Reaction-Reactor Modeling and Kinetics of 2, 4-D Oxidation Using Ozone and UV Radiation

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

Cartagena

Congreso; 1st Latin American International Conference on Semiconductor Photocatalysis, Solar Energy Conversion, & Advanced Oxidation Technologies; 2013

2,4-dichlorophenoxyacetic acid (2,4-D) is a systemic herbicide, utilized in agriculture and forest plantations. Due to its refractoriness to degradation, powerful oxidation methods are required for an efficient decomposition of this compound. Ozone based methods may involve two different major oxidative species: ozone and OH radicals. Ozone can also react through the hydroxyl radical generated during its decomposition in water. OH generation is usually promoted at high pH, with the addition of hydrogen peroxide or UV irradiation. Thus, during ozonation in water, both molecular ozone and hydroxyl radical coexist, having different reactivity and selectivity. A kinetic model for the decomposition of 2,4-D and its two main decomposition products, namely 2,4-dichlorophenol and 2-clorohydroquinone, using ozone without and with the presence of UVC radiation has been developed. The reactor and radiation field were modeled, taking into account the corresponding mass balances and the radiation model. The by-products generated can cause several series and in parallel reactions that compete with the process of absorption of photons when radiation is applied. The model includes (i) direct photolysis, (ii) separately, isolated molecular ozone attack (using t-BuOH as radical scavenger), (iii) when ozone acts in the presence of the hydroxyl radicals that produces its own decomposition and (iv) the enhanced reaction rate when UVC radiation is incorporated inside the reactor. The results of the direct photolysis indicate that these reactions are not of great importance. Isolating the action of molecular ozone oxidation, it could be observed that this reaction does not reach out results sufficiently prominent and useful, when they are compared with the case in which ozone acts in the company of hydroxyl radicals which are produced during its own decomposition. Finally, the presence of not very high levels of UVC radiation increases remarkably the process rate. Working in the most appropriate range of operating conditions within the span of the explored variables, this approach achieved complete mineralization after almost ten hours of reaction. Nevertheless, before it completed ninety minutes of reaction, all the chlorinated compounds have been decomposed and the balance of chlorine theoretically separated from the organic compounds and the chloride liberated into the solution closed with an error of less than 3 %. Very likely, long before ten hours of reaction time, toxicity essays could indicate that the process has already produced water of acceptable characteristics for the environment. These results, which from the method devised in their production have an intrinsic character, are completely suitable to realize any scaling-up or reactor design at the wanted size and configuration.