Silicate Dissolution Kinetics in Organic Electrolyte Solutions

Project Introduction

Organic compounds in natural waters are thought to interact at the silicate surface and accelerate mineral weathering processes. Natural and contaminating organic acids produce protons which contribute to proton promoted mineral weathering. Organic acid anions can also complex with metal ions in solution, lowering metal activity and increasing the apparent solubility of the mineral. The major impact of organic acids, however, may be that metal-organic complexes can form at the solid-solution interface, weakening cation-oxygen bonds, thus catalyzing the dissolution reaction.

Chemical weathering by organic acids may be an important process in oil field formation waters and soils where there are high concentrations of complexing organic compounds. Organic ligand concentrations in the bulk groundwater solution are lower, but they still might be responsible for a significant amount of mineral weathering. In groundwaters contaminated by organic compounds, however, the rate of weathering processes will be greatly accelerated by the presence of organic ligands produced by metabolizing microbes.

The overall dissolution reaction involves the multi-step process of initial rapid exchange of cations for protons at the mineral surface, followed by a slow, rate determining hydrolysis and subsequent detachment of silica and alumina species from the remaining framework. The extent of the reaction and identity of the process depend on the environmental conditions and the organic species present.

Background

Natural and contaminating organic compounds are reported to play an important role in geochemical processes. Organic ligands are suggested to chelate and mobilize heavy metals, sorb to mineral surfaces, and enhance the dissolution and the precipitation of inorganic minerals in aqueous systems (e.g., Bennett, 1991). Wide variety of organic compounds in natural and waste waters can act as complexing agents for metal ions (Smith and Martell, 1974-1982). The presence of significant concentrations of organic acid anions has been revealed from analyses of subsurface oilfield water (e.g., Surdam et al., 1984; Surdam and MacGowan, 1987) and are thought to enhance the formation of secondary porosity.

Dissolved organic compounds are suspected of interacting with a wide variety of inorganic solutes in natural waters. Dissolved organic substances can mobilize and transport relatively insoluble metals in natural waters ( e.g., Stumm and Morgan, 1981). Many investigators have found that organic acids enhance the dissolution of aluminosilicate minerals or quartz (Huang and Keller, 1970; Tan, 1980; Bennett et al., 1988; Bennett, 1991; Welch and Ullman, 1993). Grandstaff (1986) found that some organic acids accelerate olivine dissolution and that the relative reactivity of organic compounds is related to the ability of the organic ligand to form complexes with metals. Dissolved organic compounds produced by biodegrading crude oil were shown to be actively etching quartz and aluminosilicate minerals (Bennett and Siegel, 1987; Hiebert and Bennett, 1992). Huang and Longo (1992) found evidence of enhanced development of secondary porosity in feldspar dissolution in organic-bearing aqueous solutions at reservoir temperatures (95C/1 bar and 100C/88 bar).

Previous researchers have examined, to varying degrees, the kinetics of silicate dissolution, the dissolution of silicates in the organic/inorganic electrolyte solutions, and the mechanisms of silicate dissolution. While the reaction kinetics of aluminosilicate hydrolysis in inorganic aqueous solutions has been examined in detail (e.g., Berner, 1978; Aagaard and Helgesson, 1982; Helgesson et al. 1984; Chou and Wollast, 1985; Rimstidt and Dove, 1986; Anbeek, 1992; Wollast and Chou, 1992), mechanisms of dissolution at the molecular level are often difficult to distinguish due to difficulties in isolating the competing reactions and processes in the system.

The general dissolution reaction of silicates in inorganic aqueous systems involves multi-step process of initial rapid exchange of cations (K, Na, and/or Ca) for protons at the mineral surface, followed by a slow, rate determining hydrolysis (formation of activated complex) and subsequent detachment of silica and alumina species from the remaining framework (e.g., Aagaard and Helgeson, 1982). A number of studies have demonstrated that mineral hydrolysis occurs via surface complexes formed by the adsorption and desorption of protons and/or ligands from solution (Blum and Lasaga, 1988; Carroll-Webb and Walther, 1988; Brady and Walther, 1989). Destruction of framework bonds are known to occur through decomposition of these surface complexes when they are in the activated state. Therefore, the overall dissolution rate can be expressed as

Rate=k[S],

where S is the surface concentration of the reaction precursor (surface species which is in equilibrium with the activated complex) and k is constant. Also the reaction rate depends on pH and temperature of the reactants. The pH-dependency of hydrolysis is thought to be controlled by the acid-base properties and bonding in the metal-oxygen bond, and the mechanism of hydrolysis (e.g., Casey and Bunker, 1990).

Dissolution Kinetics in Inorganic Electrolyte Solutions

Various conceptual models have been suggested in order to interpret the weathering mechanism of slightly soluble minerals in aqueous solutions. The models are based on experimental observations that the dissolution of most oxides and silicates at low temperature is a surface-controlled process.

It has been suggested that the dissolution kinetics of most slightly soluble oxides and silicates is controlled by the concentration of adsorbed charged species at the mineral surface produced by these reactions, in particular by H+ and OH- (e.g., Chou and Wollast, 1984; Knauss and Wolery, 1986; Carroll-Webb and Walther, 1988; Brady and Walther, 1989, 1990; and many others). Surface protonation-deprotonation and other charged ligand surface complex reactions were considered reaction steps preceeding the rate controlling elementary reaction. Based on transition state theory (Eyring, 1935), these reactions are thought to increase the concentration of activated complexes in a rate determining detachment reaction (Aagaard and Helgeson, 1982; Helgesson et al., 1984; Wieland et al., 1988). This is likely to occur by weakening cation-oxygen bonds at the mineral or oxide surface through bond polarization by the charged surface complexes (Zinder et al., 1986). Therefore, the rate of oxide mineral dissolution directly related to the concentrations of charged surface complexes produced by surface adsorption reactions (Furrer and Stumm, 1986; Brady and Walther, 1989; Xie and Walther, 1992). However, it is not possible to predict the order of the rate relative to the concentration of the surface species, or the order of a given detachment reaction, from surface coordination chemistry (Brady and Walther, 1989).

In simple solutions, variations in hydrolysis rate with pH are controlled by the acid-base properties of bridging oxygens, or terminal hydroxyl oxygens at the mineral surface (Casey and Bunker, 1990). Variation in surface charge concentration with solution pH is characteristic for a given oxide surface, just as the extent of protonation is characteristic of a dissolved oxyacid at a given pH (Casey and Bunker, 1990). The acid base reactions on the mineral surfaces are synonymous with the sorption of hydroxyl or hydrogen ions onto the oxide surface. The sorption reactions saturate the material with a net surface charge, which can be measured in a potentiometric titration or through studies of electrophoretic mobility (e.g., Furrer and Stumm, 1986).

Although the dependency of the surface speciation on pH and ionic strength of the solution, resulting in the increase or decrease in dissolution rates, has been observed by many investigators (e.g., Casey and Bunker, 1990; Xie and Walther, 1992), the ionic strength effect on surface speciation and dissolution rate is not clearly defined. In simple solutions, in theory, increase in ionic strength causes increase in the activity of neutral species like silica. The silica dissolution rates, as a result, should decrease due to the increase in the degree of saturation. However many investigators have reported increase in quartz dissolution rates by increasing the ionic strength (e.g., Dove and Crerar, 1990; Bennett, 1991). Considering that the ionic strength effect varies by the electrolyte species in the solution and the nature of the surface reaction sites, the increased dissolution rates with increasing ionic strength may be attributed to the interaction between surface species and the specific electrolyte species (e.g., Na+ or K+).

Dissolution Kinetics in Organic Electrolyte Solutions

Field observations and laboratory experiments have shown that aluminosilicate dissolution is enhanced by dissolved organic acids .

Amrhein and Suarez (1988) found that the presence of complex-forming organic ligands resulted in a dissolution rate that increased linearly with decreasing pH. Welch and Ullman (1992) found the rates of plagioclase dissolution in solutions containing organic acids were up to ten times greater than the rates determined in solutions containing inorganic acids at the same acidity.

When conditions are acidic (pH < 5), the accepted mechanism for enhanced solubility is by organic-aluminum complexation, surface coordination of organic acids, or acid-catalyzed hydrolysis (e.g., Stumm and Furrer, 1987). At neutral pH, aluminum mobility and silicate solubility are near minimum and organic-complexes are relatively unstable (Driscoll et al., 1985; Martell and Motektaitis, 1989). Therefore, aluminum complexation does not seem to greatly enhance silicate dissolution at circum-neutral pH. However, Bennett and Siegel (1987) found that dissolved organic compounds produced by biodegrading crude oil were actively etching quartz and aluminosilicate minerals in ground water at pH 7, where the concentration of dissolved silicon approached amorphous silica equilibrium, and suggested that organic-acid-silica complexes decreased the activity of monomeric silicic acid, thereby increasing the apparent solubility of quartz. Organic-acid-silica complexes were also experimentally shown to increase quartz solubility and dissolution rate at circum-neutral pH. Bennett et al. (1991) suggested that the interaction depended on organic-acid concentration and species and appeared to be strongest with organic acids found primarily in anoxic, microbially active environments.

However, the specific surface interaction by which organic substances enhance the dissolution of silicate minerals is not well identified. Grandstaff (1986) suggested solution complexes of aluminum decrease the activity of monomeric aluminum and accelerate silicate dissolution by decreasing the rate of the reverse precipitation reaction. Coordination of organic ligands onto silicate surfaces is also recognized as a strong influence in the dissolution of silicates (e.g., Stumm and Furrer, 1987; Wieland et al., 1988). The mechanism of interaction may consist of inner-sphere coordination of an organic ligand resulting in weakened crystal framework bonds due to charge transfer or other inductive effects (e.g., Bennett, 1991).

Organic acid-aluminum complexes have been closely examined because of their role in mineral weathering and in aluminum speciation in acidic surface waters (e.g., Stumm and Furrer, 1987; Nordstrom and May, 1989). Aluminum is presumed to be complexed primarily by multifunctional organic acids via bidentate chelate, forming a ring structure that incorporates two Al-O-C bonds (e.g., Stumm and Furrer, 1987; Wieland and Stumm, 1992). A further difficulty is the separation of the effects of aluminum complexation in solution and on the surface. Aluminum-organic complexes in solution will decrease the saturation state, thus increasing the net forward rate by decreasing the first order reverse rate. Simultaneously, aluminum complexes at the surface will enhance the forward rate constant. The two effects are difficult to distinguish, but represent a fundamental definition of the dissolution mechanism.

The role of organic acids in complexing silica in natural waters is not well documented. Hydrogen bonded complexes involving an Si-O-H–O-C linkage are thought to occur in aqueous system and on silica surfaces (Iler, 1977). Iler (1979) describes the chemistry of a silica-catecholate complex, and suggests that a 1:3 complex forms, with the silicon metal center coordinated with three catechol ligands in an octahedral geometry. Bennett et al. (1988) report on a hydrogen bonded complex between silicic acid and citric acid in aqueous systems at neutral pH. Bennett (1991) suggested a stable 5-member ring complex between silicic and organic-acid anions forming Si-O-C ester bond based on molecular modeling.

Questions

While it is established that organic acids accelerate the dissolution of silicates in aqueous system, the mechanisms and environmental controls are not well understood. First, the dominant mechanism of organic-chelate–enhanced aluminosilicate dissolution at any one time is not clear, when potentially four reactions may be occurring: proton-catalyzed hydrolysis, solution complexation of aluminum, solution complexation of silica, or surface adsorption, coordination, and catalysis. The dominant mechanism may vary with temperature, solution composition, mineral surface chemistry, and reaction extent. Second, effects of inorganic electrolytes and ionic strength on silicate dissolution kinetics and on silica- and metal-organic complex equilibria are not well established. Third, while the temperature dependency of reaction rates in inorganic electrolyte solutions is well established, it is not known for organic-electrolyte systems.

Research Objectives

The goal of this research is to separate and quantify the competing effects of organic and inorganic electrolytes on silicate rock-water interactions at low temperature. This will be done by examining concurrently the parallel mechanisms of inorganic-acid catalyzed hydrolysis, continuum ionic strength effects on surface reaction rate, organic complexation of aluminum, and silica in solution, and surface-complex interactions. Dissolution experiments will be conducted to investigate the surface interaction between organic/inorganic electrolytes and mineral surface occurring in various aqueous systems. Active organic-acid complexation reactions and the dissolution rates of the silicates will be determined based on the composition of the effluent solutions. Surface investigation of minerals before and after the experiments will be performed and the dissolution effects by organic/inorganic electrolytes will be characterized.

Three objectives of this research are to:

1. Investigate the dissolution kinetics of the silicate-water-organic electrolyte system as a function of pH, temperature, organic/inorganic species, and electrolyte concentration.
2. Determine the mobility of silica, aluminum, and some transition metals under various conditions of pH, organic solutes, inorganic solutes, and ionic strengths.
3. Examine the chemistry of silica-, aluminum-, and transition metal-organic complexes as a means of investigating the nature of surface interactions on various aluminosilicate and silicate minerals in organic-rich solutions.

Approach

While there is strong circumstantial evidence that some organic acids complex silica and enhance the dissolution of aluminosilicates, little direct information on mechanisms and structures has been reported.

Establishment of dissolution kinetics on various mineral surfaces under various system-conditions is required to construct a generalized dissolution mechanism which will explain the enhanced dissolution in natural organic-rich solutions. The stability of aluminum and silica complexes in the pH range of 5-8, the most common pH range in natural waters, are required to be established. The surface speciation on the mineral surface under various pH, ionic strength, and electrolyte species should also be clearly identified. The ionic strength effect on the silica-, aluminum-, transition metal-organic acid anion complex reaction must be clarified in order to distinguish the surface interaction from net ionic behavior of the cation in the solution. For this purpose computer aided molecular modeling will be useful. Further, an evaluation of activation energy as a means of evaluating the nature of the activated complex is also needed, requiring experiments to be run at multiple temperatures. Also in order to identify the degree of enhancement of dissolution rates by the presence of organic acids, a systematic comparison of activation energies between inorganic electrolyte system and organic electrolyte system is required.

Dissolution experiments will be conducted using flow-through, completely mixed reactors. Dissolution rates for aluminosilicate minerals (albite, microcline, andalusite, and kyanite) and two end members of the system (quartz and gibbsite) will be examined as a function of temperature, pH, organic/ inorganic electrolyte concentration, and organic/inorganic electrolyte species. These experiments are designed to separate the parallel mechanisms of acidic hydrolysis, solution complexation, and surface interactions. Surface titrations will be conducted to identify the surface speciation on the mineral surfaces in electrolyte solutions. Surface framework destruction characteristics will be examined by SEM and BET method. Spectroscopic techniques will be used to characterize the aluminum-organic, silica-organic, transition metal-organic complexes both in solution and surfaces.

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