Effects of humic acid, nitrate and sulfate on hexavalent chromium removal by abiotic and biotic zero-valent iron

Humic acid, nitrate, and sulfate in the environment may affect the performance of abiotic and biotic zero-valent iron (Fe 0 ) on the remediation of heavy metal-contaminated soil. In this study, a synergistic system combining Fe 0 and microbes was established to remediate Cr(VI)-contaminated soil at low and relatively high concentrations of humic acid, nitrate, and sulfate with Fe 0 as a control. In the abiotic Fe 0 system, humic acid inhibited Cr(VI) removal by occupying iron surface with its oxygen-containing functional groups and generating iron or calcium-containing precipitates on Fe 0 surface. However, microbes could effectively degrade humic acid to promote their growth, iron corrosion, and secondary mineral production for enhanced Cr(VI) removal. As for nitrate, it showed slightly promoted effect on iron corrosion in abiotic Fe 0 , but competed with Cr(VI) for cathodic hydrogen from iron corrosion for biological denitrification and thereby significantly inhibited Cr(VI) removal in biotic Fe 0 . Different from denitrification, hydrogenotrophic desulfuration promoted active mineral generation such as iron sulfide and green rust owning to biomineralization. These minerals were beneficial for Cr(VI) adsorption and reduction. Thus, in the biotic Fe 0 system, the presence of 10 mmol kg −1 sulfate led to a higher Cr(VI) removal of 79.5% as compared to that of 48.4% in the system without sulfate. The results indicated that it is necessary to deeply understand the potential effects of organic substrates and electron acceptors on Cr(VI) removal in soil when using abiotic and biotic Fe 0 before their application.