Application of Ozone plus UVC radiation for degrading organic compounds

LOVATO MARTÍN CASSANO

Santa Fe, Argentina

Workshop; 2nd Latin American Pesticide Residue Workshop; 2009

Pesticide pollution of environmental waters is a pervasive problem, with widespread ecological consequences. Whatever the concentration level detected, pesticides have to be removed either to protect our water resources or to achieve drinking water quality.Ozone can be used for degradation of organic pollutants in water. It is a powerful oxidant, and an important advantage is that it does not introduce new substances into the aqueous media. Ozonation has been recognized as especially efficient for pesticide removal. It could be applied alone or in combination with Hydrogen Peroxide, UV radiation, or granular activated carbon absorption. Two reactions of ozone with dissolved organic substances can be distinguished in water: a highly selective attack of molecular ozone, at low pH, whereas free radicals from ozone decomposition can also react non-selectively with organic compounds. Free radicals can be produced in the aqueous media from ozone by pH modification, or by combining Ozone with hydrogen peroxide or UV radiation from a low pressure mercury lamp.In this work, Dichloroacetic acid (DCA) was selected as model compound, and O3 and O3/UV as oxidation methods. DCA is a pollutant that can be found in water as DBPs. and can also be detected in wastewaters resulting from the degradation of chlorinated compounds.It was developed a comprehensive mathematical model, which represents the reaction of decomposition of DCA using a kinetic scheme that considers all the parallel paths of attack of O3, in addition to the photolysis of O3 and spontaneous decomposition in water. For this purpose, it becomes necessary to produce kinetic models that represent each of these processes and validate them experimentally.Experimental conditions were varied: (a) Use of UV radiation, (b) Concentration of dissolved O3, (c) Initial concentration of DCA. In each experimental run concentrations of DCA, chloride, dissolved O3, H2O2 (formed in the photolysis of O3), and pH at defined intervals of time were measured.For this model compound, it was concluded that the use of O3/UV is much more effective than O3 alone, the concentration of dissolved O3 does not significantly influence the degradation of DCA and that the contaminant initial concentration inversely affects the decomposition of the acid. Through the proposed reaction simplified mechanism for decomposition of O3 in water and degradation of the pollutant model (DCA) and its comparison with experimental data it is possible to obtain the kinetic parameters of the whole system.