| Iron is one of the most abundant metal element in the earth’s crust.Iron-bearing clay minerals account for up to half of the iron in soils and sediments.The variation of valence state of structural iron in clay minerals affects their physicochemical properties(e.g.specific surface area,water-solubility and cation/anion exchange capacity,etc.),which could further influence plant growth,long-term storage of organic matter and migration and conversion of nutrient substance,contaminant and heavy metal in the environment.Humic acid(HA)is a kind of organic macromolecule generated from the decomposing remains of dead plants and animals by microbes.It accounts for up to 50%-80% of organic matter in ecological system.Currently,most studies applied anthraquinone compound as surrogate of HA to study the impacts of redox activity on electron transfer during bioreduction.And little is known about the effects of real HAs on bioreduction of clay minerals.In this study,the bioreduction of smectite in the presence of HA by Shewanella oneidensis MR-1,and the bioreduction of typical organic pollutant nitrobenzene with MR-1 and coexisting HA and clay mineral were investigated.The bioreduction of clay minerals(NAu-2 and SWy-2)could be enhanced by the Elliott soil humic acid(ESHA),Suwannee River humic acid(SRHA)and artificially prepared Aldrich humic acid(AHA),respectively.Characterization of clay mineral reduction products with transmission electron microscopy,X-ray diffraction and scanning electron microscopy indicated that HA facilitated the transformation of smectite to illite during bioreduction.After sorptive fractionation of ESHA by NAu-2,the unbound ESHA fraction demonstrated even higher stimulating effects on Fe(Ⅲ)reduction than whole ESHA.The electrostatic interaction between HA and clay mineral particles could enhance the aggregation of NAu-2 particles and decrease the exposure of mineral surface site.HA could also improve clay minerals bioreduction via Fe(II)complexation,which prevented the coverage and accumulation of Fe(II)on clay and cell surfaces.In addition to electron shuttle activity,the adsorption and Fe(II)complexation properties of HA also contribute to bioreduction of clay mineral.The addition of HA or clay mineral separately could both improve nitrobenzene bioreduction by MR-1,through electron shuttling or biogenic Fe(II)associated with the clay mineral.Significant promotion of nitrobenzene reductionby MR-1 was observed with lowerconcentration(0.1 g/L)of NAu-2,whereas poorer stimulating effects appeared at higher NAu-2 concentrations(1.0 and 5.0 g/L).Higher concentrations of NAu-2 might compete with nitrobenzene for biogenic electrons.The coexistence of ESHA and NAu-2 could not synergistically facilitate nitrobenzene bioreduction,possibly due tothe decrease of free HA concentration and available surface sites on clay mineral after adsorption of ESHA by NAu-2..Satisfying repeated stimulation of nitrobenzene bioreduction was obtained with systems containing coexisting ESHA and NAu-2.The nitrobenzene reduction efficiency maintained at around 80% in three cycles of continued operation. |
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