Behaviors of Structural Fe(II) of Nontronite and Aqueous Fe(II) on Cr(VI) Removal in the Presence of Citrate

Abstract

Structural Fe(II) in clay minerals and aqueous Fe(II) is known to reduce Cr(VI) to Cr(III), but the behaviors of structural Fe(II) and aqueous Fe(II) on Cr(VI) removal in presence of organic acid are poorly understood. The objective of this study is to reveal the relationships between structural Fe of nontronite (NAu-2), aqueous Fe(II), and citrate on Cr(VI) removal. The effects of aqueous Fe(II) and citrate on Cr(VI) removal by NAu-2 were studied. The results indicated that aqueous Fe(II) and citrate can enhance the Cr(VI) removal. The aqueous Fe(II) formed the Fe-Cr precipitates on the surface of NAu-2, while the citrate can inhibit the formation of Fe-Cr precipitates. The NAu-2 before and after reaction was characterized by scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The Cr(VI) removal by NAu-2 in presence of aqueous Fe(II) and citrate was by adsorption and reduction. The citrate induced the dissolution of NAu-2 and accelerated the structural Fe(III)/Fe(II) redox cycle in the NAu-2, while the aqueous Fe(II) formed the Fe-citrate complex in the solution and competed with structural Fe for citrate. This paper suggested that the structural Fe and aqueous Fe(II) played different roles on Cr(VI) removal in presence of citrate.