Surface controlled dissolution rates of gypsum in aqueous solutions exhibit nonlinear dissolution kinetics

Abstract

The effective dissolution rates of gypsum are determined by mixed kinetics, where the rate constants of dissolution at the surface and the transport constant of molecular diffusion of dissolved material are similar. To obtain the surface reaction rate law it is necessary to know the transport constant. We have determined the surface rate law for monocrystalline selenite by using a rotating disc set-up, where the transport coefficients are well known. As a result, up to a calcium concentration of 0.6 · c eq , we find a nearly linear rate law R s = k sl (1− c s / c eq ) n 1 , where c s is the total calcium concentration at the surface and c eq the equilibrium concentration with respect to gypsum, n 1 = 1.2 ± 0.2, and k sl = 1.1 · 10 −4 mmol cm −2 s −1 ± 15%. We also employed batch-experiments for selenite, alabaster and gypsum rock samples. The result of these experiments were interpreted by using a transport constant determined by NaCl dissolution experiments under similar physical conditions. The batch experiments reveal a dissolution rate law R s = k sl (1− c s / c eq ) n 1 , k sl = 1.3 · 10 −4 mmol · cm −2 s −1, n 1 = 1.2 ± 0.2 for c ≤ 0.94 · c eq. Close to equilibrium a nonlinear rate law, R s = k s2 (1− c s / c eq ) n 2 , is observed, where k s2 is in the order of 10 mmol · cm −2 s −1 and n 2 ≈ 4.5. The experimentally observed gypsum dissolution rates from the batch experiments could be accurately fitted, with only minor variations of the surface reaction constant obtained from the rotating disk experiment and the transport coefficient from the NaCl dissolution batch experiment. Batch experiments on pure synthetic gypsum, reveal a linear rate law up to equilibrium. This indicates inhibition of dissolution in natural samples close to equilibrium, as is known also for calcite minerals.