Experimental kinetic study of the smectite-to-illite transformation

The <20 mu m size fraction of a bentonite from the Serrata de Nijar deposit (Almeria, southeastern Spain) was hydrothermally treated to study the kinetics of the smectite-to-illite transformation, in order to estimate the performance life of a bentonite barrier in a high level nuclear waste repository. The bentonite studied consisted of randomly interstratified illite/montmorillonite with 15% illite. Minor amounts of other minerals (3% quartz, plagioclase, and cristobalite) were also present. The run conditions of the hydrothermal treatments were combinations of the following variables: KCl concentration 0.025, 0.05, 0.1, 0.3, 0.5, and 1 mol L(-1); temperature 60, 120, 175, and 200 degrees C; time 1, 5, 15, 30, 90, and 180 days. The solid:solution ratio was 1:5. Pressures were those corresponding to water vapor. The solid reaction products were analyzed by means of XRD, DTA/TG, FTIR, and NMR. The final solutions were chemically analyzed for Si and exchange cations (K, Ca, Mg, and Na), and pH. XRD detected some transformation (up to 15%), while DTA/TG, FTIR, NMR, and exchange cation analysis did not show any appreciable transformation. This leads to the conclusion that analysis of the amount of illite in illite/smectite, in hydrothermally treated samples, by means of XRD can be inaccurate. Transformation to illite was observed, however, when aqueous silica concentrations were examined. These concentrations yielded the kinetic expression - dS/dt = kK(1/4)S(n), where S is the fraction of smectite in illite/smectite, t time, k the rate constant, and K potassium concentration in solution. The exact value for n could not be determined, although data from this and other studies suggest n > 1. The activation energy of the process is similar to 7 kcal mol(-1), suggesting a solid transformation mechanism. Based on these results, smectite seems to offer a safe barrier for nuclear waste.