Discrimination among iron sulfide species formed in microbial cultures

A quantitative method for the study of iron sulfides precipitated in liquid cultures of bacteria is described. This method can be used to quantify and discriminate among amorphous iron sulfide (FeSamorph), iron monosulfide mi nerals such as mackinawite or greigite (FeSmin), and iron disulfide mineral s such as pyrite or marcasite (FeS2min) formed in liquid cultures. Degradat ion of iron sulfides is performed using a modified Cr2+ reduction method wi th reflux distillation. The basic steps of the method are: first, separatio n of FeSamorph; second, elimination of interfering species of S such as col loidal sulfur (S-c(o)), thiosulphate (S2O32-) and polysulfides (S-x(2-)); t hird, separation of FeSmin; and fourth, separation of FeS2min. The final pr oduct is H2S which is determined after trapping. The efficiency of recovery is 96-99% for FeSamorph, 76-88% for FeSmin, and > 97% for FeS2min. This me thod has a high reproducibility if the experimental conditions are rigorous ly applied and only glass conduits are used. A well ventilated fume hood mu st be used because of the toxicity and volatility of several reagents and p roducts. The advantage relative to previously described methods are better resolution for iron sulfide species and use of the same bottles for both in cubation of cultures and acid degradation. The method can also be used for Fe/S stoichiometry with sub-sampling and Fe analysis. (C) 2000 Elsevier Sci ence B.V. All rights reserved.