Promoted iron corrosion and enhanced phosphate removal by micro-electric field driven zero-valent iron

Abstract

Zero-valent iron (Fe⁰) is restricted in phosphate removal due to the formation of a passive P-Fe layer on its surface. A micro-electric field (0.20 mA cm^-2) was employed in Fe⁰ column to facilitate iron corrosion for enhanced phosphate removal with a Fe⁰ column as the control. The performance of two columns was compared by batch experiment at a Fe⁰ filling rate of 10 vol% with quartz sand as dispersing media. The stability and reusability of micro-electric field driven Fe⁰ (MFD-Fe⁰) column was estimated by cyclic test. Solid phase analysis showed promoted iron corrosion, iron ion generation, and secondary mineral production such as lepidocrocite and magnetite in the MFD-Fe⁰ column. Since iron ions tended to precipitate with phosphate, and iron minerals provided reaction sites for phosphate adsorption, the MFD-Fe⁰ column achieved an enhanced phosphate removal of 94.1%, 2.8 times higher than that of the Fe⁰ column. The increase of current density from 0 to 0.20 mA cm^-2 significantly improved phosphate removal from 24.5% to 94.1%, further demonstrating the promoting effect of micro-electric field on iron corrosion. The MFD-Fe⁰ column also possessed excellent stability and reusability. It only showed a slight decrease of phosphate removal from 94.1% to 89.7% in eight cycles. It restored a phosphate removal capacity of 97.4% as compared to the initial MFD-Fe⁰ column by eluting iron (hydro)oxides on Fe⁰ and quartz sand surfaces with sulfuric acid. This study indicated that MFD-Fe⁰ is a promising method to remove phosphate from water and an alternative strategy for overcoming Fe⁰ passivation.

Keywords: Iron corrosion; Iron oxide; Phosphate; Precipitation; Zero-valent iron.