Abstract: Natural semiconducting minerals have excellent solar photocatalytic properties. In this study, we selected natural wolframite as the research object. We performed mineralogical and photocatalytic experiments using natural wolframite from three mining areas: Wuming (WM), Limu (LM) and Chongyi (CY). We used X-ray diffraction, Raman and infrared spectroscopy, and electron probe microanalysis to analyze the structure and composition of natural samples. The main mineral phases were identified as natural wolframite in the form of (Fe, Mn)WO4 with decreasing Fe/Mn molar ratios at 7.1, 0.9 and 0.3 for WM, LM and CY mines, respectively. The forbidden band widths for the three mines were 1.5, 1.6 and 1.7 eV, respectively, indicating a good visible light response. Photocatalytic experiments were performed at pH 7. The concentrations of wolframite and methylene blue (MB) were 1 g/L and 5 mg/L, respectively, with 0.01 mol/L H2O2 in the degradation solution. The results showed that the degradation efficiency of WM wolframite was the highest, equaling to 1.1 and 1.6 times that of LM and CY wolframite, respectively. Free radical ·OH was detected in all solutions during the reaction by electron paramagnetic resonance (EPR), with the stronger signal coming from WM wolframite. ·OH was demonstrated as the major reactive oxygen species by using different scavenger in the photocatalytic reaction. The experimental results further showed that the rate of MB decolorization degradation was up to 99% (after 3 hours) in the experimental group where both wolframite and H2O2 were used under light. In the control group, wolframite or H2O2 was used and the MB degradation rates were only 7% and 31% after 3 hours, respectively. Under darkroom condition, the MB removal rate was 34% with additions of wolframite and H2O2. Degradation of MB in wolframite under different H2O2 concentrations was analyzed and found to conform to quasi-first-order kinetics, indicating the degradation process was independent of catalyst content, and H2O2 was more likely to act as an electron acceptor. According to our analysis, under sunlight, the catalytic efficiency of wolframite from all producing areas was positively correlated with Fe content and negatively correlated with band gap width, for which the suggested reaction mechanism involves MB oxidative degradation by ·OH generated by photocatalytic and Fenton reactions. Our study presented a new method of utilizing natural minerals for environmental pollution remediation.

Key words: wolframite, tungstate, natural semiconductor minerals, visible light photocatalysis, pollutant degradation