Shedding light on the performance of magnetically recoverable TiO2/Fe3O4/rGO-5 photocatalyst. Degradation of S-metolachlor as case study

Recalcitrant contaminants are not usually removed in conventional wastewater treatment plants. Therefore, they are transferred to the water resources that receive treated wastewaters and their presence can cause health and environmental issues. Herbicides are among these compounds. In particular, S-metolachlor (MTLC) is specifically of high concern because its molecule incorporates a chlorine atom that contributes to its toxicity. For its removal, a magnetically recoverable photocatalyst, TiO2/Fe3O4/rGO-5, was synthesised following a hydrothermal method. The performance of TiO2/Fe3O4/rGO-5 has been experimentally assessed and compared to TiO2 and TiO2/rGO-5 catalysts. A characterisation of the materials properties was carried out including adsorption isotherms of MTLC that provided the maximum adsorption capacity of the materials (qm), being 140.85 ± 5.14 mg g−1 for TiO2/Fe3O4/rGO-5. Furthermore, the ternary composite exhibited good recoverability from liquid media after four consecutive cycles thanks to its magnetic character (magnetic saturation of 13.85 emu g−1). Photocatalytic degradation of MTLC started after a dark adsorption step following first order kinetics (0.0197 ± 1.2 × 10−4 min−1 for the degradation of 100 mg L−1 of MTLC with 0.5 g L−1 of TiO2/Fe3O4/rGO-5) similar to the rate of appearance of chloride in solution; after total removal of the solubilized MTLC the chloride concentration in the solution continued increasing with zero-th order kinetics up to the value corresponding to the total MTLC concentration. This second step in the chloride formation was attributed to the degradation of adsorbed MTLC. Specific experiments in the presence of scavengers of reactive oxygen species (ROS) were carried out shedding light on the degradation mechanisms. It was concluded the predominant role of free hydroxyl radicals in the photocatalytic degradation in all the investigated materials, whereas the presence of rGO in the composite photocatalysts improved their electronic conductivity, enhancing the activity of superoxide radicals. The results of this work provide important information for further development of photocatalysis.These results are part of the R&D projects RTI2018-099407-B-I00 and RTI2018-093310-B-I00, funded by MCIN/AEI/10.13039/501100011033 and “ERDF A way of making Europe”. Carmen Barquín is also grateful for the FPI contract PRE2019-089096