Kinetic study of colored species formation during paracetamol removal from water in a semicontinuous ozonation contactor.

Paracetamol aqueous solutions, when ozonized, acquired a strong red coloration depending on the applied ozone dose and the initial pH of the aqueous solution. Then, this color loses intensity and turns to yellow. Color formation is favored when operating at initial pH0 = 12.0 and ozone flow-rate 4.2 mg/min. A mechanism describing color formation was proposed, being the main pathway involved an initial paracetamol hydroxylation to yield 3-hydroxyacetaminophen followed by the formation of 2-amino-5-hydroxyacetofenone. Then, these compounds are degraded to colored oxidation by-products. A model describing color evolution was also proposed, considering first-order kinetics for both color formation and degradation. The corresponding kinetic constant values were determined to be kf = 0.01 (1/min) and kd = 0.03 pH -0.055 (1/min), respectively. A relationship between aromaticity loss and color changes during the reaction has been estimated considering the parameter α=kA/kf, being α = 1.62 pH + 3.5 and the first-order rate constant for aromaticity loss given by kA = 0.0162 pH + 0.035 (1/min).

UV/H2O2 chemical oxidation for high loaded effluents: a degradation kinetic study of LAS surfactant wastewaters.

This paper describes a laboratory study conducted to elucidate the possibility of treating high loaded solutions of surfactants through an Advanced Oxidation Process. Synthetic solutions of linear alkylbenzene sulfonates are treated in this work as this is a model compound commonly used in the formulation of detergents, with a great presence in urban and industrial waste-waters. The application of UV combined with hydrogen peroxide to oxidise a surfactant effluent is shown to be suitable as a primary oxidation step since conversions of around 50% of the original compound are achieved in the most favourable cases. Initially, the influence of the operating variables on the degradation levels is analysed in this work. A kinetic model that takes into account both the contributions of direct photolysis and radical attack is also worked out. Direct photolysis is performed to determine the quantum yield in the single photodecomposition reaction. Additionally, the rate constant of the reaction between hydroxyl radicals and LAS in the oxidising system H2O2/UV is determined for different operational conditions. Finally, the contribution of each oxidation pathway is quantified; a higher contribution of the radical reaction than that of the direct photolysis was found in all cases.