Effects of Humic Acid and Nitrate, Sulfate on Cr(Vi) Removal in Soil Using Zero-Valent Iron and Microbes

Abstract

A collaborative system combined Fe 0 and microbes was established to remediate Cr(VI)-contaminated soil at low and relatively high concentrations of typical organic substrate (humic acid) and electron acceptors (nitrate and sulfate)  with Fe 0 as a control. In the abiotic Fe 0 system, humic acid inhibited Cr(VI) removal by occupying the iron surface with its structural oxygen-containing functional groups. However, humic acid could be an effective substrate for selected microorganisms to promote their growth and secondary iron mineral production, hence an enhanced Cr(VI) removal was achieved with increasing humic acid concentration in the biotic Fe 0 system. As for nitrate, it competed with Cr(VI) for cathodic hydrogen from anoxic Fe 0 corrosion, thereby inhibiting the reduction of Cr(VI). Different from denitrification, biological sulfate reduction promoted the generation of highly active minerals such as iron sulfide and green rust owning to the interaction between microorganisms and Fe 0 . These minerals were beneficial for Cr(VI) reduction and precipitation. Thus, in the biotic Fe 0 system, a 0.6 time higher Cr(VI) aq removal rate of 79.5% was obtained when sulfate concentration reached 10 mmol kg -1 in comparison with that of 48.4% in the system without sulfate. Our study demonstrated that it is necessary to deeply understand the potential effects of organic substrates and electron acceptors on Cr(VI) removal in soil using biotic Fe 0 before its application.

Keywords: zero-valent iron, Chromium, humic acid, nitrate, sulfate