Influence of phosphate, citrate and nitrilotriacetic acid on the removal of aqueous hexavalent chromium by zero-valent iron at circumneutral pH

Abstract

The influence of phosphate, citrate and nitrilotriacetic acid (NTA) on the removal of aqueous hexavalent chromium (Cr(VI)) by zero-valent iron (ZVI) at circumneutral pH was investigated. The addition of phosphate, citrate and NTA caused an increase in the removal of aqueous Cr(VI) by ZVI at circumneutral pH. Such enhancement should be attributed to the increased Fe(II) and Fe(III) concentrations at circumneutral pH. Fe(II) could form dissolved Fe(II)-complexes with these three ligands. While the addition of phosphate and citrate could improve the solubility of the ferric iron hydroxides, only NTA could form the dissolved Fe(III)–NTA complex. The results of SEM indicate that the presence of phosphate, citrate and NTA diminished the inactivating effect of the resulting passivation layer. In addition, only citrate could react with Cr(VI) in the presence of Fe(III) with illumination, which is attributed to a photocatalytic reaction. For this developed system, the effective removal of Cr(VI) by ZVI at circumneutral pH could be performed.