Effect of crystalline phase of MnO2 on the degradation of Bisphenol A by catalytic ozonation

In recent decades, manganese oxide (MnO2) has been an attractive catalyst for organic pollutants removal from wastewater by ozonation. However, the effect of the MnO2 crystalline phase on Bisphenol A ozonation in water has never been investigated. In the present study, three crystalline phases of MnO2 were synthesized by hydrothermal method, including alpha-,gamma-, and delta-MnO2, the last generating three different kinds. The as-prepared MnO2 were evaluated in the ozonation of Bisphenol A (BPA) and correlated with their physicochemical properties obtained by different characterization techniques. The activity of MnO2 polymorphs for TOC removal followed the order of gamma-MnO2 > delta 3-MnO2 > delta 1-MnO2 > alpha-MnO2 > delta 2-MnO2. The catalytic activity of MnO2 strongly depended on the presence of active sites such as density of the surface oxygen vacancy, Mn+3/Mn+4 ratio, and OH-groups on the catalyst surface. The catalysts with the highest amount of oxygen vacancies (but the minor density of the surface oxygen vacancy) and OH-groups were delta 3-MnO2 and gamma-MnO2, respectively, demonstrating excellent catalytic activity on BPA removal. The effect of stabilities of delta 3-MnO2 and.-MnO2 was also investigated in this research. Furthermore, the toxicity bioassay based on Lactuca sativa seeds demonstrated that by-products generated during conventional and catalytic ozonation were non-toxic. The elimination of the model mixture of emergent compounds in four types water were also tested with delta 3-MnO2 and gamma-MnO2, obtaining mineralization values around 47-59 % and 40-55 %, respectively, at 120 min compared with 18-23 % of conventional ozonation.