Cu(II) adsorption mechanism on pyrite: an XAFS and XPS study

The surface of a <38 µm ground fraction of pyrite was analysed using x-ray photoelectron spectroscopy (xps), x-ray absorption fine structure spectroscopy (XAFS) and inductively coupled plasma atomic emission spectroscopy (ICPAES) prior to and after Cu(II) adsorption under different pH and Cu(II) concentration conditions. Under all conditions copper is found to be coordinated to sulphur on the surface of pyrite with an average bond length of 2.27±0.02 Å. A Cu–O bond with a length of 2.00 Å due to Cu(OH)2 precipitation was also observed for a sample with copper adsorbed at pH 8.5 from a solution initially containing 2.84 × 10−4 mol m−2 Cu (all concentrations given are normalized against the surface area of pyrite in solution). The Cu 2p XPS signals for the samples with adsorbed copper show the presence of Cu(I) ions on the pyrite surfaces in all cases. For the 2.84 × 10−4 mol m−2 Cu solution at pH 8.5 the pyrite was observed to have both a Cu(I) and a Cu(II) XPS component. The ground pyrite surfaces showed increases in the oxidized S22− (Eb = 163.5 eV) signal on copper adsorption with minor increases in Fe(III) oxy- or hydroxy-species observed on pyrite surfaces exposed to long periods of adsorption. It is proposed that the reduction of Cu(II) to Cu(I) on the surface of the pyrite is accompanied by the oxidation of the S22− species present in the pyrite surface. Solution analysis in conjunction with surface analysis has indicated that a 1 : 1 exchange of Fe(II) ions for Cu(II) ions did not occur during copper adsorption and thus an ion-exchange mechanism is ruled out.