Interaction of Macroaggregates and Hg in Coastal Waters (Gulf of Trieste, Northern Adriatic Sea)

Since a great quantity of dissolved Hg in natural waters was shown to be bonded onto colloidal organic matter (COM), Hg speciation, transport, biological availability and cycling are affected by interactions between Hg and COM. They are in large proportion dependent on the chemical composition and structure of COM. Macroaggregates (macrogels), produced episodically in the northern Adriatic by agglomeration of dissolved organic macromolecules—mostly heteropolysaccharides—of prevalently phytoplankton (diatom) origin, offer a rare opportunity to study these topics. 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 fraction. 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 FTIR. The highest carbohydrate content and the lower Cₒᵣg./Nₜₒₜ. 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 a 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 constituents. This scenario was confirmed during a degradation experiment showing the preservation of Hg associated with higher MW fraction containing organic N constituents. Conversely, Hg in lower MW fractions, composed mostly of carbohydrates and subjected to degradation, is released into solution. The higher Hg concentration and lower Hg/Cₒᵣg. ratio observed in matrix, characterized by the important presence of lipids in addition to polysaccharides and proteins, suggests the possible interaction between macrogel and HgS particles originating from Idrija (NW Slovenia) mining district.