Reactions of sulfur dioxide on calcium carbonate single crystal and particle surfaces at the adsorbed water carbonate interface

Sulfur dioxide reactions with calcium carbonate interfaces at 296 K in the presence and absence of adsorbed water result in the formation of adsorbed sulfite and sulfate. The extent of reaction is significantly enhanced, approximately five- to ten-fold for particulate and single crystal CaCO₃ (calcite), respectively, in the presence of adsorbed water between 30 and 85% RH. Atomic force microscopy following the reaction shows that adsorbed water facilitates surface reactivity by enhancing the mobility of surface ions, giving rise to the formation of nanometer sized product crystallites approximately 1 nm in height. Simultaneous with the formation of these crystallites is pitting and etching of the underlying substrate, which occurs preferentially in the vicinity of monoatomic surface steps. In the absence of water, there is little pitting and no evidence for the formation of crystallites. X-Ray photoelectron core and valence band spectra confirm the presence of two sulfur adsorbed species, SO²⁻₃ and SO²⁻₄, with nearly equal amounts of SO²⁻₃ and SO²⁻₄ in the absence of adsorbed water and approximately five times more SO²⁻₃ relative to SO²⁻₄ in the presence of adsorbed water. From these data, it is proposed that the nanometer-sized crystallites are composed primarily of CaSO₃.