Epsilon Archive for Student Projects

Abstract

Transformation of inorganic mercury (Hg) to methyl mercury (MeHg) is a biotic process that is proposed to be conducted primarily by sulphate reducing bacteria (SRB). The formation of MeHg is of great interest because it is the mercury species that biomagnifies to the greatest extent in the aquatic food webs; about 90 % of all Hg in fish is MeHg. Our understanding of factors involved in the production of MeHg is very important for making a reliable risk assessment. The purposes of this study were to investigate the different factors that control the solubility of Hg and the production of MeHg in a contaminated lake sediment.

The lake Ala-Lombolo, Kiruna municipality, is well-known to be contaminated by mercury from different sources. Sediment cores were sampled at three different locations (N, SW and SE) in the lake and divided in three different layers; 0-5, 5-10 and 10-15 cm. To minimize the contact with air, 1 litre plastic buckets were completely filled with sediment as fast as possible and then sealed and put in a cooling bag during the 8 hour transport to Umeå. In the laboratory, samples where homogenized in a N2-filled glove-box and divided into sub-samples for determination of total concentrations of Hg-tot, MeHg, C, N, S and Fe in the sediment and Hg, MeHg, organic and inorganic carbon, pH, dissolved H2S, Cl and SO4 in the pore water. The potential methylation/demethylation rates where determined in sediment sub-samples after incubation 48 hours at 23 ˚C.

The lake is highly contaminated with a total Hg concentration varying between 20 and 60 mg kg-1 in the solid phase and between 0,05 and 0,77 nM in the pore water. For MeHg the concentration ranges from 6,6 to 29,2 µg kg-1 in the solid phase and from 0,00019 to 0,0014 nM in the pore water. The depth distribution of MeHg differs from Hg, with the highest concentration of MeHg in the top sediment layer (0-5 cm). The likely reason for that is a sedimentation of energy rich organic matter from dead algae, which is stimulating sulphate reducing bacteria, at the surface. As a result, Hg is transformed to MeHg. This conclusion was supported by incubation experiments, verifying that the methylation rates were highest in the top 0-5 cm sediment layer. The solubility of Hg is known to be controlled by the complexation with thiol groups in dissolved organic matter (DOM) and by inorganic bisulfides (HS-). In Ala-Lombolo relatively high concentrations of bisulfides, indirectly regulated by the formation of FeS (s), controls Hg solubility. At 5-10 and 10-15 cm depths the formation of neutral dissolved neutral Hg-sulphides and bi-sulphides control the methylation rates. In contrast to Hg, the solubility of MeHg was not controlled by inorganic suphides alone, indicating that also organic thiols contributed to the solubility of MeHg. Incubation experiments showed that the long term build-up of MeHg in the sediment, reflected by the % MeHg/Hg-tot ratio, was strongly positively correlated to the formation of methyl mercury during 48 hours under laboratory conditions, reflected by the specific methylation rate constant (Km, d-1). This suggests that the % MeHg/Hg ratio in sediments may be a good proxy for the short term methylation rate and thus a good parameter for risk assessment analyses.