Hg mobility at the sediment-water interface was investigated at three sampling points located in the Gulf of Trieste (AA1, CZ) and Grado Lagoon (BAR). Experiments were conducted under laboratory conditions at in situ temperature using a dark flux chamber simulating an oxic-anoxic transition.Temporal variations of dissolved Hg and MeHg as well as O2, NH4+, NO3-, PO43-, H2S, Fe2+ and dissolved inorganic (DIC) and organic carbon (DOC) concentrations were monitored simultaneously. Our results show low benthic Hg effluxes and even influxes in oxic conditions and effluxes in the anoxic phase of the experiment. MeHg release was less noticeable (low or absent) in the oxic phase, probably due to similar methylation and demethylation rates, but high in the anoxic phase of the experiment. In this context, the MeHg flux was linked to sulphate reduction and dissolution of Fe (and Mn) oxyhydroxides, and formation of sulphides and carbonates. The impact of DOM on Hg and MeHg oxic and anoxic fluxes was of minor importance, suggesting that the majority of dissolved Hg and MeHg are not linked to DOM. In the reoxigenation phase, the concentrations of MeHg and Hg dropped rapidly probably due to re-adsorption onto Fe (Mn) hydroxides and enhanced demethylation. Sediments, especially during anoxic events, should be, hence, considered as a primary source of MeHg for the water column in the northern Adriatic.
COBISS.SI-ID: 2307919
Among the northern Adriatic lagoons, Pialassa Baiona (P.B.) Lagoon, located near the city of Ravenna (Italy), received between 100 and 200 tons of Hg, generated by an acetaldehyde factory in the period 1957-1977. Further east, the Grado (G.L.) Lagoon has been mainly affected by a long-term Hg imput from the Idrija mine (western Slovenia) through the Isonzo River since the 16th century. Hg cycling at the sediment-water interface of the two lagoons was investigated and compared by means of an in situ benthic camber, estimating diffusive Hg and Methyl-Hg fluxes in the summer season. Major chemical features in porewaters and sediment solid phase were also measured. The daily integrated flux for MeHg was extremely low in P.B. accounting to only 7% of the corresponding flux in G.L. while the Hg fluxes were similar. A selective sequential extraction revealed that the stable crystalline cinnabar HgS is the predominant Hg fraction in G.L. The environmental conditions in P.B. seem to prevent the potential risk of Hg transfer to the aquatic trophic chain.
COBISS.SI-ID: 2426703
The chemical speciation of macroaggregate Hg in the Gulf of Trieste (northern Adriatic Sea) was studied using filtration and centrifugation discriminating between matrix (»water insoluble«) and interstitial water (»water soluble« colloidal) fractions. The colloidal fraction was subsequently ultrafiltered through membranes with a nominal pore size of 30, 10 and 5 kDa cutoff sequentially (in a »cascade fashion«), and fractions analyzed for carbohydrate, Corg. and Ntot. contents, and used for FTIR. The highest carbohydrate content and the lower C/N ratio were associated with higher molecular weight (MW) fraction ()30 kDa) suggesting that aminopolysaccharides and glycoproteins can be important constituents of this fraction. Analyses of MW of permeates, assayed by high-pressure size exclusion chromatography (HPSEC), revealed that they are composed of macromolecules of quite rather narrow MW distribution. Analyses of Hg concentrations revealed the highest concentration in high MW fraction in connection with the highest content of organic N consituents. This scenario was confirmed during a degradation experiment showing the preservation of Hg associated with higher MW fraction. Conversely, Hg in lower MW fractions, composed mostly of carbohydrates and subjected to degradation, is released into solution. The higher Hg concentration observed in matrix, characterized by the important presence of lipids in addition to polysaccharides and proteins, suggests the possible interaction between macrogel and HgS originating from Idrija (NW Slovenia) mining district.
COBISS.SI-ID: 2415439