Mechanisms of Catalytic Ozonation for the Removal of Low Molecular Weight acids

Abstract

Catalytic ozonation has been widely applied for the treatment of municipal and industrial wastewaters. However, the mechanism of catalytic ozonation is still unclear due to the controversial results reported in the literature with this limiting the optimization of this technology. In this study, we extended the mechanistic understanding of catalytic ozonation process via investigation of organic oxidation and O3 decay in the presence of a wide variety of catalysts including commercially available Fe–impregnated activated carbon, CuO and Cu–Al layered double hydroxides. We also investigated the influence of salinity as well as the matrix on the performance of the catalytic ozonation process. The Fe–impregnated activated carbon catalyst enhances O3 decay with this generating hydroxyl which enhanced the formate oxidation at pH 3.0 compared to that observed in the presence of O3 alone. The involvement of hydroxyl radicals in formate oxidation by the catalytic ozonation process is supported by the observation that the rate and extent of formate oxidation decreases in the presence of tert-butanol and Cl– (which are known bulk hydroxyl radicals scavengers under acidic conditions). Moreover, the oxidation of formate mostly occurs in the solid–liquid interface and/or the bulk solution with adsorption playing no role in the overall oxidation. The catalyst is not active at pH 7.3 and 8.5 suggesting that only the protonated iron oxide surface sites generated strong oxidant(s) on interaction with O3. A mechanistic kinetic model has been developed to adequately explain O3 decay and formate oxidation during catalytic ozonation process. In the presence of CuO and Cu–Al layered double hydroxides, oxidation of oxalate mostly occurs on the catalyst surface via interaction of surface oxalate complexes with surface−located oxidants. In contrast, the oxidation of formate occurs in the bulk solution as well as on the surface of the catalyst. Measurement of O3 decay kinetics coupled with fluorescence microscopy image analysis corresponding to 7−hydroxycoumarin formation indicates that while surface hydroxyl groups in Cu−Al layered double hydroxides facilitate slow decay of O3 resulting in the formation of hydroxyl radicals on the surface, CuO rapidly transforms O3 into surface−located hydroxyl radicals and/or other oxidants. Futile consumption of surface−located oxidants via interaction with the catalyst surface is minimal for Cu−Al layered double hydroxides; however, it becomes significant in the presence of higher CuO dosages. Based on our understanding of the process, a kinetic model has been built and adequately explains the experimental results obtained. In the study of influence of matrix on performance of ozonation and catalytic ozonation processes, our results reveal that the rate of ozone self−decay is considerably faster in phosphate buffer compared to carbonate buffered solution with this effect stemming from the differing hydroxyl radicals scavenging capacities of the buffering ions. Interestingly, while the nature of the buffer used affects the rate of organic oxidation in conventional ozonation, the overall extent of oxidation of formate and oxalate is the same for different buffering ions. The results obtained also indicate that the carbonate radicals generated as a result of carbonate ion – hydroxyl radical reaction can oxidize formate and oxalate however the oxidation of these organics by phosphate radicals appears to be minimal. The presence of phosphate ions also affects the surface chemistry of the two Cu–based catalysts tested here with phosphate ions inhibiting catalyst mediated O3 decay and sorption of the target organic compounds on the catalyst surface. This inhibition of organic sorption and O3 decay decreases the performance of the catalytic ozonation process in the presence of phosphate ions. The presence of salts (particularly chloride ions) reduces the rate and extent of degradation of humic–like substances and low molecular weight neutrals (typical pollutants present in reverse osmosis concentrates of coal chemical wastewater) during catalytic ozonation using a commercially available Fe−loaded Al2O3 catalyst. Scavenging of aqueous O3 by chloride ions and/or transformation of organics (particularly humics) to more hydrophobic form as a result of charge shielding between adjacent functional groups and/or intramolecular binding by cations inhibits the bulk oxidation of organics to a measurable extent. While the scavenging of aqueous hydroxyl radicals at the salt concentrations investigated here was minimal, the accumulation of chloride ions in the electric double layer near the catalyst surface, particularly when pH<pHpzc, results in more significant scavenging of surface associated hydroxyl radicals, thereby decreasing the performance of the catalytic ozonation process. We also discuss the caveats associated with the application of tert-butanol as a hydroxyl radicals scavenger in ozone–related studies. Our results show that tert-butanol may not be able to access surface located •OH formed during catalytic ozonation. Furthermore, tert-butanol may also interfere with the adsorption of organics on the catalyst surface and decrease the adsorptive as well as concomitant oxidative removal of organics via non radical mediated pathways (if important). In addition, TBA scavenging results are inconclusive for mildly ozone reactive compounds due to switching from O3/•OH mediated oxidation in the absence of tert-butanol to O3 driven oxidation in the presence of tert-butanol. The presence of tert-butanol may also decrease the rate of O3 decay with the increased stability of O3 in the presence of tert-butanol facilitating (i) direct oxidation of ozone−reactive organics in the bulk solution and/or (ii) diffusion of O3 to the surface and subsequent surface−mediated oxidation of organics. Overall, the results presented in thesis provide important insights into the catalytic ozonation process. The experimental methods and the kinetic modelling tools developed in this work can be used to gain mechanistic insights into catalytic ozonation process using other catalysts. Furthermore, the kinetic models developed here can be coupled with the hydrodynamics using computational fluid dynamics tools to predict and optimize the performance of full scale catalytic ozonation reactors.