On the interpretation of closed system mineral dissolution experiments: comment on “Mechanism of kaolinite dissolution at room temperature and pressure part II: kinetic study” by

Abstract

Huertas et al. (1999) reported the results of a series of constant-pH kaolinite dissolution experiments performed at 25°C in closed system reactors. The authors of this study chose to interpret these experiments assuming that the dissolution rates of kaolinite in their reactors, after a certain initial elapsed time, were time independent. The purposes of this comment are to (a) recall evidence available in the literature demonstrating that constant-pH kaolinite dissolution does not attain a constant rate in closed system reactors; (b) present a simple equation describing solution composition evolution during closed system multioxide mineral or glass dissolution experiments; and (c) demonstrate that Huertas et al.’s (1999) data are consistent with kaolinite rates that are proportional to cAl nAl where cAl refers to the concentration of aqueous aluminum, and nAl denotes a constant. Over the years both closed and open system reactors have been used to measure mineral and glass dissolution rates. Open system reactors such as mixed-flow reactors (e.g., Berger et al., 1994) or fluidized bed reactors (e.g., Chou and Wollast, 1985; Wogelius and Walther, 1990) provide dissolution rates at fixed solution compositions directly from the measured mass flux of the departing reactive fluids. In contrast, closed system reactors such as that used by Huertas et al. (1999) yield dissolution rates indirectly from the temporal aqueous concentration evolution of the reactive fluid. Two examples illustrating measured solution compositions taken from Huertas et al. (1999) as a function of elapsed time are given in Figure 1. The behavior illustrated in this figure is typical of the temporal aqueous concentration evolution during the closed system stoichiometric dissolution of a mineral. Early during each experiment the concentrations of metals liberated by dissolution increase at atypically fast rates; with time, the metal release rate slows, and therefore, plots of metal concentration versus time exhibit a parabolic pattern. A commonly adopted method for interpreting these observations, as well as that adopted by Huertas et al. (1999), is to assume that the observed initial rapid dissolution rates stem from temporary processes such as the relatively rapid dissolution of fine particles or high-energy surfaces sites present on the larger grains. The contributions of these relatively rapidly dissolving surfaces are assumed to be significant only during the early part of the experiment, and by the end of the experiment dissolution is presumed to proceed at a constant rate. Such an interpretation is predicated on the assumption that the overall dissolution reaction has a zeroth reaction order with respect to its constituent metals (e.g., constant-pH kaolinite dissolution rates are independent of aqueous Al and Si concentration). Although this seems to be the case for single-oxide minerals such as quartz (Berger et al., 1994), it is not generally the case for multioxide minerals and glasses; the constant-pH dissolution rates of kaolinite (Devidal et al., 1992, 1997; Devidal, 1994; Oelkers et al., 1994), alkali feldspars (Chou and Wollast, 1985; Oelkers et al., 1994; Gautier et al., 1994), kyanite (Oelkers and Schott, 1999), basaltic glass (Oelkers et al., 1999), enstatite (Oelkers and Schott, in press), forsterite (Pokrovsky and Schott, 2000), and smectite (Cama et al., 2000) were all observed to decrease in response to the increasing aqueous concentration of at least one of their constituent metals. In the absence of a true dissolution plateau, constant pHs far from equilibrium dissolution rates are consistent with the empirical expression given by (see also Lasaga et al., 1994; Oelkers, 1996)