ELUTION BEHAVIOR OF PHOSPHATE CONTAINED IN Mg/Fe AND Zn/Fe LAYERED DOUBLE HYDROXIDES

Advanced Materials Development and Performance (AMDP11) International Journal of Modern Physics: Conference Series Vol. 6 (12) 156-161 World Scientific Publishing Company DOI: 1.112/S11951318 ELUTION BEHAVIOR OF PHOSPHATE CONTAINED IN Mg/Fe AND Zn/Fe LAYERED DOUBLE HYDROXIDES Masashi Kurashina *, Tomohiro Amatsu, Takaaki Ochi, Nozomi Ohigashi, Eiji Kanezaki Department of Chemical Science and Technology, Faculty of Engineering, University of Tokushima, 2-1 Minamijosanjima-cho, Tokushima, 77-856, Japan * kurasina@chem.tokushima-u.ac.jp Layered double hydroxide (LDH) is a layered hydroxide and exchangeable anion is intercalated in its interlayer. Application of the LDH as a controlled-release material of interlayer anions has become of interest, thus it is important to clarify the elution behavior of interlayer anions. We synthesized hydrogenphosphate-intercalated Mg/Fe and Zn/Fe LDH and elution of phosphate from these LDH were tested in deionized water, sodium chloride solution, sodium sulfate solution, and sodium carbonate solution. For Mg/Fe LDH, the amount of eluted phosphate increased with time and reached to maximum that increased as higher concentrate solution was used. The elution of phosphate from Mg/Fe LDH could be described by the pseudo second-order equation. This elution behavior was explained as ion-exchange reaction of phosphate with sulfate or carbonate in tested solution by means of kinetic simulation using Runge-Kutta method. In the eluted solution, metal ions contained in the LDH were detected and its amount depended on ph of the tested solution, that is, amounts of eluted Mg and Zn ions were small at higher ph (ca. 1) for Mg/Fe and Zn/Fe LDH respectively, but large amount of Zn ion was detected when 2.3 mol l 1 carbonate solution (ph = 13) was used. Thus elution of phosphate was caused by two main reactions: ion exchange and decomposition of the LDH. Keywords: Layered Double Hydroxide, Phosphate, Ion Exchange, Elution. 1. Introduction Layered double hydroxide (LDH) is a kind of anionic clays. It is a layered hydroxide contained two kind of metals and the general formula is represented as M(II) 1 x M(III) x (OH) 2 (A y ) x/y nh 2 O with M(II), M(III), and A y are represent a divalent and trivalent metal, and a interlayer anion, respectively. 1 The formula is abbreviated to M(II)/M(III)-A LDH. The positive charge of the hydroxide layer M(II) 1 x M(III) x (OH) 2 x+ is compensated by the negative charge of the exchangeable interlayer anion. In recent years, LDHs have been investigated to apply its interlayer anion as a useful material such as a drag delivery system. To achieve it, it is necessary to control the release ratio of the interlayer anion, some of bactericide 2 and pesticide 3 had been investigated. Phosphate is one of the useful materials that are essential element for crop plants and exhaustible resource. Many phosphate-intercalated LDHs had been synthesized but release ratios of phosphate were unmentioned. In this report, we studied Mg/Fe-HPO and Zn/Fe-HPO 156

Elution Behavior of Phosphate Contained in Mg/Fe and Zn/Fe Layered Double Hydroxides 157 LDH, and the elution of phosphate from the LDH was tested in deionized water, sodium chloride solution, sodium sulfate solution, and sodium carbonate solution. 2. Experimental Section The Mg/Fe-NO 3 LDH was synthesized by reference to the literature. The Mg/Fe-NO 3 LDH (.1 g) was added to the KH 2 PO solution ( mmol l 1, 125 ml, adjusted at ph = 9.8-1. using KOH pellets) and the suspension was stirred for 2 h at 3 C. The suspension was filtered and the solid washed with hot ( C) deionized water. After dried under vacuum,.68 g of Mg/Fe-HPO LDH was yielded. The Zn/Fe-Cl LDH was synthesized by reference to the procedure of Zn/Fe-SO LDH in the literature. 5 The Zn/Fe-Cl LDH (.5 g) was added to the KH 2 PO solution (13 mmol l 1, 2 ml, adjusted at ph = 1 using NaOH pellets) and the suspension was stirred for a day at 3 C. The suspension was filtered and the solid washed three times with deionized water. After air dried,.6 g of Zn/Fe-HPO LDH was yielded. Elution tests were carried out as follows. The 1 ml eluting solvent was warmed at 3 C in an isothermal bath. The 1. mg of Mg/Fe-HPO or Zn/Fe-HPO LDH was added to the solvent and stirred at 3 C. After stirred for the predetermined time, all amount of the mixture was filtered using Whatman 5 filter paper. In the eluted solution, contents of Mg, Zn, and Fe were measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES), and P was measured by molybdenum blue method. 3. Results and Discussions We synthesized Mg/Fe-HPO and Zn/Fe-HPO LDH by the ion-exchange of NO 3 in Mg/Fe-NO 3 LDH and Cl in Zn/Fe-Cl LDH with HPO ion, respectively. Elemental analyses of each sample revealed that the metals and phosphate contents of the Mg/Fe- HPO LDH and Zn/Fe-HPO LDH were not constant by synthesis batch. The estimated formulas and mass fractions of each sample were listed in Table 1. The differences of the mass fractions arise from a little difference of the Mg(II)/Fe(III) ratio, the amount of the ion-exchanged phosphate, and the hydration water. On the elution test, the percentages of eluted components were calculated based on the elemental analysis data. At the synthesis of Mg/Fe-HPO and Zn/Fe-HPO LDH, main component (97.2%) in the phosphate solution at ph = 1 is HPO because pk a of H 3 PO are 1.83, 6.3, and 11.5. 6 Thus phosphate contained in the Mg/Fe-HPO and Zn/Fe-HPO LDH were considered as hydrophosphate HPO. The crystal structures of synthesized samples were determined by powder X-ray diffraction (XRD). Synthesized Mg/Fe-NO 3, Mg/Fe-HPO, Zn/Fe-Cl, and Zn/Fe-HPO LDH are crystalize in the rhombohedral system with a = 3.12 Å and c = 2. Å, a = 3.11 Å and c = 23.8 Å, a = 3.12 Å and c = 23.7 Å, and a = 3.13 Å and c = 23. Å, respectively. In general, the basal spacing of LDH (one-third of c axis) depend on the size of the interlayer anion. Although the anion size of HPO is larger than both NO 3 and Cl, the interlayer spacing of Mg/Fe-NO 3 and Zn/Fe-Cl LDH were very similar to that of Mg/Fe-HPO and Zn/Fe-HPO LDH, respectively. Nevertheless no change of the

158 M. Kurashina et al. interlayer spacing, the amount of the phosphate revealed by the elemental analysis was too large to be only caused by the surface adsorption. We deduced the phosphate contained Mg/Fe-NO 3 and Mg/Fe-HPO LDH are intercalated in interlayer spacing with rearrangement of the HPO molecular packing. Table 1. The estimated formulas and mass fractions of synthesized LDH. Mass fraction Abridgment / Formula Mg or Zn Fe HPO Mg/Fe-HPO (1) / Found 17.% 1.8% 8.5% Mg.73 Fe.27 (OH) 2 (HPO ).9 (NO 3 ).9 1.25H 2 O Calcd. 17.1% 1.6% 8.3% Mg/Fe-HPO (2) / Found 19.1% 17.9% 5.9% Mg.71 Fe.29 (OH) 2 (HPO ).6 (NO 3 ).17.36H 2 O Calcd. 19.1% 17.9% 6.% Zn/Fe-HPO (3) / Found.6% 1.6% 9.6% Zn.78 Fe.22 (OH) 2 (HPO ).11.38H 2 O Calcd..5% 1.7% 9.2% Deionized water,, and SO aqueous solutions (., 8.85 mmol l 1 ) were used for elution tests of Mg/Fe-HPO (1) LDH. Time dependence of the eluted phosphate is shown in Fig. 1 as the concentration and the eluted percentage (eluted mole number 1 / mole number contained in 1. mg of the LDH). For every solution, the amount of eluted phosphate increased with time and reached to maximum. The maximum amount of the eluted phosphate increased as the ion concentrated in the elution solvent, 8.85 mmol l 1 solution especially eluted 9.% of phosphate in 2 h. Carbonate is known that it has specific affinity for LDH 7 thus it efficiently exchanged with the phosphate in LDH. The XRD patterns of filtered solids after the elution tests showed nearly same that of before the tests, except for low crystallinity. The interlayer anions cannot be identified by the basal spacing because Mg/Fe LDH showed same XRD patterns regardless of interlayer anions, thus we deduced the filtered solids were mixture of unreacted Mg/Fe- HPO and ion-exchanged Mg/Fe- or SO LDHs. Elution concentration of HPO / mol l 1 (a) 8 1 2 3 5 6 12 7 2 8 Elution time / h 1 8 Elution percentage of HPO / % Elution concentration of HPO / mol l 1 (b) 8 1 2 3 5 6 12 7 2 8 Elution time / h 1 8 Elution percentage of HPO / % Fig. 1. Time dependence of the elution of HPO in Mg/Fe-HPO (1) LDH. The elution solvents are (a) deionized water: *, aq. ( :. mmol l 1, : 8.85 mmol l 1 ), and (b) SO aq. ( :. mmol l 1, : 8.85 mmol l 1 ). Solid line represents simulated behaviors. These elution behaviors could be described by the pseudo second-order equation that has been often used to explain the adsorption rates of clays. 8, 9 The pseudo second-order kinetic rate equation is expressed as: d d e 2 (1)

Elution Behavior of Phosphate Contained in Mg/Fe and Zn/Fe Layered Double Hydroxides 159 where q t and q e are the amounts of phosphate eluted at time t and equilibrium ( mol l 1 ), respectively, and k is the pseudo second-order rate constant for the adsorption process ( mol 1 l h 1 ). The parameters in the pseudo second-order equation were calculated from Eq. (1) and are presented in Table 2 with coefficient of determinations R 2. Table 2. The parameters of the pseudo second-order equation and coefficient of determinations R 2. Elution solvent q t (t = 2 h) / mol l 1 q e / mol l 1 k / mol 1 l h 1 R 2 Deionized water 61.8 62..17 1.. mmol l 1 62.5 62.7.18 1. 8.85 mmol l 1 67.7 67.8.17 1. SO. mmol l 1 66.7 67..9 1. 8.85 mmol l 1 83.5 83.7..999 Magnesium ion as well as phosphate was eluted in the elution test of Mg/Fe-HPO (2) LDH. In 2 h elution-tested solution, determined percentages of eluted magnesium ion are presented in Table 3 with ph of the elution solvents. The amount of eluted magnesium indicates the decomposition of LDH. The Fe ion was below measurable limits in the elution test because neutral or basic ph of the elution solvent make Fe(III) ion into insoluble Fe(OH) 3. The dissolved amount of the LDH was large when deionized water and SO aq. were used and small when was used. This dissolution mainly caused by ph of the elution solvent because eluted magnesium was decreased to 17.2% at high ph = 1. (adjusted using NaOH) compare to 32.5% at low ph = 6. in case of SO aq. (8.85 mmol l 1 ). Table 3. Percentages of eluted magnesium ion with simulated parameters in 2 h elution-tested solution. Elution solvent ph Elution of Mg Calculated k S / h 1 Simulat ed elution of Mg Deionized water 6.1 29.% 2.5 1 2 31.1%. mmol l 1 9.8 18.2% 1.1 1 2 15.7% 8.85 mmol l 1 1. 12.2% 1.1 1 2 11.7% SO. mmol l 1 6.3 3.% 2.5 1 2 31.1% 8.85 mmol l 1 6. 32.5% 2.5 1 2 29.8% 8.85 mmol l 1 adjusted ph using NaOH 1. 17.2% 2.5 1 2 One of the reaction model to explain the pseudo second-order equation is single-site model that is assumes that an adsorption ion is adsorbed on one site of the adsorbent surface. 1, 11 The following equation describes the adsorption process: a A A S d A A S, d A d a A A S d A A S (2) in which A, S, A S, k a (A), and k d (A) represent the adsorption ion, the vacant site, the occupied site, and adsorption and desorption rate constants of A, respectively. The rate of attachment of A to S is directly proportional to the product of the concentration of A in aqueous solution, [A] and the concentration of vacant sites [S]; while the rate of

1 M. Kurashina et al. detachment is only directly proportional to the concentration of A attached on site, [A S]. We suppose desorption of the phosphate in LDH is expressed in the same manner as: a HPO HPO 2 S HPO S, d HPO d d HPO a HPO HPO S d HPO HPO S (3) in which k a (HPO ), and k d (HPO ), [HPO ] represent the adsorption and desorption rate constants of HPO, and the concentration of HPO, respectively. Additionally, we assume the rate of decomposition of LDH is only directly proportional to the concentration of vacant sites, [S]. The rate of concentration change of vacant sites [S] is expressed as following equation with consideration about Eq. (2) and (3): S d S decomposed LDH, d S d d A d S S d d HPO d in which k d (S) represent the decomposition rate constants of S. By the substitution A with the anion in the elution solvent, the elution mechanism of Mg/Fe-HPO is described using the Eq. (2)-() thus these elution behaviors were simulated. These differential equation were calculated by Runge-Kutta method with the initial conditions [S] = [A S] = [HPO ] = mol l 1 and [HPO S] = 88.5 mol l 1. The [A] is concentration of the anions in the elution solvent, for the deionized water [A] is always mol l 1. The calculated parameters in these equations are presented in Table. and simulated elution behaviors are displayed in Fig. 1. Estimated decomposed percentage of LDH in these elution tests are listed in Table 3. This reaction mechanism represents the elution behaviors with the parameters only depended on the kind of the adsorption anions, not its concentration. The small value of the k d ( ) in contrast with k d (HPO ) and k d (SO ) suggests specific affinity of carbonate for LDH. We suppose the value of the k d (S) have relations with ph of the elution solvent. Actually, the vacant site S cannot be isolated because S is a cationic site of the surface of LDH, thus S is surrounded by anions, such as OH. Therefore, high concentration of OH should stabilize S. As a result, the value of k d (S) is smaller at carbonate than at sulfate and deionized water. Table. Calculated parameters in the elution test of Mg/Fe-HPO LDH. HPO SO k a / mol 1 l h 1 5. 1 3 2.5 1 5.5 1 5 k d / h 1 2.8 1.2 3.8 1 1 () For Zn/Fe-HPO (3) LDH, determined percentages of eluted phosphate and zinc ion after 2 h elution test are presented in Table 5 in deionized water, NaCl (.5 mol l 1 ), and (.25,.1,.5, 2.3 mol l 1 ) aqueous solutions with ph of the eluted solutions. In deionized water and NaCl aq. elution of phosphate were mainly caused by decomposition of the Zn/Fe-HPO LDH. As well as LDH patterns were detected, XRD analysis of filtered solids revealed generation of Zn 3 (PO ) 2 H 2 O for deionized water and an unidentified solid that has d =.9 Å diffraction for NaCl aq. These decomposed products were not detected in aq. and eluted phosphate and zinc ion were increased with concentration higher. Different from magnesium, zinc ion is dissolved in

Elution Behavior of Phosphate Contained in Mg/Fe and Zn/Fe Layered Double Hydroxides 161 basic solution as well as acidic, thus high ph did not suppress the decomposition of the LDH. In the case of 2.3 mol l 1 (nearly saturated) solution, 67.6% of LDH was dissolved due to high ph = 13.2 although 99.9% of phosphate was eluted. Thus the value of (HPO Zn) means pure contribution of ion exchange. Conditions of compatibility in these solutions were.5 mol l 1 aq. with 66.3% ion exchanged. Table 5. Eluted phosphate and zinc ion after 2 h elution test of Zn/Fe-HPO (3) LDH.. Conclusion Elution of phosphate from Mg/Fe-HPO and Zn/Fe-HPO LDHs were caused by two main reactions: ion exchange and decomposition of the LDH. Carbonate promoted the ion exchange of phosphate. The decomposition of the LDH depended on ph: basic condition (ph = 1-12) suppressed the decomposition of the LDH though too high ph = 13 dissolved the Zn/Fe-HPO LDH. Elution behavior and stable condition of LDH vary according to metal ions, thus investigation of optimal condition is important to apply the LDH to a controlled-release material of interlayer anions. Acknowledgments Elution solvent ph Elution of HPO Elution of Zn (HPO Zn) Deionized water 9.7% 8.5% 1.2% NaCl.5 mol l 1 17.3% 1.1% 7.2%.25 mol l 1 1.8 5.8%.8% 5.%.1 mol l 1 11.2 55.7% 2.% 53.7%.5 mol l 1 11.5 76.7% 1.% 66.3% 2.3 mol l 1 13.2 99.9% 67.6% 32.3% This study was supported by the Japan Science and Technology Agency project to develop "innovative seeds". References 1 V. Rives, Layered Double Hydroxides: Present and Future. (Nova Science Publishers, Huntington, N.Y., 1). 2 Q. Zhenlan, Y. Heng, Z. Bin, and H. Wanguo, Colloids Surf. Physicochem. Eng. Aspects 38, (1-3), 16 (9). 3 D.-p. Qiu, and W.-g. Hou, Colloids Surf. Physicochem. Eng. Aspects 336, (1-3), 12 (9). W. Meng, F. Li, D. G. Evans, and X. Duan, Mater. Res. Bull. 39, (9), 1185 (). 5 W. Meng, F. Li, D. G. Evans, and X. Duan, J. Porous Mater. 11, 97 (). 6 Kagaku Binran (Handbook of Chemistry), Pure Chemistry 5th Ed. (Maruzen, Tokyo, ). 7 N. Iyi, and T. Sasaki, J. Colloid Interface Sci. 322, (1), 237 (8). 8 E. Bulut, M. Özacar, and İ. A. Şengil, J. Hazard. Mater. 15, (1-3), 613 (8). 9 A. Behnamfard, and M. M. Salarirad, J. Hazard. Mater. 17, (1), 127 (9). 1 C.-I. Lin, and L.-H. Wang, J. Chin. Inst. Chem. Eng, 39, (6), 579 (8). 11 S. Azizian, J. Colloid Interface Sci. 276, (1), 7 ().