In this study, the sorption of Hg(II) onto pyrite was investigated as a function of the aqueous solution pH. X-ray photoelectron spectroscopy (XPS) was used to characterize the surfaces, and to identify the adsorbed species when possible. After 12 h of equilibration at a given initial pH (between 6 and 10.5), slabs of pyrite were in contact with Hg(II) solution (210 Β΅M, constant ionic strength fixed with 0.01 M NaNO 3 ) for 24 h. The final pH ranged between 4.1 and 3.6. The solid samples were then dried and transferred into the XPS machine for surface analyses (Hg 4f, S 2p, O 1s and Fe 2p levels).
Taking advantage of the differential charge effect when the surface was partly covered with Fe(III) oxyhydroxides, it was shown that Hg could be sorbed onto both pyritic zones and oxidized patches. On pyritic zones where no charge effect was observed, the Hg 4f 7=2 level was pointed out at 100.7 eV, ruling the presence of Hg(0) at the surface out. The S 2p level analyses clearly showed that the main component was S 2 2-, with some traces of polysulphur. Neither S(II) nor S(VI) were detected in any experiment, excluding the formation of HgS, sulphate and thiosulphate. Most observations were understood as the formation of two surface complexes between mercury and either the pyritic functional groups or the oxyhydroxide sites.