DISSOLUTION KINETICS OF SULFATE MINERALS: LINKING ENVIRONMENTAL SIGNIFICANCE OF MINERAL-WATER INTERFACE REACTIONS TO THE RETENTION OF AQUEOUS CrO42- IN NATURAL WATERS

by BOSE, SWETA

Dissolution as a function of solution undersaturation (?) 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 ?. 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 ?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 nonlinearity in the case of barite was controlled by changes in step speeds with ?. Similar dissolution rate behavior also evident on powdered barite in mixed flow reactor system establishes the dissolution rate non-linearity in terms ? to be the characteristic property of barite. Celestite (001) dissolution was also studied in terms of Sr:SO4 by AFM to examine the mineral's reactivity under nonstoichiometric solute conditions. At ? = 0.63, reaction kinetics were investigated by measuring and step speeds. Application of a theoretical model, describing step speed as a function of Sr:SO4 indicated that both step speeds reached maxima at Sr:SO4 = 1. This implied that the rate of SO42- ion attachment was equal to that of the Sr^2+ ion to the kink sites. Laboratory experiments on barite dissolution in the presence of CrO42-(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 ?M CrO42- and MFR experiments on powdered barite at 25°C quantified ~ 2 times lowering in dissolution rates at 5 ?M CrO42-. Transport of aqueous CrO42- (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.