Role of humic acid in the transformation of hexavalent chromium in a sulfidated ferrihydrite system

Ferrihydrite (Fh) often coexists with organic matter (i.e., humic acid (HA)) in the environment;however, its impacts on the transformation of hexavalent chromium (Cr(VI)) is poorly understood during the sulfidation of Fh. Upon exposed to 2 mM sulfide for 12 h, the total amount of Fe(II) (Fe(II)tot, 0.78 mM) in treatments with HA (300 mg HA/L) was higher than that (0.67 mM) in treatments without HA, since HA could enhance the solubility of Fe(II). After then, the Cr(VI) was reduced by sulfidated Fh. Aqueous Cr(VI) concentration (Cr(VI)aq) declined from 0.67 to 0.43 mM with the increase of HA concentration from 0 to 400 mg/L, which was partly ascribed to the inhibition of surface pas-sivation by HA. Moreover, the increase in Fe(II) during the sulfidation of Fh also exerted a strong impact on the trans-formation of Cr(VI) subsequently. In addition of HA, batch experiments suggested that EDTA could also promote the formation of Fe(II) (Fe(II)tot, 0.80 mM), rendering a lower Cr(VI)aq (0.59 mM) in EDTA-300 treatments. This study fur-ther demonstrated that HA played an important role in the transformation of Cr(VI), hence providing a theoretical basis for in-situ remediation of Cr(VI) in the future.

Automated summary

Fh often coexists with HA in the environment, but its impact on the transformation of Cr(VI) during sulfidation of Fh is poorly understood. Addition of HA increased Fe(II) concentration and inhibited surface passivation, leading to a lower Cr(VI)aq concentration. EDTA addition also promoted Fe(II) formation and resulted in lower Cr(VI)aq concentration in EDTA treatments. This study demonstrated the important role of HA in Cr(VI) transformation and provided a theoretical basis for in-situ remediation of Cr(VI) in the future.