(544b) Modeling Free-Radical Reactions for Hydrogen Peroxide/Ultraviolet (Hp/Uv) Oxidation: Optimizing Membrane Process Efficiencies in Drinking Water Treatment

Advanced oxidation process (AOPs) are widely used in the destruction of synthetic organic chemicals (SOCs) including pesticides, solvents, industrial chemicals, and petroleum organics present in drinking water supplies. The combination of hydrogen peroxide with UV radiation (HP/UV) has emerged as a powerful technology in water treatment applications. This research addresses the modeling of the HP/UV process kinetics for the decomposition of an organic pesticides in the presence of natural organic matter (NOM). The presence of NOM is implicated in the fouling of membrane processes used in water purification. An important aspect is the efficient and cost-effective application of the process that requires a good understanding of the complex kinetics and mechanisms of the associated free-radical reactions. The model involves the fundamental aspects of free-radical kinetics associated with various reactions including those of the hydroxyl and hydroperoxyl radicals, and the photolytic reactions associated with UV radiations and hydrogen peroxide during the oxidation of SOCs and NOM. More importantly, the model also includes UV radiation intensities and photolytic kinetics, providing the means for including energy requirements required for activating and promoting free-radical reaction mechanisms. The modeling of process kinetics in the presence of NOM has far-reaching implications in their decomposition into smaller molecules and generation of disinfection byproducts (DBPs) such as aledehydes and ketones. In the present study, alachlor, an acetanilide pesticide is employed as a model compound, whose destruction is evaluated under different conditions in the presence of NOM. Parameters influencing the process destruction efficiency were investigated through a systematic approach of combining experimental data acquisition with a kinetic modeling technique. A free-radical reaction kinetic model was developed for the HP/UV process to predict the concentrations of all principal species, including alachlor, NOM, hydrogen peroxide, carbonate species, and intermediate radicals. The proposed model, that addresses all the important aspects of previous models, incorporates additional features such as gradual pH decrease during the oxidation period (attributed to NOM mineralization), and uses a lumped parameter approach for quantification of NOM. The model involved simultaneous ordinary differential equations for the rate expressions of the principal species. The model equations were solved using algorithms based on backward differentiation formulas (Gear's methods) and Runge-Kutta methods, and the two techniques were compared. The model provided insight into the complex kinetics and reaction mechanisms involved in the HP/UV system under the influence of NOM, and the oxidation products of alachlor. The modeling approach further facilitated optimization of process parameters for a given water quality matrix to achieve efficient and cost-effective decontamination. The alachlor decomposition efficiency was experimentally evaluated as a function of pH, NOM concentrations, total carbonate concentration, hydrogen peroxide concentrations, and the UV intensity. Furthermore, the study evaluated the adverse effects of NOM and carbonate species as free-radical scavengers on the decomposition kinetics. The study showed that larger hydrogen peroxide dosage achieved very marginal improvements in alachlor decomposition efficiency in the presence of NOM and carbonate species. The investigation further demonstrated that the HP/UV process was very effective in the destruction of alachlor in the presence of NOM, and that it could be applied for the destruction of other SOCs as well. Model sensitivity studies also provided a qualitative evaluation of the influence of various process variables on alachlor decomposition.