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

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): ${\mathrm{k}}_{\mathrm{a}\mathrm{r}\mathrm{o}\mathrm{m}}=0.0005\mathrm{p}{\mathrm{H}}^2-0.0106\mathrm{p}\mathrm{H}+0.0707$; ${\mathrm{k}}_{\mathrm{c}\mathrm{o}\mathrm{l}\mathrm{o}\mathrm{u}\mathrm{r}}=0.0011\mathrm{p}{\mathrm{H}}^2-0.02\mathrm{p}\mathrm{H}+0.1125$ and k_NTU= 0.06 min⁻¹.

GRAPHICAL ABSTRACT

KEYWORDS:

• Aromaticity
• colour
• pH
• sulphamethoxazole
• turbidity

Acknowledgements

The authors acknowledge for technical and human support provided by SGIker, Phytotron Service of UPV/EHU and European Union funding (ERDF and ESF).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article.

Additional information

Funding

This work was supported by the University of the Basque Country UPV/EHU through the Project ‘Environmental Studies and Energy Efficiency (GIU20/056)’.