Bioreduction Of Ferrihydrite-montmorillonite-organic Matter Complexes

Organic matters(OM)are closely associated with Fe(hydr)oxide minerals such as ferrihydrite(Fh)in soil environment.Microbial reduction can induce mineral transformation and destabilization of Fh-OM complexes.However,little is known about the involvement of clay minerals,which are also abundant in soil environment,affecting the behavior of Fh-OM complexes during bioreduction.In this study,montmorillonite(SWy-2)as a representative clay mineral was used to synthesize Fh-(SWy-2)-OM co-precipitates and Geobacter sulfurreducens was introduced as a reducing agent.The fate of OM during bioreduction were characterized by various techniques on a molecular level.Results showed that montmorillonite mineralogy played an important role in the bioreduction process.G.sulfurreducens could utilize mineral-associated OM as electron donors.Addition of SWy-2 deccreased the initial reduction rate by increasing the repulsive forces between microbes and mineral surfaces and covering the available electron donors/acceptors,but SWy-2 increased the reduction rate at later stage and promoted the total reduction extent through alleviating the passivation of mineral and cell surfaces,inhibiting the formation of secondary minerals and increasing the bioavailability of mineral-associated OM.Reduction triggered reductive release of sorbed OM into solution,compounds with more carboxyl groups,aromatic structure and large molecular weight were preferentially retained in the solid due to stronger interaction with minerals.Microbial carbon also contributed significantly to the OM pools in both aqueous and solidassociated phase.Our results provide new insights into fate and transformation of mineral-associated OM in anoxic soil environment.