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Impact of water solutes on the formation of nitrite under Vacuum UV(VUV) advanced oxidation of nitrate-rich water Han, Mengqi

Abstract

Advanced oxidation processes (AOPs) are promising treatment options for the degradation of micropollutants in water. Vacuum-UV (VUV), as one such AOPs with UV photons under 200 nm, can directly photolyze water to produce •OH without extra oxidant or catalyst. However, a potential challenge in treating micropollutants by VUV, is the formation of nitrite in nitrate-containing water. Nitrate absorbs 185 nm photons, leading to the potential formation of nitrite. Given the greater toxicity and more stringent regulatory limits of nitrite on its concentration in drinking water, it is essential to examine the mechanisms of nitrite formation during VUV AOP. This research focused on understanding the effect of common solutes present in water, including dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), chloride, and sulfate on the formation of nitrite in nitrate-containing water during VUV photolysis. Experimental work involved kinetic studies using a custom-made benchtop UV/VUV collimated beam setup. Water samples containing solutes of interests at different concentrations were irradiated and collected for analysis at set intervals, corresponding to UV254 fluences of up to 1200 mJ/cm2. The results indicated that the formation of nitrite follows a very complex mechanism and is not simply dependent upon the concentration of nitrate. Among the solutes evaluated, DOC and chloride had the greatest impact on nitrite formation. The presence of DOC, through its scavenging of •OH, resulted in increased formation of nitrite. Chloride, on the other hand, led to a significant reduction in nitrite formation due to its competition with nitrate for absorbing VUV photons. Unlike chloride and DOC, sulfate and DIC, at concentrations commonly present in water, had little impact on nitrite formation. Their impact was only evident at extremely high concentrations to slightly reduce nitrite formation. When all the solutes, i.e., DOC, DIC, sulfate, and chloride, were present simultaneously, the effect of DOC was more dominant and eventually increased nitrite formation. Finally, dissolved oxygen was determined to decrease nitrite formation through the scavenging of ∙H and hydrate electrons. The details of the experimental and mechanistic studies can provide scientific guidance towards more effective and optimized application of VUV technology for drinking water treatment.

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