Enhancement to micropollutant degradation and reduction of halogenated by-product formation in the UV-LED/chlorine process by adding persulfate

The UV/chlorine process has emerged as an advanced oxidation process (AOP) for micropollutant degradation in drinking water treatment and potable water reuse. It produces a diverse spectrum of reactive species including hydroxyl radicals (HO·) and reactive chlorine species (RCS), which complement each other in the micropollutant degradation. However, halogenated by-products are formed in the UV/chlorine AOP. UV light emitting diodes (UV-LEDs), which are mercury-free, compact and robust, are promising alternatives to conventional mercury-vapor UV lamp, and show potential use in the UV-based AOPs. At neutral and slightly alkaline pHs, the photolysis of chlorine and the consequential radical formation increased with increasing wavelength from 265 to 300 nm by UV-LEDs, and the radical yields were higher than that by the conventional LP-UV lamps. This study investigated the degradation of four micropollutants (i.e., nitrobenzene, benzoic acid, carbamazepine and 1,4-dimethoxybenzene) and halogenated by-product formation in the UV-LED/chlorine AOP at three wavelengths (i.e., 265, 285, and 300 nm) and neutral pH, with or without persulfate additions. The degradation of the four compounds was enhanced by adding a small amount of persulfate (e.g., 100 µM). The observed rate constants of the degradation of the four compounds in the UV-LED/chlorine/persulfate system were higher than the sum of the corresponding rate constants observed in the UV-LED/chlorine alone and UV-LED/persulfate alone systems (i.e., UV-LED/chlorine/persulfate > UV-LED/chlorine + UV-LED/persulfate), indicating that the enhancement to the micropollutant degradation was not simply due to the generation of radicals from the UV photolysis of persulfate. The enhancement was attributed to the transformation of RCS to HO· and sulfate radicals (SO4-·), which were more reactive toward the tested micropollutants, in the presence of persulfate. In addition, in the presence of persulfate, the formation of total organic chlorine (TOCl) from natural organic matter (1 mg/L as C) was reduced by 20–30% in the UV-LED/chlorine AOP. It also suggested the transformation of the halogen radicals (i.e., RCS) to the non-halogen radicals (e.g., HO· and SO4-·). The establishment a dynamic model to describe the distribution of the radicals in the UV-LED/chlorine process in the presence of persulfate is on-going and the effects of UV wavelength, solution pH and chlorine/persulfate ratios will also be evaluated and presented.