Reaction-Reactor Modelling and Kinetics of Dichloroacetic Acid (DCA) Oxidation Using Ozone and UV Radiation

M. E. LOVATO, C. A MARTIN, A. E. CASSANO

San Diego

Conferencia; 17th International Conference on Advanced Oxidation Technologies for Treatment of Water, Air and Soil; 2011

Dichloroacetic Acid (DCA) is a water pollutant very often found as a DPB of natural water chlorination resulting from the combination of free chlorine with humic substances regularly present. In this work, a detailed reaction-rector model of the kinetics of DCA degradation employing ozone with UVC radiation was developed. The description differentiates the existence of illuminated and dark sections of the reacting system, as well as shows the application of the differential concept of chemically active species radiation absorption coefficients and those corresponding to other components that only participate in the characterization of the total existing radiation field. The model must represent the degradation of DCA when UVC radiation is employed alone, when ozone is applied in the absence of UVC radiation and when the combination of these two reactions is occurring together (O3+UVC). In addition, the model must show that the presence of hydrogen peroxide as an important by-product affects the existing radiation field and competes with ozone for absorption of the supplied photons. A distinction is made between the mass balances for ozone (operating in a well-stirred, recirculating, continuous flow reactor) and all other intervening species (operating in a well mixed, recirculating batch reactor). The complete sequence presented here includes three parallel reactions: (i) direct photolysis, (ii) direct ozonation and (iii) ozonation plus UVC radiation, being this last reaction the most important one. The reaction shows great sensitivity to the irradiation rates and very little effects by changes in ozone concentration (within the range of the explored variables). It was found that radiation absorption by DCA does not interfere with the one corresponding to ozone. Conversely, a small influence of radiation absorption by hydrogen peroxide -a reaction by-product- can be observed during the first stages of the degradation process. Considering the very fast hydrolysis of phosgene in aqueous media, leaving aside hydrogen peroxide, it can be safely said that there are no stable reaction intermediaries. This study reaches one single 31 steps sequence that, by suitable cancellation of the reaction paths that specifically do not take part in each of the corresponding processes, reproduces the kinetics of direct photolysis or direct ozonation without UV radiation. Three sets of ordinary differential equations resulting from the mass balances of each one of the three parallel reactions are reported. Upon estimation of the missing kinetic constants for DCA, simulation results always show good agreement with experimental data.