Mechanistic study of phosphorus adsorption onto Iron Z-A: Spectroscopic and experimental approach

Abstract

Iron was incorporated into an LTA type zeolite using the sol-gel hydrothermal method to form Iron-zeolite-A (Iron-Z-A), and its phosphate adsorption-desorption efficiency were analyzed. Samples were characterized by EDS, SEM, XRD, EPR, FT-IR XPS, and Raman to ensure the apt synthesis of Iron-Z-A and to interpret the mechanism of adsorption-desorption of PO43- in an aqueous solution. EPR and XPS analysis confirmed that the iron was doped as Fe3+ in the LTA structure. The XPS peak shift (Fe-2p), FT-IR band shift, and intensity change (-OH) confirmed the existence of the ligand exchange mechanism. In the adsorption phase at pH 5, the derivative of phosphate (H2PO4-) acts as a ligand and interacts with OH of Fe on the zeolite surface to form "Iron-zeolite (oxy) hydroxide bound phosphate". In the desorption phase at pH 10, phosphate ligand is detached and get mixed in the aqueous phase as HPO42-. The EDS data, Si-O-Al band shift and intensity change in FT-IR and XPS peak intensity change proved the contribution of Al in the process of adsorption. The data of adsorption fitted well with the Langmuir's isotherm and pseudo-second-order kinetic model. The amount of PO43- adsorbed was a function of adsorbent's surface area regardless of concentration. The amount of PO43- being adsorbed by the metal ions was found to be 382.296 mg PO43-/g Fe and 56.296 mg PO43-/g Al.