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 k_f = 0.01 (1/min) and k_d = 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=k_A/k_f, being α = 1.62 pH + 3.5 and the first-order rate constant for aromaticity loss given by k_A = 0.0162 pH + 0.035 (1/min).