Properties of As and S at NiAs, NiS, and F1-xS surfaces, and reactivity ofniccolite in air and water

Niccolite (NiAs), NiS thigh temperature form), and pyrrhotite (Fe1-xS) are structurally related, with sulfur and arsenic sixfold coordinated. Their fr acture or cleavage necessarily produces surface As and S species of lower c oordination. Some sp(3) hybridized As and S ions likely form at these surfa ces, as observed for As at GaAs and S at pyrite surfaces. Upon relaxation, lone pairs of electrons occupy As and S dangling bonds, with the lone pair directed away from the surface and poised to react with adsorbates. Accordi ng to the valence shell electron pair repulsion (VSEPR) model, the lone pai r should repel the three hybrid orbitals forming metal-As or metal-S bonds, causing tetrahedral angles to contract and As and S ions to "protrude" fro m the surface. X-ray photoelectron spectroscopic (XPS) study of niccolite (NiAs) surfaces fractured under high vacuum (10(-9) torr) indicate that arsenic is more rea ctive than Ni toward residual gases of the XPS analytical chamber, as evide nt from development of small As1+ and As3+ photopeaks in the As(3d) XPS spe ctrum. Hydroxide observed in the O(1s) XPS spectrum suggests formation of A s(OH)(3). The O(1s) spectrum also revealed adsorbed H2O and atomic oxygen r adical (produced by dissociation of O-2) at the NiAs surface. Initiation of arsenic oxidation probably involved reduction of adsorbed atomic oxygen ra dicals, followed by hydration to produce hydroxyl, and finally to produce t he observed As(OH)(3) surface species. There was no evidence for Ni reactio n with residual gases. Reaction of NiAs with air over 30 h yields a thin (similar to 10 Angstrom) oxidized overlayer containing Ni(OH)(2), arsenite, and arsenate. Similarly thin oxidized layers, composed of oxyhydroxides and sulfates, are produced on millerite and pyrrhotite surfaces. All three surfaces are largely passiv ated toward air after only a few hours reaction. A thicker oxidized overlayer (similar to 120 Angstrom thick) containing the same secondary products as the air-ixidized surface is produced on NiAs af ter 7 days reaction with aerated, distilled water. The accumulation of arse nite and arsenate salts at the niccolite surface distinguishes it from mill erite and pyrrhotite surfaces reacted with aerated solutions. Ni- and Fe-su lfate salts produced from these latter two minerals are highly soluble, hen ce do no accumulate at the mineral surfaces whereas Ni-arsenate land appare ntly Ni-atsenite) salts are less soluble and accumulate on oxidized niccoli te surfaces along with Ni(OH)(2).