Role of Fe, Na and Al in Fe-Zeolite-A for adsorption and desorption of phosphate from aqueous solution

In this study, Fe-Zeolite-A as a phosphate adsorbent was synthesized by incorporating iron into the framework of Zeolite-A using ammonium iron citrate as the Fe³⁺source. The adsorption (in acidic condition) and desorption of phosphate (in alkaline condition) from an aqueous solution was repetitively performed for 18 times in batch test using Fe-Zeolite-A. The rate of adsorption and desorption of PO₄^3- was much faster (consistently) than any of the reported study so far. The crystalline phase of pristine zeolite changed to amorphous after one adsorption phase and ultimately transformed into a highly amorphous phase after 18 adsorption/desorption cycles. It was due to the formation of more active sites on the surface of the zeolite by a release of atoms, breaking of bonds and deposition of metal and phosphate compounds on the surface (by rigorous acid/base treatment). Increase in active sites enhanced the sorption efficiency of Fe-Zeolite-A. With the help of microscopic and spectroscopic techniques, it was found that Fe³⁺, Al³⁺and Na⁺ metal ions of the Fe-Zeolite-A were involved in the adsorption/desorption of PO₄^3-. Fe³⁺ ion exhibited ligand exchange mechanism by exchanging OH ions with PO₄^3-. Al³⁺and Na⁺ exhibited interactions like precipitation, hydrogen bonding, and diffusion respectively to adsorb PO₄^3-. Fe³⁺ metal ion dominated over other metal ions by ligand exchange principle, making the sorption process a highly reversible one. The adsorbent showed quantitative adsorption/desorption capacity even after continuous 18 cycles indicating a higher level of reusability.