The ability of the metal-reducing facultative anaerobe Shewanella oneidensis to reduce soluble Fe(III)-citrate in anaerobic conditions and produce reactive oxygen species (ROS) in aerobic conditions has been optimized to degrade organic contaminants via the so-called microbially-driven Fenton reaction (MFR). In this process, hydroxyl radicals produced during the oxidation of Fe(II) by O2 or H2O2 degrade organic contaminants. As iron is mostly present under the form of iron (oxyhydr)oxide minerals in natural or contaminated environments, however, MFR has to take place at the mineral-water interface to be exploited in bioremediation strategies. Preliminary studies with 1,4-dioxane as model organic contaminant demonstrated that contaminant degradation was not as efficient when using 2L-ferrihydrite compared to Fe(III)-citrate as iron source in the microbially-driven Fenton reaction, suggesting that organic ligands not only increase solubility and bioavailability of Fe(III) but also enhance ROS oxidant yields from Fe(II)-bearing minerals through surface interactions. The objective of this study was to examine the effect of organic ligands on the mechanism of oxidant generation by MFR with iron (oxyhydr)oxides. Duplicate batch incubations of S. oneidensis with Fe(III)-citrate, ferrihydrite, or ferrihydrite amended with citrate as model organic ligand were compared to abiotic controls in otherwise identical conditions. Results confirmed that 1,4-dioxane degradation by MFR with ferrihydrite is not as efficient as with ferric citrate. In turn, addition of citrate to ferrihydrite significantly enhanced Fe(III) reduction and 1,4-dioxane degradation rates compared to the ferrihydrite-only incubations. These findings suggest that iron-binding organic ligands such as citrate: 1) increase the bioavailability of Fe(III) from iron (oxyhydr)oxides; 2) affect the mechanism and kinetics of Fe(II) oxidation; and 3) enhance hydroxyl radical production and 1,4-dioxane degradation rates. Overall, these experiments demonstrate that MFR may be exploited as bioremediation technique in contaminated subsurface environments and provide insights into the potential importance of MFR in natural environments.

Role of organic ligands in the microbially driven Fenton reaction with iron (oxyhydr)oxides

ACS Fall 2021

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