Dissolution kinetics of sulfate minerals: Linking environmental significance of mineral-water interface reactions to the retention of aqueous chromate in natural waters

Dissolution as a function of solution undersaturation (O) was studied on both celestite and barite (001) by the aid of atomic force microscopy (AFM). Both the sulfates exhibited non-linear reactivity trends showing increasing dissolution rates with decreasing O. In the case of celestite, the dissolution rate non-linearity was attributed to a changeover in reaction mechanism. At higher undersaturations below a critical saturation state of O crit ∼ 0.1, dissolution occurred both along the existing step edges and also via the creation of new steps. At conditions near saturation states dissolution took place exclusively by removal of ions from existing step edges. On the other hand dissolution rate non-linearity in the case of barite was controlled by changes in step speeds with O. Similar dissolution rate behavior also evident on powdered barite in mixed flow reactor system establishes the dissolution rate non-linearity in terms O to be the characteristic property of barite. Celestite (001) dissolution was also studied in terms of Sr2+ SO2-4 by AFM to examine the mineral's reactivity under nonstoichiometric solute conditions. At O = 0.63, reaction kinetics were investigated by measuring ⟨010⟩ and ⟨120⟩ step speeds. Application of a theoretical model, describing step speed as a function of Sr2+ SO2-4 indicated that both step speeds reached maxima at Sr2+ SO2-4 = 1. This implied that the rate of SO42- ion attachment was equal to that of the Sr2+ ion to the kink sites. Laboratory experiments on barite dissolution in the presence of CrO 42-(aq) exhibited substantial lowering in dissolution rates due to adsorption of CrO42- onto surface reactive sites. AFM studies on barite (001) at 70°C showed dissolution rates ∼2.3 times lower at 1000 muM CrO42- and MFR experiments on powdered barite at 25°C quantified ∼2 times lowering in dissolution rates at 5 muM CrO42-. Transport of aqueous CrO 42- (4mM) through powdered barite in one dimensional plug flow reactor showed delayed chromate peak arrival times compared to that of a tracer (Na+) at all flow rates. The efficiency of barite being able to retain aqueous chromate appears to be a possible means to clean chromate contaminated waste waters.