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. TiO 2 -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 TiO 2 for the pH 9, 0 °C treatment conditions. This mercury could be driven off the TiO 2 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 TiO 2 surface followed by nucleation of the reduced species. The TiO 2 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 TiO 2 catalyst.

A surface science perspective on TiO2 photocatalysis

“…Dependences on pH have also been observed in various photoreduction reactions [1001,1142,[1298][1299][1300]. In many cases, the influence of pH on photochemical reactions relates to adsorbate stabilities and not necessarily to electron transfer events [993,1290,1298,1299,[1301][1302][1303][1304][1305]. In this sense, surface coverages can be manipulated with pH, resulting in either favorable or unfavorable conditions.…”

Preparation of Porous TiO2 Sheets by Aqueous Tape Casting and their Photocatalytic Activation

“…Titania, owing to its versatile properties, has been used to prepare sensors, solar energy cells, and so forth. Significantly, titania is also a promising photocatalyst, and it has an increasingly important status in the field of environmental protection 7–11 . As a photocatalyst, TiO 2 is mainly used in the form of powder and thin films.…”

Enhanced photocatalytic reduction of Hg(II) in aqueous medium by 2-aminothiazole-modified TiO2 particles

“…The most promising methods of treatment include sorption, ion exchange, precipitation, liquid emulsion membranes and photoreduction. 5 Photocatalysis using TiO 2 as photocatalyst appears very promising for complete wastewater treatment. This 454 Cristante et al J. Braz.…”

“…The most promising methods of treatment include sorption, ion exchange, precipitation, liquid emulsion membranes and photoreduction. 5 Photocatalysis using TiO 2 as photocatalyst appears very promising for complete wastewater treatment. This semiconductor has been widely used due to its nontoxicity, high stability under irradiation and the possibility of using natural UV-light in the photoreduction of mercuric ions to metallic Hg, calomel or HgO, depending on the nature and concentration of the starting salt, which may be recycled.…”