Microwave and UV–Vis assisted photocatalysts preparation for efficient sunlight-driven degradation of wastewater emerging contaminants

Abstract

Samarium-doped TiO2 was prepared as a semiconductor with a high percentage of photoactive TiO2 phases (anatase and brookite), and optimum response to UV-A or natural solar light for the degradation of emerging contaminants in effluent from wastewater treatment plant (WWTP). To this end, the hydrothermal method assisted by microwave, ultraviolet, and visible radiation (MHM UV–Vis), still little explored in the literature, was chosen because of the synergy of the different radiations in the material synthesis stage. As a desired result, samarium-doped TiO2 produced a mixture of anatase (77 %) and brookite (23 %) polymorphs, quantified by the Rietveld refinement method with a high BET specific surface area (137.5 m2 g−1). Even with moderate bandgap energy (Egap = 3.3 eV), induced defects and oxygen vacancies produced intermediate levels in the band structure capable of trapping electrons with energies of 1.87 eV, 2.02 eV, and 2.12 eV in shallow traps, discussed in detail based in photoluminescence (PL) analysis and X-ray photoelectron spectroscopy (XPS). Under UV-A radiation by 120 min, ∼100 % ofloxacin (OFX, 1 mg L−1) and 70 % of diclofenac (DCF, 1 mg L−1) were degraded when present in a sample of effluent from a WWTP (TOC = 71.55 mg L−1). The oxidation of contaminants in the mixture was achieved by hydroxyl radical (HO•) and superoxide (O2 •-) formation in the valence band (VB) and conduction band (CB) of the semiconductor, respectively. The potential (eV) of the BV and BC was determined by electrochemical analysis. Using principal component analysis (PCA), the best synthesis conditions (microwave power and heat treatment temperature) allowed us to obtain the best photocatalyst (STiM_400_400), which maintained the degradation efficiency for up to 5 cycles. Analyses of LC-MS/MS were performed to identify the transformation products of OFX and DCF after 120 min.