Description:BACKGROUND

FIELD OF THE DISCLOSURE
[0001] Various embodiments of the disclosure relate generally to hydrogels. More specifically, various embodiments of the disclosure relate to hydrogels for adsorption of metal ions from an aqueous solution and methods of making the same.
DESCRIPTION OF THE RELATED ART
[0002] Water pollution is a major environmental concern, especially the contamination of water bodies by metal ions. Industrial activities, such as mining, electroplating, and chemical manufacturing, often discharge heavy metals like lead, mercury, cadmium, and chromium into water sources. The heavy metal ions are highly toxic, non-biodegradable, and can accumulate in living organisms, leading to severe health hazards, including neurological damage, kidney failure, and even cancer. Alkaline earth metal ions such as calcium (Ca), magnesium (Mg), strontium (Sr), and barium (Ba) are also of particular concern. The alkaline earth metals are commonly found in natural water sources due to geological formations and their concentrations are further elevated due to industrial discharges. Calcium and magnesium metal ions contribute to water hardness, which can lead to scale formation in pipes, boilers, and other water-handling equipment causing economic losses due to increased energy consumption and maintenance costs.
[0003] Several methods have been developed to address the removal of metal ions from water. Chemical precipitation is one method involving the addition of chemical reagents in water to form insoluble metal compounds, which may be removed through filtration or sedimentation. However, chemical precipitation may require high chemical usage leading to secondary pollution.
[0004] Ion-exchange is another method where specific metal ions are replaced with less harmful ions. For example, heavy metal ions which are present in the water may be replaced with more benign sodium or potassium ions. However, prolonged exposure to sodium or potassium ions at concentrations exceeding the recommended human intake could potentially be fatal. Filtration using semi-permeable membrane is another method to separate metal ions. However, filtration methods involve high energy consumption and operational costs.
[0005] Adsorption has emerged as a more favorable alternative due to its numerous advantages. Adsorption is highly effective at removing even trace amounts of metal ions from water, making it suitable for treating low-concentration pollutants. Further, adsorption does not typically generate harmful by-products or secondary pollution, and the adsorbents can often be regenerated and reused.
[0006] Recently, hydrogels have emerged as a promising class of adsorbent materials for removal of metal ions from water. Hydrogels are three-dimensional, cross-linked polymer networks capable of absorbing large amounts of water while maintaining structural integrity. The high water content and porous nature make hydrogels excellent candidates for capturing and immobilizing metal ions, from aqueous solutions.
[0007] Several approaches have been explored to enhance the metal ion adsorption capacity of the hydrogels. Incorporating functional groups in hydrogels, such as carboxyl, hydroxyl, or amino groups, which can selectively bind to metal ions has been attempted. Further, the physical properties of the hydrogel such as surface area, and porosity may influence the adsorption of metal ions. Hydrogels that exhibit higher adsorption efficiency, and greater capacity for a wide range of metal ions, under varying environmental conditions is desirable.
[0008] Limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of described systems with some aspects of the present disclosure, as set forth in the remainder of the present application and with reference to the drawings.
SUMMARY
[0009] According to embodiments of the present disclosure, a hydrogel for adsorption of metal ions is provided. The hydrogel comprises a polymer matrix comprising a first polymer, a second polymer, a polyphenol compound, and a crosslinking agent. The first polymer and the second polymer are crosslinked with the crosslinking agent and the polyphenol compound is bound to the first polymer. The hydrogel further comprises a chelating agent bound to the second polymer.
[0010] In one embodiment, a system for removing metal ions from an aqueous solution comprising the hydrogel is provided.
[0011] In some embodiments, the hydrogel has a calcium ion removal efficiency, or magnesium ion removal efficiency of more than 95% from the aqueous solution containing calcium ions, magnesium ions, or both at a concentration of 1000mg/L.
[0012] In another embodiment, a process for water softening comprising contacting the hydrogel with an aqueous solution is provided. The hydrogel adsorbs metal ions from the aqueous solution.
[0013] In yet another embodiment, a method of preparing a hydrogel is provided. The method comprises providing a first polymer in an aqueous solution comprising a second polymer at a temperature in a range of 60°C to 80°C to form a first mixture. The method further comprises adding a polyphenol compound and a crosslinking agent to the first mixture and adjusting a viscosity of the first mixture by adding water to form a second mixture. The method further comprises forming a shaped body from the second mixture. The shaped body is provided in a solution comprising a chelating agent to form the hydrogel.
[0014] In yet another embodiment, a method of forming a hydrogel bead is provided. The method comprises providing a first polymer in an aqueous solution comprising a second polymer at a temperature in a range of 60°C to 80°C to form a first mixture. The method further comprises adding a polyphenol compound and a crosslinking agent to the first mixture and adjusting a viscosity of the first mixture by adding water to form a second mixture. The second mixture is poured into a solution comprising pentasodium triphosphate to form a spherical body. The method further comprises washing and drying the spherical body to form a dehydrated spherical body. The method further comprises providing the dehydrated spherical body in a solution comprising a chelating agent to form the hydrogel bead.

BRIEF DESCRIPTION OF DRAWINGS

[0015] FIG. 1 is a flow chart that illustrates a method of producing a hydrogel, in accordance with an exemplary embodiment of the disclosure;
[0016] FIG. 2 is a bar chart displaying type of hydrogel against hardness removal in percentage;
[0017] FIG. 3 is a bar chart displaying initial hardness concentrations against hardness removal in percentage; and
[0018] FIG. 4 is a bar chart displaying dosage of hydrogel against hardness removal in percentage.
[0019] Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description of exemplary embodiments is intended for illustration purposes only and is, therefore, not intended to necessarily limit the scope of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

[0020] The following description illustrates some exemplary embodiments of the disclosed disclosure in detail. Those of skill in the art will recognize that there are numerous variations and modifications of this disclosure that are encompassed by its scope. Accordingly, the description of a certain exemplary embodiment should not be deemed to limit the scope of the present disclosure.
[0021] The term “comprising” as used herein is synonymous with “including,” or “containing,” and is inclusive or open-ended and does not exclude additional, unrecited elements, or process steps.
[0022] All numbers expressing quantities of ingredients, property measurements, and so forth used in the specification are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth herein are approximations that may vary depending upon the desired properties sought to be obtained.
[0023] These and other features and advantages of the present disclosure may be appreciated from a review of the following detailed description of the present disclosure, along with the accompanying figures in which like reference numerals refer to like parts throughout.
[0024] The term “hydrogel”, as used herein refers to a water-insoluble three-dimensional crosslinked polymer network or polymer matrix having a high-water absorption capacity.
[0025] The terms “removal efficiency” (RE) or “metal ion removal efficiency”, as used herein refers to a percentage of metal ions removed from an aqueous solution and is calculated as a percentage of difference between an initial concentration (Co) of ions in solution and final concentration (Cf) of ions in solution to the final concentration of ions in solution.
[0026] According to embodiments of the present disclosure, a hydrogel for adsorption of metal ions is provided. The hydrogel comprises a polymer matrix comprising a first polymer, a second polymer, a polyphenol compound, and a crosslinking agent. The first polymer and the second polymer are crosslinked with the crosslinking agent and the polyphenol compound is bound to the first polymer. The hydrogel further comprises a chelating agent bound to the second polymer.
[0027] The first polymer comprises polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), polyacrylamide (PAM), poly(N-vinylcaprolactam) (PVCL), poly(acrylic acid) (PAA), poly(methacrylic acid) (PMA), polyethylene oxide (PEO), poly(2-hydroxyethyl methacrylate) (PHEMA), polylactic acid (PLA), polyglycolic acid (PGA), or combinations thereof. In one embodiment, the first polymer comprises PVA.
[0028] The second polymer comprises chitosan, poly-L-lysine (PLL), polyethyleneimine (PEI), quaternized cellulose, trimethyl chitosan (TMC), chitin, cationic guar gum, poly(diallyldimethylammonium chloride) (PDADMAC), copolymer of dimethylaminoethyl methacrylate, butyl methacrylate and methyl methacrylate, or combinations thereof. In one embodiment, the second polymer comprises chitosan.
[0029] The choice of the first polymer and the second polymer is based on their ability to form a stable hydrogel. The term “stable hydrogel” as used herein, refers to a hydrogel that is water-insoluble while being water-swellable and has desired mechanical stability for intended application. The second polymer of the hydrogel is preferred to be a biopolymer or a natural polymer. Biopolymers are abundant in nature, and they are biodegradable thus enhancing the environment-friendliness of the resultant hydrogel. Typically, synthetic polymers such as the first polymers of the present disclosure have superior mechanical stability than the second polymers comprising biopolymers. A hydrogel formed using the first polymer and the second biopolymer may have improved mechanical stability than a hydrogel formed using the first and second polymer, individually. The mechanical stability and water-insoluble nature of the hydrogel are due to the formation of a 3-dimensional polymer network or polymer matrix that constitutes the hydrogel. In some embodiments, the first polymer may include functional groups such as hydroxyl groups, carbonyl groups, and lactam groups which may form covalent bonds or hydrogen bonding with the functional groups of the second polymer. In certain embodiments, the crosslinking between the first polymer and the second polymer is enhanced by using the crosslinking agent to form the polymer matrix. For example, PVA molecules can form hydrogen bonds with chitosan, improving the mechanical strength, water absorption, and improving an acid resistance of chitosan. The crosslinking between PVA and chitosan is further enhanced by using a crosslinking agent. The crosslinked first polymer, and the second polymer may have a net negative charge and localized negative charges at the molecular level for favorable electrostatic interactions with the cationic (positively charged) metal ions.
[0030] In some embodiments, a ratio of weight percent of the first polymer to the second polymer is in a range of 0.5:1 to 1:2. In one embodiment, the ratio of weight percent of the first polymer to the second polymer is 1:1.
[0031] The crosslinking agent comprises comprises glutaraldehyde, formaldehyde, carbodiimides, genipin, dialdehyde starch, N,N’-methylenebisacrylamide, epichlorohydrin, oxalic acid, tannic acid, or combinations thereof. The crosslinking agents form bonds between the first polymer and itself, or between the second polymer and itself, or combinations of the first polymer and the second polymer with itself to form a crosslinked three-dimensional polymer matrix. In one embodiment, the crosslinking agent is glutaraldehyde.
[0032] The crosslinking agent is present in the hydrogel at a weight percent in a range of 1 % to 5% to the total weight percent of the hydrogel.
[0033] The polyphenol compound comprises epigallocatechin gallate (EGCG), epicatechin, epigallocatechin, epicatechin-3-gallate, or combinations thereof. In one embodiment, the polyphenol compound is EGCG.
[0034] The polyphenol compound is covalently bound to the first polymer through hydroxyl groups of the polyphenol compound and forms part of the polymer matrix of the hydrogel. The hydrogel being porous the water molecules diffuse into the polymer matrix thus swelling the polymer matrix. The metal ions present in the water are adsorbed and retained within the hydrogel due to favorable electrostatic interactions, hydrogen bonding, and covalent bonds between the metal ion and functional groups present in the polymer matrix. The polyphenol compound additionally has binding sites exposed at a surface of the polymer matrix to which the metal ions are easily bound.
[0035] A weight percent of the polyphenol compound to the total weight of the hydrogel is 15%, or less than about 15%. In one embodiment, a weight percent of the polyphenol compound to the total weight of the hydrogel is in a range of 10% to 15%.
[0036] The chelating agent comprises ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), cis-1,2-cyclohexanediaminetetraacetic acid (CDTA), nitrilotriacetic acid (NTA), hydroxyethylethylenediaminetriacetic acid (HEDTA), or combinations thereof. In one embodiment, the chelating agent is EDTA. The chelating agents are known to chelate metal ions to form water soluble chelating agent-metal ion complexes. By being part of the hydrogel which is insoluble in water, the metal ions are bound to the chelating agents and are easily removed from the aqueous solution.
[0037] The chelating agents are bound to the second polymer. A weight percent of the chelating agents that may be bound to the second polymer to form the hydrogel depends on the concentration and binding sites available at the second polymer. In one embodiment, the weight percent of the chelating agent to the total weight of the hydrogel is 20%, or less than 20%. In some embodiments, a weight percent of the chelating agent to the total weight of the hydrogel is in a range of 10% to 20%.
[0038] In some embodiments, the hydrogel has a desired porosity expressed in terms of pore volume and pore diameter. The porosity of the hydrogel is defined as the total space or volume available within the pores of the hydrogel. The pore volume is in a range of 0.001 cubic centimeters per gram (cc/g) to 0.003 cc/g. The hydrogel has a pore diameter in a range of 2 nanometers (nm) to 4 nm. The pore volume and pore diameter are determined using gas physisorption studies. Other conventional techniques such as small-angle scattering techniques (X-ray or neutrons), or mercury intrusion porosimetry (MIP) may be utilized to determine porosity.
[0039] In some embodiments, the hydrogel exhibits antimicrobial property. In one embodiment, the antimicrobial property of the hydrogel is attributed to the presence of the polyphenol compound. Biopolymers such as chitosan are also known for antimicrobial property.
[0040] In one embodiment, the first polymer of the hydrogel is PVA, the second polymer is chitosan, the polyphenol compound is EGCG, and the chelating agent is EDTA.
[0041] In some embodiments, a system for removing metal ions comprising the hydrogel is provided. In one embodiment, the system is a water filtration unit, where the water filtration unit comprises the hydrogel in the form of beads, or in the form of a membrane. In one embodiment, the hydrogel beads are loaded in a vertical column where the water containing metal ions flows in from the top and the hydrogel adsorbs the metal ions present in the water. In another embodiment, the system comprises a horizontal filtration bed comprising the hydrogel beads where the water containing metal ions flows over the bed to absorb the metal ions from the water.
[0042] The inventive hydrogel has a net negative charge to adsorb the positively charged (cationic) metal ions. Further, the polyphenol compound and the chelating agent can covalently bind the metal ions. In one embodiment, the metal ion is a heavy metal ion. Examples of heavy metal ions include ions of metals including lead, zinc, mercury, nickel, manganese, copper, chromium, iron, cadmium, cobalt, tin, and combinations thereof. In some embodiments, the metal ion is an alkaline earth metal ion that may contribute to the hardness of the water. Examples of alkaline earth metal ions include ions of calcium (Ca), strontium, barium, and magnesium (Mg). In preferred embodiments, the metal ion is calcium, magnesium or combinations thereof.
[0043] Hydrogels containing chelating or complexing agents are known for heavy metal ion adsorption. The chelating agents, such as EDTA, are known for selective heavy metal ion adsorption over alkali or alkaline earth metal ions. The selective adsorption of the heavy metal ions is due to the higher charge densities of the heavy metal ions and their relatively small ionic radii. Further, heavy metal ions are more polarizable than alkali or alkaline earth metal ions leading to stronger covalent bonding or electrostatic attraction with the chelating agents. Additionally, the heavy metal ions due to availability of d-orbitals have multiple coordination sites which results in stronger bonding interaction with chelating agents.
[0044] Typically, the electrostatic interactions between alkali metal ions or alkaline earth metal ions and hydrogel adsorbents are much weaker compared to interactions between heavy metal ions and hydrogel adsorbents of prior art. Alkali metal ions and alkaline earth metal ions have larger ionic radius and a lower charge density compared to the heavy metal ions.
[0045] In the present invention, the adsorption of metal ions especially alkaline earth metal ions has been improved by judicious selection of hydrogel composition whereby the negative charges on the hydrogel are enhanced for stronger interaction with the positive metal ions irrespective of the charge density and size of the metal ion. Further, the presence of the polyphenol compound and the chelating agent provide covalent binding sites for metal ions. The adsorption of the metal ions is further enhanced by having the desired porosity that allows for the diffusion of water into the polymer matrix. The porosity and the swelling behavior of the hydrogel increase a contact time and a contact surface available for adsorption thereby enhancing the adsorption of metal ions. Thus, the porosity of the hydrogel, the overall negative charge on the polymer matrix, and the chelating abilities of the polyphenol compound and the chelating agent contribute to its strong affinity for metal ions.
[0046] The inventive hydrogel can function in a broad pH range for metal ion adsorption. In one embodiment, the aqueous solution containing the metal ions has a pH in a range of 3.2 to 8.5. The pH range can be fine-tuned by varying the composition of the hydrogel. For example, chitosan is known for poor performance at acidic pH, while chelating agents such as EDTA exhibit peak performance at alkaline pH. The polyphenol compound for example EGCG works best at acidic conditions. Therefore, by modifying the concentration of each of the components namely, the first polymer, the second polymer, the polyphenol compound, and the chelating agent of the hydrogel the pH range of operation may be broadened, or fine-tuned for efficient removal of metal ions.
[0047] FIG. 1 is a flow chart 100 that illustrates a method of producing a hydrogel through exemplary steps 102 through 106, according to embodiments of the present disclosure. At step 102, a first polymer is provided in an aqueous solution comprising a second polymer at a temperature in a range of 60°C to 80°C to form a first mixture. In one embodiment, the second polymer is dissolved in water under acidic conditions to form a homogeneous solution. The first polymer is added to the homogeneous solution with mixing over a duration of time in a range of 4 to 8 hours to form the first mixture.
[0048] At step 104, a polyphenol compound and a crosslinking agent are added to the first mixture. A viscosity of the first mixture is adjusted by adding water to form a second mixture. The polymer matrix of the hydrogel is formed, at step 104, in the second mixture where the first, polymer, second polymer, the polyphenol compond are part of the polymer matrix.
[0049] At step 106, a shaped body is formed from the second mixture. The shaped body may have a regular shape, or irregular shape. It is preferred for certain applications such as water treatment to have a large surface for greater contact between the hydrogel and water. In one embodiment, the hydrogel may be formed as a film or a membrane where the second mixture is poured over a suitable substrate. The second mixture may be cast or coated on a surface to form a shaped body in the form of film. Example casting or coating methods include, but are not limited to, spin coating, dip coating, and spray coating. In another embodiment, the second mixture is cast to form a layer over a substrate. The casting may be performed using a doctor blade to form a uniform layer of desired thickness. However, other methods for forming layers or films as known in the art may be utilized, such as dip coating. The substrate comprising the layer is immersed in a coagulating bath to obtain the film.
[0050] In yet another embodiment, the hydrogel is formed in the shape of a spherical body to form hydrogel beads. A spherical shaped body has larger surface area compared to other regular shapes such as a square or a rectangle. In one embodiment, forming the shaped body at step 106 includes pouring the second mixture into a solution containing pentasodium triphosphate to form the spherical body. The term “pouring the second mixture” refers to providing the second mixture in pentasodium triphosphate to form the spherical body. The second mixture is added drop-wise in pentasodium triphosphate to form the spherical body. A dimension of the spherical body may be controlled by gravitational considerations such as height at which the second mixture is poured into the pentasodium triphosphate solution, and viscosity of the pentasodium triphosphate solution. In one embodiment, the spherical body has a diameter in a range of 1 millimeter (mm) to 8 mm. The spherical body has a diameter in a range of 1 to 4 mm in dehydrated state, and can swell up to 8 mm in water. The spherical shaped body is washed in deionized water to remove any excess, or unattached first polymer, the second polymer, crosslinking agent, and the polyphenol compound and dried to form dehydrated spherical body.
[0051] At step 108, the shaped body is provided in a solution comprising a chelating agent to form the hydrogel. In embodiments, where the shaped body is a sphere, the dehydrated spherical body from step 106 is provided in a solution comprising the chelating agent to produce hydrogel beads.
[0052] The first polymer, the second polymer, the crosslinking agent, the polyphenol compound and the chelating agents are as described previously.
[0053] In another embodiment, a process for water softening comprising contacting the hydrogel produced using the method illustrated in FIG. 1 is provided. The term “water softening”, as used herein refers to the removal of dissolved calcium or magnesium ions from water. The term “water softening” is also termed as “hardness removal”, and relates to removal of calcium ions, or magnesium ions, or both. The water containing more than 200 milligrams per litre (mg/L) of dissolved calcium ions or magnesium ions is termed as hard water. The portable water or drinking water should have a minimum total hardness of 150 mg/L. The process comprises contacting the hydrogel with an aqueous solution to adsorb metal ions from the aqueous solution.
[0054] The process is a batch process, or a continuous process. For example, a horizontal tank bed may be operated in a batch mode, where a batch of water is made to contact for a period of time to soften the water and is replaced with the next batch of water to be softened.
[0055] The hydrogels of the present invention can be regenerated once the metal ion removal efficiency falls below accepted range on prolonged use. In one embodiment, the hydrogels are regenerated by washing in deionized water. In another embodiment, the hydrogel is regenerated by providing it in a solution comprising the chelating agent.
EXAMPLES
[0056] The present disclosure will now be described in greater detail by the
following non-limiting examples. It is understood that one skill in the art will envision additional embodiments consistent with the disclosure provided herein.
EXAMPLE 1
Preparation of polymer matrix comprising polyvinyl alcohol (PVA), chitosan, and EGCG:
[0057] About 2 grams (g) of chitosan was dissolved in 50 milliliters (ml) of 1% acetic acid solution in water at 70 °C until homogeneous. About 2 g of polyvinyl alcohol (PVA) powder was added and mixed for 6 hours to form a first mixture. About 0.5 g of epigallocatechin gallate (EGCG) powder and 1% glutaraldehyde (GA) were added, and the consistency of the first mixture was adjusted with an additional 50 mL of deionized water to form a second mixture. The second mixture was dropped into a solution of pentasodium triphosphate (Na5P3O10) to form spherical beads. Excess Na5P3O10 was removed by thoroughly washing the beads using deionized water Finally, the spherical beads were dried and stored at room temperature.
EXAMPLE 2
Preparation of hydrogel beads comprising EDTA
[0058] About 1 g of ethylenediamine tetraacetic acid (EDTA) was dissolved in 100 mL of deionized water, followed by the addition of 1 g of N-hydroxysuccinimide (NHS) and 0.5 g of 1-ethyl-3(3-dimethylaminopropyl-carbodiimide hydrochloride (EDC) to activate the carboxyl groups of EDTA for 2 hours with stirring to form activated EDTA solution. 5 grams of the spherical beads from Example 1 were immersed in the activated EDTA solution and stirred for 5 hours to form the hydrogel beads. The hydrogel beads comprising EDTA were prepared through an amination reaction between the carboxyl groups of EDTA and the amino groups of chitosan. The hydrogel beads were washed thoroughly with deionized water and then vacuum-dried for storage at room temperature.
Hardness or metal ion removal studies
[0059] The hardness removal efficiencies in percentage of various hydrogels were recorded by providing the hydrogels in a solution containing calcium carbonate (CaCO3) having the same initial concentration of 1000 mg/L. The calcium ion concentration of a solution may be determined from titration with EDTA, or by atomic absorption spectroscopy (AAS). Figure 2 is a bar chart 200 depicting hardness removal efficiencies of various hydrogels. Bar diagram 202 corresponds to the hardness removal efficiency of a hydrogel prepared from PVA and chitosan. The hardness removal efficiency of a hydrogel prepared from PVA, chitosan and EGCG is shown in bar diagram 204. Bar diagram 206 corresponds to the hardness removal efficiency of hydrogel prepared using PVA, chitosan, and EDTA. The hardness removal efficiency of hydrogel produced according to embodiments of the present invention (Example 2) is presented as bar diagram 208.
[0060] The hydrogels made solely from chitosan (CS) and PVA (bar diagram 202) had a removal efficiency of about 40%. By incorporating epigallocatechin gallate (EGCG) having negatively charged functional groups, into the CS-PVA hydrogel the removal efficiency increased to approximately 61% at the same initial hardness concentration of calcium ions. By coating the CS-PVA hydrogel beads with EDTA, removal efficiency increased to around 72%, which is higher than that achieved with EGCG incorporation alone. The inventive hydrogel comprising CS, PVA, EGCG, and EDTA performed the best at a removal efficiency of approximately 85% at high initial calcium ion concentration of 1000 mg/L. The synergistic effect resultant from combination of EDTA and EGCG resulted in excellent adsorption of positively charged metal ions when compared to EDTA and EGCG alone.
[0061] A set of calcium carbonate solutions was prepared having initial concentrations of 200 mg/L, 400 mg/L, 600 mg/L, 800 mg/L, and 1000 mg/L, respectively. FIG. 3 is a bar chart 300 of the initial hardness concentration against hardness removal efficiency at neutral pH, after a contact time of 4 hours. At a lower concentration of 200 mg/L, corresponding to bar diagram 302, the hardness removal is around 98%. However, when the concentration is increased to 1000 mg/L (bar diagram 310), the hardness removal slightly decreases to around 85% due to the saturation of binding sites on the hydrogel beads. At concentrations of 400mg/L (bar diagram 304), 600 mg/L (bar diagram 306), and 800 mg/L (bar diagram 308), the hardness removal was 93%, 92%, and 90% respectively. The study also revealed hardness removal efficiency reaches near equilibrium at around 4 hours, after which the removal efficiency does not significantly increase.
[0062] FIG. 4 is a bar chart 400 displaying dosage of hydrogel against hardness removal in percentage. As shown in FIG. 4, by increasing the dosage of hydrogel beads from 10 g/L to 50 g/L, the hardness removal efficiency increased from approximately 80% to 95%. The study was conducted using calcium carbonate solution having an initial concentration of 1000 mg/L at neutral pH, and after a contact time of 4 hours.
[0063] It is to be understood that the above description is intended to be illustrative, and not restrictive. Furthermore, many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. Although the present disclosure has been described with reference to specific exemplary embodiments, it will be recognized that the disclosure is not limited to the embodiments described, but can be practiced with modification and alteration within the scope of the appended claims.

, Claims:CLAIMS

We claim,
1. A hydrogel for adsorption of metal ions, the hydrogel comprising:
a polymer matrix comprising a first polymer, a second polymer, a polyphenol compound, and a crosslinking agent, wherein the first polymer and the second polymer are crosslinked with the crosslinking agent, and wherein the polyphenol compound is bound to the first polymer; and
a chelating agent bound to the second polymer.
2. The hydrogel as claimed in claim 1, wherein the first polymer comprises polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), polyacrylamide (PAM), poly(N-vinylcaprolactam) (PVCL), poly(acrylic acid) (PAA), poly(methacrylic acid) (PMA), polyethylene oxide (PEO), poly(2-hydroxyethyl methacrylate) (PHEMA), polylactic acid (PLA), polyglycolic acid (PGA), or combinations thereof.
3. The hydrogel as claimed in claim 1, wherein the second polymer comprises chitosan, poly-L-lysine (PLL), polyethyleneimine (PEI), quaternized cellulose, trimethyl chitosan (TMC), chitin, cationic guar gum, poly(diallyldimethylammonium chloride) (PDADMAC), copolymer of dimethylaminoethyl methacrylate, butyl methacrylate and methyl methacrylate, or combinations thereof.
4. The hydrogel as claimed in claim 1, wherein the crosslinking agent comprises glutaraldehyde, formaldehyde, carbodiimides, genipin, dialdehyde starch, N,N’-methylenebisacrylamide, epichlorohydrin, oxalic acid, tannic acid, or combinations thereof.
5. The hydrogel as claimed in claim 1, wherein the polyphenol compound comprises epigallocatechin gallate (EGCG), epicatechin, epigallocatechin, epicatechin-3-gallate, or combinations thereof.
6. The hydrogel as claimed in claim 1, wherein the chelating agent comprises ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), cis-1,2-cyclohexanediaminetetraacetic acid (CDTA), nitrilotriacetic acid (NTA), hydroxyethylethylenediaminetriacetic acid (HEDTA), or combinations thereof.
7. The hydrogel as claimed in claim 1, wherein the first polymer is PVA, the second polymer is chitosan, the polyphenol compound is EGCG, and the chelating agent is EDTA.
8. The hydrogel as claimed in claim 1, wherein a ratio of weight percent of the first polymer to the second polymer is in a range of 0.5:1 to 1:2.
9. The hydrogel as claimed in claim 1, wherein a weight percent of the polyphenol compound to the total weight of the hydrogel is 15%, or less than 15%.
10. The hydrogel as claimed in claim 1, wherein a weight percent of the chelating agent to the total weight of the hydrogel is 20%, or less than 20%.
11. The hydrogel as claimed in claim 1, wherein the hydrogel has a pore diameter in a range of 2 nanometers (nm) to 4 nm.
12. The hydrogel as claimed in claim 1, wherein the hydrogel exhibits antimicrobial property.
13. The hydrogel as claimed in claim 1, wherein the metal ion comprises iron (Fe), lead (Pb), cadmium (Cd), zinc (Zn), copper (Cu), mercury (Hg), nickel (Ni), tin (Sn), cobalt (Co), manganese (Mn), chromium (Cr), iron (Fe), calcium (Ca), magnesium (Mg), strontium (Sr), barium (Ba), or combinations thereof.
14. The hydrogel as claimed in claim 13, wherein the metal ion comprises calcium, magnesium, or combinations thereof.
15. A system for removing metal ions from an aqueous solution comprising the hydrogel as claimed in claim 1.
16. The system as claimed in claim 15, wherein the hydrogel has a calcium ion removal efficiency, a magnesium ion removal efficiency, or both of more than 95% from the aqueous solution comprising calcium ions, magnesium ions, or both at a concentration of 1000mg/L.
17. The system as claimed in claim 15, wherein the aqueous solution has a pH in a range of 3.2 to 8.5.
18. A process for water softening comprising contacting the hydrogel as claimed in claim 1 with an aqueous solution, wherein the hydrogel adsorbs positively charged metal ions from the aqueous solution.
19. The process for water softening as claimed in claim 18, wherein the process is a batch process, or a continuous process.
20. A method of preparing a hydrogel comprising:
providing a first polymer in an aqueous solution comprising a second polymer (102) at a temperature in a range of 60°C to 80°C to form a first mixture;
adding a polyphenol compound and a crosslinking agent to the first mixture (104) and adjusting a viscosity of the first mixture by adding water to form a second mixture;
forming a shaped body (106) from the second mixture; and
providing the shaped body in a solution comprising a chelating agent (108) to form the hydrogel.
21. The method as claimed in claim 20, wherein the method comprises forming a hydrogel bead comprising steps of:
pouring the second mixture into a solution containing pentasodium triphosphate to form a spherical body;
washing and drying the spherical body to form a dehydrated spherical body; and
providing the dehydrated spherical body in the solution comprising the chelating agent to form the hydrogel bead.