Visible-light-activated photocatalytic degradation of rhodamine B using WO₃ nanoparticles

Semiconductor photocatalysis is touted to be one of the most efficient and cost-effective methods of degrading organic pollutants in various water matrices. Herein, highly agglomerated WO₃ nanoparticles were synthesized via a facile acid precipitation method and tested on rhodamine B dye as the model pollutant. The physicochemical properties of the particles were investigated using various characterization techniques which include X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) and zeta potential measurements. The effects of calcination temperature, initial pH, catalyst loading and initial pollutant concentration were investigated. The results showed that under optimum conditions of 300 °C calcination temperature, 5 g L^-1 catalyst loading, 5 ppm initial pollutant concentration and a pH of 9.5, the catalyst achieved an excellent degradation efficiency of 96.1% after 4 h of visible light irradiation. The degradation tests revealed a strong dependence on initial pH with acidic pHs favouring adsorption and alkaline pHs favouring photocatalysis. The degradation kinetics followed the Langmuir-Hinshelwood model for catalyst loadings of less than 10 g L^-1, which typically describes heterogenous photocatalytic surface reactions. Scavenging experiments revealed that reactive superoxide and hydroxyl free radicals were the primary drivers for rhodamine B dye degradation.