Removal of trace level aqueous mercury by adsorption and photocatalysis on silica–titania composites

Silica–titania composites (STCs) were applied to trace level mercury solutions (100 μg/L Hg) to determine the degree of mercury removal that could be accomplished via adsorption and photocatalysis. STCs are a porous, high surface area silica substrate (>200 m²/g), manufactured using sol–gel methodology, impregnated with TiO₂ nanoparticles. The performance of this material along with its precursors, silica and Degussa P25 TiO₂ were compared. Under adsorption alone (no UV illumination), STCs were able to achieve approximately 90% removal of mercury, which is comparable to that of Degussa P25. Silica without TiO₂ performed poorly in comparison and was minimally affected by UV illumination. Contrary to expectations, the performance of Degussa P25 was not largely changed by UV irradiation and the STC was detrimentally affected under the same conditions. It was concluded that elemental mercury was formed under UV irradiation with or without the presence of TiO₂ due to photochemical reactions, decreasing the mercury removal by STC. Additionally, the primary particle size of the STC was reduced to increase mass transfer. The result was improved Hg removal under adsorption and photocatalysis conditions. Improved adsorption kinetics were also achieved by altering the STC pore size and TiO₂ loading.