Synergistic Effect of Copper Ion on the Reductive Dechlorination of Carbon Tetrachloride by Surface-Bound Fe(II) Associated with Goethite

The dechlorination of carbon tetrachloride (CT) by Fe(II) associated with goethite in the presence of transition metal ions was investigated. X-ray photoelectron spectroscopy (XPS) and X-ray powder diffraction (XRPD) were used to characterize the chemical states and crystal phases of transition metals on solid phases, respectively. CT was dechlorinated to chloroform (CF) by 3 mM Fe(II) in 10 mM goethite (25.6 m² L^-1) suspensions. The dechlorination followed pseudo-first-order kinetics, and a rate constant (k_obs) of 0.036 h^-1 was observed. Transition metal ions have different effects on CT dechlorination. The addition of Ni(II), Co(II), and Zn(II) lowered the k_obs for CT dechlorination, whereas the amendment of 0.5 mM Cu(II) into the Fe(II)−Fe(III) system significantly enhanced the efficiency and the rate of CT dechlorination. The k_obs for CT dechlorination with 0.5 mM Cu(II) was 1.175 h^-1, which was 33 times greater than that without Cu(II). Also, the dechlorination of CT by surface-bound iron species is pH-dependent, and the rate constants increased from 0.008 h^-1 at pH 4.0 to 1.175 h^-1 at pH 7.0. When the solution contained Cu(II) and Fe(II) without goethite, a reddish-yellow precipitate was formed, and the concentration of Fe(II) decreased with the increase in Cu(II) concentration. XPS and XRPD analyses suggested the possible presence of Cu₂O and ferrihydrite in the precipitate. Small amounts of aqueous Cu(I) were also detected, reflecting the fact that Cu(II) was reduced to Cu(I) by Fe(II). A linear relationship between k_obs for CT dechlorination and the concentration of Cu(II) was observed when the amended Cu(II) concentration was lower than 0.5 mM. Moreover, the k_obs for CT dechlorination was dependent on the Fe(II) concentration in the 0.5 mM Cu(II)-amended goethite system and followed a Langmuir−Hinshelwood relationship. These results clearly indicate that Fe(II) serves as the bulk reductant to reduce both CT and Cu(II). The resulting Cu(I) can further act as a catalyst to enhance the dechlorination rate of chlorinated hydrocarbons in iron-reducing environments.