Coexisting oxidation and reduction of chloroacetaldehydes in water by UV/VUV irradiation

Haloacetaldehydes (HALs) are the third largest disinfection by-product (DBP) ubiquitously detected in finished drinking water and have relatively higher toxicity than currently regulated DBPs. To efficiently alleviate them, this study investigated a green, chemical-free technology by using ultraviolet/vacuum ultraviolet (UV/VUV) on degrading three refractory chlorinated HALs (Cl-HALs). The results indicate that the rates of Cl-HALs decomposition in tap water irradiated by UV/VUV were 23-70 times higher than those irradiated by UV, proving that VUV instead of UV played the key role in degrading Cl-HALs. Increasing Cl-HALs dosage, pH, and dissolved oxygen (DO) all decreased the Cl-HALs degradations significantly, and the rates in tap water were apparently lower than those in ultrapure water. Unlike previous studies, this study proved that both oxidation and reduction were present during the VUV process. Photooxidation via oxidative radicals like •OH mineralized Cl-HALs, leading to substantial drops of total organic carbon; photoreduction via reductive radicals like •H dehalogenated Cl-HALs, resulting in formation of considerable intermediate organics (e.g., formic acid and acetic acid). No matter what pathway, the mass balances of chlorine were always maintained, meaning that dehalogenation occurred instantaneously rather than sequentially. Although the overall photodegradation rates dropped with rising pH and DO, photoreduction was increased with rising pH while photooxidation was elevated with rising DO. The results hence provide insights to better understand the VUV technology in controlling micropollutants in water.