A two-stage Fe(VI) oxidation process enhances the removal of bisphenol A for potential application

Ferrate (Fe(VI)) has been extensively studied as a green oxidant to treat wastewater. But Fe(VI) oxidation still faces several challenges for application, such as the sensitivity of Fe(VI) to pH and the restrictions on the Fe(VI) utilization efficiency for pollutant elimination at low concentration levels. This study proposed a two-stage Fe(VI) oxidation process to enhance the bisphenol A (BPA) removal for potential applicability, consisting of the adsorption by CNTs of stage I and the degradation by Fe(VI) of stage II. The Fe(VI) utilization efficiency in the two-stage process (0.848) was higher than that in one-stage processes (0.727) and Fe(VI) alone system (0.504) at pH 9. In stage I, the adsorption process had good compliance with the Langmuir isotherm model and pseudo-second-order kinetic model. In stage II, the effective utilization of low-concentration Fe(VI) was 2.45 times more than Fe(VI) alone, and the reduction of reaction volume was beneficial to further enhance utilization. The probe experiments (sulfoxide) and the degradation experiments of other electron-donating/withdrawing pollutants (e.g., atrazine, benzoic acid) demonstrated that Fe(IV) and Fe(V) were major oxidizing species in the two-stage process. The regeneration experiments showed that CNTs still had acceptable adsorption and catalytic capabilities after five cycles. Finally, the intermediate products in the two-stage process were detected and four possible degradation pathways of BPA were proposed. These findings were meaningful for the practical application of Fe(VI) oxidation to overcome the conditional limitation and improve the utilization. [Display omitted] •Two-stage process was designed combining CNTs adsorption and Fe(VI) oxidation.•The utilization of low-concentration Fe(VI) was enhanced >2.45 times.•CNTs could enrich the pollutants and promote the formation of Fe(IV) and Fe(V).•Fe(IV) and Fe(V) played the major role in the two-stage process.•The two-stage process had a certain resistance to changed environmental conditions.