Applications of O3/UV Advanced Oxidation Technologies for the degradation of 2,4-D in water: Elucidation of Oxidation Pathways and Reaction Kinetics

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

Jacksonville, Florida

Conferencia; 18th International Conference on Advanced Oxidation Technologies for Treatment of Water, Air and Soil (AOTs-18)"; 2012

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 toxic 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, 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. Electrophilic attack by molecular O3 usually occurs with atoms that have a negative charge density and nucleophilic carbons, with multiple bounded constituents. Although ozonation leads to the elimination of many organic compounds in aqueous solution, this is not necessarily accompanied by total mineralization. In aqueous phase, the photolysis of ozone produces hydrogen peroxide as primary product, which then can be photolyzed into hydroxyl radical directly and can react with aqueous ozone to produce an OH radical. The OH radical produces a faster and especially non-selective oxidation with most solutes. Several publications made important contributions related to many reaction intermediaries. However, several doubts and gaps still exist with regard to the whole mechanism, of the chemical transformations that lead to that result. In addition, until now, the separate contributions of ozone and OH radical on 2,4-D degradation has not been established. In order to unravel a reaction mechanism, the role of ozone and OH radicals should clearly be isolated. In this work, an OH radical scavenger (t-BuOH) was added in certain experimental runs in order to establish a clear reaction mechanism for the degradation of 2,4-D by O3 alone, evaluating the contribution of each oxidative species in terms the contaminant degradation rate, formation of aromatic and aliphatic intermediates and final products, TOC removal and dechlorination efficiency. Based on the identified compounds, a reaction mechanism for 2,4-D ozonation was elucidated. With this mechanism, a kinetic model and the corresponding reaction parameters have been obtained.