Impact of Microbial Iron Oxide Reduction on the Transport of Diffusible Tracers and Non-diffusible Nanoparticles in Soils
AbstractIn situ bioremediation to achieve immobilization of toxic metals and radionuclides or detoxification of chlorinated solvents relies on electron donor additions. This practice promotes microbial Fe(III)-oxide mineral reduction that could change soil pore structure, release soil colloids, alter matrix surface properties, and cause the formation of secondary (i.e., reduced) Fe-mineral phases. These processes in turn may impact rates of bioremediation, groundwater quality, and ultimately contaminant fate. Continuous flow columns packed with water-stable soil aggregates high in Fe-oxides were infused with artificial groundwater containing acetate as electron donor and operated for 20 or 60 days inside an anoxic chamber. Soluble Fe(II) and soil colloids were detected in the effluent within one week after initiation of the acetate addition, demonstrating Fe(III)-bioreduction and colloid formation. Br-, 2,6-difluorobenzoate (DFBA), and silica-shelled silver nanoparticles (SSSNP) were selected as diffusible tracer, low-diffusible tracer, and non-diffusible nanoparticles, respectively, to perform transport experiments before and after the active 20-day bioreduction phase, with an aim of assessing the changes in soil structure and surface chemical properties resulting from Fe(III)-bioreduction. The transport of diffusible Br- was not influenced by the Fe(III)-bioreduction as evidenced by identical breakthrough curves before and after the introduction of acetate. Low-diffusible DFBA showed earlier breakthrough and less tailing after the bioreduction, suggesting alterations in flow paths and surface chemical properties of the soils. Similarly, non-diffusible SSSNP exhibited early breakthrough and enhanced transport after the bioreduction phase. Unexpectedly, the bioreduction caused complete retention of SSSNP in the soil columns when the acetate injection was extended from 20 days to 60 days, though no changes were observed for Br- and DFBA during the extended bioreduction period. The large change in the transport of SSSNP was attributed to the enhancement of soil aggregate breakdown and soil colloid release causing mechanical straining of SSSNP and the exposure of iron oxide surfaces previously unavailable within aggregate interiors favorable to the attachment of SSSNP. These results demonstrate that microbial activity can affect soil properties and transport behaviors of diffusivity-varying solutes and colloids in a time dependent fashion, a finding with implication for interpreting the data generated from soil column experiments under continuous flow.\n\nHighlightsO_LIFe(III)-bioreduction causes time-dependent aggregate breakdown and colloid release.\nC_LIO_LIShort-term bioreduction alters soil aggregate surface chemistry and tracer transport.\nC_LIO_LIElectron donor amendment enhances transport of nanoparticle tracer.\nC_LI