Insights into chloroacetaldehydes degradation by 254 nm ultraviolet: Kinetics, products, and influencing factors

Haloacetaldehydes (HALs) are a type of disinfection byproducts (DBPs) frequently found in disinfected drinking water, and their effective control is important to safeguard human health. To evaluate the appropriateness of using ultraviolet technology for drinking water treatment, this study comparatively examines the photolysis of three chlorinated HALs (Cl-HALs), including monochloroacetaldehyde (MCAL), dichloroacetaldehyde (DCAL), and trichloroacetaldehyde (TCAL), under varying operational and environmental conditions by 254 nm ultraviolet (UV254). Results show that they can be degraded within 2 h at an initial concentration of 20 μg/L with pseudo-first-order reaction rates ranked as MCAL > DCAL > TCAL. Chloride was the major product and total organic carbon decreased substantially, indicating that UV irradiation of Cl-HALs mainly underwent mineralization. MCAL was not formed during photolysis of DCAL or TCAL, suggesting that the carbon-chloride bonds in Cl-HALs were cleaved simultaneously rather than sequentially. In terms of influencing factors, adding free chlorine into solution apparently enhanced Cl-HALs photolysis, whereas increasing Cl-HALs dosage, nitrate, bromide, or iodide inhibited Cl-HAL photolysis. Under their combined effects, the photolysis rates of Cl-HALs dosed into tap and swimming pool waters were lower than those dosed into ultrapure water. These findings hence provide insights for better understanding the treatability of UV254 for Cl-HALs control.