Enhancing Photocatalytic Degradation of Methylene Blue and Malachite Green under Solar Light Irradiation: Mechanistic Insights and Efficiency of BiVO4 and Sr-Doped BiVO4 Nanoparticles

Water pollution from untreated dye discharge has become a significant concern for all living organisms and the treatment of polluted water has become a priority. In this study, a monoclinic–tetragonal hetero-structured BiVO4 and strontium doped (15, 20 and 25 wt.%) BiVO4 NPs were synthesized using a simple co-precipitation method. The structural, optical and morphological properties of the catalysts were characterized using XRD, FTIR, UV-DRS, FESEM, TEM, PL and BET analysis. The XRD pattern exhibited the retentions of crystalline structure of BiVO4 after Sr doping. The Sr-BiVO4 exhibited a reduced band gap energy of 2.3 eV, significantly lower than that of the pristine BiVO4. Additionally, Sr-BiVO4 had an increased surface area of 16.2 m2/g nearly twice that of the pristine material. Photocatalytic activities of the prepared samples were evaluated with changing parameters such as pH, initial concentration, catalyst dose and agitation time towards the degradation of methylene blue (MB) and malachite green (MG) dyes under solar light irradiation. The results showed that Sr-BiVO4 achieved a degradation efficiency of 91% for methylene blue and 94% for malachite green at pH ~ 9 with a 20 mg catalyst dose in 120 min. The dye degradation followed a pseudo-first-order kinetic model and the catalyst demonstrated an excellent stability even after five consecutive runs. The active species test carried out using p-benzoquinone, isopropyl alcohol and Na-EDTA revealed that superoxide radicals played a key role in the degradation mechanism.