The non-oxidative dissolution of galena nanocrystals: Insights into mineral dissolution rates as a function of grain size, shape, and aggregation state

The acidic, non-oxidative dissolution of galena (PbS) nanocrystals has been studied in detail using transmission electron microscopy (TEM) to follow the evolution of the size and shape of the nanocrystals before and after dissolution experiments, X-ray photoelectron spectroscopy (XPS) to follow particle chemistry, and dissolution rate analysis to compare dissolution rates between nanocrystalline and bulk galena. Dissolution characteristics were also studied as a function of nanocrystal access to bulk vs. confined solution due to the degree of proximity of next-nearest grains. Nearly monodisperse galena nanocyrstals with an average diameter of 14.4 nm were synthesized for this study, and samples were exposed to pH 3, deoxygenated HCl solutions for up to 3 h at 25 °C. Detailed XPS analysis showed the nanocrystals to be free of unwanted contamination, surface complexes, and oxidative artifacts, except for small amounts of lead-containing oxidation species in both pre- and post-dissolution samples which have been observed in fresh, natural bulk galena. Depending on the calculation methods used, galena nanocrystals, under the conditions of our experiments, dissolve at a surface area normalized rate of one to two orders of magnitude faster than bulk galena under similar conditions. We believe that this reflects the higher percentage of reactive surface area on nanocrystalline surfaces vs. surfaces on larger crystals. In addition, it was shown that {1 1 1} and {1 1 0} faces dissolve faster than {1 0 0} faces on nanocrystals, rationalized by the average coordination number of ions on each of these faces. Finally, dissolution was greatly inhibited for galena nanocrystal surfaces that were closely adjacent (1-2 nm, or less) to other nanocyrstals, a direct indication of the properties of aqueous solutions and ion transport in extremely confined spaces and relevant to dissolution variations that have been suspected within aggregates.