Promotion of Humic Acid Transformation by Abiotic and Biotic Fe Redox Cycling in Nontronite

The redox activity of Fe-bearing minerals is coupled with the transformation of organic matter (OM) in redox dynamic environments, but the underlying mechanism remains unclear. In this work, a Fe redox cycling experiment of nontronite (NAu-2), an Fe-rich smectite, was performed via combined abiotic and biotic methods, and the accompanying transformation of humic acid (HA) as a representative OM was investigated. Chemical reduction and subsequent abiotic reoxidation of NAu-2 produced abundant hydroxyl radicals (thereafter termed as ·OH) that effectively transformed the chemical and molecular composition of HA. More importantly, transformed HA served as a more premium electron donor/carbon source to couple with subsequent biological reduction of Fe(III) in reoxidized NAu-2 by Geobacter sulfurreducens, a model Fe-reducing bacterium. Destruction of aromatic structures and formation of carboxylates were mechanisms responsible for transforming HA into an energetically more bioavailable substrate. Relative to unaltered HA, transformed HA increased the extent of the bioreduction by 105%, and Fe(III) reduction was coupled with oxidation and even mineralization of transformed HA, resulting in bleached HA and formation of microbial products and cell debris. ·OH transformation slightly decreased the electron shuttling capacity of HA in bioreduction. Our results provide a mechanistic explanation for rapid OM mineralization driven by Fe redox cycling in redox-fluctuating environments.