Kinetics mineral dissolution experiments

Oelkers E. H., Schott J., and Devidal J.-L. (2001) 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 Huertas et al. (1999). Geochim. Cosmochim. Acta 65, 4429-4432. 

ABSTRACT Atmospheric carbon dioxide is trapped within magnesium carbonate minerals during mining and processing of ultramafic-hosted ore. The extent of mineral-fluid reaction is consistent with laboratory experiments on the rates of mineral dissolution. Incorporation of new serpentine dissolution kinetic rate laws into geochemical models for carbon storage in ultramafic-hosted aquifers may therefore improve predictions of the rates of carbon mineralization in the subsurface. 

Figure 13.5. Transport vs surface controlled dissolution. Schematic representation of concentration in solution, C, as a function of distance from the surface of the dissolving mineral. In the lower part of the figure, the change in concentration (e.g., in a batch dissolution experiment) is given as a function of time, (a) Transport controlled dissolution. The concentration immediately adjacent to the mineral reflects the solubility equilibrium. Dissolution is then limited by the rate at which dissolved dissolution products are transported (diffusion, advection) to the bulk of the solution. Faster dissolution results from increased flow velocities or increased stirring. The supply of a reactant to the surface may also control the dissolution rate, (b) Pure surface controlled dissolution results when detachment from the mineral surface via surface reactions is so slow that concentrations adjacent to the surface build up to values essentially the same as in the surrounding bulk solution. Dissolution is not affected by increased flow velocities or stirring. A situation, intermediate between (a) and (b)—a mixed transport-surface reaction controlled kinetics—may develop.

The effects of pH on sorption isotherms have been studied extensively particularly with oxide surfaces (Anderson and Rubin, 1981 Sposito, 1984), but pH effects in sorption kinetic studies have not received equal attention. In contrast, pH effects in mineral-dissolution kinetic experiments have received a great deal of attention (e.g., Chou and Wollast, 1984 Stumm, 1986 Stone, 1987a,b). 

The main objective of this chapter is to review experimental work on organic acids and mineral dissolution under diagenetic conditions. Consequently, emphasis is placed on experiments conducted at elevated temperatures (70-100°C). The primary focus is on the effects of carboxylic acids on feldspar solubility and dissolution kinetics as they relate to potential aluminum mobility and creation of secondary porosity. 

Do the kinetic rate constants and rate laws apply well to the system being studied Using kinetic rate laws to describe the dissolution and precipitation rates of minerals adds an element of realism to a geochemical model but can be a source of substantial error. Much of the difficulty arises because a measured rate constant reflects the dominant reaction mechanism in the experiment from which the constant was derived, even though an entirely different mechanism may dominate the reaction in nature (see Chapter 16). 

Nonlinear Precipitation of Secondary Minerals from Solution. Most of the studies on dissolution of feldspars, pyroxenes, and amphiboles have employed batch techniques. In these systems the concentration of reaction products increases during an experiment. This can cause formation of secondary aluminosilicate precipitates and affect the stoichiometry of the reaction. A buildup of reaction products alters the ion activity product (IAP) of the solution vis-a-vis the parent material (Holdren and Speyer, 1986). It is not clear how secondary precipitates affect dissolution rates however, they should depress the rate (Aagaard and Helgeson, 1982) and could cause parabolic kinetics. Holdren and Speyer (1986) used a stirred-flow technique to prevent buildup of reaction products. 

The distances, from the column inlet, at which the dissolution of the primary minerals and precipitation of secondary minerals occur should give a clear indication of the differences between the local equilibrium and kinetic assumptions. If the distances observed in the experiments are close to the distances predicted assuming equilibrium and the fronts relatively sharp, then the rate of reaction of the minerals must be relatively fast. If fronts that are more diffuse were observed in the experiments, this would suggest that the rates of reaction of the minerals are slow. 

Numerous experiments have been conducted to study the interactions of organic acids and minerals in weathering and diagenetic processes. Most work has focused on feldspar dissolution by water-soluble organic acids. It has been proposed that simple carboxylic acids (e.g., acetic and oxalic) may complex with aluminum and increase the solubility and dissolution kinetics of feldspar. This would provide a mechanism for mobilizing aluminum and creating secondary porosity. In addition, dissolved organic species may act as proton donors and pore-fluid pH buffers. 

To evaluate this hypothesis, experimentalists have been trying to provide data on mineral solubility and dissolution kinetics as a function of temperature, pH, and fluid composition. Experimental techniques and results of published studies are summarized below along with new data from our laboratory, including the results of pH-buffered, flow-through experiments that track variations in pore-fluid chemistry through time (Reed 1990 Reed and Hajash 1990, 1992 Franklin 1991 Franklin et al. 1990, 1991). 

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