Adsorption of phosphate ions on novel inorganic ion exchangers

The demand for cost effective adsorbents is widespread recognition through drinking water-liquid wastes treatment technologies. The novel ion exchangers based on individual and mixed hydrous oxides (ZrO2·xH2O, Fe2O3·Al2O3·xH2O and Fe2O3·2Al2O3·xH2O) have been obtained by sol-gel method from easily available and cheap raw materials and employed for adsorption of H2PO4− from aqueous systems. High specific surface area of the materials was found: 404, 396 and 388 m2 g−1 for ZrO2·xH2O, Fe2O3·Al2O3·xH2O and Fe2O3·2Al2O3·xH2O correspondingly. Adsorbents were characterized by potentiometric titration (anion exchange and cation exchange capacities (on H and OH− ions)), zeta-potential and pore volume studies. pH effect of phosphate ions sorption on the double hydroxides of Fe and Al was stronger than on the hydrated zirconium dioxide, however, all investigated sorbents are capable to work in the pH range from 3 to 10 having sufficient sorption capacity. Increasing the ionic strength of the solutions with adding electrolyte NaCl increased sorption of phosphate ions on ZrO2·xH2O. The presence of Ca2+ in the solution increased sorption capacity at the higher equilibrium concentration of phosphate ions >40 mg P l−1. Isotherms of the phosphate ions sorption on Fe2O3·Al2O3·xH2O and Fe2O3·2Al2O3·xH2O were obtained at the pHs 3, 6 and 9. Isotherms fitted the Langmuir model. Data on the kinetics of phosphate sorption fit well to the pseudo-second-order model. It is supposed that mechanism of phosphate ions adsorption onto individual oxides hydrated is not only ion exchange between P(V) ions and hydroxide groups, present on the surface, but it is also a result of nucleophylic SN2-i-replacement of OH groups via formation of intermediate six-centered complex with the release of water molecules.