Oxidation of theophylline by Ferrate (VI) and formation of disinfection byproducts during subsequent chlorination

This work investigated the degradation of theophylline (TPL) of environmental concerns by ferrate (Fe(VI)) and the effect of Fe(VI) pre-oxidation on formation of disinfection by-products (DBPs) from TPL during subsequent chlorination. TPL exhibited an appreciable reactivity toward Fe(VI) with apparent second-order rate constants (k_app) ranging from 5.7 to 2480 M⁻¹ s⁻¹ in the pH range from 6 to10. The pH-dependence of k_app could be well simulated by a proposed kinetic model including the generation of an intermediate between Fe(VI) and TPL. A tentative transformation pathway was proposed including the hydroxylation of the imidazole ring as well as the cleavage of C[dbnd]N and C–N bonds. The presence of humic acid (HA) apparently reduced the degradation rates of TPL with Fe(VI) even considering the competitive consumption for Fe(VI) by HA. A two channel model successfully explained the effect of HA on the TPL degradation by Fe(VI), where oxidation intermediate of TPL was either reduced back to parent by HA or irreversibly transformed to final products. Compared to direct chlorination of TPL, Fe(VI) pre-oxidation at low dosage enhanced the formation of carbonaceous disinfection by-products (C-DBPs, e.g., 1,1-dichloro-2-propanone, 1,1,1-trichloro-2-propanone, and dichloroacetic acid) during subsequent chlorination. However, a high dosage of Fe(VI) pre-oxidation could decrease the formation of C-DBPs and nitrogenous disinfection by-products (N-DBPs, e.g., dichloroacetonitrile, trichloronitromethane). The effectiveness of Fe(VI) for elimination of TPL in natural water was confirmed, and Fe(VI) pre-oxidation at high doses reduced the formation of DBPs in natural water during subsequent chlorination.