EXPERIMENTAL KINETIC-STUDY OF THE SMECTITE-TO-ILLITE TRANSFORMATION

The <20 mu m size fraction of a bentonite from the Serrata de Nijar de posit (Almeria, southeastern Spain) was hydrothermally treated to stud y the kinetics of the smectite-to-illite transformation, in order to e stimate the performance life of a bentonite barrier in a high level nu clear waste repository. The bentonite studied consisted of randomly in terstratified illite/montmorillonite with 15% illite. Minor amounts of other minerals (3% quartz, plagioclase, and cristobalite) were also p resent. The run conditions of the hydrothermal treatments were combina tions 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 reacti on 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 ana lysis of the amount of illite in illite/smectite, in hydrothermally tr eated samples, by means of XRD can be inaccurate. Transformation to il lite was observed, however, when aqueous silica concentrations were ex amined. 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 thi s and other studies suggest n > 1. The activation energy of the proces s is similar to 7 kcal mol(-1), suggesting a solid transformation mech anism. Based on these results, smectite seems to offer a safe barrier for nuclear waste.