Utilização de óxidos de zinco dopados com manganês na fotodegradação de corante têxtil

Abstract

The treatment of dye-containing textile wastewater remains a major environmental challenge, making advanced oxidation processes promising alternatives for pollutant removal. In this work, pure ZnO and Mn-doped ZnO containing 1, 5 and 10 mol% Mn²⁺ were synthesised sol-gel modified method and evaluated regarding their structural, morphological, optical, textural and photocatalytic properties. The materials were characterised by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–Vis spectroscopy, Brunauer–Emmett–Teller (BET) surface area analysis and photocatalytic degradation of methylene blue under mercury vapour lamp irradiation. XRD results confirmed that all samples exhibited the hexagonal wurtzite structure without detectable secondary phases. Mn incorporation reduced the average crystallite size from 17.47 nm to approximately 12 nm, increased the specific surface area from 12.03 to 19.10 m² g⁻¹, and decreased the optical band gap from 3.17 to 2.82 eV, while promoting a red shift of the absorption edge towards the visible region. Despite these modifications, pure ZnO exhibited the highest photocatalytic performance, achieving approximately 50% methylene blue degradation after 240 min of irradiation, whereas the Mn-doped samples showed lower degradation efficiencies and progressively reduced apparent rate constants. The results indicate that Mn doping improved the structural, optical and textural properties of ZnO; however, it also favoured charge carrier recombination, which outweighed the benefits associated with the reduced band gap and increased specific surface area. Therefore, the photocatalytic performance of ZnO is governed by the balance between light absorption, efficient charge carrier separation and minimisation of electron–hole recombination rather than solely by the reduction of the band gap or the increase in surface area.