For reactions to occur on the pyrite surface in aqueous systems, it is a prerequisite that reactants are able to sorb within the Stern layer. Using electrophoresis and looking at changes in zeta-potential, it is possible to determine if an aqueous species sorbs within the Stern layer.
Here we report initial results of a comprehensive study to determine the development of surface charge on pyrite as a function of solution composition. Our measurements were conducted on crushed, acid-washed pyrite from Huanzala (Peru). In reducing NaCl solutions the isoelectric point (i.e.p.) is found at pH 2.2 (25°C). At higher pH the zeta potential is negative. The addition of ferrous iron (500 mM) changes the zeta-potential drastically. We observe two charge reversals. Up to pH 5 we find a positive potential, from pH 5 to 8.5 it is negative and above 8.5 it is positive again. The addition of sulfidic sulfur results in a more negative zeta-potential than observed in simple NaCl solutions. This is consistent with other reducing reagents. We report good reproducibility for the system investigated. The electrophoresis experiments give information on the bulk surface properties of the mineral. In order to investigate the surface functional groups on an atomic scale sorption of water onto pyrite has been studied in using High Resolution Electron Energy Loss Spectroscopy (HREELS). See abstract Strongin and coworkers. The combination of the two mentioned techniques is used to develop better defined surface site-binding models for sulfides.