Impact of Anaerobic Bioremediation on Soil Structure, Colloid Formation, and Contaminant Transport.

Anaerobic bioremediation is widely applied to create anaerobic subsurface conditions designed to stimulate microorganisms that degrade organic contaminants and immobilize toxic metals in situ.  Anaerobic conditions that accompany such techniques also promotes microbially mediated Fe(III)-oxide mineral reduction.  The reduction of Fe(III) could cause soil structure breakdown, formation of clay colloids, and alternation of soil surface chemical properties.  These processes could then impact bioremediation and the migration of contaminants.  Column experiments were conducted to investigate the impact of anaerobic bioreduction on soil structure, hydraulic properties, colloid formation, and transport of three tracers (bromide, DFBA, and silica shelled silver nanoparticles).  Columns packed with inoculated water stable soil aggregates were placed in anaerobic glovebox, and artificial groundwater media was pumped into the columns to simulate anaerobic bioreduction process for 3 weeks.  Decent amount of soluble Fe(II) accompanied by colloids were detected in the effluent from bioreduction columns in a week after initiation of bioreduction treatment, which demonstrated bioreduction of Fe(III) and formation of colloids.  Transport experiments were performed in the columns before and after bioreduction process to assess the changes of hydraulic and surface chemical properties through bioreduction treatment.  Earlier breakthrough of bromide and DFBA after treatment indicated alterations in flow paths (formation of preferential flow paths).  Less dispersion of bromide and DFBA, and less tailing of DFBA after treatment implied breakdown of soil aggregates.  Dramatically enhanced transport and early breakthrough of silica shelled silver nanoparticles after treatment supported the above conclusion of alterations in flow paths, and indicated changes of soil surface chemical properties.