Original ArticlesKinetics of feldspar and quartz dissolution at 70–80°C and near-neutral pH: effects of organic acids and NaCl
Introduction
Silicates are the most abundant primary minerals, a dominant component of sedimentary rocks, and precursors for soil horizons. The dissolution kinetics and formation of secondary minerals which characterize silicate weathering and diagenesis must be understood over the wide range of conditions (i.e., temperature, pH, ionic strength) encountered in natural environments. Significant quantities of organic acids (OAs), particularly low molecular weight carboxylic acids, have been measured in soils and sedimentary basin fluids Carothers and Kharaka 1978, Hanor and Workman 1986, Fisher 1987, MacGowan and Surdam 1990, Lundegard and Kharaka 1994. However, many OA species are very short-lived in the diagenetic environment due to rapid mineralization via thermal degradation or biodegradation reactions. In soil horizons, OAs are favored substrates for microbial metabolism and growth, leading to rapid loss of OAs during transport to the water table (Berner and Berner, 1987). There is strong evidence from laboratory experiments that OAs can enhance both the solubility and dissolution rates of aluminosilicate minerals (e.g., Huang and Keller 1970, Surdam et al 1984, Surdam et al 1989, Amrhein and Suarez 1988, Bevan and Savage 1989, Manning et al 1991, Welch and Ullman 1996).
Feldspars dissolve by surface-controlled mechanisms that depend on the concentration of activated surface species. Rates of feldspar hydrolysis are strongly affected by the presence of dissolved species such as the alkali cations (Na+, K+, Li+) and Al, which may adsorb onto mineral surfaces, compete with H+ for reactive sites, or affect the formation and stability of rate-controlling activated surface complexes Nesbitt et al 1991, Stillings et al 1995, Oelkers and Schott 1995. Organic acids also promote feldspar dissolution by surface-controlled mechanisms; however, the effects of dissolved cations, variable ionic medium, and saturation state on the organic ligand-promoted dissolution of feldspars, has not been well studied. The dissolution kinetics of quartz is also strongly affected by both organic acids and dissolved alkali cations Bennett et al 1988, Bennett 1991. These effects have been shown to vary with temperature, pH, the concentration of cations and OAs, and solution saturation state Dove and Crerar 1990, Bennett 1991, Berger et al 1994.
While there is now general concensus that low molecular weight di- and tricarboxylic acid anions such as oxalate and citrate can enhance the dissolution of aluminosilicates, several important questions remain to be resolved about OA-promoted reaction mechanisms. Although several researchers have concluded that oxalate can enhance feldspar dissolution, different models have been advanced for the stoichiometry and mechanism of the organic acid-promoted reaction Amrhein and Suarez 1988, Huang and Longo 1992, Welch and Ullman 1993, Welch and Ullman 1996, Franklin et al 1994, Blum and Stillings 1995, Stillings et al 1996. To some extent, these differences can be ascribed to differences in experimental conditions Manley and Evans 1986, Mast and Drever 1987, Amrhein and Suarez 1988, Franklin et al 1994, Stillings et al 1996, Welch and Ullman 1996. Different mechanisms characterize different regimes of pH, temperature, and saturation state (reaction affinity, ΔG) Furrer and Stumm 1986, Dove and Crerar 1990, Burch et al 1993, Gautier et al 1994, Hellmann 1994, Chen and Brantley 1997, making it difficult to isolate the effects of OAs alone from other variables.
Several general properties of OA-promoted dissolution of feldspars have been widely recognized including increased solubility and dissolution rate with increasing OA concentration, increasing number of carboxylic functional groups, and increasing Al content of the mineral (see review by Blum and Stillings 1995, Welch and Ullman 1996). The majority of previous experiments have been conducted in dilute acidic solutions and have been aimed at: (1) determining if organic acids enhance the dissolution of aluminosilicates independent of their role in pH buffering; (2) characterizing the effects of different types of OA; and (3) determining the pH dependence of organic ligand-promoted dissolution rates (at pH < 5). Natural waters, however, are characterized by complex compositions, variable saturation states, and pH values in the near-neutral range (pH 5–8). Importantly, both Al solubility and proton-promoted dissolution effects are lowest in this pH range. Thus, effects of OA on aluminosilicate mineral dissolution would be most pronounced in the near-neutral pH region.
The present study was initiated to resolve some uncertainties about OA-promoted reactions of silicate minerals by investigating the effects of two of the more commonly studied OA anions, oxalate and citrate, on the solubility and rate of dissolution of feldspars and quartz under well-controlled and consistent conditions at near-neutral pH. Experiments were designed to dampen the effects of competing and overlapping variables by employing constant buffered pH, fluid:solid ratio, and mineral compositions. A constant ionic medium was selected to reduce effects of solution-dependent variables such as mineral surface charge and surface/solution speciation. Further, the study of both pure SiO2 (quartz) and various feldspar compositions under the same conditions permitted evaluation of Al versus Si mobilization by OAs. Finally, the effects of OAs on precipitation of secondary Al–silicates was explored by following solution chemical evolution into the near-equilibrium region in batch experiments.
Both feldspars and quartz dissolve by surface-controlled mechanisms dominated by hydrolysis reactions. Recent and comprehensive reviews of feldspar Blum and Lasaga 1991, Blum and Stillings 1995, Brantley and Stillings 1996 and quartz (Bennet and Casey, 1994) dissolution mechanisms are available in the literature. In general, under acidic conditions, feldspars dissolve by a combination of ion-exchange reactions between charge-balancing cations (Na+, K+, Ca2+) and protons (H+, H3O+), and hydrolysis of Al–O and Si–O framework bonds Murphy and Helgelson 1987, Wollast and Chou 1988, Blum 1994, Hellmann 1995, Oelkers and Schott 1995, Brantley and Stillings 1996. Briefly, hydrolysis involves adsorption of H+ or H3O+ and formation of activated surface complexes. The rate of dissolution is proportional to the concentration of activated surface species or adsorbed/exchanged protons (e.g., Murphy and Helgelson 1987, Blum and Lasaga 1991, Brantley and Stillings 1996). Factors which may influence the formation of these complexes or otherwise affect the chemistry or speciation of the mineral surface (e.g., pH, alkali cations), for example, adsorption onto surface sites, may affect mechanisms and rates of dissolution Knauss and Wolery 1988, Dove and Crerar 1990, Berger et al 1994.
Organic acids enhance aluminosilicate mineral dissolution by decreasing the activation energy for the rate-limiting steps in hydrolysis by forming surface complexes with Al, and possibly Si Furrer and Stumm 1986, Bennett and Casey 1994. Aluminum may be complexed by a number of organic species in solution including oxalate, malonate, citrate, and fulvic acids Castet et al 1992, Palmer and Bell 1994, Wood et al 1994, Knauss and Copenhaver 1995; and on the surfaces of minerals Fein and Hestrin 1994, Fein and Brady 1995, Fein et al 1995. OA–metal surface complexes destabilize (via bond polarization effects) key Si–O framework bonds and thereby increase the rate of hydrolysis and release of Si from the mineral structure Furrer and Stumm 1986, Amrhein and Suarez 1988, Bevan and Savage 1989, Bennett 1991, Bennett and Casey 1994.
Under either acidic or basic conditions, feldspar dissolution rates are strongly dependent on pH and are thus dominated by proton- or hydroxyl-promoted effects. By contrast, it has been shown that feldspar dissolution in the near-neutral pH range is independent of pH Chou and Wollast 1985, Knauss and Wolery 1986, Murphy and Helgelson 1987, Amrhein and Suarez 1992 and may occur via a distinctly different mechanism. Thus, it is in this pH region that organic acids may have the greatest effect on aluminosilicate dissolution. Despite the near-neutral pH range (pH 5.7–7.5) of most natural waters, few studies have investigated feldspar dissolution kinetics under these conditions. This is due, in part, to very slow reaction kinetics at neutral pH and to the associated low measurable concentrations of dissolved Al and Si, which introduce greater uncertainty into rate expressions and mechanistic models for this pH region (Chen and Brantley, 1997).
It has been suggested that proton- and organic ligand-promoted mechanisms act simultaneously, but do not compete with one another and are thus additive Furrer and Stumm 1986, Amrhein and Suarez 1988. The ligand-promoted dissolution rate RL is thus obtained by subtracting the proton-promoted rate RH from the total dissolution rate. RL is proportional to the concentration of adsorbed ligand or activated surface species and has been described by models and equations of the general form: where the ligand-promoted rate RL is equal to the ligand-promoted rate constant kL multiplied by LS, usually defined as either the concentration of adsorbed ligand or dissolved ligand concentration, raised to some power m (e.g., Furrer and Stumm, 1986). Very few values of kL and m are available for conditions above 25°C and above pH 5.
Refinements of the above model have recently been put forth Welch and Ullman 1992, Welch and Ullman 1996, Stillings et al 1996. However, these relations are based only on experimental data at 25°C and do not handle dissolution behavior in the near-neutral pH range (>pH 5) well (Welch and Ullman, 1992). Also, the contribution of proton-promoted effects to the ligand-promoted reaction, and the effects of protonation of ligands on ligand adsorption are not clear.
Dissolved cations and bulk solution chemistry (e.g., ionic strength, mineral saturation states) may strongly affect the dissolution kinetics of feldspars and quartz Dove and Crerar 1990, Nesbitt et al 1991, Burch et al 1993, Stillings and Brantley 1995. In general, the addition of dissolved alkali cations such as Na+ and K+ decrease feldspar dissolution rates due to competition with protons for reactive surface exchange sites (Stillings and Brantley, 1995). This mechanism has been invoked to explain decreased rates of dissolution of albite (Chou and Wollast, 1985), K–feldspar (Stillings and Brantley, 1995) and labradorite (Nesbitt et al., 1991) in inorganic acidic solutions with added cations (e.g., Na+, K+, Ca2+, Al3+). Aqueous Al has also been reported to decrease steady-state dissolution rates of alkali feldspars at both low and high pH Chou and Wollast 1985, Gautier et al 1994, Oelkers and Schott 1995. Chen and Brantley (1997) concluded that Al3+ inhibits albite dissolution at low pH by competing with H+ for surface adsorption sites.
The presence of alkali cations has an opposite effect on quartz dissolution rates. Quartz dissolution rates are increased in the presence of alkali cations at low ionic strength (up to 0.1 m) Dove and Crerar 1990, Berger et al 1994. Bennett (1991) studied the effects of Na+ and K+ on quartz dissolution in inorganic electrolyte solutions and comparable mixed organic–electrolyte systems (2–20 mM citrate and oxalate) at pH 7 and 25–70°C and observed that OAs increased both the dissolution rates and solubility of quartz, independent of the rate enhancement caused solely by alkali cations.
Theoretical and experimental studies indicate that the effects of aqueous cations on the dissolution rates of feldspars and quartz vary as a function of pH, temperature, and mineral composition (Dove and Crerar 1990, Brantley and Stillings 1996, Strandh et al 1997 and references therein). Oelkers and Schott (1995) found no dependence of the steady-state dissolution rate of anorthite on the aqueous Al3+, Ca2+, or Si concentration at pH 2.4–3.2 and 45–95°C, and attributed this to a difference in reaction mechanism for Al-rich plagioclase relative to the more Si-rich alkali feldspars. Amrhein and Suarez (1992) concluded that K+ decreases labradorite dissolution rates by interfering with H+ exchange at the mineral surface and that other common dissolved cations such as Ca2+, Li+, Na+, and Mg2+ may be expected to have only minor effects on reaction rates via this mechanism due to their large hydrated radii, and hence, their inability to fit into potential exchange sites in the mineral structure. Eick et al. (1996) found no appreciable effect of Na+ (0.005–0.05 M) on the dissolution rate of synthetic aluminosiliate glasses at pH 7 and 25°C, which may reflect the lesser importance of H+–cation exchange processes in the near-neutral pH range, but does not necessarily confirm the hypothesis of Amrhein and Suarez (1992) that Na+ will be ineffective due to size constraints. For example, Stillings and Brantley (1995) observed decreasing K–feldspar dissolution rates with increasing Na+ concentration at pH 3 where H+ exchange processes should be significant, which implies that Na+ may indeed interfere with H+ exchange sites at the feldspar surface.
Most of the studies cited here, and on which many mechanistic models are based, were conducted at extremes of pH where feldspar dissolution rates are strongly pH-dependent and where competition between H+ or OH− and other dissolved species may be expected to play a dominat role in controlling the dissolution mechanism. In the near-neutral pH range, feldspar dissolution is essentially independent of pH Chou and Wollast 1985, Hellmann 1994, Chen and Brantley 1997 and also, there is less H+ available to compete with other cations for mineral surface sites. Additionally, Al3+ is highly insoluble in this pH region, thus the effects of other common dissolved species (e.g., Na+, Ca2+, K+) on the dissolution reaction may become more important. The near-neutral pH condition of many natural waters increases the potential for cation–OA competition during aluminosilicate dissolution and therefore merits further investigation.
Section snippets
Preparation of solids
Albite, labradorite (An70), and orthoclase feldspars were obtained from Ward’s Natural Science Establishment. The minerals were first coarse-crushed and visible impurities were removed. The separated material was ground and sieved to obtain the 38–106 μm size fraction. Grains were wet-sieved and ultrasonically cleaned in ethanol to remove fine-grained material produced during grinding. The cleaned feldspar was then dried at 80°C followed by treatment with a Franz isodynamic magnetic separator
Results
Three separate series of feldspar dissolution experiments were carried out at 80°C for periods of 577, 840, and 1824 h each. Quartz dissolution experiments were conducted at 70°C and lasted for up to 1624 h. The progress of mineral dissolution reactions was followed in terms of the release of Si and Al from the minerals to solution as a function of time. Experiments on labradorite dissolution in solutions with 0–20 mM oxalate and 0.02–2.2 M sodium (as NaCl or NaCl + Na–acetate) provide insight
Feldspars
The effects of oxalate and citrate on feldspar solubility and dissolution rates are in general agreement with the results of previous studies. These include the similarity in dissolution behavior of the alkali feldspars with similar Al content (albite and orthoclase), and the greater solubility of the more Al-rich feldspar, labradorite, in the presence of OAs Huang and Keller 1970, Manley and Evans 1986, Amrhein and Suarez 1988, Welch and Ullman 1993, Welch and Ullman 1996, Stillings et al 1996
Conclusions and geologic implications
Organic acids may play a dominant role in silicate mineral weathering and diagenesis at conditions of near-neutral pH, where proton-promoted dissolution effects are minimal. Results of laboratory experiments on feldspar and quartz dissolution in organic acid solutions at pH 6 demonstrate that oxalate and citrate (0.5–20 mM) increase rates of feldspar and quartz dissolution by up to 2–3-fold. The constant ionic medium and buffered pH of solutions used in these experiments allows evaluation of
Acknowledgements
British Petroleum (U.K.) is thanked for providing support for the experimental work in the form of an extramural research award to L. M. Walter. This research was also supported by NSF Grant No. EAR-9628196 to the University of Michigan Electron Microbeam Analysis (EMAL) facility. T. J. Huston is thanked for providing analytical support and consultation throughout the course of this project. J. B. Fein, J. R. O’Neil, and an anonymous reviewer are thanked for helpful reviews of this manuscript.
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