The fate of Hg-O in natural waters

Several aspects of the aquatic chemistry of elemental mercury (Hg-0) are vital to the understanding of the fate of this metal in the environment, yet have scarcely been studied. Reactive dissolution of metallic mercury is shown to be dependent on the metal concentration in solution, the metal oxidation rate, particulate material concentration, nature and concentration of other ions, and pH. When using 1 g L-1 of Hg-0 in distilled water open to the atmosphere, the concentration of soluble mercury increases as a function of time, reaching 5.4 mu g L-1 of total Hg in the steady state. From this, 3.2 mu g L-1 were due to Hg2+ formed via oxidation. In lake water, results showed an inhibition in the reactive dissolution process, and the total metal concentration in water was 3.1 mu g in the steady state. This inhibitory effect was attributed to particulate material. In seawater, the total concentration of soluble Hg increases as a function of time, reaching a peak of 17.8 mu g L-1 after 10 h. After this, the soluble concentration dropped, to 4.8 mu g L-1. Experiments performed at different values of pH (4.0; 7.0 and 9.0), showed that the dissolution of the metal occurred to a higher extent at pH 4.0. Adsorption studies of both mercuric ions and elemental mercury species onto particulate material showed a dependence on the surface area, following the sequence 400 mesh > 200 mesh > sediment in natura. The implications of such findings are discussed, taking into consideration the Amazonian scenario.