SOLID-PHASES IN THE CYCLING OF MANGANESE IN EUTROPHIC LAKES - NEW INSIGHTS FROM EXAFS SPECTROSCOPY

We have combined EXAFS spectroscopy with other methods such as x-ray d iffraction, electron microscopy, and porewater analyses to study the m ineralogical transformation of poorly crystallized manganese minerals at the oxic/anoxic boundary of the eutrophic Lake Sempach (Switzerland ). Manganese oxide formed in the water column was identified as H+-bir nessite offering a large amount of vacancies in the birnessite layers for cation sorption. EXAFS spectroscopy reveals reduced manganese in t he top 2 mm of the sediment. Therefore, the reduction of MnO2 must occ ur at similar rates as the sedimentation which is 2.5 mmol m(-2) d(-1) during summer. EXAFS-spectroscopy and microprobe analyses show that M n is associated with authigenic (Ca,Mn)CO3 and (Fe,Mn)(3)(PO4)(2)8H(2 )O-particles. The relative amount of Mn in these two phases was quanti tatively determined by EXAFS spectroscopy, yielding 55-60% Mn incorpor ated in carbonate and 40-45% Mn in phosphate particles. The average co ncentration of Mn amounts to 23 mol% in carbonate particles and 16 mol % in phosphate.With respect to the solid solutions, (Ca,Mn)CO3 and (Fe ,Mn)(3)(PO4)(2)H2O, the porewater remains oversaturated from spring u ntil autumn. Thus, dissolution of these phases is very unlikely. The r emobilisation of Mn from the sediment into the deep water is, therefor e, thought to be governed by other processes such as adsorption and de sorption of Mn(II) on calcite surfaces. Copyright (C) 1997 Elsevier Sc ience Ltd.