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Vacuum-UV (VUV) irradiation is a potentially viable advanced oxidation process (AOP) for water treatment and potable reuse applications due to high generation of hydroxyl radicals. Under potable reuse conditions, free chlorine and inorganic chloramines are present in the AOP feed water and could possibly participate in the photochemical reactions. The focus of this study is to determine the photochemical parameters of these species at 185 nm, which are necessary for accurate kinetic modeling of the VUV process. The measured molar absorption coefficients at 185 nm for both free chlorine and chloramines are significantly higher than the values reported at 254 nm, implying that high concentrations of these oxidants could act as an inner filter to the VUV photons. Furthermore, kinetic modeling of the process showed that both hypochlorous acid and hypochlorite ion can undergo direct photolysis at 185 nm with quantum yields of 0.7 and 0.1 mol/einstein, respectively, while the direct photolysis of chloramines at this wavelength is negligible. Overall, water is the major absorber of 185 nm photons in the VUV AOP and addition of free chlorine enhances radical generation in the system. Also, low concentration of chloramines relevant to potable reuse only slightly decreases the treatment efficiency through shielding and radical scavenging reactions.

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