Nanosized Ferrihydrite Colloids Facilitate Microbial Iron Reduction under Flow Conditions

Recent studies indicated that nanoparticulate minerals in the environment exhibit a higher reactivity than their respective bulk materials. Lately, this has also been reported for microbial iron reduction using nanosized iron oxides as electron acceptors. However, these results have been obtained under small-scale, static batch conditions. The study presented here implies a flow regime within porous medium under low salt conditions providing information on the retention of nanosized iron oxide and the long-term sustainability of their reduction. Goethite, present as coating on quartz sand as a default electron acceptor, was reduced by Geobacter sulfurreducens and discharged as Fe2 + up to a maximum concentration of 0.6 mM in the column effluent, finally representing 10% of the supplied ferric iron in total. Adding ferrihydrite colloids via the influent to goethite-coated quartz sand led to partial adsorption of the colloids, but also to a high reactivity with a maximum Fe2 + discharge of 1.2 mM. In total, 1.4 mmol out of 2.6 mmol ferrihydrite colloids were reduced. Upon addition of ferrihydrite colloids to the influent of a parallel column containing quartz sand without goethite coatings, the maximum concentration of discharged Fe2 + accounted also for 1.2 mM, and 0.4 mmol out of 0.6 mmol ferrihydrite colloids were reduced in total. The column experiments demonstrated that ferrihydrite colloids are highly reactive and bioavailable for microbial reduction under approximated in situ conditions.