Improved extraction of acid-insoluble monosulfide minerals by stannous chloride reduction and its application to the separation of mono- and disulfide minerals in the presence of ferric iron

Abstract

Metal sulfides, which are important indicators of sulfur cycling, are usually divided into two categories according to sulfur chemical valence: (1) monosulfides (S2−) and (2) disulfides (S22−). The two sulfur species are separated and quantified by a sequential-extraction method. Specifically, monosulfides are extracted as acid-volatile sulfide (AVS) using 6 M HCl prior to the extraction of disulfides using acidic CrCl2, which is defined as chromium-reducible sulfur (CRS). However, the conventional AVS procedure does not result in the quantitative extraction of S2− from the acid-insoluble metal monosulfide, copper sulfide (CuS). Consequently, residual sulfur in CuS (CuS-S) may be extracted as CRS resulting in the inaccurate separation of these two sulfur species. In this study, we used stannous chloride (SnCl2) to improve CuS-S recovery in the AVS procedure and permit the separate extraction of sulfur from CuS and pyrite (FeS2), the most abundant disulfide in nature. Our results show that the addition of SnCl2 increased the recovery of CuS-S as AVS from less than 36% to as high as 92% in the absence of pyrite and Fe3+ and 89% in the presence of pyrite and Fe3+. In addition, based on the observed correlation between the concentration of SnCl2 and the dissolution of FeS2, we identified the appropriate concentration of SnCl2 needed to avoid the dissolution of FeS2 in the AVS procedure. SnCl2 also minimized the oxidation of CuS-S by Fe3+ released from ferric minerals during the extraction of AVS. Based on the results of a series of sequential-extraction experiments, we show that an amendment of SnCl2 in the AVS procedure followed by CRS permits the quantitative separation of CuS-S and FeS2-S while also preventing interference by Fe3+. Our method will find application in research concerned with the fate of metals and the biogeochemistry of sulfur in the environment.