Bromide Affects the Degradation of Naphthalene by Uv/Chlorine Process in Seawater: Kinetics, Radical Contribution, and Transformation Pathway

Naphthalene (Nap) is one of the polycyclic aromatic hydrocarbons frequently detected in freshwater and seawater. However, the occurrence of bromide in seawater affects the performance of the UV/chlorine process. Therefore, the UV/chlorine process was set up in this work, and the effect of bromide on Nap degradation and corresponding radical contributions under different pH and water matrix components were studied. The results showed that the degradation of Nap during the UV/chlorine process followed pseudo first-order kinetics, and the degradation rate constant increased from 0.00032s-1 to 0.00036 s-1 when 0.8 mM bromide was present at pH 8. Interestingly, the effect of bromide on the degradation of Nap by the UV/chlorine process was pH-dependent. The presence of bromide inhibited the degradation of Nap at pH 6 and 10, while promoting the degradation of Nap at pH 8. In addition, the coexistence of 540 mM chloride and 0.8 mM bromide significantly promoted the degradation of Nap at pH 8, consistent with faster degradation of Nap in seawater compared to ultrapure water. Humic acid (HA) enhanced the degradation of Nap at low concentration (i.e., 500 μg/L), while a higher concentration of HA inhibited the degradation at pH 8. Based on the probe experiments, it was found that reactive halogen species (RHS) are the main radicals contributing to the degradation of Nap in seawater. Meanwhile, the intermediate products of Nap were analyzed by GC-MS, and the degradation of Nap mainly included hydroxylation, epoxidation, and ring opening. The evaluation of toxicity during Nap degradation based on software Ecosar2.0 showed that toxicity of intermediate products increased at first and then decreased. This study provided a promising technique to treat contaminated seawater containing Nap.