Enhancing the synergistic effect by introducing Cu(Ⅱ)-PMS into the underwater bubble plasma treatment process for efficient degradation of emerging contaminants in water

This paper presents a novel type of Underwater Bubble Plasma-Divalent Copper Salt-Peroxymonosulfate (UBP-Cu(Ⅱ)-PMS) for the efficient degradation of emerging contaminants in water. The results demonstrate that Cu(Ⅱ)-PMS can be effectively activated during UBP treatment process, generating two highly reactive free radicals ([rad]OH and ·SO₄^_) that promote the degradation of high-concentration tetracycline hydrochloride (TC). A comparative analysis of UBP's performance in activating persulfate (PS), PMS, and sulfite (S(IV)) for the synergistic degradation of TC revealed that PMS and UBP exhibited the strongest synergistic effect. When PMS is 0.5 g/L, the activation performance peaked, with the UBP-PMS system achieving an impressive 87.22 % degradation efficiency of TC within 20 min, which is 30.02 % higher than that of the single UBP system. Furthermore, upon introducing Cu(Ⅱ) into the UBP-PMS system, with PMS at 0.5 g/L and Cu(Ⅱ) at 0.05 g/L, the degradation efficiency of TC reached 95.73 %, and energy efficiency was measured at 1.5509 g·(kWh)⁻¹ within 20 min, surpassing other treatment systems. Free radical trapping experiments indicated that the primary reactive species responsible for TC degradation in UBP-Cu(Ⅱ)-PMS co-catalytic strategy include [rad]OH, ONOOH/·O₂^_, ·SO₄^_ and ¹O₂. Fenton-like reaction enhances the concentration of [rad]OH in the solution, while oxygen-sulfur radical reaction produces ·SO₄^\_. The conversion between different valence states of Cu primarily involves electron transfer. Possible degradation pathways were proposed based on DFT calculations and LC-MS analysis during the synergistic treatment, and the toxicity of by-products generated during the degradation process was evaluated using T.E.S.T. software. Ultimately, a synergistic degradation mechanism for the UBP-Cu(Ⅱ)-PMS strategy was proposed. This study introduces a new activation strategy involving UBP-Cu(Ⅱ)-PMS and provides valuable insights into the degradation performance and mechanisms of emerging contaminants.