Adsorption, photocatalysis, and photochemistry of trace level aqueous mercury.

摘要

Mercury (Hg) is ubiquitous in the environment and can lead to detrimental impacts for humans and ecosystems. From the literature, semiconductor photocatalysis showed promise as an aqueous Hg removal technology. To further this concept, high surface area photocatalytic adsorbents, SiO2/TiO2 composites (STC), were synthesized by adding TiO2nanoparticles to a liquid sol that was catalyzed by HNO3 and HF acids. The resulting materials were characterized by SEM, nitrogen adsorption-desorption, streaming potential, XRD, diffuse reflectance and TiO2surface area analyses. Approximate characteristics include an isoelectric point of 3, TiO2particle size of 30 nm, and a band gap energy of 3.2 eV. Surface areas of the composites ranged from 167 to 630 m2/g. Available TiO2surface area of the composite was dependent upon TiO2loading.;STCs and their precursors, silica and Degussa P25 TiO2, were applied to trace level Hg solutions (100 mug/L Hg) to determine the degree of Hg removal that could be accomplished via adsorption and photocatalysis. Under adsorption alone, STCs were able to achieve approximately 90% Hg removal. Silica without TiO2performed poorly and was not affected by UV illumination. Additionally, Hg removal was optimized by altering primary particle size, STC pore size, and TiO2loading. Contrary to expectations, the performance of DP25 was not significantly improved by UV irradiation and the performance of the STC was degraded under the same conditions. Elemental Hg was formed under UV irradiation due to photochemical reactions, decreasing the Hg removal by STC.;Photochemical transformations of Hg in batch reactors were investigated. The effect of purge gas (including rate and bubble size), UV irradiation wavelength, initial Hg concentration and time on Hg removal from prepared solutions were studied. Nitrogen purge with 254 nm UV irradiation resulted in the greatest net production of elemental Hg following a pseudo first order rate constant of 0.18 s-1 reaching over 99% removal. As oxygen was introduced into solution, the quantity of elemental Hg volatilized decreased but still resulted in significant Hg losses through volatilization up to 90% in 60 minutes. Most importantly, the loss of elemental Hg from solution is dependent upon the gas purge rate and bubble size.