Photodegradation of 2-methyl-4-chlorophenol in a KrCl exciplex flow-through photoreactor: a kinetic study

Abstract

The removal of 2-methyl-4-chlorophenol (MC), the main photoproduct from 4-chloro-2-methylphenoxyacetic acid oxidation, has been studied with an advanced oxidation treatment and, for this purpose, a KrCl exciplex flow-through photoreactor was tested. Preliminary results show that the use of UV treatment leads to practically total MC degradation in 10, 60 or 90 min for the initial concentrations of 0.2, 1.0 or 2.0 mM, respectively. However, to attain higher elimination of the main photoproducts, 2-MH and 2-MB, addition of an oxidizing agent, such as hydrogen peroxide, is necessary. With molar ratios H2O2/MC/1 or higher, smaller concentrations of photoproducts are obtained after the treatment. Additional chemical analysis, such as chemical oxygen demand (COD) suggests that although COD is smaller after the treatment than before, for the highest H2O2 concentrations an excess of H2O2 remains in the medium, which increases this parameter, being required longer treatment times. Total phenols, pH, and residual chlorine also confirm these results. Besides, a photodegradation mechanism, based on the formation of an unstable intermediate compound that quickly degrades to the photoproducts, was also proposed. A kinetic model was developed and the good level of approximation between experimental and theoretical conversion values for all the experiments confirms the validity of the model.