oa Mercury Removal Using Titanium Dioxide Photocatalysis: Nanoparticle Versus Nanotube

Mercury is one of the most detrimental by-products of industrial activities such as fossil fuel combustion and mining. In this study, photocatalytic reduction of mercury (II) to elemental mercury in two types of titanium dioxide (TiO2) was investigated. Photocatalysis involves a material's ability of creating an electron-hole pair after its exposure to solar radiation with photon energy higher than that of the applied photocatalyst, resulting in photogenerated free-radicals at the positive holes and electrons at the conduction band. These radicals can efficiently reduce or oxidize certain contaminants. Photocatalysis is a convenient method for reducing mercury (II) since it utilizes inexpensive chemicals and solar energy, an energy source found abundantly in Qatar. Laboratory experiments were conducted in both batch and continuous flow systems and two different types of titanium dioxide were evaluated for Hg(II) reduction; commercially available nanoparticles and lab-synthesized nanotubes. A microwave digestion technique was used to synthesize the nanotubes. The surface properties of the two types of TiO2 were characterized using surface analysis techniques, including: transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). To achieve efficient hole-transfer, formic acid was selected as the hole-scavenger in the experiments. Control experiments were performed in the absence of light to assess the efficiency of photocatalysis versus direct adsorption. Effects of pH, amount of hole-scavengers, initial mercury concentration, photocatalyst dose, and irradiation time on Hg(II) removal were evaluated. Preliminary results indicate that mercury removal efficiency using synthesized nanotubes is higher than that when the commercially available nanoparticles were used. While the nanoparticles removed about 80% of the initial mercury (II) concentration, synthesized nanotubes removed approximately 97% after 30 minutes of reaction time. At low concentrations of titanium dioxide, results showed that formic acid enhanced the removal of mercury (II) by 20%. In the absence of light, results showed near-to-no adsorption of Hg(II) on titanium dioxide, while a small amount (4%) of adsorption occurred in the presence of formic acid. This implies that there was no complexation of mercury with TiO2 particles or formic acid, and that photocatalytic reduction was the main mechanism for Hg(II) removal.