Kinetics of Haloacetaldehydes’ Reactions with Radicals Produced in Radiolysis of Water

Halogenated acetaldehydes (HALs) are abundant disinfection byproducts (DBPs) in drinking water with some congeners being more cyto/genotoxic than EPA-regulated DBPs. Advanced oxidation/reduction processes (AOPs/ARPs) are an attractive option for eliminating HALs from water sources; however, kinetic data for their reactions with AOP hydroxyl radicals (HO•) and ARP hydrated electrons (eaq–)/hydrogen atoms (H•), are limited. Using electron pulse radiolysis bimolecular rate constants at pH 7 and 24–25 °C for five important HALs (chloro-, dichloro-, trichloro-, bromo-, and dibromoacetaldehyde) were determined. HO• radicals react by hydrogen atom abstraction from the gem-diol (hydrate) form with kHO• values of (1.36 ± 0.07) × 109, (1.77 ± 0.01) × 109, (3.07 ± 0.14) × 109, (1.11 ± 0.01) × 109 and (3.01 ± 0.21) × 109 M–1 s–1, respectively. Corresponding HAL keaq- rate constants monitored at 720 nm were (2.45 ± 0.33) × 109, (5.81 ± 0.45) × 109, (1.50 ± 0.08) × 1010, (9.05 ± 0.53) × 109 and (1.56 ± 0.12) × 1010 M–1 s–1. These structure-dependent kinetics were consistent with electron reductive dehalogenation. Chlorinated HAL kH• values measured by electron paramagnetic resonance methods were 100× slower than for eaq–. These data successfully modeled the steady-state 60Co irradiation of an O2-saturated chloroacetaldehyde solution, validating their use for computer optimization of HO•-driven AOPs to mitigate HALs at larger scale.