Ligand-Promoted 1,4-Dioxane Degradation during Microbially Mediated Iron Redox Cycles

The carcinogenic cyclic ether compound 1,4-dioxane is a widespread contaminant that is commonly remediated using hydroxyl radicals ((OH)-O-center dot). In subsurface environments, the redox cycling of Fe(III) oxides facilitated by dissimilatory iron-reducing microorganisms, ambient reducing agents, and O-2 is ubiquitous. These processes may produce (OH)-O-center dot by the Fenton reaction without exogenous reagents. Fe(III) organic ligands may also affect the solubility and bioavailability of Fe(III) and enhance (OH)-O-center dot production and contaminant degradation. In this study, the role of ligands on (OH)-O-center dot production and 1,4-dioxane degradation was investigated during Fe(II) oxygenation experiments at circumneutral pH and during the microbially mediated redox cycling of iron with Fe(III) oxides as the starting substrate. Degradation of 1,4-dioxane via successive abiotic Fe(II) oxygenation under circumneutral conditions was enhanced by sixfold in the presence of solubilizing ligands without exogenous H2O2 addition, whereas 1,4-dioxane degradation was completely inhibited by (OH)-O-center dot scavengers. Microbial Fe(III) reduction was enhanced by 100% and 1,4-dioxane degradation rates were enhanced by one to two orders of magnitude after the addition of ligands to incubations with Fe(III) oxides exposed to redox oscillations. These results indicate that solubilization of Fe(III) increases its bioavailability during microbial Fe(III) reduction. Complexation of Fe(III) also accelerates (OH)-O-center dot production by the Fenton reaction, slows down (OH)-O-center dot consumption by Fe(II), and eventually enhances (OH)-O-center dot production and 1,4-dioxane degradation. These findings highlight the importance of ligands for (OH)-O-center dot production in subsurface environments where such reactions have been observed and provide a new process that could be exploited to optimize (OH)-O-center dot-dependent in situ bioremediation strategies.