Mercury distribution between particulate and dissolved states in wetlands in California, USA
Elke Süß (08/2006)
Support: Britta Planer-Friedrich
publication: Freiberg Online Geoscience FOG Vol.15
Mercury released from soil and sediment contaminated by historic gold mining is a major environmental concern in the San Francisco Bay area. To better understand the release mechanisms, six soil and sediment samples from the Coastal Range and the Sierra Nevadas with total mercury concentrations between 1 and 36 μg/g were subject to batch experiments under varying conditions. The effects on mobility of mercury due to pH, ionic strength, dissolved
organic matter (in different concentrations and of varying chemical qualities), simple organic ligands (mercaptoacetic acid, salicylic acid, EDTA), and inorganic ions (chloride, calcium) were investigated.
Cinnabar was identified as the major mercury source in most of the soils and sediments by sequential extraction. Leaching experiments confirm that the water soluble mercury fraction is small (9 to 350 ng/L) and thus non-critical in terms of drinking water standards. However, its dissolution can further lead to increased methylation and biomagnification of neurotoxic methylmercury in the food web. A general increase in mercury release was observed with increasing pH, attributed to dissolution of soil organic matter, and with decreasing ionic strength, attributed to colloid stabilization. Higher DOC concentrations and higher reactivity of dissolved organic carbon caused a non-linear increase in dissolved mercury. The effects of adding different organic matter isolates seem to be a synergetic effect of various chemical properties of these isolates. Among the organic ligands, mercaptoacetic acid caused major mercury release, attributed to the strong complexation potential of its thiol group. Polyvalent cations strongly inhibit mercury release, because of their surface complexation of the minerals, whereas anionic inorganic ligands such as chloride form mercury complexes, favoring mercury dissolution. The results from the investigations confirm the general notion that among all ligands found in natural systems, dissolved organic matter is the most important in influencing dissolution, mobility, and bioavailability of mercury under natural conditions.