Project Summary (2000-2005)

Chlorinated ethylenes constitute a class of common environmental contaminants of groundwater at superfund sites that have been targeted for application of cleanup technologies. Among different treatment methodologies, in situ abiotic remediation that relies upon redox processes holds promise as an effective and economical alternative to "pump-and-treat" strategies or microbial-dependent remediation. In principle, chlorinated ethylenes can be efficiently dechlorinated to innocuous carbonaceous products and inorganic chloride, if reductive and oxidative dechlorination chemistry is applied in sequence. In situ reductive treatment of chloroorganics has been considerably advanced with the introduction of permeable reactive treatment walls filled by particulate zero-valent iron. The reductive treatment is best suited for the highly chlorinated members of the chloroethylene class and currently requires support by oxidative treatment (air sparging) to degrade less chlorinated chloroorganics and non-chlorinated petroleum hydrocarbons. The present proposal investigates a novel oxidative approach that relies upon catalytic oxygenation/dechlorination of chloroethylenes, mediated by specific iron reagents, in the presence of sacrificial amounts of dioxygen and a reducing agent (usually metallic Fe or Zn) or hydrogen peroxide alone. The specific targets of this work are to: (i) develop iron-based oxidizing systems to perform oxidative dechlorination of chloroethylenes in aqueous media, by examining the fundamental chemistry of relevant iron complexes and intermediates in the course of activating dioxygen or hydrogen peroxide to unravel an active oxidant that attacks chloroethylenes; (ii) provide mechanistic understanding of the dechlorination process, by examining the nature of the active oxidant involved, via spectroscopic investigation of intermediates and kinetic analysis, and by exploring the manner by which this oxidant adds to chloroethylenes (concerted, step-wise (radical, carbocationic)); and (iii) explore heterogenized versions of the iron reagents to be tested in column dechlorination studies in conjunction with zero-valent iron.