Degradation of Herbicide 2,4-D employing Ozone and UV Radiation: A complete kinetic scheme

M.E. LOVATO, M.B. GILLIARD, A.A. MUCHIUTTI, M. J. RAMB, C. A. MARTÍN, A. E. CASSANO

Córdoba

Congreso; XI Encuentro Latinoamericano de Fotoquímica y Fotobiología; 2012

Universidad Nacional de Rio Cuarto

This work describes the degradation of herbicide 2,4-dichlorophenoxyacetic Acid (2,4-D) in aqueous solution employing Ozone (O3) based Advanced Oxidation Processes (AOP). 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 H2O2, UV irradiation or by reaction with organic compounds itself. Thus, during ozonation in water, both molecular ozone (O3) and hydroxyl radical coexist, having different reactivity and selectivity. The mayor objective was to find a comprehensive reaction pathway for 2,4-D oxidation. Addition of an OH radical scavenger such as tertiary butanol (t-BuOH) allowed to separately evaluate the contribution of each oxidative species. In order to perform a comparative study, three sets of experimental runs were carried out: (a) Ozonation reactions (molecular O3+OH), (b) O3/UV reactions (OH enhancement), (c) O3+t-BuOH (OH inhibition). The efficiencies of the different processes were evaluated, taking into account three criteria: (a) percentage conversion of 2,4-D, (b) mineralization rate, and (c)release of chloride ion. The employed photo-reactor was a cylinder made of Teflon closed in both extremes with two flat, circular windows made of quartz. Two different types of radiation sources were used: (i) Two Philips TUV lamps, 15 W each, (ii) two Heraeus NNI 40/20 lamps, 40 W each. All lamps are low pressure mercury lamps, with one single significant emission wavelength at 253.7 nm. Two different ozone concentrations were utilized (0.1 and 0.23 mM). In presence of t-BuOH, 2,4-D conversions were lower than those obtained for the same dissolved ozone concentrations, in absence of scavenger. Conversely, the addition of UV radiation improves 2,4-D degradation. For processes involving UV radiation complete dechlorination was found. For the ozonation process, dechlorination ratios were higher than when inhibiting OH radical reactions. In all cases, TOC removal was significantly lower than the corresponding 2,4-D degradation. The best TOC conversions were reached for O3/UV processes, and the poorest results were obtained for O3+t-BuOH. A reaction mechanism for 2,4-D oxidation was elucidated. Using specific mass spectrometric analysis applied to the degradation reaction of 2,4-D and afterwards, working in the same way, with its first aromatic intermediary (2,4-dichlorophenol) it was possible to identify the components that were included in the mechanistic proposal. When using t-BuOH only the direct reactions of molecular ozone can take place, and some intermediates were not identified. The intermediaries generated from previous compounds that neither have double C=C bonds nor aromatic rings in their molecular structure have not been found.