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
Metal sulfides, which are important indicators of sulfur cycling, are usually divided into two categories according to sulfur chemical valence: (1) monosulfides (S 2− ) and (2) disulfides (S 2 2− ). 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 CrCl 2 , which is defined as chromium-reducible sulfur (CRS). However, the conventional AVS procedure does not result in the quantitative extraction of S 2− 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 (SnCl 2 ) to improve CuS-S recovery in the AVS procedure and permit the separate extraction of sulfur from CuS and pyrite (FeS 2 ), the most abundant disulfide in nature. Our results show that the addition of SnCl 2 increased the recovery of CuS-S as AVS from less than 36% to as high as 92% in the absence of pyrite and Fe 3+ and 89% in the presence of pyrite and Fe 3+ . In addition, based on the observed correlation between the concentration of SnCl 2 and the dissolution of FeS 2 , we identified the appropriate concentration of SnCl 2 needed to avoid the dissolution of FeS 2 in the AVS procedure. SnCl 2 also minimized the oxidation of CuS-S by Fe 3+ 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 SnCl 2 in the AVS procedure followed by CRS permits the quantitative separation of CuS-S and FeS 2 -S while also preventing interference by Fe 3+ . Our method will find application in research concerned with the fate of metals and the biogeochemistry of sulfur in the environment.
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