Degradation of bromothymol blue and methyl green from aqueous media by Photo-Fenton: comparison between UV-lamp and sun irradiation

The degradation of two textile dye molecules was studied using photochemical processes, both in the absence and presence of light. Various methods were employed, including photolysis/UV, combined H2O2/UV photolysis, Fe2+/UV treatment, Photo-Fenton/UV at 350 nm and Photo-Fenton with solar irradiation. The decolorization efficiency of dyes in aqueous solution was evaluated for two specific dyes: Bromothymol Blue (BTB) and Methyl Green (MG). These experiments were carried out in batch mode. The results demonstrated a synergy between light irradiation and the presence of Fenton’s reagents, such as hydrogen peroxide and divalent iron. In addition, it was demonstrated that direct solar irradiation can be used without specific devices to achieve high efficiency at low cost. In the first part, we checked the impact of the various operating parameters. Reaction efficiencies were compared for the same system in the dark and under the assistance of an artificial or solar light source. In the second part, we studied the parameters of the Photo-Fenton process, such as the initial pH of the solution, the initial concentrations of oxidant, iron catalyst, and dye under irradiation from either light source. Whereas the mere photolysis without Fenton’s reagents allowed decolorization yields below 26 %, addition of the oxidant (H2O2) or the catalyst (Fe(II) species amplified the treatment efficiency. However, the presence of both H2O2 and Fe(II) under light irradiation was shown synergetic with yields ranging from 72 to 85 % depending on the dye worked and the light source: because of its broader spectrum in the UV domain, solar irradiation led to the highest decolorization yields. The above results were obtained for well-defined proportions of dye and reagents: for a 20 mg/l dye solution, Fe(II) catalyst concentration equal to 10−3 M, peroxide concentration of 5.10−2 M and a pH of 3. These conditions allowed optimal production of OH· radicals, allowing high efficiency in systems using solar irradiation.