Intrinsic kinetics of the oxidative reaction of dichloroacetic acid employing hydrogen peroxide and ultraviolet radiation

In a previous work it has been shown that the combination of H2O2 and low wavelength UV radiation is a suitable process for degrading dichloroacetic acid (DCA). The final result provided a validated and complete reaction scheme. That proposal included two possible ways for the hydroxyl radical to react with DCA [Zalazar, C., Labas, M., Brandi, R., Cassano, A., 2007. Dichloroacetic, acid degradation employing hydrogen peroxide and UV radiation. Chemosphere 66, 808-815]. This work was directed to a single objective: to derive, from the previous reaction sequence, a mathematical model able to represent the kinetics of DCA oxidation and validate its predictive quality with experiments. This representation of the reaction must include all the required variables for an ulterior reactor design and scale-up and, consequently, the kinetic model parameters must be independent of the shape, size and configuration of the laboratory reactor. Working with a complete set of experimental runs that included all the involved variables, the unknown kinetics parameters of the DCA degradation were obtained by comparing predicted concentrations by the model (represented by a set of two ordinary differential equations and two algebraic equations coupled with a mass and a radiation balance inside the reactor) with the experimental values, employing a multi-parameter non-linear regression analysis. Experimental values confirmed the validity of the proposed mechanism. Additionally, an optimal concentration ratio of hydrogen peroxide with respect to DCA was obtained (r = CH2O2/C-DCA approximate to 8). The intermediate results of the numerical solution of the complete system of differential and algebraic equations representing the proposed complete reaction mechanism were useful to find simplified, analytical expressions for the reaction rates of DCA and H2O2. The obtained rates resulting from these simplifications were compared with those of the complete system showing a very satisfactory concordance. This outcome is, at the same time, a clear indication of the significant influence of the HO2 center dot radical in the reaction evolution. (C) 2007 Elsevier Ltd. All rights reserved.