Efficient degradation of bisphenol A using High-Frequency Ultrasound: Analysis of influencing factors and mechanistic investigation

The environmentally friendly technology of high-frequency ultrasonic processes with the mostly concerned frequencies of 200 kHz and 400 kHz were applied and investigated systematically for BPA degradation in both pure water and river water mediated conditions. Investigations were carried out examining the effects of crucial parameters determining the system efficiency and BPA degradation mechanisms. Results proved that BPA was efficiently degraded via the oxidation by •OH. The 400 kHz ultrasonic process demonstrated much stronger capability in generating •OH and corresponding higher efficiency in BPA degradation based on electron spin resonance analysis. Increasing solution volume could increase the radical yields, and the temperature increase rate (dT/dt) showed linear relationship with k_obs. In addition, the degradation of BPA with different concentrations was for the first time found controlled by different limiting factors, in which the degradation only with higher concentrations (≥0.02 mM) conformed to the Langmuir–Hinshelwood mechanism. More acidic condition and the bulk temperature of about 40 °C favored BPA degradation. The presence of NO₃ ⁻, SO₄ ²⁻, and Cl⁻ demonstrated minor influence while CO₃ ²⁻, HCO₃ ⁻ and NO₂ ⁻ showed significant inhibition, and the effects of different anions also depended on their concentrations. The degradation rate decreased 23% when river water was used as the matrix. Furthermore, both hydroxylation of the benzene ring and attack on the connecting carbon were found to be the reaction pathways, and the latter was proposed to be more dominant.