Sims studies of oxidation mechanisms and polysulfide formation in reacted sulfide surfaces

The surface oxidation of metal sulfides in air and aqueous solution is of c entral importance in mineral separation and environmental control of acid m ine drainage. Mechanisms of oxidation, dissolution and surface restructurin g have been extensively studied using XPS. High binding energy components i n S 2p XPS spectra have been attributed to metal-deficiency formation of po lysulfide S-n(2-), elemental sulfur and electronic defect structures (ie Cu (I)/ZnS). The assignment of these components in S 2p XPS spectra has, howev er, left significant uncertainties particularly in the formation of S-S bon ding in polysulfide species requiring confirmation from other surface analy tical techniques. The use of static ToF-SIMS has provided a new avenue for identification of these species and their development in oxidation of the sulfide surfaces. F or the iron sulfides, there is a systematic change in the FeS2/FeS fragment ratio from troilite (FeS) through pyrrhotite (Fe-1-S-x) to pyrite (FeS2) w ith ratios varying from 0.59, 1.2 to 32 respectively. Similarly high ratios for FeSn/FeS are found for pyrite compared with pyrrhotite and troilite mi rrored in the S-n/S fragment ratios. Changes in surface oxidation, represen ted in atomic concentrations and S 2p XPS spectra, are seen in the ToF-SIMS signals for S-n/SOn ratios in the same iron sulfide sequence. These mass m arkers, reflecting increased S-S bonding, increase in surfaces after oxidat ion giving further confidence in XPS assignment to polysulfide species. Freshly cleaved galena PbS surfaces reacted in pH8 aqueous solution for inc reasing periods of time have also shown a systematic increase in S-n/S rati os with increasing at.% of oxidised S-n(2-) species from XPS spectra. Stati stical analysis of oxidised galena has shown that the ratios (PbO+)-Pb-206/ Pb-206(+) and (PbOH+)-Pb-208/Pb-208(+) directly reflect the degree of oxida tion of the surface lead species whilst the O-S-, S-/total - ion yield and SO3-/S- are the best measures for following the oxidation of sulfur species . Results from these ratios suggest that initial air oxidation takes place predominantly on the S sites rather than Pb sites but, in solution at pH9, both sites are oxidised. The ToF-SIMS results appear to directly reflect the surface chemistry of th e metal and sulfur species and are not consistent with recombination or fra gmentation of secondary neutral or ionic species. The results strongly sugg est increasing polymerisation of S-S species with increasing oxidation in a ccord with the XPS assignment to polysulfide of increasing chain length. (C ) 2000 Published by Elsevier Science Ltd. All rights reserved.