Atmospheric cycling of mercury: an overview

During Lhe last decade it has become evident that mercury emissions to air are dispersed within the regional scale. This means that much of the emitted mercury is deposited within 1,O00 to 2,000 km from the sources. Recent data indicate that lhe global background of atmospheric mercury is increasing by about 1% per year. lt has been estimated that the mercury deposition over Scandinavia must be decreased by about 80% to reach a balanced situation. Due to the serious environmental damage caused by merculy, it is thus essenti that mercury emissions are substantially decreased in most countries.

Photochemical behavior of inorganic mercury compounds in aqueous solution

Laboratory experiments were carried out by irrading several species of divalent mercury in aqueous solution with broadband light (wavelength >290 nm). Hg(OH)2 was found to be the most photoreactive species and its photolysis first-order rate constant was determined to be 1.2 x 1O-4 s-1 (n = 6). The reaction quantum vield was estimated to be 0.14. The photolysis rale constant and lifetime were calculated to be 3 X 1O -7s-1 and 600h under summer conditions in the Northern Hemisphere (Stockholm’ 5 latitude). The photoreduction of HgS22- results in the formation of Hg° and HgS(s). The potential environmental significance of these processes is discussed.

The retention of gaseous mercury on flyashes

The main objective of the present investigation was to study the adsorption of mercury on solid materials, such as flyash and calcium compounds, and thereby assess which parameters are relevant in influencing mercury retention. Ii is known that elemental and oxidized mercury are adsorbed by activated carbon and flyash. Oxidized mercury is more easily adsorbed than elemental mercury. The degree of adsorption and the influence of different trace gas components, however, are unknown. It is therefore of interest to study te influence of trace gases, such as S02 and HCI, on these adsorption processes. The retention of mercury by solid materials captured on fabric filters was investigated using a flue gas generator. The solid materials, activated carbon, flyashes, silica sand, lime, and gypsum were exposed to mercury-containing flue gas in a 1 to 3 mm thick layer supported by a standard fabric filter aL 150°C. The mercury retention capacity of the solid material was judged from measurements of the mercury concentration both upstream and downstream of the fiber. The effect on the retention of mercury by addition of HC1 and S02 was studied. The results indicate that it is possible to retain mercury on flyashes in general, and activated carbon in particular, while sand and calcium compounds showed negligible retention capacity. The carbon content in the flyashes is the crucial factor which determines mercury retention. High residual carbon content is reflected in a high retention of mercury on the flyashes. The results showed that mercury adsorption on flyashes and activated carbon was enhanced by oxidizing trace gases such as HCI and decreased by reducing species Iike S02. This is in agreement with the hypothesis that oxidized mercury is more easily captured on the solids than elemental mercury.