Coprecipitation and adsorption of Mn(II) with mackinawite (FeS) under conditions similar to those found in anoxic sediments

The metastable iron sulfide mineral mackinawite commonly forms in anoxic sediments in the region where elevated concentrations of dissolved Mn2+ are also observed. There is, consequently, a potential for significant interactions between dissolved Mn and mackinawite. We have experimentally investigated adsorption of Mn2+ on mackinawite and its coprecipitation with mackinawite using chemostat techniques. The very fine-grained nature of mackinawite results in adsorption being quantitatively (generally > 90%) much more important than coprecipitation. Adsorption of Mn2+ follows a Langmuir-type isotherm and decreases substantially with increasing salinity. The partition coefficient for Mn2+ with mackinawite, in low ionic strength solutions at 25°C extrapolated to zero reaction rate, is 4.8 (±0.5) × 103. It approximately doubles for every 20°C increase in temperature, and, over the range of conditions investigated, was found to be close to independent of pH and the mole fraction of MnS. The partition coefficient is 1.8 (±0.35) × 10−3 in seawater of S = 35 at 25°C at a slow reaction rate (0.5 μmol L−1 min.−1). It also decreases by about a factor of 3 at high precipitation rates, possibly because of the formation of a greater proportion of amorphous-FeS rather than mackinawite or an inability of the sorption reaction to keep up with the rate of formation of new surfaces.