The ozone-activated peroxymonosulfate process for the removal of a mixture of TrOCs with different ozone reactivity at environmentally relevant conditions: Technical performance, radical exposure and online monitoring by spectral surrogate parameters

• Enhanced radical exposure during O 3 /PMS than during O 3 , even within the IOD phase. • O 3 /PMS more efficient than O 3 at short reaction times and high ozone concentrations. • Oxidation by PMS plays a role at prolonged reaction and low ozone concentrations. • UVA 254 and fluorescence intensities decrease similarly during O 3 and O 3 /PMS. • Surrogate-based models predicting TrOCs removal are different between O 3 and O 3 /PMS. Secondary effluent from wastewater treatment plants contains (bio)recalcitrant trace organic contaminants (TrOCs) that are discharged into the environment. Ozonation can offer a solution for TrOCs removal from effluent and has been extensively investigated with respect to reaction mechanisms, performance and control strategies in view of full-scale application. The ozone-activated peroxymonosulfate process (O 3 /PMS) is a relatively new and promising upgrade of ozonation for which research under real conditions is, however, still lacking. Therefore, this work focuses on the removal of 11 TrOCs with different ozone reactivity at environmentally relevant concentrations in effluent by using O 3 /PMS. At short reaction times, the hydroxyl radical exposure is approximately 2 times higher in O 3 /PMS than during ozonation at O 3 concentrations exceeding the instantaneous ozone demand (IOD). The radical production during O 3 /PMS is even noticeable at ozone concentrations lower than the IOD, which indicates that radical reaction pathways are more important during the IOD phase in O 3 /PMS than during ozonation. At longer reaction times, also direct PMS oxidation enhances the removal of some TrOCs at low ozone concentrations (< IOD). However, the extra TrOCs removal during O 3 /PMS is overall limited compared to ozonation, i.e. up to 24% extra removal at 0.5 min reaction time and the highest ozone concentration (12.3 mg/L). In a second part of this research, spectral surrogate models based on UV absorbance at 254 nm (UVA 254 ) and fluorescence were developed for the prediction of TrOCs removal. The intensity of both surrogates decreased in a similar way as a function of the applied ozone concentration in both the ozonation and O 3 /PMS process. However, the regression parameters of the surrogate models and particularly the location of the inflection point showed differences between both advanced oxidation processes.