Bisphenol A degradation by chlorine dioxide (ClO₂) and S(IV)/ClO₂ process: Mechanism, degradation pathways and toxicity assessment

Recently, it has been reported that chlorine dioxide (ClO₂) and (bi)sulfite/ClO₂ showed excellent performance in micropollutant removal from water; however, the degradation mechanisms and application boundaries of the two system have not been identified. In this study, bisphenol A (BPA) was chosen as the target contaminant to give multiple comparisons of ClO₂ and S(IV)/ClO₂ process regarding the degradation performance of contaminant, generation of reactive species, transformation of products and toxicity variation. Both ClO₂ and S(IV)/ClO₂ can degrade BPA within 3 min. The BPA degradation mechanism was mainly based on direct oxidation in ClO₂ process while it was attributed to radicals (especially SO₄^·-) generation in S(IV)/ClO₂ process. Meanwhile, the effect of pH and coexisting substances (Cl⁻, Br⁻, HCO₃⁻ and HA) were evaluated. It was found that ClO₂ preferred the neutral and alkaline condition and S(IV)/ClO₂ preferred the acidic condition for BPA degradation. An unexpected speed-up of BPA degradation was observed in ClO₂ process in the presence of Br⁻, HCO₃⁻ and HA. In addition, the intermediate products in BPA degradation were identified. Three exclusive products were found in ClO₂ process, in which p-benzoquinone was considered to be the reason of the acute toxicity increase in ClO₂ process.