Formation of chlorinated biphenyls, diphenyl ethers and benzofurans as a result of Fenton-driven oxidation of 2-chlorophenol

Abstract

Homogeneous catalytic Fenton oxidation proved to be very efficient in the degradation of high concentrations (3.9 mM) of 2-chlorophenol (2-CP) in aqueous matrices. When using [H 2O 2] 0/[2-CP] 0 substoichiometric molar ratios of 4 and 16, the detected aromatic intermediates included mainly chlorinated benzenediols, with the virtual absence of condensation products of higher molecular weight. At even lower substoichiometric ratios of [H 2O 2] 0/[2-CP] 0 (⩽2.2), hydroxylated chlorobiphenyls, hydroxylated chlorodiphenyl ethers and hydroxylated chlorinated dibenzofurans were formed in addition to chlorinated benzenediols. The aromatic intermediates were identified as trimethylsilyl ethers and dimethyl-t-butyl silyl ethers. A reaction scheme was proposed to describe the formation of aromatic intermediates based on coupling reactions of resonance-stabilized 2-CP radicals generated by electrophilic attack of reactive hydroxyl radicals. The pattern of aromatic intermediates identified in the Fenton solutions coincided well with that predicted on the basis of oxidative coupling reactions. In addition to coupling of stabilized radicals, aromatic intermediates can be formed by addition of organoradicals onto neutral analyte molecules. The findings presented in this contribution are considered crucial for the design and optimization of Fenton-based remediation devoted either to wastewater treatment under economically feasible conditions or to in situ groundwater treatment where poorly controlled reaction conditions prevail.