ORGANIC CONTAMINANTS DESTRUCTION USING THE UV/FREE CHLORINE PROCESS: MECHANISMS AND MODELING
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Advanced oxidation processes (AOPs) are effective technologies to oxidize recalcitrant organic contaminants in the aqueous phase. The UV/free chlorine process has gained attention as a promising AOP technology, and it generates various reactive radicals (i.e. HO∙, Cl∙, Cl2-∙ and ClO∙) at room temperature and pressure. These electrophilic radicals eventually mineralize refractory organic contaminants into CO2 and H2O. Compared with other common AOPs (e.g. UV/H2O2 and UV/Persulfate processes), the UV/free chlorine process has many advantages, for example (1) it has much lower chemical reagent costs; (2) it has higher energy efficiency; (3) it is only slightly impacted by chloride ions (Cl-) (We found Cl- significantly inhibits the effectiveness of the UV/Persulfate process). For large scale applications, understanding the degradation mechanisms is critical to the design of the UV/free chlorine process that has the lowest energy consumption and greatest toxicity reduction. A number of related studies have shed light on the degradation of some selected organic compounds (e.g., atrazine, naproxen, etc.). However, these previous studies of the UV/free chlorine process have not comprehensively examined the mechanistically complex radicals-initiated chain reactions. Many researches have conducted experiments to determine the degradation mechanisms. However, these experimental studies are very time consuming and expensive. With respect to developing kinetic models that can simulate the reaction pathways in the UV/free chlorine process, most studies have used simplified lumped reactions or invoked the simplified pseudo steady state assumption because the rate constants between reactive radicals and organic compounds are unknown. Accordingly, conducting experiments and developing simplified kinetic models would be impossible to fully elucidate the oxidation mechanisms of all organic contaminants that may be found in the aqueous phase (Chemical Abstracts Service lists about more than 147 million compounds). To overcome the above-mentioned challenges, we developed a first principles-based kinetic model to predict the oxidation of organic compounds in the UV/free chlorine process. First, we collected photolysis and chemical reactions that describe the oxidation of target organic compounds from literature. Second, we developed a rate constants estimator to predict the rarely reported second-order rate constants between reactive radicals and organic compounds (i.e. kHO∙/R, kCl∙/R, kCl2-∙/R and kClO∙/R). kHO∙/R was estimated by the group contribution method (GCM). kCl∙/R, kCl2-∙/R and kClO∙/R were estimated by using the genetic algorithm that was fit to our experimental data (i.e. experimental observed time-dependent concentration profiles of target organic compounds). Third, we developed a stiff ordinary differential equations solver using Gear’s method to predict the time-dependent concentration profiles of target organic compounds, and our prediction results agreed with our experimental data for various operational conditions. Accordingly, our first principles-based kinetic model was successfully verified using our experimental data. Based on our UV/free chlorine kinetic model, we developed four quantitative structure activity relationships using Hammett constants of organic compounds and our predicted rate constants. We then determined relative contribution of these reactive radicals and photolysis, and, we found ClO∙ was the dominant radicals for organic contaminants oxidation. We also optimized the operational conditions (i.e. UV intensity and free chlorine dosage) that has the lowest energy consumption. Furthermore, we successfully implemented graph theory to develop a computerized pathway generator, which was built based on the predefined reaction mechanisms from experimental observations. The pathway generator can automatically predict all possible reactions and byproducts/intermediates that are involved in the degradation of target organic contaminants during the UV/free chlorine process (e.g. the degradation of TCE involves more than 200 byproducts /intermediates and more than 1,000 reactions). Therefore, the pathway generator significantly advances our understanding about the degradation pathways. However, we have noticed that it is difficult to estimate the rate constants of all possible involved reactions at current stage, because we only have very limited amount of experimental data (e.g., we do not have data on peroxyl radicals reactions) to develop a GCM. Consequently, future work will mainly focus on developing new methods (e.g. quantum chemistry) to estimate the rate constants of all possible involved reactions, and then predicting the time-dependent concentration profiles of byproducts. Finally, we investigated the disinfection byproducts (DBPs) and disinfection byproducts formation potentials (DBPFPs) in the UV/free chlorine process. In practical applications, natural organic matter can react with residual free chlorine to produce toxic DBPs. As a result, both the micropollutants and the DBPFPs must be decreased. Therefore, we need determine the controlling factor (i.e., organic contaminant destruction or DBPFPs reduction) in the design of the UV/free chlorine system. Overall, our study can be used to design the most cost-effective UV/free chlorine process.