Revealing the role of individual components in the high performance photocatalytic mineralization of mixtures of organohalogen pollutants

Abstract

In this work, we address the photocatalytic mineralization of single-component solutions and mixtures of three organohalogenated compounds, dexamethasone (DEX), S-metolachlor (MTLC) and dichloroacetic acid (DCA) using a low-cost TiO2/rGO composite. A detailed experimental analysis has evidenced the positive influence of the presence of DCA on the degradation, and mineralization kinetics, as well as on the release of ions into solution, which is explained by the acidic pH of the solutions containing this compound. Mineralization curves of the aqueous mixtures follow two distinct kinetic trends, the first is assigned to the breakdown of the primary compounds and proceeds with second order kinetics (k = 0.58 × 10−3 mM−1 min−1 at neutral pH, and k = 1.19 × 10−3 mM−1 min−1 at pH 4 and the tertiary mixture). In the second zone, which progresses at a slower rate and is attributed to the mineralization of intermediate compounds, all mixtures have kinetic constants within the range (9.7 ± 0.7) × 10−3 min−1. The progress in the release of halogen ions (F- and Cl-) and NH4+, together with the changes in the chemical composition of the catalyst determined by XPS, allow to conclude that no organohalogenated intermediates remain in solution after treatment. Furthermore, the lack of phytotoxicity of the treated mixtures is confirmed with germination indices (GI), reaching GI values above the control point for the time of complete mineralization; this together with the stability of the catalyst after 5 reuse cycles highlights the green and sustainable nature of the selected photocatalytic remediation process.