Arsenic mobilization through microbially mediated deflocculation of ferrihydrite

This study examined the potential impact of microbially mediated reduction of Fe in the Fe(III)-(hydr)oxide mineral ferrihydrite on the mobility of As in natural waters. In microcosm experiments, the obligately anaerobic bacterium Geobacter metallireducens reduced on average 10% of the Fe(III) in ferrihydrite with varying sorbed As(V) surface coverages, which resulted in deflocculation of initially micron-sized As-bearing ferrihydrite aggregates to nanometersized colloids. No reduction of As(V) to As(III) was observed in microcosm samples. Measurement of Fe and As within operationally defined particulate, colloidal, and dissolved fractions of microcosm slurry samples revealed that little Fe or As was released from ferrihydrite as dissolved species. Microbially induced deflocculation of ferrihydrite in the presence of G. metallireducens was correlated with more negative zeta potential of ferrihydrite nanoparticles suggesting that G. metallireducens mediated As mobilization through alteration of ferrihydrite surface charge. TEM analysis and solution chemistry conditions suggested formation of a magnetite surface layer through topotactic recrystallization of ferrihydrite (2LFH) driven by sorbed Fe(II). The formation of nanometer-sized As-bearing colloids through microbially mediated reduction of Fe-(hydr)oxides has the potential to increase human As exposure by enhancing As mobility in natural waters and hindering As removal during subsequent drinking water treatment.