Role of the propagation reactions on the hydroxyl radical formation in ozonation and peroxone (ozone/hydrogen peroxide) processes
Graphical abstract
Introduction
Ozonation and peroxone (O3/H2O2) processes have been widely used for the elimination of biological refractory compounds in the effluents of municipal wastewater treatment plants (WWTPs) (Wert et al., 2009, Gerrity et al., 2011, Zimmermann et al., 2011, Pisarenko et al., 2012, Lee et al., 2013). As a selective oxidant, O3 reacts rapidly (k > 100 M−1 s−1) with organic compounds containing electron-rich moieties (ERMs) such as phenols, amines, and olefins (von Sonntag and von Gunten, 2012). Therefore, micro-pollutants containing ERMs can be eliminated efficiently during ozonation (Lee et al., 2013, von Sonntag and von Gunten, 2012). However, the ozone-refractory organic micro-pollutants (k < 10 M−1 s−1) such as atrazine, tris(2-carboxyethyl)phosphine, and tri-n-butyl phosphate are also degraded to some extent (Lee et al., 2013, Pocostales et al., 2010), and this is ascribed to the formation of the non-selective OH via the complex chain reactions of O3 with water matrix including initiation, propagation, and termination reactions (von Gunten, 2003). The ERMs present in dissolved organic matter (DOM) such as amines and phenols initiate O3 decomposition into OH (Buffle et al., 2006a, Buffle et al., 2006b, Buffle and von Gunten, 2006). Hereafter, the OH formed further reacts with DOM leading to the formation of carbon centered radicals, which yield peroxyl radicals upon O2 addition. Some of these radicals (e.g., α-hydroxylalkylperoxyl radicals) can generate HO2/O2− quickly (von Sonntag and Schuchmann, 1991), which then reacts with O3 to form O3− and finally OH again. These reactions in sequence are termed as propagation reactions (in brief, OH + DOM + O2 → O2−, O3 + O2− → O3− → OH) (von Gunten, 2003).
To better predict the elimination of ozone-refractory organic micro-pollutants, an understanding of the OH formation is important. However, the OH formed is quickly consumed by the water matrix (e.g., DOM and carbonate/bicarbonate), and the steady-state concentration of OH is too low to be measured directly. For this reason, a Rct concept describing the ratio of OH to O3 has been developed by Elovitz and von Gunten (1999), and it is obtained from the ratio of measured OH exposure and O3 exposure. Although the Rct concept provides most valuable information regarding the contribution of the OH route to the oxidation of micro-pollutants, it does not describe the OH formation in absolute terms.
The “tert-Butanol (tBuOH) assay” is often used to determine the OH yield (Ф, Ф = Δ[OH]/Δ[O3], i.e., mol OH formed per mol O3) on absolute terms, based on the known chemistry that the OH reacts with tBuOH producing stoichiometric formaldehyde (Flyunt et al., 2003, Nöthe et al., 2009). During ozonation of wastewater effluent, the Ф values were determined to be 12%∼24% (Lee et al., 2013, Pocostales et al., 2010, Nöthe et al., 2009). However, tBuOH is present at such a large excess that it tends to scavenge all available OH formed and subsequently inhibits the propagation reactions. Thus, the OH is primarily produced though the initiation reactions when using the “tBuOH assay”. It was reported that in waters with high DOM content (>3 mg/L), O3 decomposition was controlled by the propagation reactions (Acero and von Gunten, 2001). The theoretical Ф value in the propagation reactions for O3 consumption (i.e., O3 + O2− → O3− → OH) is 100% (Naumov and von Sonntag, 2011), which implies that more OH should be generated when the propagation reactions become dominant. Therefore, Ф values described previously may be underestimated by the “tBuOH assay”.
The role of propagation reactions in OH formation during the O3/H2O2 (peroxone) process has received limited attention. The peroxone process has been widely used to enhance the oxidation of the ozone-refractory organic micro-pollutants (Staehelin and Hoigné, 1982). However, many studies have shown that the H2O2 addition is of little advantage in wastewater treatment (Wert et al., 2009, Gerrity et al., 2011, Pisarenko et al., 2012, Pocostales et al., 2010). Simulations conducted by Pocostales et al. (2010) concluded that fast reactions of O3 with the water matrix competed out the relatively slow reaction of O3 with H2O2, and that the contribution of H2O2 to the OH yield became important only at high O3 doses. However, this analysis was conducted by assuming the separate kinetics of O3 with water matrix vs. H2O2. In fact, the reaction of O3 with H2O2 may affect O3 consumption by the water matrix via propagation reactions. The contribution of the propagation reactions to Ф values in the peroxone process is also ignored when using the “tBuOH assay”.
The objective of the current study was to evaluate the relative contributions of propagation reactions to O3 consumption and OH formation in ozonation and peroxone processes. O3 consumption via the propagation reactions was estimated based on a kinetic analysis of O3 decay. A competition method which accounted for the OH exposure and the scavenging capacity of the water matrix was then used to determine the Ф values in ozonation and the influence of the propagation reactions on the Ф values was evaluated. The role of H2O2 in peroxone process was also reevaluated comprehensively by considering the propagation reactions.
Section snippets
Materials
All the reagents used were of analytical grade. Stock solutions were prepared in MilliQ water (18.2 MΩ/cm) from a Millipore system. Concentrated O3 stock solutions were made by continuously bubbling O3/O2 mixture (generated by oxygen-fed ozonator (TOGC2B, Degrémont)) through 4 °C MilliQ water. The concentrations of the O3 stock solutions were determined by UV spectrophotometer (DR5000, Hach) taking ε(260 nm) = 3300 M-1s−1 for O3 (von Sonntag and von Gunten, 2012). The H2O2 stock solutions were
Complex reactions of ozone with water matrix
It has been well reported that the kinetics of O3 decomposition in wastewater effluents are mainly broken up into two phases (i.e., the initial rapid phase and the secondary slower phase) (Nöthe et al., 2009, Buffle et al., 2006a, Buffle et al., 2006b). This was also observed in this study (as shown in Figure (S1)). These results showed the apparent O3 decay rates at different time scales. However, the information on the contribution of the complex reactions (including direct, initiation,
Conclusions
This work has shown that the propagation reactions played an important role on the OH formation of ozonation and peroxone processes undertaken in wastewater effluents. Kinetics analysis of ozone decay in wastewater effluents indicated that >25% of ozone was consumed via the propagation reactions which generated OH with yield of 100%. Then the overall OH yield during ozonation was determined by the competition method to be 33%∼58%, which was much higher than that (6–24%) as determined by the “t
Acknowledgments
This work was financially supported by the National Science & Technology Pillar Program, China (2012BAC05B02), the Funds for Creative Research Groups of China (51121062), the National Natural Science Foundation of China (51178134 & 51378141), the Foundation for the Author of National Excellent Doctoral Dissertation of China (FANEDD, 201346), and the Funds of the State Key Laboratory of Urban Water Resource and Environment (HIT, 2013DX05).
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