This study presents an investigation of the biogeochemical processes regulating the distribution of trace elements Co and Pb at the oxic-anoxic transition in the water column of Paul Lake (MI). The research focused on the interactions of metals with hydrous oxides of manganese and iron using the natural environment as the laboratory.

Biological processes are responsible for major chemical changes in the water column. O2(aq), CO2tot, and pH profiles reflect the combined effect of photosynthesis and mineralization of organic matter. Sulfate disappears below the oxycline and is accompanied by the formation of sulfides, but the precipitation of metal sulfides was not observed. Manganese oxides (MnOx) and hydrous iron oxides (Fep) form two distinct peaks in the mixolimnion and in the suboxic waters, respectively. Mn and Fe particles were characterized by Transmission Electron Microscopy (TEM). MnOx form manganese crusts around bacteria, while Fep forms complex entities with fibrils of exopolysaccharides (EPS).

The analytical speciation of Co at the microparticle level and in bulk water samples shows that the cycling of Co is regulated by MnOx. There is a concurrent remobilization of Mn2+ and Co2+. The Co:Mn ratio obtained on individual microparticles is rather constant (~ 2%), and is greater than that of the dissolved species demonstrating that Co is preconcentrated in the solid phase.

The analytical speciation of dissolved Pb shows that it is completely complexed in the monimolimnion, probably by an organic ligand. TEM analyses and batch reactor experiments show that Pb is scavenged by an Fep-EPS entity. The latter experiment demonstrates that Pb is removed by co-entrainment during the oxidation of iron rather than by adsorption subsequent to particulate iron formation.

A dynamic model was built to predict the distribution of Fe and Pb in the water column. It was based on a mass balance approach depicting transport and reaction. Simulations indicate that the processes regulating the cycling of Pb depend on the good understanding of the mechanisms affecting the distribution of iron. This exercise confirms the importance of good observations and field data to characterize and predict the distribution of trace elements in aquatic systems.