Haloacetaldehydes (HALs) and haloacetonitriles (HANs) are groups of carcinogenic disinfection by products (DBPs) present in water supplies, of which trichloroacetaldehyde (TCAL) and dichloroacetonitrile (DCAN) are frequently detected. The efficiency of ultraviolet (UV) irradiation processes in the removal of DBPs depends strongly on the contribution of direct and indirect photolysis. Significant gaps exist in research regarding kinetics of photodegradation in multi-solute systems. Therefore, in this study the efficiency of vacuum UV (VUV) and UV-C processes was tested on batch photodegradation with synthetic waters containing either TCAL or DCAN and bi-solutes. A radical scavenger test was performed to determine the presence of OH radicals. The VUV (185 + 254 nm) degraded TCAL and DCAN more effectively than UV-C (254 nm), achieving absolute elimination after 30 min (>99.9%, 113 mW/cm2) for TCAL, but only an 84% reduction in DCAN after 120 min of irradiation at fluence of >450 mW/cm2. The experimental results demonstrate that the main mechanism in TCAL reduction was indirect photolysis, but for DCAN it was direct photolysis by VUV photolysis. When indirect photolysis dominated, HALs and HANs in the mixture competed for OH radicals under VUV photolysis. A degradation pathway study indicated that TCAL was degraded and transformed to formic acid, while DCAN was dechlorinated by OH radicals. Overall, this study confirms that the VUV process is more effective than UV-C in photodegrading carbonaceous DBPs.
Nakboon et al. (Sun,) studied this question.