Mechanistic insights on the catalytic ozonation of trimethoprim in aqueous phase using geopolymer catalysts produced from mining waste

The catalytic properties of low-cost ceramic aluminosilicate polymers (geopolymers) containing metakaolin and different amounts of magnetic mining waste, up to 75% w/w, were investigated for the degradation of the antibiotic drug trimethoprim (TMP) in water in the presence of ozone. In the presence of the geopolymers, the removal of TMP increased by up to 11% to reach 71% conversion, while its mineralization increased by 4% to reach 40%. Moreover, the catalyst also decreased the steady-state ozone saturation level of the solution. Reactive species (ROS) trapping experiments demonstrated that superoxide (O2 center dot-) had the most significant impact on the removal rate (55% decrease), followed by singlet oxygen (1O2) (30% decrease). The 20 intermediate reaction products identified by mass spectroscopy was consistent with a four-branch reaction mechanism by superoxide attack on the TMP molecule including i) the methylene bridge rupture, ii) the oxidation of the diaminopyrimidine amine with further hydroxylation and carbonylation, iii) the hydroxylation of diaminopyrimidine amines and iv) 1,2,3-trimethoxybenzene demethylation. XRF, FTIR, ss-NMR and XPS analyses of the geopolymers before and after the reaction also demonstrated the robustness of the geopolymers catalysts during the ozonation reaction. Overall, this study demonstrates that inexpensive geopolymers produced from mining waste could be used in catalytic ozonation reactions for water decontamination.