Albite, dissolution

Figure 7.3. Changes in (a) pH, where pH is the pH of the input solution, and (b) the concentrations of Si with time for albite dissolution (100-200 ju.m size fractions) and pH 5.68 water as the input solution. [From Chou and Wollast (1984), with permission.]...

Chou and Wollast (1984, 1985) employed a fluidized-bed reactor to study albite dissolution with time. Figure 7.3 shows a short-term experiment run at room temperature and pressure using water as the input solution. There is a fast nonstoichiometric dissolution early in the reaction period that decreases rapidly until a steady state is approached. Linear kinetics and stoichiometric dissolution prevail later. If the pH of the input solution is changed, however, there is an increase in dissolution rate (Fig. 7.4) similar to the beginning of an experiment (Fig. 7.3). 

The net rate of dissolution and precipitation of a given phase is a function of AG, the driving force for reaction. For example, for albite dissolution, AG is defined for the reaction... 

In some cases, even a single equation such as (60) or (61) is inadequate to describe dissolution over a range of chemical affinity. For example, Burch et al (1993) suggested that the rate of albite dissolution at 80 °C near pH 8.8 (Figure 10) can be written as a sum of parallel rates ... 

Chen Y. and Brantley S. L. (1997) Temperature- and pH-dependence of albite dissolution rate at acid pH. Chem. Geol. 135, 275-292. 

Initial dissolution of primary silicates is typically incongruent that is, the stoichiometric ratio of elements released to solution is not the same as that found in the bulk phase of the mineral. An excellent example of incongruent dissolution (Fig. 7-13) is presented by Chou and Wollast (1984). They reacted albite with aqueous solutions in a fluidized bed reactor and maintained solution concentrations of the reaction products below saturation for potential secondary products. Even so, the molar ratio of Na/Si initially released to solution was almost an order of magnitude higher than that of the bulk albite. Dissolution incongruence has posed a particularly difficult theoretical problem for researchers working on mineral dissolution problems. 

Figure 9. Logarithmic plot of albite dissolution rate versus its surface charge.

KINETICS This keyword begins a block of data which identifies kinetic reactions and supplies parameters used in each reaction, such as activation energies, duration of reactions, number of steps, etc. In Table 11.2 only one kinetic reaction is specified, for albite dissolution. For comparison with the react example, we use the same amount of mineral, and the same kinetic parameters. The current amount (moles) of mineral, which will decrease during dissolution, is identified as -m, and the initial amount of mineral, in this case the same quantity, is identified as -mO. phreeqc measures... 

The results from phreeqc look much like those in Figure 11.2, with some differences in the Saturation Indices, because of the different data used. A more complex example of albite dissolution based on the work of Sverdrup (1990) involving temperature effects, CO2 concentration, and organic decomposition is included in the database supplied with phreeqc.3... 

For instance, the albite dissolution maybe represented as the formation of activated complex and its subsequent dissociation ... 

Figure 2.33 Albite dissolution rate vs. pH at temperature between 25 and 300 °C. The data at 100, 200 and 300 °C (rhombs, triangles and squares) are after HeUman, 1994 at 25 °C (circles) are after Chou and WoUast, 1985. Grey curves are results of nonlinear regression. Dashed straight lines are results of piecewise-linear regression at 100 °C. Black curves are sums of piecewise-linear regression (Palandri and Kharaka, 2004)...

A third explanation is that the different rates could reflect differences in the pH of the waters. The rate of feldspar dissolution is enhanced at pH > 7 (e.g. Knauss Wolery 1986). This fact might explain the observation that the alteration is more intense at DH-3/26 m where the sampled water had a pH of 8.9-9.3, than at DH-4/80 m where the water had a pH of 6.8. Knauss Wolery (1986) found a rate of albite dissolution some five times faster at the higher pH levels. However, it follows from this hypothesis that waters in different fractures have acquired distinct pH values. Once again, this observation may reflect the differing natures of water/rock interactions in different fractures, which may in turn be linked to variable hydraulic properties of different fractures. 

The predictions of albite reaction with the young fluid anticipated complete albite dissolution in the first half of the column by end of the experiment. Tobermorite and mesolite were... 

Franklin (1991) also observed that both acetate and oxalate increased albite dissolution rates. The kinetics of albite dissolution in acetate solutions as a function of pH were investigated by using acetate buffer solutions having pHi values of 2.5, 3.5, 4.7, and 5.5. Values for pHf were 3.4, 3.8, 4.7, and 5.3, respectively. As expected, the difference between initial and final pH increased with increased departure from the acetate pK at 100 °C of = 4.9. This reflects the decreased buffer capacity of solutions farther away from the pK. Concentration data vs. time (Fig. 3) show that, at essentially constant total acetate, apparent dissolution rates for Na, Al, and Si increased with decreasing pHj from 5.5 to 2.5. 

Dissolution rates for albite in acetate-oxalate solutions (Fig. 4) were significantly higher than those in acetate solutions and distilled water. The effect of pH on dissolution rate in the acetate-oxalate system was investigated using 4200 ppm acetate-500 ppm oxalate solutions with pHj values of 2.2, 3.2, 4.4, and 5.4. The pHf values were 2.8, 3.3, 4.6, and 5.1, respectively. Initial pH values were slightly lower than those for the acetate buffer alone because of addition of the oxalic acid. Apparent dissolution rates for Na, Al, and Si increased significantly as pH decreased from 5.4 to 2.2 (Fig. 4). H" " activity appears to be an important factor in albite dissolution kinetics in acetate-oxalate solutions however, the rates do not show as large a pH dependence as that observed in the acetate experiments (Fig. 3). 

Partial rate laws obtained for albite dissolution in acetate-oxalate solutions (Franklin 1991) indicate that dissolution rate is less dependent on pH relative to oxalate concentration. This suggests that oxalate adsorption and Al-oxalate complex formation are dominant processes controlling dissolution. Dissolution rate constants for acetate and acetate-oxalate solutions are as much as an order of magnitude higher than those for distilled water (pHf... 

Oxalate and acetate significantly increase feldspar dissolution rates at elevated temperatures (Bevan and Savage 1989 Franklin 1991 Franklin et al. 1991). Oxalate has a greater effect on the dissolution rate than acetate and measured rates increase with increasing oxalate at essentially constant pH. At essentially constant total oxalate, dissolution kinetics increase with decreasing pH, similar to the recent observations at low temperatures (Amrhein and Suarez 1988). At pH — 4.4-4.7, oxalate and acetate significantly increase the albite dissolution rate over that observed for distilled water or C02-charged water (Fig. 6). 

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