Enhanced abatement of phenolic compounds by chlorine in the presence of CuO: Absence of electrophilic aromatic substitution.

It has been demonstrated that chlorine predominately reacts with phenolic compounds through an electrophilic aromatic substitution, yielding chlorinated phenols. Previous studies showed that copper oxide (CuO), a water pipe corrosion product, can catalytically enhance the reactivity of chlorine and its disproportionation. In this study, kinetics and mechanisms for the reactions of chlorine with phenolic compounds in the presence of CuO were investigated. CuO at 100 mg/L increases the apparent second-order rate constants (kapp) for reactions of chlorine with phenol, chlorophenols, bromophenols, iodophenols, 2,6-dimethylphenol, acetaminophen, and 4-hydroxybenzoic acid at pH 7.6 and 21 °C by up to 50 times. For the same reaction conditions, increasing CuO concentrations from 0 to 200 mg/L increase the kapp of phenol chlorination from ∼42 to 608 M-1 s-1. In general, a stronger enhancement of the chlorine reactions with phenols was observed in the pH range of 6.6-7.6 than 7.6-9.0, indicating that CuO more readily activates hypochlorous acid. Moreover, CuO significantly changes the pathway for phenol chlorination. Yields of chlorophenols decreased from 98 % to < 5 % as the CuO concentration increased from 0 to 100 mg/L. Non-chlorinated compounds (e.g., catechol, 2,3-dihydroxymuconic acid, maleic acid, and oxalic acid) are major transformation products. Model simulations suggest a pre-equilibrium step with the formation of a CuO-HOCl complex as the rate-limiting step for the overall reactions. Heterogeneous chlorination processes with limited formation of chlorinated phenols tend to be predominant for CuO concentrations > ∼ 5 - 36 mg/L for various phenols. These findings have implications for the transformation of phenolic compounds during chlorination in copper-containing water distribution systems.