Dissolution of forsteritic olivine at 65°C and 2<pH<5

Abstract

Dissolution experiments with forsteritic olivine (Fo₉₁) were conducted in a batch reactor at a temperature of 65°C between pH 2 and 5. Constant pH was maintained by using a pH-stat technique. The following dissolution rate law for forsteritic olivine at 65°C and acid pH was derived based on the experimental results normalized to the initial surface area:

$$\text{r=10}{}^{\mathrm{-8.51}}\text{(a}{}_{\mathrm{<mtext>H+</mtext>}}\text{)}{}^{0.70}$$

where r is rate in mol forsteritic olivine/cm²/s. Our results, combined with data from the literature for forsteritic olivine dissolution at 25°C, show that the pH-dependence of forsteritic olivine dissolution is temperature-dependent. As temperature increases, the dissolution rate of forsteritic olivine becomes more pH-dependent, which is consistent with a surface protonation model for dissolution at pH<pH_pznpc. The activation energy of dissolution, E_a, has been estimated based on our results and literature data at 30±4 kcal/mol. We also observed that at pH between 2 and 4 and at 65°C, the release rates of Mg, Si and Fe were stoichiometric. However, at pH 5, the release rate of Fe was slower than that of Mg and Si, probably due to oxidation of the mineral surface. Results of several dissolution experiments in the presence of Al in solution (pH 3, 0.03–10 ppm Al) show that the dissolution rate of forsteritic olivine under the experimental conditions is independent of Al concentration within the experimental error. This result may indicate that Al does not form strong crosslinks to the unpolymerized surface of orthosilicates such as forsteritic olivine, and therefore does not retard the dissolution of this phase under the experimental conditions.

Introduction

Extensive research on dissolution kinetics shows that for simple oxides, dissolution is controlled by surface reaction at the oxide–water interface, where the reactive solution species are H⁺, OH⁻ or other suitable ligands (e.g., Stumm and Furrer, 1987). The surface protonation behavior of simple oxides such as Al₂O₃, TiO₂ and Fe₂O₃ as a function of pH and temperature have been constrained (e.g., Berube and DeBruyn, 1968; Furrer and Stumm, 1983; Carroll-Webb and Walther, 1988; Machesky, 1989; Brady and Walther, 1992). Dissolution behavior as a function of pH is less well constrained for mixed oxides such as ortho-, ino-, phyllo-, and tectosilicates.

Several authors have argued that mineral dissolution will be pH-dependent for pH<pH_pznpc or pH>pH_pznpc (e.g., Furrer and Stumm, 1986). Here, pH_pznpc stands for pH of point of zero net proton charge, or the pH at which the concentration of positively charged surface sites equals the concentration of negatively charged surface sites of the mineral in the absence of specifically adsorbed cations or anions. In general, the pH dependence of silicates has been described by fitting the rate of dissolution for pH<pH_pznpc to a simple rate equation for rate data at constant temperature:

$$\text{log}\text{r=}\text{log}\text{k−n}\text{pH}$$

where r is the dissolution rate, k is the apparent rate constant and n is the apparent reaction order with respect to activity of aqueous H⁺.

Brady and Walther (1992)and Casey and Sposito (1992)suggested, based on thermodynamic arguments, that the pH-dependence of mineral dissolution should increase with temperature at pH<pH_pznpc. However, the effects of temperature on the pH-dependence of silicate dissolution are not well-established. For example, Hellmann (1994)published data suggesting that n increased with increasing temperature (from 100 to 300°C) for albite; however, reanalysis of his data with our data suggested to us that n may remain constant from 5° to 300°C for dissolution of this phase (Chen and Brantley, 1997). In contrast, our experiments at pH<pH_pznpc show that both diopside and anthophyllite dissolution become more pH-dependent at 90°C than at 25°C (Chen and Brantley, 1998). The increase in pH-dependence with temperature is greater for dissolution of diopside than anthophyllite over the same pH range (Chen and Brantley, 1998). If the effect of temperature on pH-dependence is related to the degree of polymerization of the silicate, we might expect to see an even stronger temperature effect on the dissolution of silicates that are even less polymerized, such as forsteritic olivine, at pH<pH_pznpc. For example, Westrich et al. (1993)report a strong change in pH dependence with temperature for several orthosilicates over the range T=25 to 65°C.

Polymerization of the dissolving silicate phase may also be important with respect to inhibition by aluminum. Several workers have shown (Chou and Wollast, 1985; Casey et al., 1988; Oelkers et al., 1994; Chen and Brantley, 1997) that Al in solution affects dissolution of feldspar, perhaps because it repolymerizes the silica-rich feldspar surface. Fewer workers have investigated Al inhibition on less polymerized silicate phases. One of the purposes of this study was to look at the pH-dependence of a simple orthosilicate by conducting an experimental study of forsteritic olivine dissolution at an elevated temperature, and to compare the results with the literature data for dissolution of this phase at room temperature. The second purpose was to test whether dissolved Al inhibits olivine dissolution.