Chemical state of Fe3+ in a Fe3+-type cation exchange resin for the removal and recovery of phosphate ions and the adsorption mechanism of phosphate ion to the resin

Abstract

To apply the Fe3+-type cation exchange resin, Fe3+-type UBK 10, which is an effective adsorbent of selenite ions, for the removal and recovery of phosphate ions (PO43−), the adsorption mechanism of PO43− to the Fe3+-type UBK 10 resin was investigated. To elucidate the aforementioned mechanism, the chemical state of Fe3+ in the Fe3+-type UBK 10 resin and the adsorption mode of PO43− into the adsorbent were examined via 57Fe Mössbauer spectroscopy and Fe and P K-edge X-ray absorption spectroscopy. The optimum pH for the adsorption of PO43− was found to be approximately 6 and the adsorbed PO43− was rapidly desorbed using 6 mol dm-3 hydrochloric acid (HCl) through the conversion of Fe3+ to FeCl4-, which is practical for the removal and recovery of PO43−. The Fe K-edge X-ray absorption near-edge structure (XANES) spectra showed, through the appearance of three isobestic points, that Fe3+ in the Fe3+-type UBK 10 resin exists as the following four species in equilibrium, depending on the pH: two monomeric hydrolytic species of ferric ions, (-S)2FeOH and (-S)Fe(OH)2, a μ-oxo dimer with corner sharing structure, (-S)2Fe-O-Fe(S-)2, and a dihydroxo dimer with edge sharing structure, (-S)2Fe-(OH)2-Fe(S-)2 (S: SO3 group in UBK 10). It should be emphasized that a PO43− can combine with a (-S)2Fe-(OH)2-Fe(S-)2 site at pH of 6, which is an optimum pH for adsorption of PO43−, forming a bidentate-binuclear configuration, (-S)2Fe-(OH)2(OPO2O)-Fe(S-)2, through the formation of Fe-O-P bonds.