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 (SO) 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 SO and monochloramine (NHCl) on the removal of 1,4-dioxane (1,4-D) for potable-water reuse. The dual oxidant effects of NHCl and SO on 1,4-D degradation were examined at various levels of oxidant dosage, chloride, and solution pH. Results showed that a NHCl-to-SO molar ratio of 0.1 was optimal, beyond which the scavenging by NHCl of HO, SO, and Cl 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 NHCl and SO was sensitive to the solution pH due to a disproportionation of NHCl at pH lower than 6 into less-photoreactive dichloramine (NHCl) and radical scavenging by NH. The presence of chloride transformed HO and SO to Cl that is less-reactive with 1,4-D, while the presence of dissolved O 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 NHCl dosage and a minimal chloride residue.