Redoxpotentiale und redoxprozesse von mangan-, eisenund schwefelverbindungen in hydromorphen böden und sedimenten

Abstract

Results of laboratory experiments with soil material saturated with sea water indicate that, as predicted by thermodynamics, manganese (III, IV)-oxides are first reduced to Mn 2+-ions (beginning at about +450 mV at pH 6.1.; E 7 ≈ +400 mV), next amorphous iron (III)-oxides are reduced to Fe 2+-ions (beginning at about +220 mV at pH 6.0; E 7 ≈ +160 mV), and finally sulphates are reduced to sulphides (beginning at about +10 mV at pH 6.0; E 7 ≈ -50 mV). Direct quantitative relations between redox potentials, pH-values and Mn 2+- (or Fe 2+-) contents of water-saturated soils and sediments and calculated redox reactions of known manganese and iron systems could not be established. The influence of organic redox systems produced by microbial fermentation processes on the measured potentials and on the reduction of manganese and iron oxides is discussed. A reduction of the oxides by microbially formed sulphides, which themselves are oxidized by this process, seems also to be possible. Therefore, sulphides do not occur as stable sulphur phase in higher amounts before all available Fe-oxides are reduced to Fe 2+-ions. Then formation of iron monosulphides takes place by precipitation of Fe 2+- ions by sulphides (H 2S, HS). In a sulphide-stabilized environment redox reactions of sulphur — especially the reaction H 2S aq = S 0 + 2 H + + 2 e − — may determined the measured potentials. The results show that the dynamics and morphology of hydromorphic soils and sediments are strongly dependent on microbial processes.