Efficient degradation of cytotoxic contaminants of emerging concern by UV/H2O2

The degradation kinetics and cytotoxicity of two commonly detected contaminants of emerging concern (CECs), diclofenac and triclosan, in UV/H ₂O ₂ system were investigated in this study. The second-order rate constants of hydroxyl radical (OH) with diclofenac (k _{OH,diclofenac}) and triclosan (k _OH,triclosan) varied at different reaction pH (5.3-8.5) in the range of 7.6-9.1 × 10 ⁹ M ^-1 s ^-1 and 7.0-4.4 × 10 ⁹ M ^-1 s ^-1, respectively. The pH plays a crucial role in the UV/H ₂O ₂ treatment for the destruction of diclofenac, triclosan, and four additional CECs (estrone, 17β-estradiol, 17α-ethynylestradiol, and bisphenol A), affecting the ionic state of the CECs (based on pK _a) and scavenging the OH by increasing the concentration of hydroxide ion. The impacts of H ₂O ₂ concentration, common inorganic ions (i.e., HCO ₃ ^-, NO ₃ ^-, Cl ^-, and SO ₄ ^2-), and natural organic matter (NOM) were studied as well. Field water samples from the local water works and Lake Harsha were utilized as reaction matrices to assess the possibility of applying UV/H ₂O ₂ to decompose diclofenac and triclosan in surface water. Cytotoxicity of diclofenac and triclosan was not reduced during treatment even though concentrations of the compounds were diminished, indicating the formation of toxic transformation products. Overall, UV/H ₂O ₂ is useful to degrade CECs, such as diclofenac and triclosan, in both Milli-Q water and field water samples, but higher UV fluence might be needed to reduce the cytotoxicity of CECs after UV/H ₂O ₂ treatment.