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 diffraction, electron microscopy, and porewater analyses to study the mineralogical 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+-birnessite offering a large amount of vacancies in the birnessite layers for cation sorption. EXAFS spectroscopy reveals reduced manganese in the top 2 mm of the sediment. Therefore, the reduction of MnO2 must occur at similar rates as the sedimentation which is 2.5 mmol m(-2) d(-1) during summer. EXAFS-spectroscopy and microprobe analyses show that Mn 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 quantitatively determined by EXAFS spectroscopy, yielding 55-60% Mn incorporated in carbonate and 40-45% Mn in phosphate particles. The average concentration 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 until autumn. Thus, dissolution of these phases is very unlikely. The remobilisation of Mn from the sediment into the deep water is, therefore, thought to be governed by other processes such as adsorption and desorption of Mn(II) on calcite surfaces. Copyright (C) 1997 Elsevier Science Ltd.