Photoreduction of Mercuric Salt Solutions at High pH

Abstract

Titanium dioxide and a 100 W mercury spotlamp were used to photoreduce 100 ppm Hg aqueous mercuric chloride solutions. The solution's basicity and temperature were varied. Two optimum photoreduction conditions were determined: pH 9, 0 °C and pH 11, 40 °C. TiO2-assisted photoreduction at these two conditions lowered the concentration of mercury left in the solution to below 200 ppb. Methodology was developed to perform an overall mercury mass balance on the process. The overall mercury balance revealed that more than 97% (average 103% ± 6%) of the mercury removed from solution was deposited as mercury metal on the surface of the TiO2 for the pH 9, 0 °C treatment conditions. This mercury could be driven off the TiO2 surface by heating to 100 °C for half of an hour under nitrogen flow. The pH and temperature information under light and dark conditions is consistent with a pH-dependent adsorption of a dissociated mercuric species by hydroxide ions on the TiO2 surface followed by nucleation of the reduced species. The TiO2 assisted photoreduction process shows promise for remediation of mercuric waste below the EPA 200 ppb mercury disposal limit as well as the potential for recycling the mercury and TiO2 catalyst.