Treatment of Vapor-Phase Organohalides with Zero-Valent Iron and Ni/Fe Reductants

Abstract

The reduction of vapor-phase cis-1,2-dichloroethylene (cis-DCE) by zero-valent iron [Fe(0)] and nickel-plated iron (Ni/Fe) was assessed as a possible treatment strategy for organohalides in landfill gas and soil vapor extraction system offgas. The rate of cis-DCE removal and the consequent formation of ethylene, ethane, and vinyl chloride were measured in both batch and column reactors. Auger Electron Spectroscopy was used to determine nickel surface loading before and a fter reaction. In batch experiments, nickel surface loadings greater than 30% increased the pseudo-first-order rate constant for cis-DCE reduction by a factor of 10–20 over that of unamended Fe(0). In column studies using Ni/Fe particles (40% nickel surface loading), 100% removal of gaseous cis-DCE was initially achieved from both anaerobic and aerobic gas streams, compared to only partial removal of cis-DCE in columns containing Fe(0). The enhanced reactivity of Ni/Fe r eductants relative to Fe(0) diminished, however, over time in both anaerobic and aerobic columns. Reactivity of Ni/Fe columns decreased to a level comparable to unamended iron within 32 days in anaerobic column experiments and 7 days in aerobic experiments. Deactivation of Ni/Fe reductants occurred as a passivation front progressed along each column. This passivation front was associated with the loss of Ni from the surface due to the growth of an iron oxide overlayer that eventually buried s urface-associated nickel throughout the entire column. Treatment of deactivated Ni/Fe reductants with 1 M HCl was successful in temporarily restoring reactivity in both aerobic and anaerobic columns.