UV/ peroxymonosulfate process for degradation of chloral hydrate: Pathway and the role of radicals

Highlights: • Chloral hydrate (CH) can be completely mineralized by UV₂₅₄/PMS process. • HO accounted for the mineralization of CH by UV₂₅₄/PMS process. • O₂⁻ transferred CH into refractory trichloroacetic acid (TCAA) in base/PMS system. • UV exposure ceased transformation of CH to TCAA and facilitated CH mineralization. In this study, kinetics, influencing factors and potential mechanisms involved in the degradation of chloral hydrate (CH) by UV/peroxymonosulfate (PMS) process were demonstrated. The degradation rate of CH could reach 89.6% by UV₂₅₄/PMS process, significantly exceeding UV₃₀₀/PMS (0.7%), UV₃₅₀/PMS (6.3%), UV₂₅₄ direct photolysis (9.0%) and PMS alone (0.0%) processes. CH degradation in UV₂₅₄/PMS system followed pseudo first-order degradation kinetics with an apparent rate constant of 0.186 min⁻¹, which was suppressed by Cl⁻ and HCO₃⁻. The optimal pH for CH degradation was around 5.0. Direct mineralization accounted for the CH degradation in UV/PMS system. Interestingly, the addition of PMS at the neutral condition before UV irradiation transferred CH into trichloroacetic acid (TCAA). The transformation efficiency of CH into TCAA at 10 min was enhanced from 2.17%–40.38% with the elevation of initial pH from 7.0–8.0. The subsequent exposure of UV lamps ceased the transformation of CH into TCAA and facilitated the direct mineralization of CH, but it did not work in the refractory TCAA degradation. Finally, it was revealed that HO predominantly participated CH degradation in UV/PMS process, while O₂⁻ was responsible for the transformation of CH into TCAA by addition of PMS before UV irradiation.