Reductive dehalogenation of hexachloroethane, carbon tetrachloride, and bromoform by anthrahydroquinone disulfonate and humic acid

Article abstract of DOI:10.1021/es00062a026

The reductive dehalogenation of hexachloroethane (C₂Cl₆), carbon tetrachloride (CCl₄), and bromoform (CHBr₃) was examined at 50 °C in aqueous solutions containing either (1) 500 μM of 2,6-anthrahydroquinone disulfonate (AHQDS), (2) 250 μM Fe²⁺, or (3) 250 μM HS^-. The pH ranged from 4.5 to 11.5 for AHQDS solutions and was 7.2 in the Fe²⁺ solutions and 7.8 in the HS^- solutions. The observed disappearance of C₂Cl₆ in the presence of AHQDS was pseudo-first-order and fit k'(CCl₄) = k₀[A(OH)₂] + k₁[A(OH)O^-] + k₂[A(O)₂^2-] where A(OH)₂, A(OH)O^-, and A(O)₂^2- represent the concentrations of the three forms of the AHQDS in solution. The values of k₀, k₁, and k₂ were ~0, 0.031, and 0.24 M^-1 s^-1, respectively. The addition of 25 mg of C/L of humic acid or organic matter extracted from Borden aquifer solids to aqueous solutions containing 250 μM HS^- or Fe²⁺ increased the reduction rate by factors of up to 10. The logarithms of the rate constants for the disappearance of C₂Cl₆ and CCl₄ in seven different experimental systems were significantly correlated; log k'(CCl₄) = 0.64 log k'(C₂Cl₄)-0.83 with r² = 0.80. The observed trend in reaction rates of C₂Cl₆ > CCl₄ > CHBr₃ is consistent with a decreasing trend in one-electron reduction potentials. The reductive dehalogenation of hexachloroethane (C₂Cl₆), carbon tetrachloride (CCl₄), and bromoform (CHBr₃) was examined at 50°C in aqueous solutions containing either (1) 500 μM of 2,6-anthrahydroquinone disulfonate (AHQDS), (2) 250 μM Fe²⁺, or (3) 250 μM HS^-. The pH ranged from 4.5 to 11.5 for AHQDS solutions and was 7.2 in the Fe²⁺ solutions and 7.8 in the HS^- solutions. The observed disappearance of C₂Cl₆ in the presence of AHQDS was pseudo-first-order and fit k′_CCl(4) = k₀[A(OH)₂] + k₁[A(OH)O^-] + k₂[A(O)₂^2-] where A(OH)₂, A(OH)O^-, and A(O)₂^2- represent the concentrations of the three forms of the AHQDS in solution. The values of k₀, k₁, and k₂ were approximately 0, 0.031, and 0.24 M^-1 s^-1, respectively. The addition of 25 mg of C/L of humic acid or organic matter extracted from Borden aquifer solids to aqueous solutions containing 250 μM HS^- or Fe²⁺ increased the reduction rate by factors of up to 10. The logarithms of the rate constants for the disappearance of C₂Cl₆ and CCl₄ in seven different experimental systems were significantly correlated; log k′_CCl(4) = 0.64 log k′_C(2)Cl₆ approximately 0.83 with r² = 0.80. The observed trend in reaction rates of C₂Cl₆ > CCl₄ > CHBr₃ is consistent with a decreasing trend in one-electron reduction potentials.