Insight into Cl2•−-dominated degradation of pharmaceuticals in high-salinity wastewater: Radical contributions, oxidation byproducts and toxicity evaluation

UV/Fenton proves to be a highly effective approach for treating high-salinity organic wastewater. Throughout the treatment, Cl2•− can significantly contribute to the degradation of organic contaminants. Our study evaluated the impact of Cl− on the degradation of four typical pharmaceuticals in the UV/Fenton system, and further investigated the overlooked transformation pathways of organic compounds mediated by Cl2•−. The results indicated that as the Cl− concentration increased from 0 to 5000 mg/L, the degradation of carbamazepine, ibuprofen, and metronidazole was significantly inhibited. Conversely, the sulfadiazine (SDZ) degradation was significantly promoted and the pseudo-first-order rate constant (kobs) increased from 6.06 × 10−3 to 1.14 × 10−2 s−1. The contribution of HO• to SDZ degradation decreased from 82.8 % to 3.0 %, whereas the contribution of Cl2•− increased from 0 % to 79.4 %. Moreover, the degradation pathways of SDZ induced by HO• or Cl2•− were compared and the toxicity of the primary byproducts was assessed. Although a high steady-state concentration of Cl2•− enhanced the removal of SDZ, intermediates with higher toxicity, such as P185, P221 and P433, were simultaneously produced and tended to accumulate, leading to lower mineralization and higher toxicity of the reaction system. Therefore, while Cl2•− may enhance the degradation of some pollutants, the transformation of organic compounds it induces could potentially pose heightened environmental risks.