Intrinsic kinetics of gypsum and calcium sulfate anhydrite dissolution : surface selective studies under hydrodynamic control and the effect of additives

The intrinsic dissolution activity of the basal (010) and edge (001) surfaces of gypsum and polycrystalline calcium sulfate anhydrite crystals has been investigated, under far from equilibrium conditions, via the channel flow cell (CFC) method with off-line inductively coupled plasma-mass spectrometry (ICP-MS) for the measurement of dissolved Ca(2+) from the crystal surface. This approach allows measurements to be made over a wide range of flow rates so that the importance of mass transport versus surface kinetics can be elucidated. Complementary quantitative modeling of the dissolution process was carried out by formulating convective diffusive equations that describe mass transport in the CFC, coupled to a boundary condition for dissolution of the crystal surface. We found that a linear rate law applied, and intrinsic dissolution fluxes were deduced. The following dissolution fluxes, J(o) = k(diss) x c(eq) were measured, where k(diss) is the dissolution rate constant and c(eq) the calcium sulfate concentration in saturated solution: 5.7 (+/- 1.4) x 10(-9) mol cm(-2) s(-1) for basal plane gypsum and 4.1 (+/- 0.7) x 10(-9) mol cm(-2) s(-1) for calcium sulfate anhydrite. Edge plane gypsum, under the experimental conditions applied, was found to dissolve at a mass transport-controlled rate. The effects of t-tartaric acid, d-tartaric acid, and sodium trimetaphosphate (STMP) as important potential additives of the dissolution process of basal plane gypsum were investigated. It was found that the tartaric acids had little effect but that STMP significantly retarded gypsum dissolution with J(o) = 1.6 (+/- 0.6) x 10(-9) mol cm(-2) s(-1) (5 mM STMP solution). The mode of action of STMP was further elucidated via etch pit morphology studies