Binary Zr-Fe Hydroxide as Phosphate Adsorbent

Transcription

1 328 J. ION EXCHANGE Article Binary Zr-Fe Hydroxide as Phosphate Adsorbent Ramesh Chitrakar*, Satoko Tezuka, Akinari Sonoda*, Kohji Sakane, Kenta Ooi, and Takahiro Hirotsu Health Technology Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Hayashi-cho, Takamatsu , Japan. (Manuscript submitted March 20, 2007; accepted May 15, 2007) Abstract Binary Zr-Fe hydroxides with Zr/Fe molar ratios of 0.33, 1.0 and 3.0 were synthesized by a coprecipitation method at room temperature. The materials are amorphous with high surface area ( m2 g-1). Phosphate adsorptive properties of the materials were investigated by a batch method. Distribution coefficients (Kd) of oxo-anions at ph 9 from a mixed anion solution show the selectivity sequence as follows: Cl-, NO3-, SO42- ƒ HPO42-. The phosphate uptakes from a single electrolyte at ph 2 are 75 mg-p g-1 for the present material, 50 mg-p g-1 for amorphous ZrO(OH)2.1.5H2O, and 28 mg-p g-1 for low crystalline akaganeite (FeOOH EH2O). The results suggest that binary Zr-Fe hydroxide can act as phosphate adsorbent. Key words: Phosphate, adsorption, iron, zirconium, seawater, wastewater 1 Introduction Phosphorus is found in the environment as phosphate. Phosphate is released to aquatic environments due to discharge of domestic wastewater, mining, industrial, and agricultural uses. The increasing phosphate concentration in surface water raises the growth of organism such as algas, which use large amount of oxygen and prevents sunlight entering water. This phenomenon is usually known as eutrophication, which has relevant effects on water quality [1]. In the medical field for patients with chronic renal failure, control of serum phosphate is important. High serum phosphate level (known as hyperphosphatemia) normally results if the kidneys do not function properly [2]. Inorganic substances like aluminum hydroxide, calcium carbonate, and lanthanum carbonate known as oral phosphate binders, are usually taken with meals to bind any phosphate that may be present in the ingested food. Hydroxides of various polyvalent metals such as AI(III), Fe(III), La(III), Mn(IV), Sn(IV), Ti(IV), and Zr(IV) are amphoteric ion exchange materials, showing both cation and anion exchange reactions depending on the ph of solution [3]. Different adsorbents such as layered double hydroxides (LDHs) [4,5], composites of ƒáalumina/potassium aluminosilicate gel [6], phosphoric acid resin loaded with Zr(IV) [7], and Cu(II) loaded polymeric ligand exchanger [8] have been studied for phosphate adsorption from aqueous solutions. In our previous studies, [9,10] we carried out the selective adsorption of phosphate from seawater on ZrO(OH)2, and low crystalline goethite (ƒ -FeOOH), and akaganeite (R-FeOOH). There has been only one study reported for the adsorption of phosphate on binary Fe-Al hydroxide [11]. The aim of the present work is to prepare binary Zr-Fe hydroxides with different Zr/Fe molar ratios and to study their high phosphate adsorption properties. (184)

2 Vol.18 No.4 (2007) Experimental 2.1 Synthesis of materials A 0.1 M(M= mol dm-3) NaOH solution (ca.1.4 dm3) was added to a mixed solution of 0.1 MZrOCl2 and 0.1 M FeCl3 (total volume 600 cm3) at Zr/Fe molar ratios of 0.33, 1.0 and 3.0. The suspension was stirred magnetically during the addition of NaOH solution till the ph reached 10, the addition time being 30 min. The suspension was settled for 1 h at room temperature, centrifuged and washed with deionized water till neutral, and finally dried at 40 Ž. The material was ground and sieved to ƒêm. Materials are abbreviated as Zr3Fe, ZrFe and 3ZrFe for Zr/Fe molar ratios of 0.3,1.0 and 3.0, respectively. 2.2 Physical measurements Powder X-ray diffraction analysis was carried out using a Rigaku X-ray diffractometer (RINT 1200) equipped with a Ni-filtered Cu Kƒ radiation (ƒé = nm) and a graphite monochromator. TG-DTA was performed on MAC Science Thermal Analyzer (2000 TG-DTA) at a heating rate of 10 Ž min-1 in air. BET surface area was determined by nitrogen gas adsorption with Quantachrome Type 1-C after degassing the material at 150 Ž for 2h. 2.3 Chemical analysis Material (0.05 g) was dissolved in 0.5 MHCl solution. Zr and Fe contents in the solution were analyzed by a Seiko Instrument Inc. ICP atomic emission spectrometer (SPS 7800). Phosphate was determined with a portable colorimeter, Model DR/700, HACH Company, USA. The analysis was performed according to the procedure described in the manual. 2.4 Point of zero charge Point of zero charge (PZC) was determined by the titration method as described [12]. Material (0.10 g) was mixed with 100 cm3 solution containing a known concentration of NaNO3, and HNO3 or KOH at different ph, and the suspension was equilibrated for 1 d at 25 Ž. After measuring the final ph of solution, uptake of H+ or OW on the material was calculated by taking the difference between test suspension and blank titration volumes at particular ph values. 2.5 Distribution coefficient (Kd) Distribution coefficient (Kd) values of oxo-anions were determined by a batch method using material (0.10 g) immersed in a mixed solution (10 cm3) of NaCl, NaNO3, NazSO4, Na2CO3 and NaHZPO4 (2 x 10-3 M of each anion) at room temperature for 3d. Anions were determined by ion chromatography. Kd value was calculated as follows: Kd (cm-3 g-1) =anion uptake (mg g-1 of solid) /final anion concentration (mg cm-1 of solution). 2.6 Adsorption of phosphate Phosphate adsorption experiments were performed by stirring a known weight of material in a known volume of solution of desired phosphate concentration by a batch method at room temperature for 3d. Model wastewater was prepared as described (phosphate = 2 mg-p dm 3, alkalinity =100 mg dm 3 as NaHCO3, Ca = 25 mg dm 3 as CaCl2 and ph = 9.0) [13]. Phosphate-enriched seawater was prepared by addition of NaH2PO4 solution to seawater to make a final phosphate concentration of 0.33 mg-p dm Characterization of materials 3 Results and Discussion X-ray diffraction patterns of uncalcined materials with different Zr/Fe molar ratios, and their heat treated materials at 700 Ž are shown in Fig. 1. Unclacined materials do not exhibit any diffraction peak, suggesting the materials to be amorphous. X-ray diffraction patterns of heat-treated 3ZrFe at 700 Ž exhibits only peaks ( 185 )

3 330 J. ION EXCHANGE characteristics of tetragonal phase of ZrO2. Other heat-treated ZrFe and Zr3Fe exhibit the peaks of mixed phases corresponding to tetragonal ZrO2 and hematite (Fe2)3). Uncalcined material do not show the presence of akaganeite phase (ƒà-feooh) even in iron rich material Zr3Fe, suggesting the materials are not a mixture of zirconium hydroxide and iron hydroxide, because, ƒà-feooh phase is obtained if FeCl3 solution is precipitated with NaOH solution at ph 10. Fig. 2 shows TG-DTA curves of materials. There is a continuous weight loss up to 400 Ž with an endothermic peak around 80 Ž due to loss of physically adsorbed water molecules and hydroxide Fig. 1. X-ray diffraction patterns. Fig. 2. TG-DTA curves. groups of the material. The crystallization temperature of Zr3Fe (691 Ž) is much higher than that of single component ZrO(OH)2 (470 Ž). The crystallization temperature of other materials shifts to a lower temperature with increase in Zr/Fe molar ratio. Chemical compositions of synthesized materials are given in Table 1. The Zr/Fe molar ratios in the starting solutions are comparable with those in the solid products, which indicate that all the metal ions are precipitated during the formation of binary hydroxides. Chemical formulae of the materials are based on balancing the electrical charges of Zr4+ and Fe3+ with 02 and OW. Materials exhibit high surface area ( m2 g-1), which may be due to amorphous phase. The PZC values of three materials are in the range of Distribution coefficient (Kd) Kd values of oxo-anions are given in Table 1. All materials show the selectivity sequence Cl-, NO3-, SO42- ƒhpo42- in concentration range of 2 x 10-3 M The materials do not show affinity for Cl-, NO3 and SO42 ions at ph A similar selectivity sequence has been observed on akaganeite [l0] Rate of phosphate adsorption The rate of phosphate adsorption on materials is slow as it requires 2d to attain equilibrium. Diffusion of phosphate ions into the lattice or matrix of the material requires a longer time to attain equilibrium. The rate is much fast on akaganeite [10] to attain equilibrium in 8h as compared to 7d on ZrO(OH)2 [9]. Table 1. Chemical compositions of binary Zr-Fe hydroxides and their Kd values. amolar ratio in solution, parenthesis denote for solid. bbet surface area. point of zero charge. (-) = not adsorbed. ( 186 )

4 Vol.18 No.4 (2007) Effect of ph on phosphate adsorption Trivalent and tetravalent metal hydroxides are amphoteric ion-exchangers showing anion exchange in acidic solution and cation exchange in basic solution. Phosphate uptake is strongly affected by ph of solution, ionic species of phosphate, and surface of material. ph titration curves of Zr3Fe, ZrFe and 3ZrFe are shown in Fig 3. ph titration data of ZrO(OH)2 and f3-feooh are alsc included for comparison [9,10]. At ph 2, the order ol phosphate uptake on different materials is,3-feooh ƒ ƒ ZrO(OH)2 ƒzr3fe ƒzrfe ƒ3zrfe. This shows that present materials are strong anion exchangers, because the presence of high concentration of Cl- ion does not suppress the phosphate uptake at ph 2. All materials show decrease it phosphate uptake with increase in ph of the solution. At ph 4, phosphate uptake on ZrO(OH)2 is comparable with binary hydroxides. But phosphate uptake is very low on,ƒà- FeOOH, suggesting a weak anion exchanger. 3.4 Phosphate adsorption isotherm Fig. 3. ph dependence of phosphate uptake on different adsorbents Weight = 0.10 g, vol. =100 cm3, concn. =100 mg-p dm 3, contact time = 3d. Adsorption of phosphate ion at equilibrium can be expressed in terms of adsorption isotherms. Fig. 4 represents results of phosphate adsorption isotherm plotted as phosphate uptake vs. equilibrium phosphate concentration. The phosphate uptake increases with increase in phosphate equilibrium concentration, and finally reaches nearly a saturation ( 90 mg-p dm 3). The curve of adsorption isotherm indicates that the material has higher affinity for phosphate ion at lower concentration ( ƒ10 mg-p dm3). The experimental data fits better to Freundlich model as compared to Langmuir. Freundlich model is applicable for heterogeneous surface of the material, and predicts an increase in the amount of the anion adsorbed on the surface Fig. 4. Phosphate adsorption isotherm on ZrFe. Weight = 0.10 g, vol. =100 cm3, concn. = mg-p dm 3, equilibrium ph = 6.5, contact time = 3d. of solid with increasing equilibrium concentration of the anion in solution. 3.5 Effects of competitive anions on phosphate adsorption Effects of Cl-, 5042-, HCO3- and CO32- ions at different concentrations (10 `104 mg dm-3) on phosphate adsorption from a solution containing 10 mg-p dm 3 were studied (Fig. 5). Phosphate adsorption increases from 50% to 70% with an increase in Cl- concentration from 10 to 100 mg dm3, and adsorption further increases to 100% at Cl- concentration of 5 ~103-1 ~104 mg dm-3. In the presence of also ( mg dm-3), phosphate adsorption is 78%, which increases to 100% with further increase in concentration. The equilibrium ph of the solution after phosphate adsorption is nearly constant ( ) on both cases. This clearly indicates that the presence of CF and ions in solution enhances the phosphate adsorption. The reason may be due to the exchange of CV or ions with the surface hydroxide groups of the material in the initial stage, and then exchange of phosphate with surface Cl- or SO42- ions also, resulting higher phosphate adsorption. Similar observations have been reported for phosphate adsorption on binary Fe-Al hydroxide in the presence of Cl- ions [13]. ( 187 )

5 332 J. ION EXCHANGE The presence of HCO3-, and CO32- ions slightly affects the phosphate adsorption at initial concentration of 100 mg dm 3, and mg dm 3, respectively. With further increase in HCO3-, and CO32- concentrations, there is no appreciable decrease in phosphate adsorption. A similar pattern of arsenate adsorption on calcined Mg- Al LDH in the presence of CO32- ion has been reported, and the reason is not clarified [14]. Fig. 5. Effects of CL- HCO3-, CO32- and SO42 - anions on phosphate adsorption with ZrFe. Weight = 0.10 g, vol. =100 cm3, concn. =10 mg-p dm 3, contact time = 3d. 3.6 Adsorption of phosphate from seawater and wastewater Phosphate uptakes were studied by stirring 0.04 g of material in 2 dm3 seawater (0.33 mg-p dm3), and 0.05 g materh in 1 dm3 model wastewater (2 mg-p dm3). The uptake results on materials with different Zr/Fe molar ratios are shown in Fig. 6. The amount of phosphate uptake is nearly same (10 mg-p g-1) frorr seawater on three materials at equilibrium ph 8.1. In the case c wastewater, the uptake is little greater at Zr/Fe=1 than at Zr/Fe=O.3 and 3.0. This may be due to a slight decrease in the equilibrium ph 8.5 for Zr/Fe=1 as compared to ph 8.7 for Zr/Fe=0.3 and Zr/Fe=3. Fig. 6. Phosphate uptake on ZrFe materials. 3.7 Mechanism of phosphate adsorption Simple anions such as Cl-, NO3- and exchange with hydroxide groups of the material through electrostatic interaction as follows: M-OH + X- + H+ -+ MOH2+...X- (in acidic solution, ph ƒphpzc) (1) where M represents the Zr-Fe matrix. The values of PZC of present materials are in the range of ,, which indicates that anion exchange reaction occurs only at ph ƒ7.8. The selectivity sequence of oxo-anions on three (188)

6 Vol.18 No.4 (2007) 333 materials from Kd values is Cl-, NO-3, SO42- ƒhpo42- at ph 9. At ph 9, the surface charge of the material is negative (phpzc =7.8), and therefore there will be no adsorption of anions through electrostatic attraction. The present Kd values also indicate that the materials do not adsorb Cl-, NO3-, and SO42-, although SO42- is tetragonal with approximately the same diameter ( nm) as that of HPO42 Phosphate adsorption on metal hydroxides can occur through ligand exchange reaction with surface hydroxide groups at different ph values [15], and therefore the reaction scheme can be written as follows: Phosphate exists in different ionic species as H2PO4 (pk1=2.15), HPO42- (pk2=7.2) and PO43- (pk3=12.33). In the ph titration study, all materials show higher phosphate uptakes at ph 2 despite the presence of large amount of Cl- ions, and the uptake can be related to ligand exchange reaction. Phosphate uptakes from seawater at ph 8.1, and wastewater at ph on materials also occur through ligand exchange reactions, because phpzc ƒ ph of the solutions. In conclusion, amorphous binary Zr-Fe hydroxides with high exchange capacity for phosphate can be prepared by a simple precipitation method. The adsorbent is selective in removing phosphate from seawater and wastewater. References 1) L. Zeng, X. Li, and J. Liu, Water Res., 38, (2004). 2) M. Gallieni, and S. Garella, Nephron, 52, (1989). 3) A. Clearfield, ed., Inorganic Ion Exchange Materials, CRC Press, Boca Raton, Florida, (1982). 4) Y. Seida and Y. Nakano, Water Res., 36, (2002). 5) R. Chitrakar, S. Tezuka, A. Sonoda, K. Sakane, K. Ooi, and T. Hirotsu, Chem. Lett., 36, (2007). 6) K. Okada, J. Temmujin, Y. Kameshima, and K. J. D. MacKenzie, Mat. Res. Bull., 38, (2003). 7) X. Zhu and A. Jyo, Water Res., 39, (2005). 8) D. Zhao and A. K. Sengupta, Water Res., 32, (1998). 9) R. Chitrakar, S. Tezuka, A. Sonoda, K. Sakane, K. Ooi, and T. Hirotsu, J. Colloid Interface Sci., 297, (2006). 10) R. Chitrakar, S. Tezuka, A. Sonoda, K. Sakane, K. Ooi, and T. Hirotsu, J. Colloid Interface Sci., 298, (2006). 11) N. I. Chubar, V. A. Kanibolotskyy, V. V. Strelko, G. G. Gallios, V. F. Samanidou, T. O. Shaposhnikova, V. G. Milgrandt, and I. Z. Zhuravlev, Colloids Surfaces A, 255, (2005). 12) C. Jing, X. Meng, S. Liu, S. Baidas, R. Patraju, C. Christodoulatos, and G. P. Korfiatis, J. Colloid Interface Sci., 290,14-21(2005). 13) S. Nagamine, T. Ueda, I. Masuda, T. Mori, E. Sasaoka, and I. Joko, Ind. Eng. Chem. Res., 42, (2003). 14) L. Yang, Z Shahrivari, P. K. T. Liu, M. Sahimi, and T. T. Tsotsis, Ind. Eng. Chem. Res., 44, (2005). 15) L. Sigg, and W. Stumm, Colloids Surfaces, 2, (1981). (189)