Reductive Sequestration of Cr(VI) and Immobilization of C during the Microbially Mediated Transformation of Ferrihydrite-Cr(VI)-Fulvic Acid Coprecipitates

Uncoveringthe coupled kinetics of Cr-(VI) reduction andimmobilization of Cr-(III) and C during dissimilatory iron reductioncontributes to the synchronous sequestration of heavy metals and organicmatter in soil environments. Cr-(VI) detoxification and organic matter (OM) stabilizationareusually influenced by the biological transformation of iron (Fe) minerals;however, the underlying mechanisms of metal-reducing bacteria on thecoupled kinetics of Fe minerals, Cr, and OM remain unclear. Here,the reductive sequestration of Cr-(VI) and immobilization of fulvicacid (FA) during the microbially mediated phase transformation offerrihydrite with varying Cr/Fe ratios were investigated. No phasetransformation occurred until Cr-(VI) was completely reduced, and theferrihydrite transformation rate decreased as the Cr/Fe ratio increased.Microscopic analysis was uncovered, which revealed that the resultingCr-(III) was incorporated into the lattice structure of magnetite andgoethite, whereas OM was mainly adsorbed on goethite and magnetitesurfaces and located within pore spaces. Fine line scan profiles showedthat OM adsorbed on the Fe mineral surface had a lower oxidation statethan that within nanopores, and C adsorbed on the magnetite surfacehad the highest oxidation state. During reductive transformation,the immobilization of FA by Fe minerals was predominantly via surfacecomplexation, and OM with highly aromatic and unsaturated structuresand low H/C ratios was easily adsorbed by Fe minerals or decomposedby bacteria, whereas Cr/Fe ratios had little effect on the bindingof Fe minerals and OM and the variations in OM components. Owing tothe inhibition of crystalline Fe minerals and nanopore formation inthe presence of Cr, Cr sequestration and C immobilization can be synchronouslyfavored at low Cr/Fe ratios. These findings provide a profound theoreticalbasis for Cr detoxification and synchronous sequestration of Cr andC in anoxic soils and sediments.