Role of moisture in adsorption, photocatalytic oxidation, and reemission of elemental mercury on a SiO2-TiO2 nanocomposite

A novel silica-titania (SiO2-TiO2) nanocomposite has been developed to effectively capture elemental mercury (Hg-0) under UV irradiation. Moisture has been reported to have an important impact on this nanocomposite's performance. In this work, the role of moisture on Hg-0 removal and reemission as well as the corresponding mechanisms was investigated. Hg-0 removal experiments were carried out in a fixed-bed reactor at 65 degrees C using air as the carrier gas. Without UV irradiation, Hg-0 adsorption was found to be insignificant, but it could be enhanced by the photocatalytic oxidation product, mercuric oxide (HgO), possibly due to the high affinity between HgO and Hg-0. Under dry conditions 95% of Hg-0 can be removed; however, increased humidity levels remarkably suppress both Hg-0 adsorption and photocatalytic oxidation. Introducing water vapor can also result in significant reemission of captured Hg-0 from the nanocomposite, which may be ascribed to the repellant effect of water vapor adsorbed on the superhydrophilic TiO2 surface. Exposure to UV light was found either to prohibit Hg-0 reemission when photocatalytic oxidation of reemitted Hg-0 prevailed or to promote Hg-0 reemission when photocatalytic reduction of HgO to Hg-0 dominated later on. The results indicate that minimization of Hg-0 reemission can be achieved by appropriate application of UV irradiation.