AN ELECTROKINETIC STUDY OF SYNTHETIC GREIGITE AND PYRRHOTITE

The isoelectric points of synthetic greigite, Fe3S4, and pyrrhotite, F e1-xS, in 10(-3)-1.1 10(-2) M NaCl solution at a(Fe) < 10(-4) and a(s) < 10(-4) were determined to be pH 3 and 2, respectively. Below pH 2 f or pyrrhotite and below pH 3 for greigite, the surfaces are positively charged; above these pH values the surfaces are negatively charged. U pon addition of sulfidic sulfur (H2S, HS-), traces of terrestrial humi c acids, or a combination of sulfidic sulfur and terrestrial humic aci ds. the surfaces are negatively charged over the entire pH range (2-11 ) studied. Because humic acid and sulfidic sulfur influence the zeta p otential of the iron sulfide surfaces, they must be adsorbed within th e shear plane of the double layer, close to the actual surface of the minerals. The isoelectric points for greigite and pyrrhotite in the ab sence of either sulfidic sulfur or humic acid are very close to those for natural pyrite (FeS2) and sphalerite (ZnS), as well as synthetic Z nS, CuS, CdS, galena (PbS), chalcopyrite (CuFeS2), and elemental sulfu r. It is plausible that the speciation of thiol surface groups (=S-H-2 (+), =S-H-0, =S-(-)) dominates the surface charge on all these phases. The increase in net zeta potential upon the addition of sulfidic sulf ur is, however, not well understood.