Kinetic modelling of water-color changes in a photo-Fenton system applied to oxidate paracetamol

Paracetamol aqueous solutions are oxidized with a photo-Fenton tecnology. The analysis of the color generation shows that it is created by aromatic intermediates of paracetamol degradation, besides iron species. These appear due to the catalyst regeneration cycle, which consist of ferric hydroxides and organometallic compounds. The nature of these intermediates is a function of the dosage of hydrogen peroxide that determines the oxidation reached. Maximum water color happens when 4.8 mol of hydrogen peroxide react with 0.66 mol of paracetamol. The colored solutions consist of mixtures of intermediates such as pyrogallol, muconic acid, hydroquinone and resorcinol. Paracetamol is degraded to intermediates that induce color in the water, reaching a maximum value (Color(max), AU at t(colormax), min). Next, color decreases until a steady state around one hour. The catalyst is oxidized in the presence of UV light to ferric hydroxides, which increase the intensity of the maximum water color following a 0.16 reaction order, Color(max)= alpha(Fecolor) (Fe2+)(0.16). The catalytic effect of iron is revealed by reducing the time for water to acquire the maximum coloration, following a 0.72 order, t(colormax) = alpha(Fet) (Fe2+)(-0.72) The temperature also causes a notable effect on the reaction, decreasing the time for maximum of color formation by a 0.5 order without affecting, however, the color intensity, t(colormax) = alpha(Tt) (T)(-0.5). Moreover, water color formation (k(f), AU(-1) min(-1)) and degradation (k(d), AU(-1) min(-1)) follow second order kinetics. The kinetic constant of color formation shows a dependence of order 0.7 with iron dosage, k(f) = alpha(fFe) (Fe2+)(0.7). Nevertheless, iron effect is minor in the color degradation. Both kinetic constants for color formation, k(f) = alpha(fT) T + epsilon(fT) and degradation, k(d) = alpha(dT) T + epsilon(dt) reveal a linear dependence with temperature. This finding denotes that the catalytic effect of temperature is greater than that of iron. (C) 2018 Elsevier B.V. All rights reserved.