Accelerated oxidation of 2,4,6-trichlorophenol in Cu(II)/H₂O₂/Cl^- system: A unique “halotolerant” Fenton-like process?

The roles of chloride in enhanced oxidative degradation of refractory organic pollutants are recently identified in the Cu(II)/H₂O₂/Cl⁻ system, but the identity of the reactive oxidants and potential conversion of inorganic chloride to organochlorine in such oxidizing environment remain obscure. Here we report that Cu(II)/H₂O₂/Cl⁻ system is a unique “halotolerant” Fenton-like process that works most efficiently in saline water among the five tested redox-active metals ions (i.e. Cr(VI), Ce(III), Co(II), Mn(II) and Cu(II)). The observed pseudo first-order rate constant for 2,4,6-trichlorophenol (TCP) degradation was linearly correlated with the elevated Cl⁻ content. The TCP degradation rate at [Cl⁻]₀ = 1000 mM by the Cu(II)/H₂O₂ system was approximately 46-fold higher than that at [Cl⁻]₀ = 5 mM. The optimal mineralization rate of TCP and percentage of absorbable organic halogens (AOX) decrease were 31.6% and 63.8%, respectively, in the tested Cu(II)/H₂O₂/Cl⁻ system. However, the detection of fused chlorinated byproducts (i.e. chloro-anthracene-pentaol, dioxine, chlorinated dibenzofuran) reminds us of cautiousness in evaluating the applicability of Cu(II)-catalyzed Fenton-like reaction, particularly while it is to be applied to the treatment of wastewater contaminated with chlorophenols. Two independent models (i.e. “Cu(III) model” and “^[rad]OH model”) were developed to describe the kinetics of Cu(II)/H₂O₂/Cl⁻ system. The failure of “^[rad]OH model” to rationalize the observed AOX decay has disproved the “^[rad]OH model” through reductio ad absurdum. The ability of “Cu(III) model” to adequately explain the experimental data demonstrates that Cu(III)-chloro complexes, rather than ^[rad]OH, is the major product resulting from reactions between Cu(I)-chloro complexes and H₂O₂ at neutral pH.