Hydroquinone enhanced Cu(II)/peroxymonosulfate for the degradation of sulfonamide antibiotics under acidic pH conditions

The Cu(II)-mediated activation of peroxymonosulfate (PMS) has been studied for the degradation of contaminants due to its ability to produce reactive species. However, the yield of reactive species generated from Cu(II)/PMS is relatively low under acidic conditions due to the slow conversion rate of metal valence states, which limits its application prospects. Herein, we found that the introduction of hydroquinone (HQ) in the Cu(II)/PMS system can accelerate the activation of PMS by promoting the redox cycling of Cu(II)/Cu(I), thereby achieving efficient destruct of sulfonamide antibiotics. The degradation rate constants of six sulfonamide antibiotics by HQ/Cu(II)/PMS system was 5–8 times of that by Cu(II)/PMS system under pH 3.0. Both hydroxyl radical (HO·) and singlet oxygen (¹O₂) participated in the degradation of sulfamethoxazole (SMX, a representative sulfonamide antibiotic), and the former played the most important role. HQ improved the reduction of Cu(II) to Cu(I) due to the electron-donating capacity of hydroxyl in HQ, thereby promoting PMS decomposition to produce more HO· under acidic conditions. The presence of humic acid (HA) and low concentration of Cl^- presented the inhibitory effect on SMX degradation, while the degradation of SMX was promoted in the presence of high concentration of Cl^-. The addition of NO₃^– and SO₄^2- has almost no effect on the removal of SMX. The degradation pathways of SMX by HQ/Cu(II)/PMS system mainly involved in the S-N cleavage and amino oxidation. The significant enhancement effects were still observed in actual wastewater treatment by Cu(II)/PMS system after introduction of HQ.