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

摘要

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 SnCl2 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.