A study of iron and manganese transformations at the O sub 2 /S(-II) transition layer in a eutrophic lake (Lake Bret, Switzerland): A multimethod approach

The usefulness of an analytical scheme based on the simultaneous use of various analytical methods for the study of Fe and Mn species at a mid-water O{sub 2}/H{sub 2}S redox transition layer has been investigated. The relative abundance of particulate, colloidal and electroactive species as well as their redox state has been determined, across the interfacial zone. The relevance of polarographic results in unmodified anoxic lake water was tested by performing measurements both directly in the field and the laboratory after sample storage. Significant differences in the signals of Fe(II) and S(-II) species have been observed. The concentration of manganese oxyhydroxide (MnO{sub x}) was measured using three different analytical techniques and the results suggest that natural lacustrine MnO{sub x} contains a reactive and a less reactive fraction. Use of the multimethod approach, has enabled the authors to demonstrate a spatially well resolved chemical make-up of the water strata at the transition layer and peaked profiles for dissolved Mn(II) and particulate and colloidal Fe(III) and Mn(IV) species. The most probable reactions between O{sub 2}, Mn(II)/Mn(IV), Fe(II)/Fe(III) and S(-II) are discussed on the basis of these results.