Kinetics of Hydroxyl Radical Production from Oxygenation of Reduced Iron Minerals and Their Reactivity with Trichloroethene: Effects of Iron Amounts, Iron Species, and Sulfate Reducing Bacteria

Reactive oxygen species generated during the oxygenation of different ferrous species have been documented at groundwater field sites, but their effect on pollutant destruction remains an open question. To address this knowledge gap, a kinetic model was developed to probe mechanisms of center dot OH production and reactivity with trichloroethene (TCE) and competing species in the presence of reduced iron minerals (RIM) and oxygen in batch experiments. RIM slurries were formed by combining different amounts of Fe(II) and sulfide (with Fe(II):S ratios from 1:1 to 50:1) or Fe(II) and sulfate with sulfate reducing bacteria (SRB) added. Extents of TCE oxidation and center dot OH production were both greater with RIM prepared under more reducing conditions (more added Fe(II)) and then amended with O2. Kinetic rate constants from modeling indicate that center dot OH production from free Fe(II) dominates center dot OH production from solid Fe(II) and that TCE competes for center dot OH with Fe(II) and organic matter (OM). Competition with OM only occurs in experiments with SRB, which include cells and their exudates. Experimental results indicate that cells and/or exudates also provide electron equivalents to reform Fe(II) from oxidized RIM. Our work provides new insights into mechanisms and environmental significance of TCE oxidation by center dot OH produced from oxygenation of RIM. However, further work is necessary to confirm the relative importance of reaction pathways identified here and to probe potentially unaccounted for mechanisms that affect abiotic TCE oxidation in natural systems.

Automated summary

This study investigates the role of reactive oxygen species, specifically center dot OH, in the oxidation of trichloroethene (TCE) in the presence of reduced iron minerals (RIM) and oxygen. Experimental results and kinetic modeling show that center dot OH production is dominated by free Fe(II) and that TCE competes with Fe(II) and organic matter for center dot OH. Further research is needed to confirm the identified reaction pathways and explore other potential mechanisms.