Reduction of Cr(VI) to Cr(III) by Fe(II) in the presence of fulvic acids and in lacustrine pore water

In this study, the efficacy of Fe(II)–dissolved organic matter (DOM) complexes to reduce Cr(VI) to Cr(III) under anoxic conditions was examined. Cr(VI) was quickly reduced (minutes) in Fe(II) solutions in the absence and presence of Suwannee River (SRFA) and Pony Lake fulvic acid (PLFA). Fe(II)–DOM solutions did not always reduce Cr(VI) faster than Fe(II)-only systems, which is attributable to the pH dependent formation of highly reactive ferrous hydroxide species that could be stronger reductants than the Fe(II)–DOM complexes. DOM composition also influenced Cr(VI) reduction kinetics as Fe(II) in the presence of SRFA exhibited significantly faster Cr(VI) reduction kinetics at pH 5, 5.5, and 6 than Fe(II)–PLFA solutions. Further, 3:1 Fe(II):Cr(IV) stoichiometry was not observed in the Fe(II) fulvic acid solutions leading us to believe that the oxidized iron produced during Cr(VI) reduction is cycled back to Fe(II) by redox active fulvic acid moieties. Cr(VI) was reduced even more quickly in anoxically extracted sedimentary pore fluids from a wetland (Old Woman Creek Estuarine Reserve Huron, OH) that possessed higher levels of Fe(II) and DOM than the amounts used in the fulvic acid experiments. These data demonstrate that abiotic Cr(VI) reduction in anoxic pore waters that are naturally high in Fe(II) and DOM occurs more quickly relative to rates reported for biotic processes and is the dominant reduction pathway in these environments.