title: Chlorinated solvent transformation by palladized zerovalent iron: mechanistic insights from reductant loading studies and solvent kinetic isotope effects.
abstract: Palladized nanoscale zerovalent iron (Pd/NZVI) has been utilized for source zone control, yet the reductant responsible for pollutant transformation and the optimal conditions for subsurface application remain poorly understood. Here, trends in Pd/Fe reactivity toward 1,1,1,2-tetrachloroethane (1,1,1,2-TeCA) and cis-dichloroethene (cis-DCE) were compared in H2O and D2O batch systems as a function of pH, chlorinated solvent concentration, Pd surface loading, Pd/Fe mass loading, Pd/Fe aging time, and zerovalent iron [Fe(0)] particle size. For Pd/NZVI, the solvent kinetic isotope effect [i.e., kobs(H2O)/kobs(D2O) or SKIE] for 1,1,1,2-TeCA and cis-DCE reduction increased substantially with Pd loading and Pd/NZVI concentration, evidence that multiple pathways exist for chlorinated solvent reduction. At low Pd loadings and Pd/NZVI concentrations with relatively small SKIEs (less than ~5), we propose that modest reactivity enhancements (≤ 10-fold) reflect more efficient electron transfer to 1,1,1,2-TeCA from Fe(0) facilitated by Pd nanodeposits. Much larger SKIEs (e.g., exceeding 100 for cis-DCE) imply the involvement of atomic hydrogen in more reactive systems with high Pd loadings and Pd/NZVI concentrations. Generally, evidence of SKIEs supporting a dominant role for atomic hydrogen was not observed for Pd/Fe prepared from micrometer-sized Fe(0), or for any size of nonpalladized Fe(0). During anaerobic aging of Pd/NZVI, decreases in the SKIE for 1,1,1,2-TeCA reduction suggest that atomic hydrogen will contribute to reactivity for only approximately 1 week after application.
authors.affiliation: Department of Chemical and Environmental Engineering, University of California , A242 Bourns Hall, Riverside, California 92521, USA.
authors.forename: Yang
authors.initials: Y
authors.lastname: Xie
authors.forename: David M
authors.initials: DM
authors.lastname: Cwiertny
mesh_terms: D002713:Chlorine / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D007554:Isotopes / Q000737:chemistry*; D007700:Kinetics; D046529:Microscopy, Electron, Transmission; D010084:Oxidation-Reduction; D010165:Palladium / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry*
chemical_list: D007554:Isotopes; D012997:Solvents; D002713:Chlorine; D010165:Palladium; D007501:Iron