of this process is to use residual heat to reduce the viscosity of the system and remove air bubbles entrained during stirring and reaction, thereby minimizing the introduction of macroscopic defects caused by air bubbles into the final hydrogel and helping to ensure the integrity and uniformity of the material structure.

[0058] With Ca 2+ With the addition of [the substance] and the proceeding of the reaction, the system undergoes a transformation from sol to gel. Macroscopically, this is manifested as a sharp increase in viscosity, eventually leading to loss of fluidity and the formation of an elastic wet gel. Microscopically, this process involves polymer chains being bridged by calcium ions, forming an infinitely large network that permeates the entire space. Through this step, the functional composite material was successfully shaped, transforming the linear and branched polymer solution into a wet gel with a stable three-dimensional network structure. Simultaneously, CNT and HTO-Fe nanoparticles were chemically bonded and physically entangled within this network, directly achieving a stable and uniform composite of the organic matrix and the inorganic functional phase. This lays the structural foundation for the subsequent synergistic photothermal, adsorption, and mechanical properties of the material.

[0059] S104. The cross-linked system was subjected to freeze-thaw cycles and then freeze-dried to obtain a calcium ion double cross-linked three-dimensional network hydrogel.

[0060] It should be noted that the cross-linked and defoamed wet gel system from S103 is injected into a mold of a specific shape and allowed to stand at room temperature for about 1 hour for initial shaping. Subsequently, it is frozen at -15°C for 10 hours, and then thawed at room temperature (approximately 25°C) for 2 hours. This freeze-thaw cycle constitutes a complete cycle. This process is a physical cross-linking enhancement method based on ice crystal-induced phase separation and polymer crystallization. During the cryogenic freezing stage, water in the system forms ice crystals. The growth of these ice crystals displaces and concentrates the polymer molecular chains, CNTs, and HTO-Fe particles into the unfrozen liquid region, forcing them into close contact. For polyvinyl alcohol (PVA), the low-temperature environment greatly promotes the orderly arrangement of hydroxyl groups (-OH) in its molecular chains, inducing PVA crystallization. These PVA microcrystalline regions formed at the ice crystal interfaces act as exceptionally robust physical cross-linking points, synergistically working with the calcium ion chemical cross-linking network formed in the S103 step to enhance the overall network structure.

[0061] The repeated formation and melting of ice crystals is equivalent to forging the gel network multiple times, which promotes the formation of more hydrogen bonds and entanglement points between polymer chains, making the network denser and more resilient. At the same time, this process helps to eliminate internal stress, making the distribution of functional nanomaterials in the matrix more uniform.

[0062] It should be noted that the number of cycles affects the mechanical strength of the final material. Preferably, the freeze-thaw cycle is performed 3 times. If the number of cycles is too few (e.g., 1-2 times), the physical cross-linking is insufficient and the gel strength is inadequate. If the number of cycles is too many, although the strength may continue to improve, the preparation cycle will be prolonged and the pore structure may become too dense, thus affecting the mass transfer performance.

[0063] The hydrogel, strengthened by the aforementioned three freeze-thaw cycles, was then freeze-dried. Freeze-drying is a gentle dehydration technique that aims to directly sublimate the frozen water (ice) in the gel under low temperature and low pressure conditions, thereby removing the water. This process removes ice crystals through sublimation, precisely leaving pores in the original ice crystal locations. This perfectly preserves and fixes the three-dimensional hierarchical porous network structure constructed by the freeze-thaw process. This interconnected porous structure plays a role in rapid water transport, ion diffusion, and light absorption in subsequent material applications. Finally, after freeze-drying, the water in the wet gel is completely removed, resulting in a dry, lightweight, and structurally stable solid calcium ion double crosslinked three-dimensional network composite hydrogel. This process avoids the pore collapse and structural shrinkage caused by surface tension in traditional thermal drying, maximizing the preservation of the gel's network integrity, high specific surface area, and the activity of the CNT and HTO-Fe functional components.

[0064] The method provided in this embodiment, through iron doping optimization, calcium ion double crosslinking, and freeze-thaw cycle enhancement, successfully prepared a composite hydrogel material with excellent mechanical strength, superhydrophilicity, and a permeable porous structure. This method not only achieves stable and uniform immobilization of nanoscale adsorbents in a macroscopic gel network, effectively solving the engineering problems of easy agglomeration and difficult recycling of powder materials, but also cleverly utilizes the introduction of carbon nanotubes to endow the material with efficient broadband photothermal conversion capabilities. The resulting material, driven by solar energy, can simultaneously achieve efficient seawater desalination and highly selective, high-capacity adsorption of lithium ions.

[0065] The prepared calcium ion double crosslinked three-dimensional network hydrogel (PGST@HTO-Fe) sample was subjected to a series of analytical tests to understand its physical and chemical properties. The specific measurement methods and instruments used are as follows:

[0066] The morphology of the hydrogel samples was characterized using scanning electron microscopy (FlexSEM1000, Shimadzu, Japan); the chemical bonding state was analyzed using Fourier transform infrared spectroscopy (FTIR, Nicolet Nexus 470); the surface elemental composition was determined by X-ray photoelectron spectroscopy (XPS, ESCALAB250xi, Thermo Fisher Scientific, USA); optical properties (including transmittance and reflectance) were recorded using a UV-Vis-NIR spectrophotometer (UV-3600Plus, Shimadzu, Japan); surface wettability was evaluated using a static contact angle analyzer (SDC-100S, Sindin, China); the compressibility of the hydrogels was tested on a universal testing machine (WDW-1D, Popwil, China); thermal images under illumination were captured using an infrared thermal imager; and the concentration of metal ions in the desalination filtrate and the Li before and after adsorption were quantitatively analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES, Thermo Fisher Scientific, USA). + content.

[0067] The testing and experimental principles involved in this application are described below:

[0068] Light-driven interface water evaporation test:

[0069] The hydrogel sample was supported on polyethylene foam, ensuring contact between the salt solution and the sample bottom surface for continuous water transfer. The device was placed on a high-precision electronic balance (0.1 mg) to continuously record changes in water mass during evaporation. Simulated sunlight irradiation was achieved using a xenon lamp (CEL-S500 / 350, CEAULIGHT, China) equipped with a standard AM1.5G solar filter. Incident light intensity was measured using a power meter (PL-MW2000, POPELIE, Beijing, China), and sample temperature was captured using an infrared thermal imager (FLUKETIS20+). The normalized evaporation rate (ER, kg•m³) was measured. -2 •h -1 Calculate using the following formula:

[0070] ;

[0071] Where Δm (kg) is the mass of water loss, and A (m 2 ) represents the evaporation area of ​​the sample, and Δt(h) represents the evaporation time.

[0072] Adsorption test:

[0073] In the adsorption test, anhydrous LiCl was first dissolved in deionized water to prepare a stock lithium solution, which was then diluted to obtain lithium solutions of different concentrations. The samples were immersed in lithium solutions of different initial concentrations and shaken at 200 rpm for 24 hours in a water bath shaker. After adsorption, the solutions were filtered through a 0.22 μm filter membrane, and the lithium content was determined using inductively coupled plasma optical emission spectrometry (ICP-OES, Thermo Fisher Scientific, USA). A Li... + The calibration curve was used to calculate the post-adsorption concentration based on the absorbance-concentration relationship. Lithium adsorption capacity (q) e Calculate using the following formula:

[0074] ;

[0075] Among them, C0 and C e (mg•L) -1 ) are the initial and equilibrium Li + Concentration, V (L) is the solution volume, m (g) is the adsorbent mass. Fe 3+ In Ti 4+ Doping at specific sites generates additional vacancies and optimizes the crystal structure, thereby significantly improving the adsorption capacity. During adsorption, Li... + H in the sieve + During exchange and desorption, H+ in concentrated hydrochloric acid... + Replace Li + This achieves efficient elution and regeneration. The reusability (R) of PGST@HTO-Fe is calculated using the following formula:

[0076] ;

[0077] Among them, Q0 and Q n (mg•g) -1 The figures are Li before and after PGST@HTO-Fe regeneration. + Adsorption capacity, where n is the number of adsorption-desorption cycles.

[0078] To evaluate the adsorption selectivity of PGST@HTO-Fe, competitive adsorption experiments were conducted using simulated seawater and actual seawater collected from the East China Sea region of Zhoushan. The partition coefficient (K...) d Calculate using the following formula:

[0079] ;

[0080] Where V(L) and m(g) are the solution volume and adsorbent mass, respectively, and C0 and C2 are the adsorbent mass and adsorbent mass, respectively. e Li + Initial and equilibrium concentrations of other metal ions. Partition coefficient K. d It is related to the adsorption capacity q e The relevant parameters.

[0081] Methylene blue photocatalytic degradation test:

[0082] In the experiment, the test sample was cut into 3cm × 3cm pieces and suspended in 50mL of a solution with a concentration of 5mg•L. -1 The surface of a methylene blue (MB) solution was subjected to photocatalytic degradation. A xenon lamp was used to simulate solar irradiation. Before the photocatalytic reaction, the sample was placed in darkness for 0.5 hours to reach adsorption equilibrium. Subsequently, photocatalytic degradation was performed under standard sunlight, with samples taken every 0.5 hours. Analysis was conducted using a UV-Vis-NIR spectrophotometer, and the actual MB concentration was determined based on the corresponding calibration curve. The photocatalytic degradation efficiency (De) was calculated using the following formula:

[0083] ;

[0084] Where C0 is the initial concentration of the MB solution, C t Let t be the concentration at time t(h).

[0085] Density Functional Theory (DFT) Calculation Methods:

[0086] First-principles calculations were performed using an open-source plane-wave quantum chemistry program based on density functional theory (DFT). Structural models of pristine HTO (titanium-based lithium-ion sieve, H₂TiO₃), iron-doped HTO (HTO-Fe), and HTO-Fe / CNT heterojunctions were constructed and fully optimized to obtain stable configurations. Based on the optimized structures, the electronic band structure and charge density difference were calculated to investigate the effects of iron doping and heterojunction formation on electronic properties and interface charge transfer. The total energy difference method was used to evaluate the Li₂ / Li₂ / Li₃ / Li� ... + The adsorption energy of Li was determined using the climbing image elastic band (CI-NEB) method. + The diffusion behavior and corresponding energy barrier are determined to obtain the lowest energy migration path.

[0087] The specific experimental process and results of this application are described below:

[0088] Morphological characteristics:

[0089] Figure 2 This application presents the SEM morphology, EDS elemental distribution map, and structural schematic diagram of the PGS, PGST, and PGST@HTO-Fe composite hydrogels. Figure 2Sub-figures (a), (b), and (c) are SEM images of PGS at scales of 5 μm, 20 μm, and 100 nm; (d), (e), and (f) are SEM images of PGST at scales of 5 μm, 20 μm, and 100 nm; (g), (h), and (i) are SEM images of PGST@HTO-Fe at scales of 5 μm, 20 μm, and 100 nm; (j) is the EDM approximation image of C, O, Ti, and Fe elements; and (k) is the FTIR spectrum of PGS, PGST, and PGST@HTO-Fe. PGS is a basic hydrogel framework formed by double crosslinking of polyvinyl alcohol, gellan gum, and sodium alginate with calcium ions; PGST is a photothermal enhanced hydrogel based on PGS with the addition of multi-walled carbon nanotubes; and PGST@HTO-Fe is a calcium ion double crosslinked three-dimensional network hydrogel, which is the final product based on PGST with further loading of iron-doped titanium-based lithium-ion sieves.

[0090] Please refer to Figure 2 The surface morphology and microstructure of PGS, PGST, and PGST@HTO-Fe were investigated using scanning electron microscopy (SEM). Figure 2 As shown in subgraphs (a) and (b), PGS exhibits an open three-dimensional (3D) porous network structure with interconnected macropores. Figure 2 The sub-graph (c) in the figure has an aperture of up to 450 nm, which is beneficial for rapid water transport. Figure 2 Subfigures (d) and (e) in the figure show the surface morphology of CNT-modified PGS. The modified PGS retains its three-dimensional network structure, but CNTs are uniformly deposited on the PGS framework. Figure 2 (See sub-figure (f)). Due to the high light absorption capacity of CNTs, the photothermal conversion efficiency of PGST@HTO-Fe is improved. Figure 2 The schematic diagrams in sub-figures (g) and (h) further demonstrate that PGST@HTO-Fe retains a three-dimensional porous structure, providing abundant specific surface area and active sites, thereby promoting solar energy capture and lithium-ion adsorption. High-magnification SEM images ( Figure 2 Subgraph (i) shows nanoparticles deposited on the PGST surface, confirming successful immobilization of the lithium-ion sieve. Figure 2As shown in subfigure (j), the spatial distribution of C, O, Ti, and Fe elements on the PGST@HTO-Fe surface was analyzed using SEM-EDS elemental mapping. C elements mainly originated from the hydrogel matrix and CNT framework, exhibiting a continuous and uniform distribution. O elements were uniformly distributed throughout the sample, consistent with the polysaccharide-based hydrogel matrix and HTO-Fe structure. Notably, Ti and Fe elements showed highly uniform distribution on the hydrogel surface, without obvious local enrichment or aggregation, and their spatial distribution overlapped well with the carbon-rich regions, indicating that HTO-Fe nanoparticles were effectively immobilized in the CNT-reinforced hydrogel network, forming a stable composite structure. These observations suggest that during the composite material preparation process, HTO-Fe did not undergo significant phase separation or large-scale aggregation, but was uniformly dispersed on the surface and near-surface regions of the PGST matrix. In summary, the combination of SEM morphology and multi-element EDS mapping provides strong indirect evidence for the uniform dispersion and stable interfacial integration of HTO-Fe and CNTs in the hydrogel network.

[0091] The results showed that GG and SA, as natural polysaccharides rich in carboxyl and hydroxyl groups, served as the main structural components of the hydrogel, via Ca... 2+ Induced double crosslinking and intermolecular interactions construct a three-dimensional network structure, endowing the hydrogel with excellent formability and hydrophilicity. The introduction of PVA, through its abundant hydroxyl groups, forms numerous hydrogen bonds with the GG / SA molecular chains, acting as a physical crosslinking agent and toughening agent, effectively improving the mechanical properties of the hydrogel. Furthermore, multi-walled carbon nanotubes (CNTs) are mainly dispersed in the polymer matrix through physical embedding and interfacial interactions. As one-dimensional high-strength nanofillers, CNTs can effectively promote stress transfer and enhance the overall mechanical properties of the hydrogel. Simultaneously, their excellent broad-spectrum light absorption and electron transport properties introduce photothermal conversion and charge transport enhancement effects into the composite material, which are crucial for subsequent photothermal evaporation and photocatalytic processes. In this composite hydrogel, GG / SA provides the structural framework, PVA contributes to toughening, and CNTs combine mechanical and functional reinforcement, forming a well-defined and synergistic composite system. This synergistic design lays a solid foundation for the stable loading and multifunctional properties of HTO-Fe.

[0092] FTIR analysis further confirmed the successful preparation of PGST@HTO-Fe. Compared with PGST, several characteristic absorption peaks of the PGST@HTO-Fe composite material showed slight shifts, indicating that the formation of the composite material led to some changes in the local chemical environment of the polymer matrix, but the overall skeletal structure remained stable. The OH stretching vibration shifted from 3330 cm⁻¹. -1 The redshift reached 3324.4 cm. -1 This indicates that the metal ions (Fe) in HTO-Fe 3+ / Li + It coordinates with the hydroxyl group; the C=O stretching vibration blue shifts from 1717.9 cm⁻¹. -1 Up to 1723.5cm -1 This confirms the chelating effect of the carboxyl group with metal ions; the CH vibration changes from 2923.1 cm⁻¹. -1 Moved to 2915.1cm -1 This further indicates a change in the chemical environment. Furthermore, at approximately 1609.2 cm... -1 Vibrational peaks associated with the unsaturated framework were observed, and these peaks shifted only slightly after the introduction of CNTs. Combined with changes in SEM morphology and optical absorption properties, these results indicate that multi-walled carbon nanotubes were successfully embedded in the hydrogel matrix without significantly altering the polymer backbone structure. Based on the above FTIR analysis, metal-ligand interactions exist between HTO-Fe and the PGS matrix, while the introduction of CNTs primarily affects the optical and interfacial properties of the material. These components synergistically construct a structurally stable composite system.

[0093] Chemical composition analysis:

[0094] Figure 3 The XPS full and fine spectra of the PGS, PGST, and PGST@HTO-Fe composite hydrogels shown in this application are presented. Please refer to... Figure 3 , Figure 3 Subplot (a) shows the full XPS spectra of PGS, PGST, and PGST@HTO-Fe; subplot (b) shows the high-resolution Ti2p XPS spectrum of PGST@HTO-Fe; subplot (c) shows the high-resolution Fe2p XPS spectrum of PGST@HTO-Fe; subplots (d), (e), and (f) show the high-resolution O1s XPS spectra of PGS, PGST, and PGST@HTO-Fe; and subplots (g), (h), and (i) show the high-resolution C1s XPS spectra of PGS, PGST, and PGST@HTO-Fe.

[0095] Please refer to Figure 3The chemical composition and bonding state of PGS, PGST, and PGST@HTO-Fe were investigated using XPS analysis. The full spectrum showed distinct C1s and O1s peaks, with no peak shift observed after the introduction of CNTs and HTO-Fe, indicating that the inherent framework of PGS was preserved. Conversely, new Ti2p and Fe2p peaks appeared only in PGST@HTO-Fe, confirming that HTO-Fe was successfully immobilized in the hydrogel network. The high-resolution Ti2p XPS and Fe2p XPS spectra of PGST@HTO-Fe showed distinct characteristic peaks at binding energies of approximately 458.6 eV (Ti2p) and 710.2 eV (Fe2p), respectively, while no corresponding signals were observed in the PGS and PGST samples, further confirming that both Ti and Fe elements originated from lithium-ion sieves. The fine spectrum of Ti2p shows a double peak at binding energies of 458.7 eV (Ti2p3 / 2) and 464.4 eV (Ti2p1 / 2), with a spin-orbit splitting energy of 5.7 eV, similar to that of TiO2. 4+ The standard binding energy is consistent; the fine spectrum of Fe2p shows double peaks at Fe2p3 / 2 (710.8 eV) and Fe2p1 / 2 (724.3 eV), further confirming that Fe... 3+ The presence of [the lithium ion sieve] and the observed elemental characteristic peaks and their chemical states are consistent with the composition of HTO-Fe, strongly demonstrating that the lithium ion sieve has been successfully chemically integrated into the hydrogel network.

[0096] By comparing the O1s XPS spectra of PGS, PGST, and PGST@HTO-Fe, significant changes in the chemical environment and composite mechanism of the materials can be observed. The O1s spectrum of the PGS (PVA / GG / SA) hydrogel shows a characteristic peak near 532.0 eV, corresponding to C=O and CO groups, originating from the carboxyl groups of sodium alginate (SA) and gellan gum (GG), and the hydroxyl groups of PVA. After introducing CNTs to form PGST, the C=O and CO peaks show a slight blue shift, accompanied by satellite peaks of physically adsorbed H2O, indicating that CNTs influence the electronic structure of the polymer through hydrophobic interactions and hydrogen bonding. In PGST@HTO-Fe, the C=O peak further shifts to 531.5 eV, the CO peak to 532.5 eV, and a distinct Ti-O peak appears at 529.8 eV. The continuous shifts of C=O and CO are attributed to Fe in HTO-Fe. 3+ / Ti 4+ The strong coordination interaction between the lithium ion sieve and the carboxyl / hydroxyl groups in the polymer alters the electron density around the oxygen atom. The presence of Ti-O bonds directly confirms the chemical anchoring of the lithium ion sieve in the hydrogel network, marking a shift from physical mixing to chemical composite.

[0097] C1sXPS spectroscopy further revealed the evolution of the chemical environment from PGS to PGST and then to PGST@HTO-Fe. All three materials exhibited a strong CC / CH peak at 284.0 eV, providing a stable reference for peak fitting. Significant changes occurred in the oxygen-containing functional groups: the binding energy of the C-OH bond gradually shifted from 286.0 eV in PGS to 286.2 eV in PGST, and finally reached 286.5 eV in PGST@HTO-Fe; the shift of the OC=O bond was even more significant, shifting from 288.0 eV (PGS) to 288.5 eV (PGST), reaching 289.0 eV in the final composite material. This systematic shift towards higher binding energies indicates a continuous decrease in the electron density around the carbon atoms. This phenomenon is attributed to a gradual, specific interaction: first, the multi-walled carbon nanotubes (CNTs) introduced in PGST interact with the PGS matrix through π-π stacking and van der Waals forces, altering the electronic environment; subsequently, the highly electronegative metal ions (Fe) in HTO-Fe... 3+ / Ti 4+ The HTO-Fe exhibits strong coordination with the carboxyl (OC=O) and hydroxyl (C-OH) groups of sodium alginate and gellan gum. These metal ions act as electron acceptors, effectively extracting electrons from carbon atoms, leading to the observed increase in C1s binding energy. The stability of the C1s bond contrasts with the changes in oxygen-containing functional groups, strongly confirming that the HTO-Fe is chemically bonded to the hydrogel matrix rather than physically blended.

[0098] To verify the successful synthesis of HTO-Fe and its state in the composite hydrogel, X-ray diffraction (XRD) analysis was performed on PGS, PGST, and PGST@HTO-Fe. The XRD pattern of the PGST@HTO-Fe composite material showed a series of diffraction peaks at 18.56°, 19.6°, 36.2°, 46.8°, 64.3°, and 66.87°. Compared with the standard card (JCPDS#33-0831), these diffraction peaks correspond to the (002), (110), (-131), (-204), (-206), and (062) crystal planes of Li2TiO3. It is worth noting that the measured diffraction peaks show a slight shift relative to the standard card. This phenomenon can be attributed to two factors: First, when HTO-Fe nanoparticles are uniformly dispersed in a large amount of amorphous hydrogel matrix, the total volume of crystalline material exposed to X-ray irradiation and capable of diffraction is significantly reduced, leading to a decrease in the absolute intensity of all diffraction peaks; second, Fe during the synthesis process... 3+ Heteroatoms may be partially embedded in the Li₂TiO₃ lattice, with differences in ionic radii (Fe 3+ With Ti 4+This process induces lattice expansion or contraction. Such subtle lattice structure modulation can sometimes enhance the ion exchange kinetics of materials. These results clearly demonstrate that HTO-Fe nanoparticles have been successfully and uniformly dispersed within the three-dimensional hydrophilic network of PGST. This structure enables both efficient lithium-ion capture and full utilization of the synergistic effect between the hydrogel's three-dimensional network and functional groups during adsorption, laying the structural foundation for subsequent excellent adsorption performance.

[0099] Performance characterization:

[0100] Figure 4 For characterization of the hydrophilicity, antifouling properties, and mechanical properties of the PGST@HTO-Fe composite hydrogel shown in this application, please refer to [reference needed]. Figure 4 , Figure 4 Subplot (a) shows the water contact angle test; subplot (b) shows the water transport test; subplot (c) shows the anti-fouling (oil resistance) test; subplot (d) shows the tensile test of PGST@HTO-Fe; subplot (e) shows the stress-strain curves of PGS, PGST, and PGST@HTO-Fe.

[0101] Please refer to Figure 4 In light-driven seawater desalination, the water transport properties of materials are crucial. For example... Figure 4 As shown, water droplets rapidly penetrated the PGST@HTO-Fe surface within 2 seconds, with a contact angle approaching 0°. When the dried hydrogel block was immersed in deionized water, PGST@HTO-Fe quickly absorbed water and swelled, exhibiting excellent hydrophilicity. To further evaluate its water transport capacity, 0.1 g of NaCl was placed on the dried PGST@HTO-Fe surface in a petri dish containing deionized water; the salt completely dissolved within 15 minutes, indicating high water transport efficiency. During continuous photothermal evaporation for 6 hours, no significant salt crystallization was observed on the PGST@HTO-Fe surface, indicating stable water transport and effective inhibition of salt accumulation. This behavior can be attributed to the interconnected hierarchical porous structure within the hydrogel, which facilitates continuous water replenishment and salt redistribution. This excellent mass transfer characteristic provides a basis for subsequent Li + Adsorption provided favorable conditions. Furthermore, PGST@HTO-Fe exhibited significant antifouling properties. For example... Figure 4 As shown in subfigure (c), a 2×2 cm hydrogel block was immersed in oil and then placed in deionized water; oil droplets slid off the surface without adhesion. This property significantly broadens the practical application scenarios of the material. To evaluate long-term oil resistance, PGST@HTO-Fe was immersed in oily contaminants for 30 days. After removal and a simple rinse with water, the material returned to its initial state, and no oil adhesion was observed. These results demonstrate that PGST@HTO-Fe exhibits excellent long-term oil resistance.

[0102] The mechanical stability of PGST@HTO-Fe was also evaluated. For example... Figure 4 As shown in subfigure (d), the hydrogel maintained its shape under loads of 300g and 500g, indicating a stable network structure. Stress-strain curves revealed significant differences in the mechanical properties of PGS, PGST, and PGST@HTO-Fe, highlighting the reinforcing effects of CNTs and HTO-Fe. The basic PGS hydrogel (PVA / GG / SA) exhibited relatively low elastic modulus and tensile strength (approximately 1.76 MPa), displaying a soft network. Introducing CNTs to form PGST significantly improved the elastic modulus and tensile strength (approximately 5.31 MPa), while also increasing the elongation at break, indicating that CNTs enhanced the network structure through physical crosslinking and load transfer. Further introduction of HTO-Fe resulted in PGST@HTO-Fe achieving the best mechanical properties, with the highest tensile strength (approximately 5.29 MPa), demonstrating that the addition of lithium-ion sieves had minimal adverse effect on tensile properties. This improvement is attributed to the coordination between HTO-Fe particles and the carboxyl / hydroxyl groups on the polymer chains, providing additional chemical crosslinking points that synergize with CNT reinforcement to maintain the three-dimensional network of the hydrogel. The fatigue resistance of PGS, PGST, and PGST@HTO-Fe samples was evaluated through bidirectional repeated bending and manual compression. PGS showed significant edge damage, while PGST and PGST@HTO-Fe remained intact and fully recoverable. The improved mechanical properties are consistent with previous FTIR and XPS analyses, confirming the successful preparation and structural stability of PGST@HTO-Fe. In summary, these results demonstrate that PGST@HTO-Fe possesses excellent tensile strength and stable cyclic compression performance, exhibiting superior mechanical properties.

[0103] Photothermal properties:

[0104] Figure 5 For the photothermal properties and optical absorption characterization of the composite hydrogel shown in this application, please refer to... Figure 5 , Figure 5 Sub-figure (a) is a schematic diagram of the solar evaporation device; sub-figure (b) is an infrared thermal image of PGS, PGST, and PGST@HTO-Fe; sub-figure (c) is the ultraviolet-visible-near-infrared absorption spectrum of PGS, PGST, HTO-Fe, and PGST@HTO-Fe; sub-figure (d) is the surface temperature change of PGS, PGST, and PGST@HTO-Fe.

[0105] Please refer to Figure 5 Xenon lamps were used to simulate solar irradiation, and the light intensity was monitored and adjusted to 1 kW•m using a photometer (PL-MW2000). -2The photo-driven evaporation performance of the material was evaluated. The sample was supported on polystyrene foam and floated on the surface of deionized water. Water mass loss during evaporation was continuously measured using a high-precision electronic balance (0.1 mg). To further investigate photothermal conversion, the sample surface temperature was recorded using an infrared thermal imager. Figure 5 The subgraph (b) is shown in the figure. Figure 5 Subplot (d) shows the temperature evolution of PGS, PGST, and PGST@HTO-Fe under standard sunlight irradiation over 0–60 minutes. After 5 minutes of irradiation, the surface temperatures of PGST and PGST@HTO-Fe rapidly increased to 36.1 °C and 35.5 °C, respectively, while PGS only reached 26.2 °C, highlighting the rapid thermal response of CNTs. After 60 minutes, the equilibrium temperatures of PGS, PGST, and PGST@HTO-Fe stabilized at 30.1 °C, 39.5 °C, and 38.6 °C, respectively. The final temperature of PGST@HTO-Fe was slightly lower than that of PGST, possibly attributed to the effect of HTO-Fe on light scattering or heat conduction; however, the temperature rise of PGST@HTO-Fe was still significantly higher than that of PGS, indicating that the photothermal performance was effectively enhanced. The synergistic effect of CNTs and the iron-doped lithium-ion sieve (HTO-Fe) significantly improved the photothermal conversion efficiency, consistent with previous structural characterization.

[0106] Excellent photothermal materials should also exhibit high solar energy absorption rates in the ultraviolet (UV), visible, and infrared (IR) bands. Ultraviolet-visible-near-infrared spectroscopy is used to quantitatively assess the solar energy absorption of samples. Figure 5 As shown in subfigure (c), PGS exhibits relatively low absorption with strong wavelength-dependent fluctuations, averaging only 53%. HTO-Fe shows significantly enhanced absorption across the entire visible light region and part of the near-infrared region, indicating that iron doping effectively broadens its photoresponse window. In contrast, CNT-containing PGST and PGST@HTO-Fe demonstrate near-saturated broad-spectral absorption in the 400-2500 nm range, with average absorption consistently exceeding 90%, and PGST@HTO-Fe reaching 94.5%. This behavior highlights the dominant role of CNTs in broad-spectral light harvesting and photothermal conversion. Notably, the absorption spectra of PGST@HTO-Fe and PGST highly overlap, indicating that the introduction of HTO-Fe did not induce significant photon absorption competition or shielding effects. Instead, CNTs and HTO-Fe exhibit a synergistic effect in light utilization, promoting photon absorption rather than competing for photons.

[0107] Light-driven evaporation performance:

[0108] Figure 6 For the solar evaporation and seawater desalination performance diagrams of the composite hydrogel shown in this application, please refer to... Figure 6 , Figure 6Subplot (a) shows the water quality changes of pure water, PGS, PGST, and PGST@HTO-Fe under one standard sunlight exposure; subplot (b) shows the evaporation rate of PGST@HTO-Fe in solutions with different salinity under one standard sunlight exposure; subplot (c) shows the evaporation rate of PGST@HTO-Fe under different sunlight intensities; subplot (d) shows the changes in ion concentration in actual seawater before and after evaporation of PGST@HTO-Fe; subplot (e) shows the evaporation rate of PGST@HTO-Fe in 10 consecutive cycles under one standard sunlight exposure; and subplot (f) shows the evaporation rate of PGST@HTO-Fe in a 15-day continuous evaporation test.

[0109] Please refer to Figure 6 To evaluate the performance of light-driven water vapor generation, hydrogel samples (1×1cm) were used. 2 The sample was placed in a beaker and exposed to simulated sunlight. Polystyrene (PS) foam was used as the insulation layer to minimize heat loss. The hydrogel portion was immersed in water, and the mass loss during the evaporation process under sunlight was recorded using a high-precision electronic balance, according to the formula... Evaporation rate was calculated. Compared with pure water and CNT-free PGS, the evaporation rate of PGST was significantly increased to 2.16 kg·m³. -2 •h -1 This demonstrates significantly improved performance. In contrast, the evaporation rate of PGST@HTO-Fe decreased slightly to 1.796 kg•m. -2 •h -1 This behavior can be attributed to the introduction of 10% iron-doped HTO-Fe, which alters the hydrogel network structure and the mass transfer behavior of water molecules. Although HTO-Fe itself possesses good photoresponsive properties, its introduction as an inorganic filler partially occupies water transport channels, increasing local interfacial resistance and causing the evaporation process to shift from being primarily photothermally limited to being constrained by mass transfer and structural factors. In contrast, PGS lacking CNTs exhibits low photothermal conversion efficiency, limiting its evaporation rate. The salt tolerance of PGST@HTO-Fe was further investigated. Figure 6 As shown in subplot (b), the evaporation rate decreased from 1.81 kg·m³ to 1.50 kg·m³ as the salinity increased from 0% to 20%. -2 •h -1 This is mainly attributed to the decrease in surface vapor pressure under high salt concentrations. Despite this decrease, the hydrogel maintains high evaporation efficiency, indicating its excellent salt resistance. Evaporation experiments were also conducted under different light intensities to assess the effect of solar irradiation. As the light intensity increased from 0.5 to 2.0 kW•m... -2 The evaporation rate increased from 1.34 to 2.53 kg·m³. -2 •h -1The increased surface temperature under stronger irradiation confirms that evaporation performance is directly dependent on incident light intensity. Further experiments were conducted using PGST@HTO-Fe to perform actual seawater evaporation, and the collected freshwater was analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES). Figure 6 As shown in subplot (d), the concentrations of all ions decreased to 10. o The concentration of salts in the water is below mg / L, meeting the drinking water quality standards of the World Health Organization (WHO) and the U.S. Environmental Protection Agency (EPA). These results demonstrate that the hydrogel-based evaporation system can effectively separate salt from water, exhibiting excellent seawater desalination performance.

[0110] To evaluate the long-term stability and reusability of PGST@HTO-Fe hydrogel under real-world conditions, evaporation experiments were conducted using seawater from the East China Sea near Zhoushan. After 10 consecutive cycles and 15 days of continuous operation, the evaporation rate stabilized at 1.7-1.8 kg·m³. -2 •h -1 Within the specified range, the attenuation rate is less than 3%. This excellent cycling stability can be attributed to the robust three-dimensional network and salt crystallization resistance of the hydrogel, ensuring reliable long-term operation. These results indicate that the PGST@HTO-Fe composite material shows great promise for light-driven seawater desalination applications.

[0111] Adsorption performance:

[0112] Figure 7 For a graph showing the lithium-ion adsorption performance of the composite hydrogel illustrated in this application, please refer to... Figure 7 , Figure 7 Subplot (a) shows the adsorption capacity of HTO and HTO-Fe; subplot (b) shows the lithium adsorption isotherm of PGST@HTO-Fe; subplot (c) shows the lithium adsorption kinetics of PGST@HTO-Fe; subplot (d) shows the lithium adsorption kinetics of PGST@HTO-Fe at different temperatures; subplot (e) shows the van der Hoff plot of adsorption thermodynamics; subplot (f) shows the adsorption capacity of PGST@HTO-Fe at different NaCl concentrations (20 mg·L⁻¹). -1 Li + Lithium adsorption capacity of PGST@HTO-Fe in solution.

[0113] Please refer to Figure 7 To compare the adsorption capacity of lithium-ion sieves before and after modification, suspensions of HTO and 10% iron-doped HTO (HTO-Fe) powder were shaken for 24 hours at 298 K and 200 rpm in a shaker. The pH of the solution was adjusted to 12, and the solid-liquid ratio was 1 g•L. -1 Data shows that the adsorption capacity of HTO-Fe reaches 35.43 mg•g. -1 The concentration was significantly higher than that of unmodified HTO (28.5 mg / g). -1This enhancement is primarily attributed to the fact that iron doping effectively widens the lattice spacing of the titanium-based lithium-ion sieve, increasing the resistance to Li-ion degradation. + The number of adsorption sites with specific recognition. To evaluate the maximum adsorption capacity of PGST@HTO-Fe, hydrogel sheets (20 mg, 1 × 1 cm) were used. 2 Immersion concentration is 20-200 mg / L -1 Li + In the solution (pH=12), shake for 24 hours at 298K and 200rpm in a shaker, according to the formula... Calculate the adsorption capacity. For example... Figure 7 As shown in subplot (b), the equilibrium adsorption capacity of PGST@HTO-Fe varies with the initial Li + The concentration gradually increased, but when the concentration approached 200 mg / L... -1 At this point, the growth rate slowed down, indicating that the adsorption sites on 10% iron-doped HTO-Fe gradually became saturated. The adsorption isotherms were fitted using the Langmuir and Freundlich models to further analyze the adsorption process. The maximum adsorption capacity calculated by the Langmuir model was 31.25 mg•g. -1 =30.21 mg•g -1 Close. Langmuir correlation coefficient (R²) 2 =0.993) is significantly higher than the Freundlich model (R² = 0.993). 2 =0.957), indicating that the PGST@HTO-Fe hydrogel is effective against Li + The adsorption follows the monolayer chemisorption mechanism.

[0114] To study the adsorption equilibrium time, PGST@HTO-Fe hydrogel sheets were immersed in an initial concentration of 100 mg•L. -1 Li + In a solution (pH=12), the solution was shaken in a water bath at 303K and 200 rpm. Samples were taken every 10 minutes for the first hour, and then every hour thereafter. The adsorption capacity-time curve is shown below. Figure 7 Subplot (c) is shown in the figure, and the data were fitted using pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models. PGST@HTO-Fe reached 83% of its equilibrium adsorption capacity within the first hour, which is attributed to the abundant available adsorption sites on the 10% iron-doped HTO-Fe, allowing Li in the solution to... + These sites can be rapidly occupied, accelerating the adsorption process. As adsorption proceeds, the number of available sites gradually decreases, and the rate slows down until it approaches equilibrium after 5 hours, with a final adsorption capacity of 25.28 mg•g. -1 The correlation coefficient R of the PFO model 2 =0.966, while the R-value of the PSO model is...2 =0.995, indicating that HTO-Fe has a positive effect on Li + The adsorption of [the substance] mainly occurs through chemisorption.

[0115] To investigate the effect of temperature on the adsorption capacity of lithium-ion sieves, PGST@HTO-Fe hydrogel sheets were shaken at 200 rpm in water baths at 303 K, 313 K, and 323 K. Figure 7 As shown in subplot (d), increasing the temperature significantly improved the equilibrium adsorption capacity, with the adsorption amount at 323 K increasing by 28.6% compared to 303 K. This result confirms that photothermal heating can effectively improve the adsorption performance of ion sieves. The adsorption kinetics were fitted using pseudo-first-order (PFO) and pseudo-second-order (PSO) models. The results show that the adsorption process at different temperatures better conforms to the PSO model, indicating that chemisorption is the dominant mechanism. The van der Hoff diagram of adsorption thermodynamics is shown below. Figure 7 As shown in subgraph (e), lnKd exhibits a good linear relationship with 1 / T (R 2 =0.95). The calculated thermodynamic parameters show that ΔH°>0 and ΔG°<0, proving that the adsorption process is spontaneous and endothermic, further supporting the dominance of chemisorption and the beneficial effect of increasing temperature. In addition, the effect of salinity on Li was investigated. + The effect of adsorption was investigated using 20 mg / L NaCl solution with a concentration of 0-0.8 M. -1 Li + The (pH=12) solution was tested in a constant temperature water bath shaker at 298K and 200rpm. The results showed that PGST@HTO-Fe maintained a concentration of 23-25 ​​mg•g. -1 The high adsorption capacity of PGST@HTO-Fe demonstrates excellent resistance to high salt interference. In summary, iron doping not only enhances the inherent adsorption performance of the lithium-ion sieve, but also works synergistically with the three-dimensional hydrogel network to enable PGST@HTO-Fe to exhibit high adsorption capacity and significant resistance to salt interference, highlighting its potential in practical seawater lithium extraction.

[0116] Figure 8 For the photothermal assisted lithium adsorption, performance optimization, and comprehensive application performance diagrams of the composite hydrogel shown in this application, please refer to... Figure 8 , Figure 8Subplot (a) shows the lithium adsorption kinetics of PGST@HTO-Fe under dark and light conditions; subplot (b) shows the lithium adsorption capacity under different light intensities; subplot (c) shows the water mass change of hydrogels with different HTO-Fe contents under one standard sunlight irradiation; subplot (d) shows the lithium adsorption capacity of PGST@HTO-Fe with different HTO-Fe contents; subplot (e) shows the lithium adsorption capacity of PGST@HTO-Fe at different pH values; subplot (f) shows the lithium adsorption capacity in LiCl solution and LiCl / MB mixed solution; subplot (g) shows the adsorption selectivity of PGST@HTO-Fe for various competing ions; subplot (h) shows the partition coefficient (Kd) of various ions when PGST@HTO-Fe adsorbs from actual seawater; subplot (i) shows the reusability of PGST@HTO-Fe.

[0117] Please refer to Figure 8 To study photothermal assisted lithium adsorption, 0.1 g (1 cm × 1 cm) PGST@HTO-Fe hydrogel was placed in 100 mL of 100 mg·L⁻¹ hydrogel. -1 Li + The solution surface is exposed to sunlight. Polystyrene (PS) foam is used as the insulation material to minimize heat loss. Figure 8 As shown in sub-figure (a), Li under illumination + The adsorption capacity increased by 7.47 mg•g compared to the dark condition. -1 The increase was 32.3%. Pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models were used to further analyze the adsorption behavior. The analysis showed that the PSO model more accurately described the adsorption process, indicating that chemisorption was dominant. To further verify the feasibility of improving the lithium adsorption capacity of PGST@HTO-Fe hydrogel through solar photothermal effect, the adsorption capacity of Li under different solar irradiance intensities was systematically compared. + Adsorption properties. For example... Figure 8 As shown in sub-figure (b), the lithium adsorption capacity continuously increases with light intensity, from 23.1 mg•g -1 Increased to 35.56 mg•g -1 These results clearly demonstrate that illumination effectively promotes the Li-to-Fe synthesis of PGST@HTO-Fe. + The adsorption process. This enhancement can be attributed to the accelerated solute diffusion in the lithium solution induced by evaporation, and the increased temperature under irradiation promoting Li adsorption. + Migration, thereby improving the equilibrium adsorption capacity and adsorption rate.

[0118] To determine the optimal HTO-Fe loading, this application systematically compared the solar evaporation performance and Li₂O₃ loading of PGST@HTO-Fe composites with different HTO-Fe doping ratios (5%, 10%, and 15%). +Adsorption capacity. The results showed that when the HTO-Fe content reached 15%, Li... + The equilibrium adsorption capacity is the largest, at 28.4 mg•g. -1 However, the corresponding evaporation rate was the lowest (1.43 kg·m³). -2 •h -1 The decrease in evaporation performance is attributed to the excessive solid particles clogging the capillary channels within the hydrogel, hindering water transport and reducing the effective evaporation area. Conversely, the highest evaporation rate, 1.84 kg·m³, was observed with an HTO-Fe loading of 5%. -2 •h -1 , and Li + The lowest equilibrium adsorption capacity was 19.53 mg•g. -1 A 10% HTO-Fe loading achieved the optimal balance between the two performance characteristics, with an evaporation rate of 1.796 kg·m³. -2 •h -1 Li + The equilibrium adsorption capacity is 25.2 mg•g. -1 This ratio not only maintains the porous structure and water transport channels required for efficient photothermal evaporation, avoiding the decrease in evaporation efficiency caused by light shielding and pore blockage due to excessive filler, but also provides sufficient lithium-ion specific adsorption sites, ultimately achieving synergistic optimization of photothermal evaporation enhancement and adsorption performance.

[0119] pH value affects the pH value of PGST@HTO-Fe hydrogel Li + A key factor in adsorption capacity. In this application, PGST@HTO-Fe samples were immersed in 100 mg•L of solution with a pH of 2-12. -1 In LiCl solution, shake in a water bath at 298 K and 200 rpm for 24 hours. Figure 8 As shown in subplot (e), the adsorption capacity increases significantly with increasing pH, from 4.9 mg•g at pH=2. -1 Increased to 23.5 mg / g at pH=12 -1 This behavior is primarily attributed to the Li of PGST@HTO-Fe. + The adsorption mechanism is ion exchange (H+). + Li + Under alkaline conditions, the OH- in the solution... - Increased concentration leads to deprotonation of the HTO-Fe surface, forming a negatively charged surface, which enhances Li through electrostatic attraction. + Adsorption. Simultaneously, the alkaline environment significantly reduced H₂. + Competition for adsorption sites makes Li + It can occupy these sites more effectively, thereby increasing the equilibrium adsorption capacity. Therefore, an alkaline environment is more favorable for Li+ Adsorption.

[0120] like Figure 8 Subgraph (f) shows a comparison of Li in PGST@HTO-Fe in a single LiCl solution and a mixed LiCl / MB system. + Adsorption performance. Specifically, 0.1 g of PGST@HTO-Fe hydrogel was placed in pure LiCl solution (100 mL, 100 mg·L⁻¹). -1 In a solution (pH=12), after exposure to sunlight for one day, Li + The adsorption capacity is 24.2 mg•g -1 5 mg•L was introduced into the system. -1 After methylene blue (MB), Li + The adsorption capacity decreased only slightly to 23.8 mg•g -1 This indicates that the presence of MB does not significantly inhibit Li + Adsorption process. This result indicates that PGST@HTO-Fe in Li + It exhibits strong resistance to interference from organic pollutants during the adsorption process. The fundamental reason for this lies in the fact that Li... + Adsorption primarily occurs at the lattice sites of the HTO-Fe lithium ion sieve, while MB and its potential degradation intermediates are relatively large organic molecules that cannot penetrate the crystal structure of the ion sieve. Therefore, MB and its potential degradation intermediates will not adsorb onto Li. + Direct competition for adsorption sites. The slight decrease in adsorption capacity observed in the mixed system may be attributed to a slight change in the pH of the adsorbent surface caused by the photogenerated hole-driven oxidation reaction during photocatalytic degradation, which would affect the adsorption sites of Li. + Diffusion and mass transfer were slightly affected. Overall, these results indicate that PGST@HTO-Fe maintains stable Li in complex aquatic environments containing organic contaminants. + The adsorption performance confirmed the good compatibility between photocatalytic pollutant degradation and photothermal-assisted lithium extraction in a single system. These findings provide experimental evidence for the synergistic integration of multifunctional water treatment and lithium resource recovery.

[0121] This application also investigated the selective adsorption properties of the PGST@HTO-Fe hydrogel. In the presence of competing ions (Na+)... + 10,770 mg / L -1 Mg 2+ 1,280 mg / L -1 K + 399 mg•L -1 The concentration is kept at the same ratio as that of actual seawater, Li + The concentration is 200 mg•L -1Under the specified conditions, adsorption experiments were conducted by shaking in a constant-temperature water bath at 298 K and 200 rpm for 24 hours. After adsorption, the solution was filtered through a 0.22 μm filter membrane, and the concentration of adsorbed ions was determined using inductively coupled plasma optical emission spectrometry (ICP-OES, Thermo Fisher Scientific, USA). The adsorption results are as follows: Figure 8 The sub-graph (g) is shown in the figure. In the presence of competing ions, PGST@HTO-Fe exhibits resistance to Li. + The adsorption capacity is 30.25 mg•g. -1 Significantly higher than that for Na + (7.37 mg•g) -1 ), K + (4.8 mg•g) -1 ) and Mg 2+ (3.14 mg•g) -1 The adsorption capacity of HTO-Fe is high. This selectivity stems from the molecular sieve effect and crystal structure of HTO-Fe, which is a Li... + It provides specific recognition sites, preferentially adsorbing lithium ions even in the presence of competing cations. To evaluate the applicability of PGST@HTO-Fe under real seawater conditions, competitive adsorption experiments were conducted using natural seawater collected from the coastal area near the campus. In the experiment, 1 L of real seawater (initial Li) was treated with 1 g of PGST@HTO-Fe. + The concentration is approximately 0.173 mg / L. -1 Simultaneously, the concentration changes of the main cations were monitored. The results are as follows: Figure 8 The subgraph (h) and Table 1 are shown in the figure.

[0122] Table 1 Separation performance of PGST@HTO-Fe in actual seawater

[0123] Metalion Ionicradius(pm) <![CDATA[C0(mg / L)]]> <![CDATA[Q e (mg / g)]]> <![CDATA[K d (ml / g)]]> <![CDATA[Li + ]]> 76 0.173 0.023 153.3 <![CDATA[K + ]]> 138 380.7 5.6 14.9 <![CDATA[Mg 2+ ]]> 72 1253.2 3.5 2.8 <![CDATA[Na + ]]> 102 10500.51 8.4 0.8

[0124] Please refer to Table 1 and... Figure 8 Subgraph (h) in the diagram, despite the presence of Li in natural seawater + The concentration is extremely low, and Na + Mg 2+ K + Even with much higher competing ion concentrations, PGST@HTO-Fe still resists Li. + It exhibits significant preferential adsorption. According to the formula... Calculation, Li + The partition coefficient Kd is 153.3 mL•g. -1 Significantly higher than Na + Mg 2+ and K + This indicates that the material exhibits excellent Li properties in practical seawater matrices. +Selectivity. These results demonstrate that PGST@HTO-Fe can effectively enrich Li under high salinity conditions. + This confirms its potential feasibility in actual seawater lithium extraction.

[0125] To evaluate the cyclic stability of the material, adsorption-desorption experiments were conducted in a constant-temperature water bath shaker. One cycle consisted of seven consecutive adsorption-desorption operations, and a total of three cycles (21 operations in total) were performed to evaluate the reusability of the adsorbent. In each cycle, the PGST@HTO-Fe hydrogel was immersed in 50 mL of Li + Solution (100 mg•L) -1 In PGST@HTO-Fe, adsorption was carried out by shaking at 298 K and 200 rpm for 24 hours. After adsorption, the hydrogel was thoroughly rinsed with deionized water, followed by desorption in 100 mL of 0.2 M HCl in a water bath shaker for 12 hours. After acid treatment, the hydrogel was washed with deionized water, and then the next cycle was performed. After the first adsorption-desorption cycle, Li in PGST@HTO-Fe... + Adsorption capacity from 24.1 mg•g -1 Decreased to 21.2 mg•g -1 The overall loss is relatively small. According to the formula... Calculations showed a reuse rate of 87.5%, indicating that PGST@HTO-Fe maintained good adsorption stability during the initial reuse phase. After the second cycle, the adsorption capacity continued to gradually decrease, but the rate of decrease was not significantly faster than in the first cycle. The adsorption capacity remained at approximately 19-21 mg / g. -1 Within this range, it indicates that the material reached a relatively stable state after the initial "adaptation" period, during which the structure and active sites tended to stabilize. After the third adsorption-desorption cycle, the adsorption capacity further decreased, dropping to approximately 12.28 mg•g after the 21st cycle. -1 This decrease may be attributed to the gradual loss of active components due to repeated acid desorption, localized structural fatigue, or partial blockage of mass transfer channels within the hydrogel network. Nevertheless, the material retains a measurable adsorption capacity. Overall, PGST@HTO-Fe exhibits a predictable, gradual decrease in adsorption capacity over three consecutive cycles, demonstrating acceptable cycling stability and indicating its potential for long-term lithium extraction applications.

[0126] Table 2 compares the performance of different photothermal-assisted lithium adsorption materials. Unlike most systems that excel in only a single metric, PGST@HTO-Fe achieves a good balance of performance across key metrics, including evaporation rate, lithium adsorption capacity, selective partition coefficient, and cycle stability. These results indicate that this material possesses significant overall advantages and holds broad application prospects in the synergistic application of photothermal evaporation and selective lithium extraction.

[0127] Table 2 Comparison of the performance of various photothermal assisted adsorbents

[0128] Materials <![CDATA[Adsorptioncapacity(mg•g -1 )]]> <![CDATA[Evaporationrate(kg•m -2 •h -1 )]]> Adsorption Time (h) <![CDATA[K d ]]> Cyclenumber Reusability DEF 18.44 1.15 (under 3sun) 3 931.23 4 47.8% CA@HTO 20.36 2.07 (under1sun) 12 92.71 20 68% DLMS 13.68 1.23 g / h (under 1.5 sun) 5 / 8 87% HTO@NMC-PS 27.6 1.32 (under 1 sun) 6 142.7 9 94.2% S-evaporator 20.09 1.51 (under1sun) 7 45.27 5 89% <![CDATA[LIG / MnO2NPs]]> 13.48 2.99 (under1sun) 15 / 10 / PGST@HTO-Fe 30.21 1.8 (undur1sun) 10 153.3 7 87.5%

[0129] Methylene blue photocatalytic degradation performance:

[0130] Figure 9 For a diagram showing the photocatalytic degradation performance of the composite hydrogel illustrated in this application, please refer to... Figure 9 , Figure 9 Subplot (a) shows the degradation rates of PGS, PGST, and PGST@HTO-Fe; subplot (b) shows the mass changes of PGS, PGST, and PGST@HTO-Fe on methylene blue solution under one standard sunlight irradiation; subplot (c) shows the solution image during the photocatalytic degradation process; and subplot (d) shows the pseudo-first-order kinetic model of photocatalytic degradation.

[0131] Please refer to Figure 9 Considering the coexistence of multiple pollutants in seawater, achieving simultaneous catalytic degradation of pollutants during photo-driven seawater desalination is of great significance. This application selects methylene blue (MB) as a model pollutant to systematically evaluate the photocatalytic degradation performance of the PGST@HTO-Fe composite material. PGST@HTO-Fe was cut into 3×3cm pieces. 2 The material, in block form, was placed on the surface of a beaker containing 50 mL of methylene blue solution (5 mg / L), allowing it to float on the solution. Before the reaction, the material was immersed in the solution in the dark for 0.5 hours to ensure adsorption-desorption equilibrium was reached. Subsequently, photocatalytic degradation was carried out under standard sunlight, according to the formula... Degradation efficiency was calculated. The results showed that after 4 hours of illumination, PGST@HTO-Fe achieved a degradation efficiency of 82.2% for MB, while the control materials PGS and PGST exhibited negligible photocatalytic activity under the same conditions.

[0132] To evaluate the photo-driven evaporation performance of PGST@HTO-Fe in contaminated water, a methylene blue (MB) solution was used for testing. Figure 9 As shown in subplot (b), PGST@HTO-Fe maintained 1.63 kg•m -2 •h -1The high evaporation rate indicates that it can maintain efficient water evaporation even in complex aquatic environments. During the photocatalytic degradation process, the color of MB gradually fades from blue to transparent. The reaction kinetics were analyzed using a pseudo-first-order model by plotting the ln(Ct / C0) versus time. The calculated rate constant for MB degradation by PGST@HTO-Fe was 0.36, significantly higher than that of PGS (0.022) and PGST (0.032), indicating that its photocatalytic activity is much faster. This enhancement is attributed to the broad-spectrum light absorption capacity of carbon nanotubes (CNTs), which absorb visible to near-infrared light and convert it into heat or electronic excitation, while HTO-Fe mainly interacts with ultraviolet light to generate electron-hole pairs. The combination of CNTs and HTO-Fe broadens the light absorption range and promotes efficient charge separation, with CNTs acting as effective electron acceptors. Therefore, the synergistic effect of CNTs and lithium-ion sieves significantly improves the photocatalytic efficiency, indicating that PGST@HTO-Fe has high efficiency in degrading organic pollutants such as MB.

[0133] Furthermore, the possible mechanism of PGST@HTO-Fe photocatalytic degradation of MB is that carbon nanotubes (CNTs) with excellent conductivity act as electron transport channels, effectively transferring photogenerated electrons (electrons). - ) Transfer from the HTO-Fe conduction band, thereby limiting e - / h + For the composite. The porous hydrogel matrix with a large specific surface area promotes the interaction between methylene blue molecules and catalytic sites (HTO-Fe and CNTs), indirectly inhibiting e. - / h + Complexation. During this process, dissolved molecular oxygen is converted into superoxide radicals (•O2). - Photogenerated holes interact with water to generate hydroxyl radicals (•OH). In h + •O2 - Under the synergistic oxidation of α and β-OH, MB ultimately decomposes into H₂O and CO₂. The main reaction process can be represented by the following equation:

[0134] HTO-Fe+hν→e - +h + ;

[0135] e - +O2→·O2 - ;

[0136] h + +H₂O→·OH+H + ;

[0137] h + +MB → intermediate product;

[0138] ·O2 - / ·OH+MB→CO2↑+H2O;

[0139] In summary, the PGST@HTO-Fe composite material effectively improves light utilization and photocatalytic rate by utilizing the synergistic effect between its components, showing broad prospects for water treatment applications.

[0140] Mechanism of iron doping and HTO-Fe / CNT heterostructure:

[0141] Figure 10 For the DFT calculation and mechanism analysis diagrams of HTO and HTO-Fe / CNT shown in this application, please refer to... Figure 10 , Figure 10 Sub-figure (a) shows the atomic configuration of HTO (Ti, O, H, and Li atoms are represented by blue, red, white, and green spheres, respectively); sub-figure (b) shows the differential charge density of HTO; sub-figure (c) shows the band structure of HTO; sub-figure (d) shows the atomic configuration of HTO-Fe (Ti, O, H, Fe, and Li atoms are represented by blue, red, white, yellow, and green spheres, respectively); sub-figure (e) shows the differential charge density of HTO-Fe; sub-figure (f) shows the band structure of HTO-Fe; sub-figure (g) shows the atomic configuration of HTO and HTO- Li adsorption energy on Fe; Subgraph (h) shows the Li diffusion barrier on HTO and HTO-Fe; Subgraph (i) shows the Li diffusion path on HTO; Subgraph (j) shows the Li diffusion path on HTO-Fe; Subgraph (k) shows the atomic configuration of HTO-Fe / CNT (Ti, O, H, Fe and Li atoms are represented by blue, red, white, yellow and green spheres, respectively, and CNTs are located at the bottom); Subgraph (l) shows the differential charge density of HTO-Fe / CNT; Subgraph (m) shows the band structure of HTO-Fe / CNT.

[0142] Please refer to Figure 10 To gain a deeper understanding of iron doping and the introduction of CNTs in Li + The roles of adsorption, photothermal synergy, and photocatalytic behavior were investigated. Density functional theory (DFT) calculations were performed on the system to study its structural stability, electronic properties, and kinetic behavior.

[0143] First, to confirm the iron-induced Li + To enhance adsorption performance and structural stability, adsorption models for pristine HTO and iron-doped HTO were constructed. The overall structures of pure HTO and 10% iron-doped HTO (HTO-Fe) were established, and their electronic properties were systematically analyzed. Results show that in both materials, Li… + All are stably adsorbed at triangular vacancy sites composed of oxygen atoms. Although iron doping does not change the Li + The basic type of adsorption site, but significantly modulates Li +The surrounding local coordination environment. Further differential charge density analysis showed that, compared with the original HTO, HTO-Fe in Li + After adsorption, a more significant interfacial charge redistribution was observed. In particular, the oxygen atoms adjacent to the Fe-doped atoms showed enhanced electron accumulation, indicating that iron doping effectively enhanced the interaction between the framework oxygen atoms and Li. + The electron attraction between the ions strengthens the Li-O ion interaction. This indicates that iron doping enhances the Li-O ion interaction by inducing local lattice distortion and modifying the electrostatic environment. + Diffusion. Electronic structure analysis further elucidated the modulating effect of iron doping on the inherent properties of the system. For example... Figure 10 As shown in subfigures (c) and (f), the band gap of HTO-Fe significantly decreases from 1.769 eV in the original HTO to 0.954 eV, indicating enhanced electronic polarizability and responsiveness, providing an electronic basis for strengthening interfacial charge redistribution and interactions. With a band gap of 0.954 eV, the theoretical optical absorption edge of HTO-Fe is redshifted to approximately 1300 nm, extending into the near-infrared (NIR) region. This means that iron doping successfully extends the optical response of HTO from the visible light region to a broad visible-near-infrared range covering most of the solar spectrum. Given that near-infrared light (780-2500 nm) accounts for more than 50% of total solar energy, HTO-Fe theoretically possesses a strong ability to capture and utilize a larger proportion of solar radiation, laying a solid physical foundation for efficient photothermal conversion. Correspondingly, Li... + The adsorption energy on the HTO-Fe surface decreased from -2.787 eV to -3.104 eV, indicating that iron doping significantly improved the thermodynamic stability of individual adsorption sites. This result is consistent with experimental observations of Li... + The adsorption capacity increased by 24.3%, which is highly consistent.

[0144] To further clarify the effect of iron doping on Li + The influence of migration kinetics, combined with diffusion path diagrams and elastic band (NEB) calculations, is applied to Li in HTO and HTO-Fe. + The diffusion behavior was compared and analyzed. For example... Figure 10 As shown in subgraph (i), in the original HTO, Li + Migration primarily occurs along interlayer oxygen channels. Diffusion pathways are relatively limited, and Li... + The diffusion barrier is as high as 2.20 eV, requiring passage through a narrow bottleneck formed by adjacent oxygen atoms. This indicates that Li in HTO... + The migration kinetics are strongly restricted. In contrast, in the HTO-Fe system, iron doping-induced local structural modulation moderately alters the Li content near the Fe sites. + The diffusion path forms a more continuous and flexible migration channel, and the diffusion path is shorter. Correspondingly, NEB calculations show that Li...+ The diffusion barrier was significantly reduced to 1.27 eV, indicating that iron doping effectively alleviated the Li diffusion barrier between adjacent adsorption sites. + The kinetic barrier to migration. The reduction in the diffusion barrier can be attributed to two key structural changes: (i) Fe 3+ Replace Ti 4+ Introducing lattice distortion and locally increasing interlayer spacing, for Li + Transport provides a larger free volume; (ii) differential charge density analysis reveals charge polarization around Fe atoms, which weakens the Li migration process. + The electrostatic repulsion between the oxygen atoms in the framework stabilizes the low-energy transition state.

[0145] The introduction of CNTs resulted in a heterojunction structure with significant interfacial coupling characteristics in the HTO-Fe / CNT system. Differential charge density analysis revealed a more significant charge redistribution during Li adsorption compared to the original HTO-Fe, with stronger charge accumulation observed near the Li adsorption sites. This indicates that the HTO-Fe / CNT surface is more readily available to donate electrons, thereby promoting charge transfer to the adsorbed Li. + Ions. Notably, the band gap of the heterojunction system further decreased to 0.673 eV, significantly enhancing the interfacial electronic response. This narrowing of the band gap lowers the energy barrier for electronic excitation, thereby promoting faster charge transfer during lithium adsorption and desorption. Simultaneously, this modulation of the electronic structure facilitates the rapid separation and migration of photogenerated carriers, providing a solid electronic basis for the efficient generation of active species in photocatalytic reactions. Iron doping modulates the electronic structure of HTO, leading to a reduced band gap and enhanced light absorption. Furthermore, the heterojunction interface between HTO-Fe and CNTs forms a favorable energy level arrangement, promoting electron migration and driving the directional separation and transport of photogenerated carriers. These theoretical results are highly consistent with the experimentally observed enhancements in photocatalytic performance.

[0146] Overall, iron doping improves the thermodynamic stability of adsorption sites and reduces Li... + The kinetic barriers to migration effectively enhance Li + Adsorption performance. Furthermore, the introduction of CNTs, by constructing a tightly coupled heterojunction, further modulates the interfacial electronic structure, providing a solid theoretical basis for the photocatalytic activity observed under illumination and photothermal assisted lithium extraction.

[0147] Example 2

[0148] Corresponding to the aforementioned embodiment of a method for preparing a calcium ion dual crosslinked three-dimensional network hydrogel, this application also provides a calcium ion dual crosslinked three-dimensional network hydrogel, wherein the calcium ion dual crosslinked three-dimensional network hydrogel comprises:

[0149] A three-dimensional network matrix composed of calcium ion-crosslinked polyvinyl alcohol, gellan gum, and sodium alginate;

[0150] Multi-walled carbon nanotubes uniformly dispersed in the three-dimensional network matrix;

[0151] And an iron-doped titanium-based lithium-ion sieve that is uniformly dispersed in the form of nanoparticles and anchored in the three-dimensional network matrix.

[0152] Example 3

[0153] Corresponding to the aforementioned embodiment of a method for preparing a calcium ion dual crosslinked three-dimensional network hydrogel, this application also provides an application of the calcium ion dual crosslinked three-dimensional network hydrogel, which is used for simultaneous seawater desalination and lithium ion extraction.

[0154] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A method for preparing a calcium ion double crosslinked three-dimensional network hydrogel, characterized in that, The method includes: A lithium source, a titanium source, and an iron source are mixed in a molar ratio of Li:Ti:Fe=2(1+x):1:x. Anhydrous ethanol is added, the mixture is stirred, and ultrasonically treated. The mixture is then calcined at 800~900℃ to obtain a precursor. The precursor is then leached with an acid solution, washed, and dried to obtain an iron-doped titanium-based lithium-ion sieve. Wherein, x=0.05~0.

15. An aqueous solution of polyvinyl alcohol, gellan gum, and sodium alginate was mixed, and then a multi-walled carbon nanotube dispersion and the iron-doped titanium-based lithium-ion sieve were added and mixed to obtain a composite precursor solution. A calcium chloride solution was added to the composite precursor solution to carry out a double crosslinking reaction; The cross-linked system was subjected to freeze-thaw cycles and then freeze-dried to obtain a calcium ion double cross-linked three-dimensional network hydrogel.

2. The method according to claim 1, characterized in that, The value of x is 0.

10.

3. The method according to claim 1, characterized in that, The aqueous solution of polyvinyl alcohol has a mass percentage concentration of 10 wt%, the aqueous solution of gellan gum has a mass percentage concentration of 2 wt%, and the aqueous solution of sodium alginate has a mass percentage concentration of 3 wt%; the mass ratio of the aqueous solutions of polyvinyl alcohol, gellan gum, and sodium alginate is 2:1:

1.

4. The method according to claim 1, characterized in that, The acid solution is a 0.75 mol / L hydrochloric acid solution, the leaching temperature is 300~350K, and the leaching time is 10~15h.

5. The method according to claim 4, characterized in that, The leaching temperature was 333K and the leaching time was 12h.

6. The method according to claim 1, characterized in that, The mass percentage concentration of the added multi-walled carbon nanotube dispersion is 2wt%, and the added iron-doped titanium-based lithium-ion sieve is 0.2g.

7. The method according to claim 1, characterized in that, The freeze-thaw cycle is performed three times.

8. The method according to claim 1, characterized in that, The lithium source is lithium carbonate, the titanium source is titanium dioxide, and the iron source is iron(III) oxide.

9. A calcium ion-double crosslinked three-dimensional network hydrogel, characterized in that, The calcium ion dual crosslinked three-dimensional network hydrogel is prepared according to the preparation method described in any one of claims 1-8, and the calcium ion dual crosslinked three-dimensional network hydrogel comprises: A three-dimensional network matrix composed of calcium ion-crosslinked polyvinyl alcohol, gellan gum, and sodium alginate; Multi-walled carbon nanotubes uniformly dispersed in the three-dimensional network matrix; And an iron-doped titanium-based lithium-ion sieve that is uniformly dispersed in the form of nanoparticles and anchored in the three-dimensional network matrix.

10. An application of a calcium ion double crosslinked three-dimensional network hydrogel, characterized in that, The calcium ion double cross-linked three-dimensional network hydrogel is used for simultaneous seawater desalination and lithium ion extraction.

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