WO2021230760A1

WO2021230760A1 - An adsorbent material, synthesis thereof, and use thereof

Info

Publication number: WO2021230760A1
Application number: PCT/QA2021/050008
Authority: WO
Inventors: Dema ALMASRI; Rachid ESSEHLI
Original assignee: Qatar Foundation For Education, Science And Community Development
Priority date: 2020-05-15
Filing date: 2021-05-15
Publication date: 2021-11-18

Abstract

The present disclosure provides a sustainable method for the synthesis of a material, layered double hydroxide, such as the non-limiting example of Zinc-LDH, to be used as an adsorbent for water treatment. The LDH material is prepared via a facile synthesis method using two chemicals, zinc acetate dihydrate and sodium hydroxide. The synthesis procedure is also prepared at a relatively low temperature and can be setup for preparation in large batches and easy scale-up. The present disclosure also provides highly efficient phosphate adsorption. The prepared LDH material is highly efficient in removing phosphate from synthetic water as well as wastewater. The present disclosure further provides effective recovery of phosphate.

Description

AN ADSORBENT MATERIAL, SYNTHESIS THEREOF, AND USE THEREOF

PRIORITY

[0001] The present application claims priority to US Serial No. 63/025,564, filed May 15, 2020, the entire contents of which are being incorporated herein by reference.

BACKGROUND

[0002] Phosphate nutrient pollution of water is a leading cause of water quality degradation and leads to eutrophication. Eutrophication is one of the most significant surface water quality problems, distinguished by the development of algal blooms, hypoxia, and shortfall in biodiversity. Furthermore, the reuse of treated wastewater, such as treated sewage effluent, for industrial purposes is gaining wide attraction for sustainable water management purposes. However, the reuse of treated wastewater with phosphate amounts higher than the recommended levels contribute towards biofouling in circulating water or corrosion on some metal surfaces in cooling systems. Therefore, phosphate removal from wastewater is imperative.

[0003] To regulate eutrophication, the United States Environmental Protection Agency (US EPA) provided a recommended limit of 0.05 mg/L for total phosphorus in streams entering lakes and 0.1 mg/L for total phosphorus in flowing water. Furthermore, phosphorus amounts at below 0.5 mg/L have been found to be the limiting value for algal growth. Therefore, it is essential to ensure phosphate amounts lie below these stringent levels in treated wastewater prior to its reuse.

[0004] Phosphate is a major nutrient required for plants. As the population surges, so does the requirement for more crop growth, which entails higher phosphate demands. Phosphate is obtained through rock mining, which is a non-renewable source and is depleting. Therefore, recovering phosphate from used adsorbents is very important.

[0005] To date, various biological, chemical, and physical treatment processes have been developed for the removal of dissolved phosphate. Biological processes, such as conventional activated sludge, can remove >97% of phosphate from water; however, these processes are less efficient in removing trace amounts of phosphate. Precipitation (e.g., as struvite), a simple but effective chemical treatment process, is limited by the complexity in sludge handling, neutralization, waste disposal, and treatment and management costs. Physical separation techniques, such as reverse osmosis or electrodialysis, have been shown to be expensive and ineffective (removes only 10% of the total phosphate). Also, conventional ion exchange processes can remove anions, however, the preference of the anion exchange resins for sulfates (over phosphates), which are present in higher concentrations in wastewater (compared to phosphate), compromises their efficiency for phosphate removal. Adsorption is reported to be one of the most effective processes for phosphate removal with advantages of low-cost, high efficiency (a wide range of concentration), and simple operation.

[0006] Sorption is one of the most attractive options for phosphorus removal due to its high efficiency, simplicity, and cost effectiveness. Numerous sorbents have been explored for the removal of phosphate and nutrients from water which include modified ion exchange resins, waste biomass, clay, iron and aluminum (hydr)oxides, lanthanum hydroxide, and layered double hydroxides.

[0007] There are various adsorbents used that are selective towards specific contaminants of interest (i.e. inorganic or organic contaminants). However, at the turn of the 21st century, the utilization of adsorbents with characteristic properties of: (i) low-cost and reasonable adsorption capabilities and (ii) environmentally friendly nature have become of high importance to researchers. A variety of adsorbents have been studied for phosphate sorption. Activated carbon is commonly used due to its high efficiency and large surface area; however, it has a higher cost relative to other adsorbents.

[0008] Recently, the development of low-cost adsorbents such as iron hydroxides, alum sludge, and sand have been gaining interest for aqueous phosphate removal. While these low cost-alternatives are available, many suffer from low phosphate removal efficiency, sorbent particle agglomeration, or difficulty in sorbent separation from water. Clays have an affinity towards phosphate adsorption and are an attractive choice due to their relative abundance and environmental compatibility, however, most have a relatively low adsorption capacity.

[0009] Layered double hydroxides (LDHs) high adsorption capacity and ability to remove a wide range of inorganic and organic anions from water. LDHs, also known as hydrotalcites, are clay minerals that exist naturally or can be synthesized under laboratory conditions.

[0010] LDHs consist of positively charged stacked sheets with negatively charged interlayer species to balance the overall charge. LDHs are typically described by the chemical formula of M²⁺(i-_X) M³⁺ _x (OH)2 (Aⁿ) _x/n x yH20, where M²⁺ are divalent cations (i.e. Mg²⁺, Zn²⁺, Ca²⁺, etc.), M³⁺ are trivalent cations (i.e. Al³⁺, Fe³⁺, Mn³⁺, etc.) and Aⁿ are interlayer anions (i.e. CO3^2', NO3^', etc.). The interlayer anions act as exchangeable anions that neutralize the positive charge, hence, providing LDHs with high anion-exchange capacity. SUMMARY

[0011] According to one non-limiting aspect of the present disclosure, an adsorbent for water treatment may comprise zinc-layered double hydroxide (Zn-LDH).

[0012] According to another non-limiting aspect of the present disclosure, the Zn-LDH may be synthesized by a facile synthesis method using zinc acetate dihydrate and sodium hydroxide.

[0013] According to another non-limiting aspect of the present disclosure, the adsorbent can be used for removing phosphate from synthetic water and/or wastewater. The Zn-LDH can adsorb phosphate onto its structure. The Zn-LDH can be regenerated with regenerants such as acetic acid and sodium hydroxide to release the phosphate from the adsorbent material. [0014] Additional features and advantages are described herein, and will be apparent from the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE DRAWING

[0015] Features and advantages of the present technology according to various embodiments including an adsorbent for water treatment and a method and use thereof described herein may be better understood by reference to the accompanying drawings in which:

[0016] FIG. 1 shows the SEM images of Zn-LDH at different magnifications (a,b) and EDS elemental analysis of Zn-LDH.

[0017] FIG. 2 shows an XRD pattern of Zn-LDH.

[0018] FIG. 3 shows an N2 adsorption-desorption isotherm of Zn-LDH.

[0019] FIG. 4 is an FTIR spectra for Zn-LDH before and after adsorption of PO4.

[0020] FIG. 5 shows the effect of contact time on phosphate removal from water (a), pseudo-first order kinetics (b), pseudo-second order kinetics (c) and intraparticle diffusion (d) model plots for the kinetic data.

[0021] FIG. 6 shows the (a) Influence of pH on phosphate adsorption by Zn-LDH, (b) phosphate speciation diagram and (c) Zeta potential of Zn-LDH.

[0022] FIG. 7 shows a phosphate adsorption isotherms on Zn-LDH.

[0023] FIG. 8 shows the (a) Effect of coexisting ions on the adsorption of phosphate and (b) removal of anions coexisting with phosphate. DETAILED DESCRIPTION

[0001] The various aspects and embodiments according to the present disclosure, as set forth herein, are illustrative of the specific ways to make and use the invention and do not limit the scope of invention when taken into consideration with the claims and the detailed description. It will also be appreciated that features from aspects and embodiments of the invention may be combined with further features from the same or different aspects and embodiments of the invention.

[0002] As used in this detailed description and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. For example, reference to “an LDH” or “a method” includes a plurality of such “LDHs” or “methods.” The term “and/or” used in the context of “X and/or Y” should be interpreted as “X,” or “Y,” or “X and Y.” Similarly, “at least one of X or Y” should be interpreted as “X,” or “Y,” or “both X and Y.” Similarly, the words “comprise,” “comprises,” and “comprising” are to be interpreted inclusively rather than exclusively. Likewise, the terms “include,” “including” and “or” should all be construed to be inclusive, unless such a construction is clearly prohibited from the context. However, the embodiments provided by the present disclosure may lack any element that is not specifically disclosed herein. Thus, a disclosure of an embodiment defined using the term “comprising” is also a disclosure of embodiments “consisting essentially of’ and “consisting of’ the disclosed components.

[0003] All ranges described are intended to include all numbers, whole or fractions, contained within the said range. As used herein, “about,” “approximately” and “substantially” are understood to refer to numbers in a range of numerals, for example the range of -10% to +10% of the referenced number, preferably -5% to +5% of the referenced number, more preferably -1% to +1% of the referenced number, most preferably -0.1% to +0.1% of the referenced number. Moreover, these numerical ranges should be construed as providing support for a claim directed to any number or subset of numbers in that range. For example, a disclosure of from 1 to 10 should be construed as supporting a range of from 1 to 8, from 3 to 7, from 1 to 9, from 3.6 to 4.6, from 3.5 to 9.9, and so forth. As used herein, wt.% refers to the weight of a particular component relative to total weight of the referenced composition.

[0024] The above problems are solved by the present invention — leading to divalent cation LDHs, trivalent cation-LDHs, and interlayer anionic LDHs — that ultimately lead to the specifically disclosed such as the non-limiting example of Zn-LDH and synthesis thereof.] [0025] Layered double hydroxides (LDHs) have high adsorption capacity and ability to remove a wide range of inorganic and organic anions from water. LDHs, also known as hydrotalcites, are clay minerals that exist naturally or can be synthesized under laboratory conditions.

[0026] LDHs consist of positively charged stacked sheets with negatively charged interlayer species to balance the overall charge. LDHs are typically described by the chemical formula of M²⁺(i-_X) M³⁺ _x (OH)2 (Aⁿ) _x/n x yH20, where M²⁺ are divalent cations (i.e. Mg²⁺, Zn²⁺, Ca²⁺, etc.), M³⁺ are trivalent cations (i.e. Al³⁺, Fe³⁺, Mn³⁺, etc.) and Aⁿ are interlayer anions (i.e. CO3^2', NO3^', etc.). The interlayer anions act as exchangeable anions that neutralize the positive charge, hence, providing LDHs with high anion-exchange capacity.

[0027] The present disclosure provides a sustainable method for the synthesis of a material, layered double hydroxide — such as the non-limiting example of Zinc-LDH — to be used as an adsorbent for water treatment. The LDH material is prepared via a facile synthesis method using two chemicals, zinc acetate dihydrate and sodium hydroxide. The synthesis procedure is also prepared at a relatively low temperature and can be setup for preparation in large batches and easy scale-up.

[0028] The present disclosure also provides highly efficient phosphate adsorption. The prepared LDH, such as the non-limiting example of Zinc-LDH, material is highly efficient in removing phosphate from synthetic water as well as wastewater.

[0029] The present disclosure further provides effective recovery of phosphate. While an LDH adsorbent efficiently adsorbs phosphate and removes it from the solution, it is very important to recover the phosphate. Phosphate recovery was tested with different regenerants at different concentrations and showed effective phosphate recovery after two regeneration cycles.

[0030] The present disclosure solves the problems of phosphate removal from wastewater and phosphate recovery. The LDH material is synthesized to be used as an adsorbent to remove phosphate from water (specifically wastewater). The used LDH will have phosphate adsorbed onto its structure. Instead of disposing of the used LDH adsorbent, it will be regenerated with regenerants such as acetic acid and sodium hydroxide to release the phosphate from the adsorbent material.

[0031] Relative to other adsorbents reported, LDH, such as the non-limiting example of Zinc-LDH, is prepared using a novel and simple method using a minimum number of precursors as well as a low synthesis temperature. LDH not only has a high anion exchange capacity for phosphate removal, it will also be regenerated to recover phosphate. Phosphate regeneration from a used adsorbent is quite challenging. The disclosed regeneration methodology uses a regenerant (glacial acetic acid) to retain the structure of the LDH to be reused again for another adsorption cycle. Glacial acetic acid is more environmentally friendly and lower in cost than the commonly used regenerant, sodium hydroxide. From the regeneration process, the phosphate released is recovered.

[0032] For Zinc-aluminum layered double hydroxides (Zn-Al-LDH), a decent phosphate adsorption capacity was achieved, and the time needed to reach equilibrium was 72 hours. The phosphate adsorption capacities reported in the prior art were similar or lower than the adsorption capacity of the disclosed Zn-LDH adsorbent. This shows that the LDH disclosed herein can provide a high adsorption capacity without the addition of aluminum to the synthesis procedure. Furthermore, the equilibrium time needed for Zn-LDH to reach the equilibrium adsorption capacity is 60 min., which is much shorter and more practical than many of the reported equilibrium times.

[0033] The present invention discloses an adsorbent for water treatment that comprises an LDH, such as the non-limiting example of Zn-LDH. The LDH may be synthesized by a facile synthesis method using zinc acetate dihydrate and sodium hydroxide. Once synthesized, the adsorbent of may be used for removing phosphate from synthetic water and/or wastewater. Specifically, the LDH adsorbs phosphate onto its structure and is regenerated with regenerants such as acetic acid and sodium hydroxide to release the phosphate from the adsorbent material. [0034] EXAMPLES

[0035] The following non-limiting examples are experimental examples supporting one or more embodiments provided by the present disclosure.

[0036] Example 1: Materials and Chemicals

[0037] All solutions were prepared with reagent grade chemicals and deionized water (Milli-Q system). Zinc acetate (Zn(OAc)2) and sodium hydroxide were obtained from Sigma- Aldrich and VWR Chemicals (Leuven, Belgium), respectively. Monopotassium phosphate (KH2PO4) was procured from Sigma-Aldrich.

[0038] Example 2: Synthesis of LDH

[0039] A layered double hydroxide (LDH) was prepared from zinc acetate via a simple nucleation process requiring only two chemicals. Zinc acetate (20g) was dissolved in 600 mL of deionized water. Approximately 1-10 g of NaOH was dissolved in 100 mL deionized water. The NaOH solution was added dropwise to the zinc acetate solution until the pH reached a range of 3-10 (±0.2) or until the pH was stable and was unaffected after the addition of the base. The mixing was carried out at 35-55°C at a rate of 300-500 rpm. The resulting slurry was aged and left to stir and the precipitate was then filtered, washed with deionized water and dried. [0004] In an embodiment, the method disclosed adjusted pH herein may comprise at least about 1 to 14, about 2 to 13, about 3 to 12, about 4 to 11, about 5 to 10, about 6 to 9, or about 7-8.

[0040] Example 3: Phosphate adsorption on an LDH

[0041 ] Batch adsorption experiments were carried out to investigate the performance of the

LDH for phosphate removal. Unless stated otherwise, 0.1-1.0 g L ¹ of the adsorbent was placed in a centrifuge tube with a 1- 10 mg/L phosphate solution. The pH of the solution was adjusted with 0.0001-1 mg L ¹ HC1 or NaOH. All samples were placed in polyethylene centrifuge tubes and shaken at a rate of 250-450 rpm using a mechanical shaker table. The effect of adsorbent dosage, contact time, solution pH, initial phosphate concentration, and co-existing ions were examined. All experiments were conducted in duplicates and at room temperature. The pH of the experiments was adjusted to a pH of between 3-10 in order to resemble the pH of real treated wastewater. The LDH dosage experiments were conducted at different adsorbent amounts ranging between 0.5 to 200 g/L. The sorbent dose of 0.5g/L was found to be the most efficient and economic dose and was used for the proceeding experiments. Kinetics experiments were conducted at time intervals ranging between 0.5 to 120 min to determine the equilibrium contact time and maximum adsorption capacity. Experiments investigating the effect of pH on the adsorption capacity of the LDH were conducted at a pH range of 3 to 10. Initial phosphate concentration experiments were carried out after the pH experiments at initial concentrations ranging between 0.5 mg L ¹ to 1000 mg L ¹ at a fixed pH of between 3 and 10. Initial and final phosphate concentrations were analyzed using a Dionex ion chromatography unit (ICS-5000+).

[0042] Example 4: LDH regeneration and phosphate recovery

[0043] Phosphate adsorption was conducted as mentioned above; 0.5 g L ¹ of the adsorbent was placed in a bottle with a 5 mg/L phosphate solution. The pH was adjusted to a value of between 3-10 and a contact time of 30-90 min. Afterwards, the LDH was filtered using a vacuum filter, washed several times, and dried. The LDH was then regenerated in a regenerate by following the same procedure used for the adsorption experiments. The regenerants used were Acetic Acid (0.00025 M), Acetic Acid (0.0005 M), NaOH(0.1 M), NaOH(0.01 M). [0005] In an embodiment, the method disclosed adjusted pH herein may comprise at least about 1 to 14, about 2 to 13, about 3 to 12, about 4 to 11, about 5 to 10, about 6 to 9, or about 7-8.

[0044] Existing LDH synthesis uses co-precipitation. This method involves the precipitation of LDH from an aqueous solution of divalent and trivalent transition metals. The precipitate is formed by the mixing of the metal precursors with a caustic solution (Urea or sodium hydroxide), followed by ageing.

[0045] According to the present disclosure, an LDH, such as the non-limiting example of Zn-LDH, is prepared using a method including a minimum number of precursors as well as a low synthesis temperature in addition to no requirement for reflux. Zn-LDH not only has a high anion exchange capacity for phosphate removal, it will also be regenerated to recover phosphate. Phosphate regeneration from a used adsorbent is quite challenging. The disclosed regeneration methodology will use a regenerant (glacial acetic acid) to retain the structure of LDH to be reused again for another adsorption cycle. Glacial acetic acid is more environmentally friendly and lower in cost than the commonly used regenerant, sodium hydroxide. From the regeneration process, the phosphate released is recovered.

[0046] The LDH prepared by the present disclosure can provide a high adsorption capacity without the addition of aluminum to the synthesis procedure. The equilibrium time needed for an LDH to reach the equilibrium adsorption capacity is 30-90 min.

[0047] Example 5: Characterization

[0048] The surface morphology of Zn-LDH was studied with a JEOL JSM-7610F field emission SEM at an accelerating voltage of 5 kV. The specific surface area of Zn-LDH was measured with a Micromeritics ASAP 2020 BET N2 (Norcross, GA, U.S.A.) surface area analyzer at 77 K. The crystallinity was analyzed with a Rigaku Miniflex-600 XRD (Chapel Hill, NC-U.S.A.), equipped with Cu-Ka lamp (l = 0.154 nm).

[0049] Example 6: Adsorption Experiments

[0050] Batch adsorption experiments were carried out to investigate the performance of an LDH, such as the non-limiting example of Zn-LDH, for phosphate removal. Unless stated otherwise, 0.5 g L ¹ of the adsorbent was placed in a centrifuge tube with a 5 mg/L phosphate solution. The pH of the solution was adjusted with 0.1-1 mg L ¹ HC1 or NaOH. All samples were placed in polyethylene centrifuge tubes and shaken at a rate of 250-450 rpm using a mechanical shaker table. The effect of adsorbent dosage, contact time, solution pH, initial phosphate concentration, and co-existing ions were examined. All experiments were conducted in duplicates and at room temperature. The pH of the experiments was adjusted to a pH of between 3 and 10 in order to resemble the pH of real treated wastewater. LDH dosage experiments were conducted at different adsorbent amounts ranging between 0.5 to 200 g/L. The sorbent dose of 0.5g/L was found to be the most efficient and economic dose and was used for the proceeding experiments. Kinetics experiments were conducted at time intervals ranging between 0.5 to 120 min to determine the equilibrium contact time and maximum adsorption capacity. Experiments investigating the effect of pH on the adsorption capacity of LDH were conducted at a pH range of 3 to 10. Initial phosphate concentration experiments were carried out after the pH experiments at initial concentrations ranging between 0.5 mg L ¹ to 1000 mg L ¹ at a fixed pH of between 3 and 10. Initial and final phosphate concentrations were analyzed using a Dionex ion chromatography unit (ICS-5000+).

[0006] In an embodiment, the method disclosed adjusted pH herein may comprise at least about 1 to 14, about 2 to 13, about 3 to 12, about 4 to 11, about 5 to 10, about 6 to 9, or about 7-8.

[0051] The adsorption capacity, qt, at a specific time t and the percent removal of phosphate were calculated based on the following equations:

% removal

where, Co (mg L ¹), and Ct (mg L ¹) denote the initial and equilibrium phosphate concentrations, respectively, V (L) is the volume of the solution, and W (g) is the mass of the adsorbent used.

[0052] Example 7: Kinetics and Equilibrium Models

[0053] In order to identify the equilibrium phosphate uptake and potential rate-controlling steps, two kinetic models (pseudo-first order, pseudo-second order models) and a diffusion model (intra-particle diffusion model) were applied to capture the adsorption process on LDH. The pseudo first order and pseudo second-order kinetics models are generally used to gain insights on the equilibrium adsorption capacity of adsorbents. The model that delivers the best fit and correlation coefficient is usually used to determine the adsorption capacity.

[0054] The pseudo-first order equation is expressed as follows: log (¾ - t) = log (

where, q_e and qt (mg.g ¹) depict the phosphate adsorption capacity at equilibrium and at time t (min), respectively, and ki (min ¹) depicts the pseudo-first order rate constant. The parameters q_e and ki can be calculated from the slope and intercept of the plot of log (qe-qt) versus t. [0055] The pseudo-second order equation is expressed as follows:

where, qt (mg.g ¹) is the amount of phosphate adsorbed at a certain time t (min), q_e (mg g ¹) is the amount of phosphate adsorbed at equilibrium, and k2 (g. mg ¹. min ¹) is the pseudo-second- order reaction rate. From the intercept and slope of the (t/qi) vs. I plot, k2 and q_e can be calculated, respectively.

[0056] The Weber and Morris intraparticle diffusion model was used to interpret the steps that occurred during the adsorption process and to determine whether intra-particle diffusion is the rate-limiting factor. The Weber and Morris intra-particle diffusion model is expressed as follows: qt = kpt° ⁵ + C (3) where, qt (mg.g ¹) is the amount of phosphate adsorbed at time t (min), k_P is the intraparticle diffusion rate constant (mg.g. min^{0 5}) and C is a constant. The values of k_P and C can be determined from the intercept and slope of the linear plot of qt versus t⁰⁵.

[0057] Well-established adsorption isotherm models (i.e. Langmuir and Freundlich) were used to fit the experimental data of LDH, such as the non-limiting example of Zn-LDH, towards PO4 removal. These equilibrium models provide an insight on the sorbate-sorbent binding interaction and the possible mechanisms of adsorption occurring. The Langmuir isotherm is based on the assumption that there exists maximum coverage based on monolayer adsorption on the active sites of the adsorption surface. A very important characteristic of the Langmuir model is the dimensionless constant ( RL ), generally known as the separation factor. The value of RL indicates whether adsorption is irreversible (RL=0), favorable (0<RL <1), linear (RL=1), or unfavorable (RL> 1). The Freundlich isotherm, on the other hand, represents a non-ideal and reversible adsorption process not limited to monolayer adsorption. This empirical model assumes a heterogeneous surface and that the amount of pollutant adsorbed increases with pollutant concentration.

[0058] The models and their parameter details are outlined in the Supplementary Information. For maintaining consistency, all phosphate in the text is represented as the orthophosphate P (V).

[0059] Example 8: Physical Morphology

[0060] The representative morphology of Zn-LDH is shown in FIG. la and FIG. lb.

[0061] The SEM Image in FIG. la shows the stacking of the Zn-LDH sheets.

[0062] FIG. lb depicts the irregularly shaped plate-like structure of the layered hydroxides particles. The elemental analysis of Zn-LDH confirms the presence of Zn in the Zn-LDH sample.

[0063] Example 9: Surface Area Analysis

[0064] The N2 adsorption-desorption curves are shown in FIG. 3. [0065] The surface area of the synthesized Zn-LDHs was found to be approximately 71.1 m²/g. The features of the isotherm resemble type IV adsorption with an H3-type hysteresis loop according to the classification of Brunauer, Deming, Deming and Teller (BDDT) and IUPAC, respectively. This is a characteristic feature of mesoporous solids and the H3 hysteresis loop also suggests the formation of slit-shaped pores by non-rigid aggregation of plate-like particles. The adsorption isotherm does not form a plateau at high P/Po values and does not show a limited uptake at a relatively high range.

[0066] Example 10: FT-IR Analysis

[0067] The FTIR spectra of Zn-LDH before and after adsorption is presented in FIG. 4.

[0068] The broad band at 3457 cm ¹ may be assigned to H-bonding stretching vibrations of the OH group involved in significant hydrogen bonding. The strong peaks at 1557 and 1398 cm ¹ may be attributed to the asymmetric and symmetric stretching vibrations of the carboxylate group from the acetate anion. The bending vibration of the interlayer water, typically expected to appear at 1600 cm ¹, can be seen to be masked by the strong band of the carboxylate asymmetric stretch. The strong peak at 1052 cm ¹ in the sample before adsorption may be assigned to OH bending vibrations. Both the 1052 cm ¹ and 921 cm ¹ peaks seem to have decreased immensely after phosphate adsorption which could be due symmetric substitution as the FT-IR wavelengths for the P-OH bond could also correspond to the mentioned two peaks.

[0069] Example 11: Effect of Contact Time and Adsorption Kinetics

[0070] Very fast phosphate adsorption kinetics can be observed in FIG. 5a.

[0071] More than 40-60% of phosphate was removed within the first 45-75 s and more than 60-80% was removed within the first 3-7 min of contact time, after which gradual phosphate adsorption was observed. In order to ensure equilibrium was met, an equilibrium contact time of 60 min was used for the remaining experiments. The kinetics data for the adsorption of phosphate using an LDH was fit using the pseudo-first order and pseudo-second order kinetics models and intraparticle diffusion model. The pseudo first order kinetics model (FIG. 5 b) gave a low correlation coefficient (R² < 0.917). The pseudo-second order model fit the data very well, and gave an R² value of 0.999. The calculated value of q_e from the pseudo- second order model was 8.92 mg/g for the 5 ppm solution. This is quite close to the equilibrium adsorption capacity of 8.85 mg/g, which confirms the validity of the pseudo-second order kinetics model.

[0072] In order to gain more insight on the rate-controlling step involved in phosphate adsorption on an LDH, the intra-particle diffusion model was used to fit the kinetic data. As shown in FIG. 5 d, the qt versus t° ⁵ plot exhibited multilinearity which indicates that the adsorption process is not dominated by intraparticle diffusion. In Table 1, the rate constant kpi was the greatest in the first linear stage. This first linear regime can be attributed to the diffusion in bulk to the external surface of an LDH driven by the initial phosphate concentration. In this stage, phosphate ions are adsorbed onto the active sites on the surface and edges of an LDH before being transported via intraparticle diffusion. The second linear regime denotes phosphate ions diffusing into the mesopores, or layers of the adsorbent, signifying that intraparticle diffusion is the rate-limiting step of this phase. The last regime denotes the slowest adsorption rate which is typically ascribed to the adsorption of the low phosphate ions in the solution to the less available adsorption sites on an LDH, reaching a saturation steady state.

[0073] The boundary layer thickness, Ci, offers an insight into the tendency of the phosphate ions to adsorb onto the adsorbent or remain in the solution. Typically, a higher Ci value indicates higher adsorption amounts. The higher Ci values for the second and third regime suggest that intraparticle diffusion and phosphate anion exchange with the host anions plays a major role in the overall adsorption process.

[0074] Table 1. Kinetic parameters for phosphate adsorption onto Zn-LDH.

Kinetic model q_e,ex_P (mg.g ¹) 8.85 Pseudo-first order qe.calc (mg.g ¹) 3.94 ki (min ¹) 0.092

R² 0.908

Pseudo-second order qe.calc (mg.g ¹) 8.92 k₂ (g. mg ¹. min ¹) 0.115

R² 0.999

Intra-particle diffusion kpi (mg.g.min⁰⁵) 1.64

Ci 3.51

R² 0.969 k_P2 (mg.g.min⁰⁵) 0.409

C₂ 6.55

R² 0.952 kp3 (mg.g.min⁰⁵) 0.063

Cs 8.21

R² 0.715 [0075] Example 12: Effect of Solution pH

[0076] The pH is an important variable affecting physicochemical reactions at the solid- liquid interface in adsorption systems. The phosphate adsorption capacity of Zn-LDH at pH values ranging from 2.09 to 9.90 was studied and the results are presented in FIG. 6a. At the low pH of 2.09, complete dissolution of the Zn-LDH adsorbent was observed after the adsorption experiments were conducted. This incident is also reflected in the data in FIG. 6a where a negligible amount of phosphate was adsorbed at pH 2. At pH 3, a considerable amount of phosphate is present in the solution as H3PO4 as illustrated in FIG. 6b, which explains the lower phosphate uptake. Phosphate adsorption was found to be the highest over a wide range of pH (4.98 to 9.01). At pH 4 to 6, phosphate mainly exists in the monovalent form, H2PO4^' (95 to 98%), while at pH values 7 to 9, phosphate exists as both H2PO4^' and HPO4^2' species. [0077] It is specifically important to note the pH_pzc (point of zero charge) in order to observe when the surface of Zn-LDH is positively and negatively charged. In general, when the zeta potential is positive, phosphate adsorption is favored, and, when it is negative, phosphate repulsion occurs. From the zeta potential plot (FIG. 6c), the pH_pzc of Zn-LDH was found to be approximately 8.3. At a pH < pH_pzc, the surface charge of Zn-LDH was found to be positive and at a pH > pH_pzc, the surface charge was negative.

[0078] While the surface charge of an LDH could interfere with the adsorption of phosphate, phosphate removal was still observed when the surface charge of an LDH was negative (pH> 8.3). This phenomenon indicates that electrostatic attraction was not the major mechanism for phosphate adsorption on an LDH. The increase in pH after phosphate adsorption at low initial pH values (2.09 to 4.98) and the persisting phosphate removal at higher initial pH support the ligand exchange mechanism (exchange of phosphate ions with hydroxyl or acetate ions). Therefore, ligand exchange and electrostatic interactions could be considered as the major mechanisms for phosphate adsorption on an LDH. Based on the pH results, it can be said that an LDH can be used at a wide pH range whilst maintaining a high adsorption capacity, making it very attractive for practical purposes.

[0079] Example 13: Adsorption Isotherms

[0080] Adsorption isotherms of phosphate on Zn-LDH are shown in FIG. 7.

[0081] The adsorption isotherm model constants and correlation coefficients derived from the fitting of the experimental data with the Langmuir and Freundlich models are presented in Table 2. The Freundlich model (R² ~ 0.998) provided a better description of the isotherm data relative to the Langmuir model (R² ~ 0.974) in terms of the higher correlation coefficient. These findings suggest that the adsorption of phosphate on an LDH tends towards multilayer adsorption on heterogeneous active sites rather than monolayer adsorption. This provides further evidence that electrostatic interaction between the LDH surface and the negatively charged phosphate species did not play a dominant role in the adsorption process as adsorption as it becomes less effective beyond monolayer adsorption. It could be said here that the adsorption of phosphate species was likely not limited to the external surface but the intercalation of the phosphate into the interlayer of the layered structure.

[0082] Table 2. Langmuir and Freundlich isotherm parameters for phosphate adsorption on Zn-LDH.

Equilibrium Zn-LDH adsorption

_ models _

Langmuir

Xm (mg.g ¹) 205.99 b 0.006

R² 0.989

Freundlich

KF (mg g- 10.49 0.417

0.998

[0083] Table 3. Comparison of various published adsorbents for phosphate removal.

[0084] Example 14: Wastewater Treatment [0085] Table 4. Phosphate removal from TSE.

[0086] It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.

Claims

CLAIMS The invention is claimed as follows:

1. An adsorbent for water treatment, the adsorbent comprising a layered double hydroxide (LDH) material comprising M²⁺ _(i-X) M³⁺ _x (OH)2 (Aⁿ) _x/n x ybhO, wherein M²⁺ are divalent cations, M³⁺ are trivalent cations, and Aⁿ are interlayer anions.

2. The adsorbent of claim 1, wherein where M²⁺ are at least one of Mg²⁺, Zn²⁺, or Ca²⁺; the M³⁺ are at least one of Al³⁺, Fe³⁺, or Mn³⁺; and the Aⁿ are at least one of CO3^2' or NO3 ^'.

3. The adsorbent of claim 1, wherein the adsorbent comprises Zn-LDH.

4. The adsorbent of claim 3, wherein a surface area of the Zn-LDH is approximately 71.1 m²/g.

5. The adsorbent of claim 1, wherein a surface of the LDH material has an isotherm resemble type IV adsorption with an H3-type hysteresis loop.

6. A method of synthesizing the LDH material of claim 1 by a facile synthesis, the method comprising: adding a NaOH solution dropwise to a zinc acetate solution until the pH of the solution reaches a range of 3-10; mixing at 35-55°C at a rate of 300-500 rpm; aging and stirring a slurry; washing and filtering a precipitate with deionized water; and drying the precipitate.

7. A method for phosphate adsorption via a M²⁺ _(ΐ-c₎ M³⁺ _x (OH)2 (Aⁿ) _x/n x yH20, the method comprising: placing 0.5 to 200 g/L of the M²⁺ _(i-x> M³⁺ _x (OH)2 (Aⁿ) _x/n x yH20 adsorbent in a centrifuge tube with a 5 mg/L phosphate solution; wherein M²⁺ are divalent cations, M³⁺ are trivalent cations, and Aⁿ are interlayer anions adjusting pH of the solution with 0.0001-1.0 mg L ¹ HC1 or NaOH to a pH of between 3-10 in order to resemble the pH of real treated wastewater; placing all samples in polyethylene centrifuge tubes and shaking at a rate of 250-450 rpm using a mechanical shaker table; and examining an effect of adsorbent dosage, contact time, solution pH, initial phosphate concentration, and co-existing ions.

8. The method of claim 11, wherein 40-60% of phosphate is adsorbed within the 45-75 seconds.

9. The method of claim 11, wherein at least 60-80% of phosphate was adsorbed within 3- 7 min of contact time.

10. The method of claim 11, wherein an adsorption equilibrium time to reach the M²⁺-, M³⁺-, and A^n— LDH equilibrium adsorption capacity of phosphate is approximately 30-90 min.

11. The method of claim 11, wherein the pH is adjusted to between 4.98 to 9.01.

12. A method of:: placing 0.1-1.0 g L ¹ of a M²⁺(i-_X) M³⁺ _x (OH)2 (Aⁿ) ^n x yH20, adsorbent in a bottle with a 1-10 mg/L phosphate solution; wherein M²⁺ are divalent cations, M³⁺ are trivalent cations, and Aⁿ are interlayer anions; adjusting the pH to a value of 3-10 using a contact time of 30-90 min; filtering the M²⁺(i-_x> M³⁺ _x (OH)2 (Aⁿ) _x/n x yH20 using a vacuum filter; washing at least one time; and drying.

13. The method of claim 16, wherein the M²⁺(i-_x> M³⁺ _x (OH)2 (Aⁿ) _x/n x yH20 with at least one regenerant; wherein the regenerant is at least Acetic Acid and NaOH, wherein the Acetic Acid is between 0.00025 M Acetic Acid to 0.0005 M Acetic Acid and the NaOH is between 0.1 M NaOH, to 0.01 M NaOH.