Comparison of TiO(2) and ZnO for Heterogeneous Photocatalytic Activation of the Peroxydisulfate Ion in Trimethoprim Degradation

The persulfate-based advanced oxidation process is a promising method for degrading organic pollutants. Herein, TiO(2) and ZnO photocatalysts were combined with the peroxydisulfate ion (PDS) to enhance the efficiency. ZnO was significantly more efficient in PDS conversion and SO(4)(•−) generation than TiO(2). For ZnO, the PDS increased the transformation rate of the trimethoprim antibiotic from 1.58 × 10(−7) M s(−1) to 6.83 × 10(−7) M s(−1). However, in the case of TiO(2), the moderated positive effect was manifested mainly in O(2)-free suspensions. The impact of dissolved O(2) and trimethoprim on PDS transformation was also studied. The results reflected that the interaction of O(2), PDS, and TRIM with the surface of the photocatalyst and their competition for photogenerated charges must be considered. The effect of radical scavengers confirmed that in addition to SO(4)(•−), (•)OH plays an essential role even in O(2)-free suspensions, and the contribution of SO(4)(•−) to the transformation is much more significant for ZnO than for TiO(2). The negative impact of biologically treated domestic wastewater as a matrix was manifested, most probably because of the radical scavenging capacity of Cl(−) and HCO(3)(−). Nevertheless, in the case of ZnO, the positive effect of PDS successfully overcompensates that, due to the efficient SO(4)(•−) generation. Reusability tests were performed in Milli-Q water and biologically treated domestic wastewater, and only a slight decrease in the reactivity of ZnO photocatalysts was observed.