In the present study, mulberry branch-derived biochar CuO (MBC/CuO) composite was successfully synthesized and used as a catalyst to activate persulfate (PS) for the degradation of bisphenol A (BPA). The MBC/CuO/PS system exhibited a high degradation efficiency (93%) of BPA, under the conditions of 0.1 g/L MBC/CuO, 1.0 mM PS, 10 mg/L BPA. Free radical quenching and electron spin-resonance spectroscopy (ESR) experiments confirmed that both free radicals ^•OH, SO₄^•− and O₂^•− and non-radicals ¹O₂ were involved in the MBC/CuO reaction system. Cl⁻ and NOM displayed negligible influence on the degradation of BPA, while HCO₃⁻ promoted the removal of BPA. In addition, the toxicity tests of BPA, MBC/CuO and the degraded BPA solution were conducted by the 5th instar silkworm larvae. The toxicity of BPA was reduced after the treatment in the MBC/CuO/PS system, and no obvious toxicity of the synthesized MBC/CuO composite was found in the toxicity evaluation experiments. This work provides a new value-added utilization of mulberry branches as a cost-effective and environmentally friendly PS activator.

The activation of persulfate (PS) by Fe(III) for the removal of environmental organic pollutants was severely limited by the low reduction rate from Fe(III) to Fe(II). In present study, we reported that reducing sulfur species (i.e., SO₃²⁻, HSO₃⁻, S²⁻, and HS⁻) under low concentration could significantly accelerate the Fe(III)/Fe(II) cycle in the Fe(III)/PS system. Under the condition of 1.0 mM Fe(III) and 4.0 mM PS, the removal performance of Fe(III)/PS system was poor, and only 21.6% of BPA was removed within 40 min. However, the degradation efficiency of BPA increased to 66.0%, 65.5%, 72.9% and 82.7% with the addition of 1.0 mM SO₃²⁻, HSO₃⁻, S²⁻, and HS⁻, respectively. The degradation efficiency of BPA was highly dependent on solution pH and the concentration of reducing sulfur species. When the reductant was excessive, the removal efficiency would be significantly inhibited due to the elimination of reactive species. This study provided some valuable insights for the treatment of organic wastewater containing these inorganic reducing ions.