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

The effective dissolution rates of gypsum are determined by mixed kinetics, where the rate constants of dissolution at the surface and the transport c onstant of molecular diffusion of dissolved material are similar. To obtain the surface reaction rate law it is necessary to know the transport consta nt. We have determined the surface rate law for monocrystalline selenite by using a rotating disc set-up, where the transport coefficients are well kn own. As a result, up to a calcium concentration of 0.6 . c(eq), we find a n early linear rate law R-s = k(sl) (1-c(s)/c(eq))(n1), where c(s) is the tot al calcium concentration at the surface and c(eq) the equilibrium concentra tion with respect to gypsum, n(l) = 1.2 +/- 0.2, and k(sl) = 1.1 . 10(-4) m mol cm(-2) s(-1) +/- 15%. We also employed batch-experiments for selenite, alabaster and gypsum rock samples. The result of these experiments were int erpreted by using a transport constant determined by NaCl dissolution exper iments under similar physical conditions. The batch experiments reveal a di ssolution rate law R-s = k(sl) (1-c(s)/c(eq))(n1), k(sl) = 1.3 . 10(-4) mmo l . cm(-2) s(-1), n(1) = 1.2 +/- 0.2 for c less than or equal to 0.94 . c(e q). Close to equilibrium a nonlinear rate law, R-s = k(s2)(1-c(s)/c(eq))(n2 ), is observed, where k(s2) is in the order of 10 mmol . cm(-2) s(-1) and n (2) approximate to 4.5. The experimentally observed gypsum dissolution rate s from the batch experiments could be accurately fitted, with only minor va riations of the surface reaction constant obtained from the rotating disk e xperiment and the transport coefficient from the NaCl dissolution batch exp eriment. Batch experiments on pure synthetic gypsum, reveal a linear rate l aw up to equilibrium. This indicates inhibition of dissolution in natural s amples close to equilibrium, as is known also for calcite minerals. Copyrig ht (C) 2001 Elsevier Science Ltd.