Association of cobalt and manganese in aquatic systems: Chemical and microscopic evidence

Abstract

The distributions of cobalt and manganese in the water column of Paul Lake, a small meromictic lake located in the upper Michigan peninsula, follow remarkably Similar patterns. At the oxycline, these elements show pronounced concentration gradients. In May 1994, Co and Mn increase from detection limits to 6 nM and 3 μM, respectively. The association between Co and Mn can be attributed to the sorption of cobalt to manganese hydrous oxides, and the redox-driven interconversion between dissolved Mn and particulate Mn-oxides species, which is often mediated by micro-organisms. In this paper, we have used a multi-method approach to the study of Co:Mn association in an aquatic system. First, using equilibrium calculations, we discuss the chemical speciation of dissolved Co and Mn in the water column. Second, we present a Transmission Electron Microscopic (TEM) characterization of individual Mn-rich micro-particles. These particles were primarily observed at the oxic/anoxic transition. Micrographs confirm the biological origin of the hydrous manganese oxides. Analytical Electron Microscopy clearly establishes that Co and Fe are scavenged by micro-organisms bearing a Mn-oxide crust. Co:Mn ratios were determined on individual particles by Energy Dispersive Spectrometry (EDS). The comparison of the average Co:Mn ratio in particles to the one in solution shows that Co is enriched in the Mn-oxides overgrowths. High resolution TEM-EDS analyses of environmental particles proves to be a powerful tool to unveil chemical associations that are difficult to deduce from changes in the water chemistry.