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

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 ofPO(4)(3)(-) 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 Fe-57 Mossbauer 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)(2)FeOH and (-S)Fe (OH)(2), a mu-oxo dimer with corner sharing structure, (-S)(2)Fe-O-Fe(S-)(2), and a dihydroxo dimer with edge sharing structure, (-S)(2)Fe-(OH)(2)-Fe(S-)(2)(S: SO3 group in UBK 10). It should be emphasized that a PO43- can combine with a (-S)(2)Fe-(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)(2)Fe-(OH)(2)(OPO2O)-Fe(S-)(2), through the formation of Fe-O-P bonds.