Sulfur isotope fractionation during experimental precipitation of iron(II) and manganese (II) sulfide at room temperature

Fractionation of stable sulfur isotopes (S-34/S-32) during the precipitation of iron(II) and manganese(II) sulfides from aqueous solutions at room temperature (approximate to 20 degrees C) was investigated using batch-type experiments. The sulfides were precipitated (1) by mixing of metal(II) chloride solutions with hydrogen bisulfide solutions, and (2) by the transformation of anhydrous metal(II) carbonates in hydrogen bisulfide solutions. It was found that the partitioning of S-34/S-32 upon metal sulfide precipitation depends on the synthesis procedure, an indication for the influence of reaction kinetics. The sulfur isotopic composition of sedimentary metal monosulfides is close to that of dissolved hydrogen bisulfide. Kinetic fractionation factors for method (1) were 0.9988 +/- 0.0007 (alpha'(FeS-HS-)) and 1.0002 +/- 0.0007 (alpha'(MnS-HS-)), and for method (2) 1.0004 +/- 0.0001 (alpha'(FeS-HS-)) and 1.0012 +/- 0.0003 (alpha'(MnS-HS-)). Therefore, sulfur isotope effects related to the precipitation of metal monosulfides are small when compared to important microbial reactions. (C) 1998 Elsevier Science B.V. All rights reserved.