Kinetic modelling of colour and turbidity formation in aqueous solutions of sulphamethoxazole degraded by UV/H2O2

Abstract

ABSTRACT The oxidation of sulphamethoxazole medicine (SMX) has been studied by means of UV/H2O2 conducting at a controlled pH between 2.0 and 12.0 and oxidant ratios of 500 mol H2O2/mol SMX. It is verified that operating at pH = 2.0 the highest rates of SMX degradation (74%) and loss of aromaticity (64%) are obtained. During the process, a strong brown tint and high turbidity are generated in the water depending on the pH, as it affects the chemical speciation of the dissociable compounds. The colour intensity of the water increases from pH = 2.0 (light brown, 3.5 NTU) to a maximum value at pH = 4.0 (dark brown, 42 NTU), when the neutral SMX species is almost 100%. Under these conditions, the formation of carboxylic acids (acetic and oxalic) and nitrate ion are minor. Conducting at higher pH, hue decreases, obtaining at pH = 12.0 a light yellow water (5 NTU) when the anionic SMX predominates. Thus, the maximum formation of nitrate ion occurs under these conditions. A pseudo-first order kinetic modelling is proposed for the loss of aromaticity and colour and turbidity formation in water, where the kinetic parameters are expressed as a function of the applied pH, being the pseudo-first-order rate constants (min−1): k a r o m = 0.0005 p H 2 − 0.0106 p H + 0.0707 ; k c o l o u r = 0.0011 p H 2 − 0.02 p H + 0.1125 and kNTU = 0.06 min−1.