Mechanistic investigations of granular iron and iron-based bimetallic reductants for treatment of organohalide pollutants

Over the past decade granular iron permeable reactive barriers (FePRBs) have established themselves as a proven alternative to conventional groundwater treatment. This study explores the chemical processes through which iron-based reductants degrade halogenated organic solvents. Identifying those factors that influence the rates of such processes may allow treatment strategies targeting these ubiquitous pollutants to be optimized.;We report on a nonlinear relationship between rates of alkyl polyhalide reduction and granular iron mass loadings, which we attribute to the thickening of a passive oxide overlayer on reductant particles in batch systems with high mass loadings. This nonlinear relationship contradicts the underlying assumptions of the surface-area-normalized kinetic model traditionally applied to iron systems, illustrating the need for alternative models if laboratory-scale data is to accurately predict pollutant fate in field-scale treatment systems. Results from this work also suggest that surface sites responsible for iron corrosion may also participate in alkyl polyhalide reduction.;Metal additives to granular iron can both enhance rates of organohalide reduction and alter branching ratios to minimize the formation of partially halogenated products. Depending upon the identity of the organohalide, however, certain additives may also inhibit reduction rates. Bimetallic reductant reactivity was found to correlate with the solubility of atomic hydrogen within the metal additive, while similar reactivity trends for our bimetals toward 1,1,1-trichloroethane reductive dehalogenation and 2-butyne hydrogenation also support a role for reactive atomic hydrogen in bimetallic systems. From a treatment perspective, bimetallic reductants are likely best applied to vinyl halide pollutants, which were considerably more susceptible to reduction than alkyl polyhalides in our reductant systems.;Rates of alkyl polyhalide reduction by granular iron generally increased with halogenation, although brominated species were substantially more reactive than their chlorinated analogues. Polyhalogenated ethanes possessing halogens on adjacent carbon centers reacted exclusively via reductive beta-elimination, while alternative reduction pathways (hydrogenolysis, reductive alpha-elimination) were only observed when multiple halogens were present only on a single carbon center. These observations, in addition to reactivity correlations developed between rate constants for organohalide reduction and carbon-halogen bond dissociation energies, may allow predictions of pollutant behavior in field-scale iron treatment systems to be improved.