Enhanced Dechlorination of Chlorinated Methanes and Ethenes by Chloride Green Rust in the Presence of Copper(II)

Abstract

The enhanced removal of carbon tetrachloride (CCl4), tetrachloroethene (C2Cl4), and trichloroethene (C2HCl3) by chloride green rust (GR(Cl)) in the presence of copper ions was investigated. X-ray powder diffraction (XRPD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the crystallization and chemical speciation, respectively, of the secondary mineral phases produced in the GR(Cl)−Cu(II) system. The addition of Cu(II) to GR(Cl) suspensions resulted in enhanced dechlorination of the chlorinated hydrocarbons examined in this study. The degradation reactions followed pseudo-first-order kinetics and the pseudo-first-order rate constant (kobs) for CCl4 (20 μM) removal by GR(Cl) at pH 7.2 was 0.0808 h-1. Addition of 0.5 mM Cu(II) completely dechlorinated CCl4 within 35 min, and the kobs was 84 times greater than that in the absence of Cu(II). Chloroform (CHCl3), the major chlorinated product in CCl4 dechlorination, accumulated at a concentration up to 13 μM in the GR(Cl) system alone, but was completely dechlorinated within 9 h in the GR(Cl)−Cu(II) suspension. Also, rapid removal of C2Cl4 and C2HCl3 by GR(Cl) was observed when Cu(II) was added. The kobs values for the removal of chlorinated ethenes were 4.7−7 times higher than that obtained in the absence of Cu(II). In addition, the kobs for PCE removal increased linearly with respect to Cu(II) concentrations in the range from 0.1 to 1.0 mM. Addition of Cu(II) at a concentration higher than 1.0 mM decreased the kobs for the removal of both C2Cl4 and C2HCl3 due to the decrease in structural Fe(II) concentration in GR(Cl) and the changes in redox potentials and pH values. Moreover, the highest removal efficiency and rate of C2Cl4 was obtained at near-neutral pH when Cu(II) was added into the GR(Cl) suspension. XPS and XRPD results showed that the Fe(II) in the GR(Cl) suspension could reduce Cu(II) to both Cu(I) and metallic Cu. These findings are relevant to the better understanding of the role of abiotic removal of chlorinated hydrocarbons during remediation and/or natural attenuation in iron-reducing environments.