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

Highlights: • A modified sequential extraction was developed for the analysis of metal sulfides. • Stannous chloride promoted the recovery of sulfur from copper sulfide. • An appropriate addition of SnCl₂ minimized FeS₂ dissolution in the AVS extraction. 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.