Iron and sulfate reduction in the sediments of acidic mine lake 116 (Brandenburg, Germany): Rates and geochemical evaluation

A combination of rate measurements of iron(III)oxide and sulfate reduction, thermodynamic data, and pore-water and solid phase analyses was used to ev aluate the relative significance of iron and sulfate reduction in the sedim ents of an acidic strip mining lake (Lake 116, Brandenburg, Germany). The r ate of sulfate reduction was determined using a S-35-radiotracer method. Ra tes of iron turnover were quantified by mass balances based on pore-water c oncentration profiles. The differences in Gibbs free energy yield from redu ction of iron and sulfate and from methanogenesis were calculated from indi vidual redox couples and concentrations of reactants to account for the inf luence of high Fe2+ concentrations and differing mineral phases. Integrated (O-20 cm) mean rates of sulfate reduction were 1.2 (pelagial), respectivel y 5.2 (littoral) mmol (m(2)d)(-1). Based on electron equivalents, the estim ated iron reduction rates reached between about 50 % (pelagial) and 75 % (l ittoral) of the sulfate reduction rates. Compared to conditions usually ass umed in the literature, in the sediments Gibbs free energy advantage of iro n reauction over sulfate reduction was reduced from +11 KJeq(-1) to a range of about +7 KJeq(-1)(ferrihydrite, "reactive iron") to -6 KJeq(-1) (goethi te). This indicates that iron reduction was thermodynamically favored to su lfate reduction only if amorphous iron(III)oxides were available and is in accordance to the high competitiveness of sulfate reducers in the sediment. While total iron concentration in the sediments was high (up to 80% of the dryweight), reactive iron only accounted for 11-38% and was absolutely and relatively diminished in the zone of iron reduction. Pore-water concentrat ion gradients and (CS)-C-137 profiles indicated that little or no bioturbat ion occurred in the sediments, probably inhibiting the renewal of reactive iron. We further hypothesize that the reactivity of the iron oxide surfaces was reduced due to adsorption of DOM, suggested by IR spectra of the DOM a nd by a surface coverage estimate using literature data. Pelagial and litto ral sediments displayed different dynamics. At the littoral relative iron r eduction rate estimates were higher, iron sulfides were not accumulated and residence times of iron oxides were short compared to the pelagial. At the littoral site reoxidation of iron sulfides probably resulted in the renewa l of reactive iron(III)oxides, possibly allowing for higher relative rates of iron reduction.