Differentiating ozone direct and indirect reactions on decomposition of humic substances

The ozonation reaction is accomplished into two pathways: direct ozone oxidation and indirect free hydroxyl radical oxidation. Both reactions are proceeding simultaneously, thus, it is difficult to separate them and study their respective reaction mechanisms. Carbonate and bicarbonate react with free radicals that will effectively inhibit the indirect free radical oxidation without affecting the direct oxidation reaction. The feasibility of using these chemicals as scavengers to inhibit the hydroxyl radicals so that the direct ozone oxidation reaction and indirect free hydroxyl radical oxidation reaction can be differentiated is presented in this paper. The results will assist in achieving a better understanding of the ozone reaction mechanism in oxidizing organic substances. In this study, synthetic samples made of commercially available humic substance are used to carry out the inhibited and non-inhibited ozone oxidation experiments. The intersecting point of the ORP (oxidation reduction potential) time series curves of the inhibited and un-inhibited reactions, occurring at 120 s after the onset of the oxidation reaction, marks the turning point at which all added scavenger chemicals have been completely exhausted. Based on the measured A254 values, percentage distributions of direct ozone oxidation and indirect hydroxyl radical oxidation are estimated. The results indicate that the contribution of indirect reaction decreases with increasing amounts of inhibiting scavengers. The on-line ORP measurement results are simulated using a modified form of the Nernst equation expressed as ORP = 11 852 − 1196 pH + 130 ln [(A254,0 − A254,t)(A254,t)−1] with R2 value of 0.99. In this equation, A254 is the absorbance at 254 nm used to index the humic substance concentration while subscript 0 and t denote the initial and the final time, respectively.