Direct oxidation of organic micropollutants by persulfate and hydrogen peroxide: A potentially misleading contribution in advanced oxidation processes

Advanced oxidation processes (AOPs) are becoming increasingly attractive for the removal of organic micropollutants (OMPs) from water. However, various aspects of AOPs need to be carefully evaluated. Most AOPs involve the introduction of oxidants such as hydrogen peroxide and persulfate, which are subsequently activated by UV irradiation, ozone, dissolved transition metals, solid catalysts, or other techniques. However, the direct reactions between OMPs and hydrogen peroxide/persulfate are rarely discussed. In this study, direct reactions between eight OMPs including acesulfame, atenolol, carbamazepine, diclofenac, metoprolol, primidone, saccharin, and valsartan acid and hydrogen peroxide/persulfate were investigated without any activation and under dark conditions. To evaluate the impact of the water matrices, removals of OMPs were compared in deionized water, tap water and wastewater effluent. The results demonstrated the influence of direct reactions on the oxidation of most OMPs to varying extents. Notably, diclofenac, a widely studied indicator contaminant, could be completely removed through direct reactions with both hydrogen peroxide and persulfate under the highest oxidant dose (2 mM) and contact time (240 min). Carbamazepine, another well-studied indicator contaminant, showed removals of approximately 30% and 55% with hydrogen peroxide and persulfate, respectively. Atenolol and metoprolol also exhibited high oxidation, particularly with persulfate, achieving removals exceeding 70%. Furthermore, the formation of selected transformation products was also studied to gain deeper insights into the reaction mechanisms.