UV Photolysis of Chloramine and Persulfate for 1,4-Dioxane Removal in Reverse-Osmosis Permeate for Potable Water Reuse

A sequential combination of membrane treatment and UV-based advanced oxidation processes (UV/AOP) has become the industry standard for potable water reuse. Chloramines are used as membrane antifouling agents and therefore carried over into the UV/AOP. In addition, persulfate (S2O82-) is an emerging oxidant that can be added into a UV/AOP, thus creating radicals generated from both chloramines and persulfate for water treatment. This study investigated the simultaneous photolysis of S2O82- and monochloramine (NH2Cl) on the removal of 1,4-dioxane (1,4-D) for potable-water reuse. The dual oxidant effects of NH2Cl and S2O82- on 1,4-D degradation were examined at various levels of oxidant dosage, chloride, and solution pH. Results showed that a NH2Cl-to-S2O82- molar ratio of 0.1 was optimal, beyond which the scavenging by NH2Cl of HO•, SO4•-, and Cl2•- radicals decreased the 1,4-D degradation rate. At the optimal ratio, the degradation rate of 1,4-D increased linearly with the total oxidant dose up to 6 mM. The combined photolysis of NH2Cl and S2O82- was sensitive to the solution pH due to a disproportionation of NH2Cl at pH lower than 6 into less-photoreactive dichloramine (NHCl2) and radical scavenging by NH4+. The presence of chloride transformed HO• and SO4•- to Cl2•- that is less-reactive with 1,4-D, while the presence of dissolved O2 promoted gaseous nitrogen production. Results from this study suggest that the presence of chloramines can be beneficial to persulfate photolysis in the removal of 1,4-D; however, the treatment efficiency depends on a careful control of an optimal NH2Cl dosage and a minimal chloride residue.