Reduction of tetrachloroethylene and trichloroethylene by magnetite revisted

Groundwater aquifers with toxic chlorinated solvents is an old problem. But today the most frequently detected volatile organic compounds (VOC) in aquifers are tetrachloroethylene (PCE) and trichloroethylene (TCE), also known as chlorinated ethenes (CE). Conventional remediation like pump and treat has a high carbon footprint, and is unlikely to meet regulated standards. To reduce the environmental impact for treating contaminated groundwater, we explored alternatives with ferrous iron [Fe(II)] bearing minerals, like magnetite to catalyze CE removal. Active research on Fe(II)-bearing minerals, have advanced our understanding for groundwater remedies for natural attenuation and engineered systems. Knowledge gaps exist towards the predictability of mineral-specific processes, and how they contribute to removal of CEs. The first field based evidence for natural non-biological degradation of a CEs with Fe(II)-bearing minerals, was from sediments containing magnetite. Magnetite with Fe²⁺/Fe³⁺ ratio (x) of 0.5 has greatest potential to reduce PCE and TCE, however, few studies report the solid-state ratio when performing experiments with magnetite. In this study, we measured PCE/TCE removal in batch reactors as a function of the Fe²⁺/Fe³⁺ ratio, solids loading, pH, and aqueous Fe(II) concentration. Our results indicate that magnetite at the most favorable ratio (x = Fe²⁺/Fe³⁺ = 0.5) are unreactive with PCE and TCE over 150 days. At pH values and Fe(II) concentrations where Fe(OH)₂ precipitation is expected, however, transformation of PCE and TCE is observed. Thus, our findings imply that magnetite may not be reactive enough to be a major contributor to abiotic natural attenuation of PCE and TCE alone, unless it is in the presence of Fe(OH)₂.