Various factors affecting photodecomposition of methylene blue by iron-oxides in an oxalate solution

The effect of various factors on the photodecomposition of methylene blue (MB) by iron oxides calcined at various temperatures in various concentrations of oxalate solutions was investigated by illuminating with UV, visible and solar radiation. Iron oxides were prepared by a gel evaporation method and calcined at 200-700 °C. XRD showed that the as-synthesized iron oxides were amorphous, but formed maghemite (γ-Fe₂O₃) at 200-400 °C and hematite (α-Fe₂O₃) at ≥500 °C. The effect of the various iron oxides, their contents, the oxalate concentration and wavelength of the light source (UV, visible and solar) were all found to strongly influence MB photodecomposition. The optimal contents of the iron oxides increased greatly from 25 to 2000 mg/L at higher calcining temperatures. The MB photodecomposition rate at each optimal iron oxide content was related to the calcining temperature in the order 700 °C < uncalcined < 500 °C < 400 °C < 300 °C. The MB degradation was confirmed to occur by visible light illumination. Excellent photodecomposition was found at pH 2-5, but the photodegradation decreased greatly at pH > 6, consistent with the presence of iron-oxalate complexes. A much higher concentration of hydroxyl radicals was generated in the present system compared with those from a commercial TiO₂ (ST-01), as determined by the coumarin method. Since this process does not require the addition of hydrogen peroxide and shows good efficiency even under solar light, it is an economically viable method for pre-treating and/or decolorizing wastewaters containing dyes.