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.

Oxidative degradation of commingled trichloroethylene and 1,4-dioxane by hydroxyl radicals produced upon oxygenation of a reduced clay mineral.

Improper disposal of chlorinated solvents such as trichloroethylene (TCE) and its stabilizer 1,4-dioxane has resulted in extensive contamination in soils and groundwater. Oxidative degradation of these contaminants by strong oxidants has been proposed recently as a remediation strategy, but specific mechanisms and degradation efficiencies are still poorly understood, especially in commingled systems. In this study, a reduced iron-bearing clay (RIC), nontronite (rNAu-2), was oxygenated to produce hydroxyl radicals (•OH) for degradation of TCE and 1,4-dioxane under circumneutral and dark conditions. Results showed that TCE and 1,4-dioxane could be effectively degraded during oxygenation of rNAu-2 in both single and commingled systems. Compared with the single compound system, the degradation rates and efficiencies of TCE and 1,4-dioxane decreased in the commingled system. The negative effect was more significant for TCE than 1,4-dioxane. The commingled TCE and 1,4-dioxane impacted the degradation pattern of each other, due to their difference in •OH scavenging efficiency, surface affinity to rNAu-2 and solubility. Moreover, solution pH, buffer type, rNAu-2 dosage, and dissolved organic matter all affected •OH production and contaminant degradation efficiency. Our findings provide new insights for investigating the natural attenuation of commingled chlorinated solvents and 1,4-dioxane by RIC in redox-fluctuating environments and offer guidance for developing possible in-situ remediation strategies.