Chemo-Dynamics of Chromate Reduction In Soils: Implications to Remediation.

The continuous influx of toxic heavy metals including chromium (Cr) into the environment leads to the contamination of both soil and water. A complex variety of biotic and abiotic pathways determine the fate and speciation of Cr in the soil. While the hexavalent Cr [Cr(VI)] is highly toxic, mobile and impacts human health, the trivalent Cr [Cr(III)] is non-toxic and less mobile. Chromium toxicity in soils can be mitigated by reduction of Cr(VI) to Cr(III) which is controlled by the presence of free Cr(VI) species in soil solution, and the supply of protons and electrons. In this study the effects of Cr(VI) adsorption (i.e. availability of free Cr(VI) species in soil solution), soil pH (i.e. supply of protons) and various electron donor carbon sources [black carbon (BC), chicken waste biochar (CWB) and cow manure (CM)] on the reduction of Cr(VI) to Cr(III) in soil were investigated. The rate of Cr(VI) reduction decreased with increases in Cr(VI) adsorption and soil pH, which is attributed to the decrease in the supply of free Cr(VI) ions and protons. Among the three different amendments tested, BC achieved the highest rate of Cr(VI) reduction followed by cow manure and biochar. The difference in Cr(VI) reduction between the amendments is attributed to the differences in dissolved organic carbon (DOC) and functional groups which provide the electrons for the reduction to proceed. The enhanced Cr(VI) reduction by BC was supported by FTIR spectroscopic analysis which showed changes in the functional groups, indicating their participation in reduction. These results demonstrated that BC can be used a potential source of electron donor for the reduction of Cr(VI), thereby mitigating Cr toxicity in contaminated soils.