Do carbonate precipitates affect dissolution kinetics? 1: Basaltic glass

Abstract

Basaltic glass dissolution rates were measured in mixed-flow reactors at basic pH and at 25 °C and 70 °C in aqueous solutions supersaturated with respect to calcite for up to 140 days. Inlet solutions were comprised of NaHCO 3 ± CaCl 2 with ionic strengths > 0.03 mol kg − 1 . Scanning Electron Microscope images show that significant CaCO 3 precipitated during these experiments. This precipitate grew on the basaltic glass in experiments performed in Ca-free inlet solutions, but nucleated and grew independently of the glass surfaces in experiments performed in Ca-bearing inlet solutions. In those experiments where CaCO 3 precipitated on the glass surface, it grew as discrete crystals; no pervasive CaCO 3 layers were observed. The lack of structural match between glass and calcium carbonate favors CaCO 3 nucleation and growth as discrete crystals. Measured basaltic glass dissolution rates based on either Si, Al, or Mg were both 1) independent of time during the experiments, and 2) equal to that of corresponding control experiments performed in NaHCO 3-free inlet solutions. Taken together, these observations show that basaltic glass dissolution rates are unaffected by the precipitation of secondary CaCO 3 precipitation. It seems therefore likely that carbonate precipitation will not slow basaltic glass dissolution during mineral sequestration efforts in basaltic rocks.