Iron(III)-Bearing Clay Minerals Enhance Bioreduction of Nitrobenzene by <i>Shewanella putrefaciens</i> CN32

Abstract

Iron-bearing clay minerals are ubiquitous in the environment, and the clay–Fe­(II)/Fe­(III) redox couple plays important roles in abiotic reduction of several classes of environmental contaminants. We investigated the role of Fe-bearing clay minerals on the bioreduction of nitrobenzene. In experiments with <i>Shewanella putrefaciens</i> CN32 and excess electron donor, we found that the Fe-bearing clay minerals montmorillonite SWy-2 and nontronite NAu-2 enhanced nitrobenzene bioreduction. On short time scales (<50 h), nitrobenzene reduction was primarily biologically driven, but at later time points, nitrobenzene reduction by biologically formed structural Fe­(II) in the clay minerals became increasingly important. We found that chemically reduced (dithionite) iron-bearing clay minerals reduced nitrobenzene more rapidly than biologically reduced iron-bearing clay minerals despite the minerals having similar structural Fe­(II) concentrations. We also found that chemically reduced NAu-2 reduced nitrobenzene faster as compared to chemically reduced SWy-2. The different reactivity of SWy-2 versus NAu-2 toward nitrobenzene was caused by different forms of structural clay-Fe­(II) in the clay minerals and different reduction potentials (<i>E</i>_h) of the clay minerals. Because most contaminated aquifers become reduced via biological activity, the reactivity of biogenic clay–Fe­(II) toward reducible contaminants is particularly important