Accelerated oxidation of 2,4,6-trichlorophenol in Cu(II)/HO/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)/HO/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)/HO/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] = 1000 mM by the Cu(II)/HO system was approximately 46-fold higher than that at [Cl] = 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)/HO/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 "OH model") were developed to describe the kinetics of Cu(II)/HO/Cl system. The failure of "OH model" to rationalize the observed AOX decay has disproved the "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 OH, is the major product resulting from reactions between Cu(I)-chloro complexes and HO at neutral pH.