Temperature effects on ozonation: Impacts on kinetics of organic amines abatement, bromate formation and disinfection

Ozonation has been applied in water treatment for disinfection and micropollutant abatement for decades. However, rate constants for predicting ozonation performance have been mostly measured at room temperature. To investigate the temperature effect on ozonation, the activation energies (E_a) for ozonation of amine moieties and bromate formation have been investigated in this study and compared to other ozone reactive organic moieties as well as disinfection by ozone. The temperature dependence of second-order rate constants for ozone reactions with model aliphatic amines and micropollutants containing amino groups as the main reactive sites has been investigated in presence of an ^•OH scavenger (tert‑butanol) resulting in activation energies, E_a of 75±10 kJ/mol. This is relatively high compared to E_a (∼30 kJ/mol) for phenols, anilines and olefins. This difference could be ascribed to the strong hydration of amine moieties in water with a higher energy input for water exchange during ozone attack. For micropollutants with high reactivity with ozone, temperature has a negligible impact on their abatement. For those with moderate or low ozone reactivity, a comparison of E_a for target compound oxidation and for ozone decrease in real water matrices is needed to assess the overall temperature effect. The temperature effect on the tradeoff between micropollutants oxidation or microorganism inactivation and bromate formation during ozonation depends on their relative E_a, and the overall E_a of bromate formation depends on the R_ct value (ratio of ^•OH exposure to O₃ exposure). For water matrices with low R_ct values (< 5 × 10^-9, e.g. lake water), where ozone reactions are more important, the ozone oxidation or disinfection efficiency (E_a = 20-100 kJ/mol) at different temperatures can be maintained without increase of bromate formation risk (E_a = ∼100 kJ/mol). However, bromate formation in high R_ct water (> 1 × 10^-8, e.g. WWTP effluent), where ^•OH dominates the reaction, may be problematic at low temperature due to its low E_a (∼30 kJ/mol) and bromate mitigation approaches may be needed.