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Feldspar dissolution in acidic and organic solutions: Compositional and pH dependence of dissolution rate
Gewbimica et Cosmochimica Acta, Vol. 60, No. 16, pp. 2939-2948, 1996 Copyright Q 1996 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/96 $15.00 + .OO
PII SOO16-7037(%)00134-2
Feldspar dissolution in acidic and organic solutions: Compositional and pH dependence of dissolution rate S. A. WELCH and W. J. ULLMAN College of Marine Studies, University of Delaware, Lewes, DE 19958, USA
Abstract-The steady-state dissolution rates of plagioclase feldspars into inorganic acid solutions in a flow-through reactor increased with Al content of the mineral from 1.4 * 10-l’ mol Si/m’/s for albite to 5.6. 10e9 mol Si/m*/s for bytownite. A similar trend was observed for minerals dissolved in neutral solutions although the rates were lower. The results of these experiments are used to develop a simple empirical equation to describe the dissolution of tectosilicates (quartz + feldspars): RH = k,az where RH is the dissolution rate of tectosilicates in acid solution, a H+ is the activity of H + ion, and kH and nH are
dependent
on
the
aluminum
fraction
in
the
1
tectosilicate
framework
1
Al = Al* and nH = -0.052 + 4.23 * . This model, with its strong Al + Si Al + Si [ [ dependence on Al fraction, suggests that tectosilicate dissolution in acid solution results primarily from attack at Al sites at the mineral surface. In acidic oxalate solutions the steady-state dissolution rates were, in some cases, up to a factor of 10 higher than dissolution rates in inorganic solutions at the same pH and appeared to have a similar dependence on pH and mineral composition, at least away from the extremes in aluminum fraction (quartz and bytownite). On the basis of the results of the experiments with acidic oxalate and previous experiments showing a linear dependence of feldspar dissolution rate on organic ligand concentration, an empirical expression for the ligand-promoted component of tectosilicate dissolution rates as measured by silica release (RL) is proposed: RL = (oh [L] - k&z;;+ + lRb(Si) where the first term describes the effect of competitive proton and ligand attack at Al sites at the mineral surface leading to silica release to solution and Hi(Si) reflects the smaller rate of attack at Si sites (oh is a factor depending on the ligand, [L]is the ligand concentration, kH and a ,++ are as given above, and n describes the pH dependence of ligand- and proton-promoted dissolution and is taken to be equal to nH away from the extremes of aluminum fraction). The strong dependence of dissolution rate in acidic organic solutions on aluminum fraction indicates that both protons and ligands attack the mineral surface at the same, presumably Al, sites. = -11.24
+ 25.98 *
1. INTRODUCTION
Solution chemistry, such as pH, organic content, saturation state, and ionic strength also affects dissolution rates. At the pH of most natural environments, 5-8, the rates of silicate and aluminum-silicate dissolution in the laboratory are low and nearly independent of pH (Chou and Wollast, 1985; Brady and Walther, 1989; Wogelius and Walther, 1991; Welch and Ullman, 1993). However, dissolution rates increase away from neutral pH. Chemical weathering rates in natural environments affected by anthropogenic acids, such as acid rain or acid mine drainage, or in areas affected by natural sources of acids, such as from vulcanism, respiration, or in microbial micro-environments (where pH of less than 3 have been measured; Parasuraman, 1995 ) , should be strongly dependent on pH. Soluble organic acids, which are common constituents of groundwater, surface water, sedimentary porewater, and oilfield formation water, increase the dissolution rate and solubility of feldspars and other minerals in the laboratory and in nature (Bennett and Siegel, 1987; Mast and Drever, 1987; Bennett et al., 1988; MacGowan and Surdam, 1988; Welch, 1991; Bennett, 1991; Wogelius and Walther, 1991; Welch and Ullman, 1993). As with inorganic acids, weathering rates of plagioclase feldspars should depend on the relative abundance of Al and Si sites at the mineral surface as the Al site is more susceptible to attack by ligands than the
Quartz and feldspars are the most abundant
rock-forming minerals at the Earth’s surface. Chemical weathering of these minerals is important for controlling groundwater geochemistry and quality and can affect the porosity and permeability in aquifers; therefore chemical weathering can impact the rates of groundwater flow and transport of contaminants. Over longer timescales the dissolution of Ca,Mg-rich silicate minerals, such as the Ca,Al-rich plagioclase, bytownite, regulates atmospheric CO2 concentration, and can therefore affect global climate (Bemer et al., 1983; Brady, 1991). The rates of chemical weathering of rock-forming minerals are controlled by both the compositional and textural characteristics of the minerals and the composition of the solutions into which they dissolve. Observations of naturally weathered feldspars (Goldich, 1938) and laboratory experiments (Casey et al., 1991; Stillings and Brantley, 1995) confirm the importance of crystal chemical characteristics and show that Ca,Al-rich plagioclases dissolve faster than Na,Si-rich plagioclases in similar weathering solutions. Textural characteristics, such as dislocation density, outcrops, and twinning and crystallographic order also may affect reaction rates as dissolution is thought to occur preferentially at high energy sites at mineral surfaces (Ho&en and Speyer, 1987; Casey et al., 1988). 2939
S. A. Welch and W. J. Ullman
2940
Si site ( Amrhein and Suarez, 1988; Blum and Lasaga, 199 I 1. Insoluble organic polymers may also inhibit dissolution by irreversible adsorption at the mineral surface. reducing the number or activity of reactive sites (Welch and Vandevivere, 1995). The saturation state of the solution with respect to the primary and secondary mineral phases may also affect weathering rates. At the initiation of the dissolution process in dilute systems, solutions should be undersaturated with respect to the primary dissolving phase and possible secondary precipitates and mineral dissolution rate should be independent of solution composition. However, as reaction proceeds and dissolution products accumulate (or if product ions are already present in the initial solution), dissolution rate decreases with the thermodynamic driving force for dissolution (Nagy and Lasaga, 1992; Burch et al., 1993 ). Other ions may reduce the dissolution rates of feldspars by competing with H+ ions for cation exchange sites at the mineral surface, one of the initial steps in the proton-promoted dissolution process or, as in the case of quartz, enhance rates by altering the structure of the silicate surface to permit higher accessibility to the water molecules needed for hydrolysis (Dove and Crerar, 1990; Stillings and Brantley, I995 ) . This paper describes the results of a series of dissolution experiments in mixed-bed flow-through reactors in acidic aqueous solution. Experiments were performed on five members of the tectosilicate family ranging in composition from quartz to bytownite to determine how mineral dissolution rates depend on pH and on mineral composition. The results of these experiment are used to develop an empirical expression describing the observed rates. The empirical model is a modification of previous models for proton-promoted dissolution (Blum and Lasaga, 1988; Amrhein and Suarez. 1988), but includes the dependence on mineral composition. Additional experiments using a reaction medium containing oxalate ion, a naturally occurring organic ligand (Graustein, 1977) known to form stable complexes with aluminum in solution and also known to catalyze silicate and aluminumsilicate mineral dissolution (Bennett et al., 1988; Welch, 1991; Harrison and Thyne, 1992; Welch and Ullman. 1993; Franklin et al., 1994; Fein and Hestrin, 1994)) were performed to determine the effect of mineral composition and pH on ligand-promoted dissolution rates. An additional empirical model based on these experiments and previous work on the dependence of dissolution rate on ligand concentration is proposed to account for the synergistic effect of protonand ligand-promoted mechanisms on tectosilicate dissolution rates, although not all required parameters can be estimated on the basis of the present experiments. Catechol, a ligand that forms strong bidentate complexes with both aluminum and silica in solution and, presumably, at Al and Si sites at the crystal surface and that catalyzes mineral dissolution reactions (Zutic and Stumm, 1984; Stumm et al., 1980; Kummert and Stumm, 1980; Baumann, 1960,1963; Tandura et al., 1986), was also used in some experiments. 2. MATERIALS 2.1. Experimental
AND METHODS
Apparatus
Reactors similar to those used by Chou and Wollast (1984) were constructed for these experiments. These
mixed-bed, flow-through reactors simulate the conditions of a partially open (to both mass and energy transfer) geochemical system. The solution in this type of reactor is kept well away from equilibrium with respect to the dissolving phase by a constant replenishment of solution from a dilute reservoir. The reactor volume is approximately 10 mL. One gram of feldspar or quartz sand was added to each reactor at the beginning of the experiment. The reactor vessel and tubing was either cleaned with 10% HCI or replaced between subsequent experiments. The recycled flow rate was held at 4 mL/ min to mix particles in the reactor. The sampling flow rate (Q) was held constant at 6 mL/h. The reactions were run at room temperature (22 5 0.X). Experiments were completed in under ten days in order to minimize the change in surface area that accompanies longer experiments (Stillings and Brantley, 1995 ). Dissolution rate was determined from the rate of silicate release from the mineral surface after the concentration of dissolved silica (Cs,.,,) in the effluent stream became constant (3-S days). These “steady-state” rates are the focus of this paper as they represent the rates of destruction of the silicate or aluminum-silicate mineral framework. Although our short experiments (days to a week) yield reaction rates up to an order of magnitude higher than longer experiment (weeks to months) they should reliably estimate the dependence of dissolution rate on important characteristics of the solution and solid in nature. Silica release rate is taken as an indicator of the mineral dissolution rate: Dissolution
Rate =
Qc G-cc - Cs,-mp) Surface Area
’
where Csl~,“Pis the concentration of silica in the input solution, if any. Molar silica release rates could be converted to molar mineral dissolution rates using the stoichiometry of the dissolving phase. We have chosen to use silica release rates rather than mineral dissolution rates in our discussion, as these and previous dissolution experiments using our reactor at room temperature are only rarely stoichiometric (Welch and Ullman, 1992, 1993). Therefore, the stoichiometric correction is not useful for predicting the rates of release of other component ions from the dissolving phase. 2.2. Experimental Solids Bulk feldspar mineral samples (Table I ) were purchased from Wards Natural Science Establishment (Rochester, NY, USA). Only samples without extensive weathering rinds or significant concentrations of other mineral phases were used. Thin sections were made from representative samples of each mineral for petrographic evaluation All of the feldspar samples were coarse grained and showed twinning. The bytownite rock sample contained traces of metal oxides, garnets, and very-fine-grained illite which occurred in cracks and along grain boundaries. The igneous labradorite A sample had some small exsolved phases within the larger feldspar crystals. Clays and iron oxides were present as accessory minerals. The metamorphic labradorite B sample had been identified as andesine by the supplier. Many of the feldspar crystals from this rock sample were zoned. Labrador&e B contained garnet, muscovite, and illite as accessory minerals. The two labradorites had nearly identical composition but were collected from different locations, and have different paragenetic histories, and presumably different crystallographic and textural characteristics as well. The albite sample contained muscovite and quartz. Quartz sand was purchased from Fisher Scientific. The
Dissolution kinetics of feldspar
2941
Table 1. Composition, aluminum fraction, specific surface area and sampling location of minerals used in
sand did not appear to contain any impurities and was used as delivered. Prior to use in these experiments each feldspar sample was crushed to a uniform grain size and cleaned of impurities and fines. The feldspars were initially crushed in a jaw crusher to a coarse sand to pebble size. Large impurities were removed by hand picking. Samples were then ground to a fine sand size using a disc grinder and sieved to separate size fractions ( <125pm, 12.5-250pm, >250pm). The 125-250pm size mineral grains used in our reactors were ultrasonically cleaned in deionized water to remove fine particles until a clear supematant was observed (approximately twenty times). Samples were dried overnight at 90°C. A magnet was used to remove magnetic minerals and iron filings introduced by the crushing procedure. Samples were further purified using a Frantz Isodynamic Magnetic Separator. The composition of the feldspars was determined by X-ray fluorescence spectrometry at X-ray Assay Laboratories (Toronto, ON, Canada). The surface areas of the starting materials (Table 1) were measured using single point B.E.T. with a krypton gas adsorbate by Quantachrome Corporation (Syosset, NY). Scanning Electron Micrographs (SEM) were taken of representative subsamples of the starting materials and of some of the minerals after reaction (Welch, 1991; Welch and Ullman, 1993). 2.3. Experimental
Solutions
Stock solutions of reagent grade oxalic acid and catechol in water ( 100 mM) were diluted for the mineral dissolution experiments and the pH of these initial reaction solutions and distilled water was adjusted using dilute hydrochloric acid, nitric acid, sodium hydroxide, or ammonium hydroxide. Based on preliminary experiments neither chloride nor nitrate ions were expected to have an effect on dissolution. Cl-, however, was found to interfere with some analytical procedures; therefore, nitric acid was used in preference to hydrochloric acid in most experiments. Ammonium hydroxide was used in preference to other bases since the ammonium ion is not a major constituent in the mineral phases under study and therefore would not directly affect the thermodynamic solubility of the mineral. Ionic strength of the input solutions ranged from approximately 0 to 2 mM. Dissolution experiments were performed in solutions ranging from pH = 3 to 10 although the principal focus of this paper is on the acidic experiments. To minimize photodegradation, the catechol solutions and experiments were kept in the dark. The reactor effluents were collected daily and filtered with a 0.4 pm Nuclepore filter before analysis. The pH was determined potentiometrically using an Orion Research Model 701A Digital Ionalyzer equipped with a Corning semi-micro combination electrode and did not differ from that of the initial solution by more than 0.2 pH units. Dissolved silica concentrations were determined using the silicomolybdenum blue method on a Technicon AutoAnalyzer II. Standard addition experiments showed that there was no significant matrix effect on the determination of silica using this method in up to 1
mM organic solutions. Aluminium, calcium, and sodium were also determined for some experiments using atomic absorption spectroscopy to verify the saturation state of the solution and determine the nonstoichiometry of the dissolution process. 3. RESULTS
Nearly all the dissolution experiments had an initial timedependent and often nonreproducible dissolution period which lasted from a few hours to a day, followed by slower and constant “steady-state” dissolution (Welch, 1991; Welch and Ullman, 1993). Even though Si release reached “steady-state,” dissolution was not stoichiometric (Welch, 1991; Welch and Ullman, 1993). Sodium and calcium were preferentially leached compared to Si. The ratio of the release of the framework ions, Al/Si, depended on solution pH and organic ligand content. In inorganic-acid solution, the Al/Si release ratios were greater than or equal to the Al/ Si in the mineral and decreased to below detection (
S. A. Welch and W. J. Ullman
s”-10 -
~0.36
-10
~~......_.._.__~___~___..~~~
\ n
,r 4
-11 ,
0.0
I
I
,
/
0.1
0.2
0.3
0.4
-12 h
0.5
Al fraction FIG. 1. Steady-state dissolution rates for quartz, albite, labradorite A and B, and bytownite dissolved in 1 mM oxalate ( * * A * . ) and 1 mM nitrate (- 0 -) at pH 3. Aluminum fraction equals Al/ (Si+Al) of the initial solid material (see Table 1).
was much less striking than in the acidic solutions. Dissolution rates in neutral catechol solutions were 2 to 8 times greater than dissolution rates in water and were much less dependent on mineral composition than the rates in oxalic acid solutions. The dependence of mineral dissolution rates in inorganic and 1 mM oxalate solutions is shown in Fig. 3. For all of the feldspars the steady-state dissolution rates increased with
2
r _/-- _--ii:,
I
??
0.1
0.2
0.3
0.4
8 10
2
4
6
8 10
FIG. 3. Log steady-state dissolution rates vs. pH for five tectosilicate minerals dissolved in inorganic (a through e) or 1 mM oxalate solutions (f through j). The exponent n in each figure is the slope of the lines shown in the acidic range (pH 3 to 7) of the curve and is equivalent to nH or nb in Table 2 and in the text.
CL:’
decreasing pH in the neutral to acid range. The dissolution rate for the labradorite B sample was nearly constant in the neutral pH range (pH 6 to 8) and then increased with decreasing pH (Fig. 3~). Quartz dissolution rate was independent of pH in the absence of oxalate (Fig. 3e and 3h). Although the presence of oxalate in solution increases the tectosilicate dissolution rates compared to inorganic solutions at the same pH, the degree of ligand-promoted enhancement of dissolution depends on pH and mineral composition. For example, the dissolution rate of bytownite in oxalate solution at pH 3 was approximately twice the rate in the
_12J-____ 0.0
6
PH
??
_----
4
Ob
Al fraction FIG. 2. Steady-state dissolution rates for quartz, albite, labradorite A and B, and bytownite dissolved in 1 mh4 solutions of oxalate (- *A* *), catechol (--W-), and in water (- 0 -) at approximately pH 6.
Dissolution kinetics of feldspar inorganic solution (Figs. 1, 3a and 3f). However, at pH 6, the dissolution rate of bytownite in oxalate was approximately 10 times the rate in inorganic solution (Figs. 2 and 3a, f). Oxalic acid had a lesser effect on the Na,Si-rich feldspars: increases in dissolution rates ranged from a factor of 1.1 to 5. 4. DISCUSSION The results of our experiments indicate that tectosilicate dissolution rates are affected by both mineral and solution composition and that the effects of pH and oxalate concentration in solution are not uniform over the range of mineral compositions. 4.1. Effect of Mineral
Composition
on Dissolution Rate
The observed rates of feldspar dissolution at neutral pH are generally consistent with those summarized in Helgeson et al. (1984). Their work, which is based on compilations of studies on alkali feldspars, shows that the hydrolysis rate of feldspars is nearly constant (lo-” ’ o.3 moles Si/m*/s) from pH = 2.9 to 8 at 25°C. Although our dissolution rates at neutral pH are approximately one order of magnitude higher than those reported in Helgeson et al. (1984) and show a systematic trend with Al content, all of our values at neutral pH fall within an order of magnitude of each other (rate =lO-lo to lo-” moles Si/m’/s). The increase in dissolution rates with increasing Al content that we observed in acidic solutions is also consistent with other studies. Casey et al. ( 1991) analyzed results of batch dissolution experiments with plagioclase feldspars at pH = 2 and 3. At pH = 2 the dissolution rate of feldspars ranged from 3 * lo-” moles Si/m’/s for albite to 4.7 - 10m9 moles Si/m’/s for bytownite. These are approximately the same values that we obtained in our experiments at pH = 3 (1.4. IO-” moles Si/m*/s for albite to 5.6. lo-’ moles Si/ m*/s for bytownite). Stillings and Brantley (1995) also found that silica release rate at pH = 3 increased approximately linearly with increasing Al content from 2.2. lo-” moles Si/m*/s for albite to 1.3. lo-” moles Si/m’/s for bytownite. Even though the trends with composition are similar, Stillings and Brantley’s (1995) rates are one to two orders of magnitude lower than ours for similar experimental conditions, presumably because their experiments ran longer than ours. They also measured an increase in surface area which results in a lower dissolution rate/area. Huang and Kiang (1972) and Manley and Evans ( 1986) also showed that Ca,Al-rich members of the plagioclase series dissolve more readily than Na,Si-rich members in organic acid solutions in batch reactors. It is not clear why there is such a large difference between the dissolution rates of the two chemically similar labradorites in our experiments in the same solutions. Differences in experimental design, (i.e., differences in flow and/or mixing rates) can produce different apparent reaction rates (Welch and Ullman, 1993; Amrhein and Suarez, 1992), although this is unlikely to be a factor in this study since experiments were done under identical flow conditions. Casey et al. ( 1991) also reported differences in dissolution rates of
2943
Na,Si-rich plagioclase from factors of 2 to 5 in replicate experiments with the same mineral or with minerals of similar composition but from different localities. Differences in reaction rates for more Ca,Al-rich plagioclases were much larger. If dissolution rate is related only to the effective surface area and mineral composition, then these two samples, which have nearly identical composition and surface area (Table 1) , should have shown very similar behavior in all solutions. Crystallographic or textural characteristics must also be important. The two labradorites were formed at different temperatures and should differ in crystallographic order and, therefore, the degree of coordination of the structural Si atoms to Al. If the rate of silica release is controlled by the proton-promoted and ligand-promoted attack at adjacent Al sites (see discussion below), these crystallographic differences could lead to the observed differences in dissolution rate. The present data, however, are insufficient to test this hypothesis. An alternative or additional explanation of these differences in dissolution rate is the difference in the number or extent of macroscopic surface features. Thin sections also showed differences in the exsolved phases and extent of zonation and twinning, all of which could lead to the observed differences in dissolution rate. 4.2. Effect of pH and Mineral Composition on Dissolution Rate The steady-state dissolution rates of the feldspars show a strong dependence on solution pH reflecting the importance of proton-promoted dissolution mechanisms. Dissolution rate decreases as pH increases in the acidic range, is fairly constant in near neutral solutions (pH 6 to 8), and then increases with pH in the basic range (Fig. fc,f,g ). This trend was observed for all of the feldspar experiments in this study and is consistent with previous studies for albite (Chou and Wollast, 1984, 1985) and other feldspars (Brady and Walther, 1989), olivine (Blum and Lasaga, 1988; Wogelius and Walther, 1991) , kaolinite and corundum (Carroll-Webb and Walther, 1988), and aluminum oxides (Furrer and Stumm, 1986). In contrast to these results, Mast and Drever ( 1987) and Helgeson et al. ( 1984) found no dissolution rate dependence on pH. The discrepancy between these two and the larger group of observations may be due to the reduced efficiency of proton-promoted dissolution in the alkali-rich feldspars studied by the latter group. The conclusions of Helgeson et al. (1984), however, were based on a compilation of a variety of studies using different methodologies. Recent work on mineral dissolution shows that differences in experimental procedures can have a significant impact on measured dissolution rates (Casey et al., 1991; Amrhein and Suarez, 1992; Welch and Ullman; 1993) which makes it difficult to rely on conclusions based on such compilations. Blum and Lasaga ( 1988, 1991) have shown that the rate of dissolution for a silicate mineral in acidic solution is related to the adsorption of excess protons on the mineral surface and is, as a result, an exponential function of hydrogen ion activity. For a given mineral and a given acid: RH = kHa2+
(1)
S. A. Welch and W. J. Ullman
2944
of surface silica sites with Al content of the mineral or that there is an increase in protonation of aluminum sites that subsequently leads to the release of surface silicate molecules to solution. This latter conclusion is more consistent with our data as Si release from quartz is essentially pH independent in the acid region (Table 2; Fig. 3e), indicating that protonation of Si sites on the mineral surface of framework silicates is not a principal mechanism leading to silica release. Brady and Walther (1989) have similarly concluded that protonation of Al surface sites is the principal mechanism of feldspar dissolution in acidic solutions and that protonation of Si sites is less important. According to their hypothesis, and consistent with our results, silica sites are destabilized by proton adsorption and possible detachment of neighboring Al tetrahedra. The model described in Eqns. 1 and 2 can be easily modified to include the rate dependence on mineral composition as both log kH and nH are strong functions of the square of the aluminum fraction [Al/( Al+%)] of the mineral (Casey et al., 1991; Fig. 4). From the rates of dissolution determined in the experiments with inorganic acids, log ku for the proton-promoted surface reaction can be estimated from a regression of the data in Fig. 4a:
-5
-6 -7 A 8
-8 -9 -10 -11 -12 II
log k, = - 11.24 + 25.98 *
(3)
The pH dependence ( nH) of dissolution rate can be estimated from the data in Fig. 4b:
I/I
0.0
nn = -0.052
+ 4.23 *
0.00 0.05 0.10 0.15 0.20 0.25
FIG. 4. (a) Logarithm of dissolution rate at pH = 0, for the inorganic experiments (kH; - 0 -) and for the oxalate experiments (/CL,; . *A. *); and (b) pH dependence of dissolution rate for the inorganic experiments (n”; - t -_) and oxalate experiAl ments (~2~; . *A. 4) vs. left of the minerals. [ Al + Si I
log RH = log kH - n,pH
z 1
(4)
The constant terms in these two empirical equations can be taken as estimates of the rate of hydrolysis for the Alfree member of the tectosilicate mineral family, quartz. The term containing aluminum fraction reflects the preferential impact of protons on aluminum sites, as discussed above. Combining Eqns. 2, 3, and 4, we can write an empirical dissolution rate equation for estimating the dissolution rate of tectosilicates in acidic to neutral solutions if the solution pH and mineral composition are known. log RH =
or equivalently
-+[ Al + Si
-11.24
+ 25.98 *
[P&i])
(2)
where RH is the mineral dissolution rate in inorganic acid solutions (as measured by silica release rate in this study), kH is the (hypothetical) reaction rate at pH = 0, pH = -log a”+ (where uH+ is the activity of H’ ion in solution), and rru is a parameter describing the variation of rate with proton activity shown for each mineral in Fig. 3.
In our experiments using inorganic acids, rzu increased systematically with the aluminum content of the minerals from 0.001 for quartz to 0.86 for bytownite (Fig. 4b). Oxburgh et al. (1994) found a similar relationship for plagioclase feldspars. If the pH relationship reflects surface protonation as suggested by Blum and Lasaga (1988), this trend indicates either that there is an increase in the protonation
-
-0.052
+ 4.23 * [&I’)
*PH
(5)
where RH is in units of moles Si/m2/s. Figure 5 shows the relationship between measured and predicted rates (using Eqn. 5) for a wide range of experiments on feldspar and
quartz dissolution in inorganic acid solutions from our work and from other flow-through and batch reactor studies for which data is available. Our model overestimates rates determined in many other mineral dissolution studies by up to an order of magnitude. This overestimation may be due to differences in the length of our experiments and those of other investigators and the differences in reactors used. A better fit to the bulk of the data reported by others could
Dissolution kinetics of feldspar
2945
Table 2. Rate constants and slope @H dependence on dissolution rate in acid solutions) for minerals dissolved in inorganic and oxalic acid solutions(EquationsI, 2, 7 and 8). Mineral
OxalicAcid
InorganicAcid log 4,
“n
be achieved by reducing the value of the constant term in the equation describing log ku ( - 11.24 in Eqns. 3 and 5) by approximately 0.5 to 1 unit to account for the higher rates of reaction that are found in our reactor compared to those of other investigators. The form of the empirical equation (Eqn. 5)) however, appears to remain the same and yields a reasonable prediction of the results of other experiments after this change. 4.3. Effect of Oxalate, pH and Mineral Composition on Dissolution Rates Organic ligands, such as oxalate ion, can also enhance the rates of silicate mineral dissolution by the destabilization of
loa k,L
L nu
the mineral surface due to the adsorption and reaction with metal sites at the mineral surface (Kummert and Stumm, 1980). Our experiments indicate that the impact of oxalate, a ligand that forms strong complexes with Al in solution, on mineral dissolution rates varies with both mineral composition and pH (Figs. 1 and 2). The impact of ligands on dissolution rate, however, is dependent on the nature of the ligand as well. In contrast to oxalate, which has a greater impact on dissolution rates of the Ca,Al-rich members of the tectosilicate family, catechol, a ligand that forms strong complexes with both Al and Si, enhances dissolution rates approximately uniformly over the whole range of tectosilicate composition. The systematic variation with pH on the impact of oxalate ion on the rates of mineral dissolution indicates that both organic ligands (oxalate in the case covered here) and protons can attack surface sites and catalyze dissolution and that these reactions proceed in parallel (Welch and Ullman, 1993; Amrhein and Suarez, 1988). Under these conditions, the Amrhein and Suarez (1988) model suggests that the overall rate of dissolution as measured in our experiments in organic acid solution, Rh, can be treated as the sum of the proton-promoted rate as given by Eqn. 1 and the ligandpromoted rate, RL: Rb = RH + RL
-12
-11
-10
-9
-8
-7
Log Measured Rate (mole Si/m*/s) FIG. 5. A comparison of measured rate vs. predicted rate using Eqn. 5. Data are from our work (- 0 - ; this study; Welch, 1991; Welch and Ullman, 1993). The other flow-through data (0) are
from Oxbnrgh et al. ( 1994), Chou and Wollast ( 1984, 1985), and Brady and Walther (1989). Batch dissolution data (V) are from Holdren and Speyer ( 1987) and Casey et al. ( 1991). The dashed line shown (---) is the line describing a one order of magnitude reduction in the rate of dissolution predicted by our experiments. The results of most reported experiments are within an order of magnitude of our predictions (see text).
(6)
In principal, therefore, the impact of the ligand-promoted dissolution alone can be determined by calculating the difference between the measured rate of dissolution in the organiccontaining acidic solutions and in solutions of the same pH but containing no ligands. Rb and RH have very similar functional dependence on pH due to the dominance of the proton-promoted mechanism of dissolution in mixed acid/ligand solutions (Figs. 1 and 2). Equations 1 and 2 can therefore be used to model mineral dissolution rates in the acidic oxalate solutions as well. In these experiments, nh (where the superscripts and subscripts indicate that dissolution has both a proton and oxalate dependence) ranges from 0.59 for bytownite to 0.22 for quartz (Table 2; Fig. 3). The more Na,Si-rich feldspars do not show a significant change in surface proton activity when organic ligands are present (as reflected in the similarity of nH and nkj. However, n for bytownite decreases from 0.86 ( =nH) in inorganic solutions to 0.59 (=nkJ in oxalate solu-
S. A. Welch and W. J. Unman
2946
tions, implying that proton activity on the bytownite surface can be affected by variables other than the proton activity in solution. In our experiments, surface proton activity also changes for quartz dissolved in oxalate (nb = 0.22) compared to dissolution in inorganic solutions ( nH - 0). Bennett et al. (1988) had nearly the opposite results for quartz dissolved in batch reactors. Oxalate enhanced quartz dissolution at neutral pH, however, quartz dissolution rate was lower in acidic oxalate solutions (pH 3) than in acidic inorganic solutions. They also observed an increase in quartz dissolution rates with increasing solution pH from 3 to 7. It is not clear why there is a discrepancy between our results and those of Bennett et al. ( 1988). Our experiments lasted a short time, lOO- 150 hours, and we may not attain steadystate rates in that time even though Si release from minerals is fairly constant. Other factors which were not controlled in our experiments, such as ionic strength and temperature, could also have affected dissolution rates. Table 2 shows that in all but the extremes of tectosilicate compositions (bytownite and quartz) the pH dependence on dissolution rate (n) is indistinguishable between the organic and the inorganic experiments (n = nH = n”,, For these cases the total dissolution rate can be written as Rh = kba’A+ = k,a’;-
+ R’-
(7)
or solving for RL: RL = (kb - k,)ais
(8)
In a previous papers concerning bytownite dissolution in oxalate and gluconate solutions, we have shown that k”, is a linear function of ligand concentration (Welch and Ullman, 1992; Vandevivere et al., 1994). Thus, a more complete kinetic formulation might be given by RL = (K~[L]
-
k,)as*
(9)
where [L] is the concentration of ligand in solution. This model and the results of the experiments on which it is based suggest that there is a competition between the ligand-promoted and proton-promoted dissolution mechanism of tectosilicate dissolution. The competition may be the result of simultaneous ligand and proton attack at the same mineral surface sites, possibly involving a protonated ligand (Welch and Ullman, 1992). Consistent with the hypothesis that proton attack occurs primarily at Al sites at the mineral surface, ligand or protonated-ligand attack is assumed to occur at the Al sites. Si release from the mineral framework is then largely the result of the processes occurring at the adjacent Al sites. This conceptual model is consistent with the evidence that dissolution rates, as determined by Si release, are a function of the Al concentration of the mineral. The results of the quartz dissolution experiments indicate that a direct attack at Si sites by the organic ligands leading to Si release to solution is also possible but may be too small to be reliably detectable in our experiments. For oxalate ion, RL,(Si) ~10-‘“.23 moles/m2/s
(10)
where the superscripts and subscripts indicate that this term may be dependent on proton and ligand concentrations as
well. A complete expression of silica release from tectosilicate dissolution should include this term as well: RL = (K~[L]
-k,)ah+
+ Rb(Si)
(11)
It is possible lR”,(Si) is substantially smaller than indicated in Eqn. 10, in part, because competition between Al and Si sites for protons and ligands may favor Al (as it does in solution), and thus minimize the direct attack at Si surface sites in aluminous tectosilicates. Although it is possible on the basis of the experiments reported here to estimate a value for kh for oxalate, substantially more experiments are needed over a range of oxalate concentrations and mineral compositions to estimate this value with confidence. Three possible explanations exist for the apparent difference in the pH dependence in the rates with and without organic ligands in the bytownite experiments. All three explanations suggest that the rate of silica release in acidic organic solutions should have been higher than observed. The first possibility is that oxalate irreversibly adsorbs to the surface under the experimental conditions, thus reducing the number of sites available for proton and ligand attack. Oxalate can also react with Ca from the mineral to form stable calcium oxalate phases (Graustein, 1977). Under these conditions there may be transport inhibition and/or a reduction in available oxalate concentration and therefore an apparent reduction in oxalate-promoted dissolution rate. Although we cannot document dissolution inhibition in the presence of oxalate, we have observed dissolution inhibition in the presence of organic polymers (Welch and Vandevivere, 1995) and, in some of our experiments using oxalate, we have observed a decrease in Ca concentration in the reactor effluent compared to similar experiment without oxalate (Welch, 1991). The second possibility is that secondary aluminum silicates may have precipitated, removing Si from solution and yielding only minimum estimates of the primary dissolution rate. Concentrations of dissolved Al and Si in the acidic organic experiments are approximately 0.5 mM, close to the equilibrium saturation levels for several aluminosilicate clay minerals in the absence of complexing ligands. No secondary precipitates were observed in photomicrographs of the reaction products of these experiments (Welch, 1991; Welch and Ullman, 1993) although the SEM method may not be sensitive enough to detect precipitates unless they are widespread across the mineral surfaces. The third possibility is that there is additional transport limitation at high surface reaction rates. Blum and Lasaga (1988) noted that transport limitation at low pH and high rates of dissolution leads to a deviation from the linear dependence of dissolution rate on pH. It may be that our inclusion of high rate measurements in the data used to calculate the pH dependence has lead to the artifactual determination of nb and kh for this mineral. Thus, the calculated values of n under these circumstances would reflect minimum estimates of the true dependence of the primary rate of dissolution on pH. If the rate at pH 6 is assumed to be correct and the pH dependence of rate is assumed to be the same in the organic acid experiments as in the inorganic experiment, then log tin would be = -4.5 and the form of the expression for Rh would be the same as for the other minerals. With
Dissolution kinetics of feldspar
the present data, there is no way to verify these estimates of kb and nb for bytownite. Our model, like that of Amrhein and Suarez ( 1988), describes the dissolution rate of feldspars in acidic oxalate solutions as the linear sum of the effects of proton-promoted dissolution and ligand-promoted dissolution. In contrast to the Amrhein and Suarez ( 1988) model which assumes that ligand-promoted dissolution occurs only at Al sites and proton-promoted dissolution occurs at Si sites, our model implies that both ligands and protons attack principally at Al sites. Amrhein and Suarez (1988) relied heavily in their modeling on the conclusions of Helgeson et al. (1984) who concluded, in contrast to our observations, that inorganic weathering rates of all feldspars are essentially constant over a wide range of pH and feldspar composition. Our modified model, which includes the possible interactions between ligands and protons at a single site at the mineral surface (Eqn. lo), appears to be a better description of our experimental results. Some additional support for the contention that ligands, such as oxalate, attack silicate mineral surfaces primarily at the same Al sites where protons attack comes from the comparison of the impact of catechol solutions on dissolution with that of oxalate. Catechol, which strongly complexes both Al and Si in solution and presumably at surfaces, uniformly increases tectosilicate dissolution across the whole range of compositions. In contrast, oxalate, which strongly complexes Al but only weakly complexes Si in solution and presumably at surface, has a significantly greater impact on the dissolution of the Ca,Al-rich endmembers of the tectosilicate family than on the Si-rich endmembers. 5. CONCLUSIONS Tectosilicate dissolution rates into inorganic and organic solutions depend on many mineralogical and solution variables. Dissolution rates are generally consistent with the Goldich weathering sequence: Ca,Al-rich feldspars weather faster than Na,Si-rich feldspars which weather faster than quartz. The pH dependence on dissolution rate, nH, increases with increasing Al content of the mineral, indicating that the Al site is preferentially attacked by protons. The dissolution rates of all feldspars increased as solution acidity increased in the acid range, although proton concentration had no effect on quartz in these experiments. Organic acid anions increase the dissolution rates of all the minerals compared to inorganic acid solutions, but have a much larger effect on more aluminum-rich minerals due to the apparent preferential adsorption at Al-sites and resulting ligand-promoted dissolution. Organic acids also enhance quartz dissolution rates, indicating that there is some small Si-organic interaction at the mineral surface as well. The Al-organic interaction is probably more important over the wide range of aluminum silicates. The dissolution models proposed here suggest that the detachment of the structural framework ion, Si, can be due to proton attack, ligand attack, or simultaneous protons and ligands attack at the same Al sites. Our results indicate that these contributions may be of varying importance depending on mineral composition, the type and concentration of ligands, and the total acidity.
2941
Acknowledgments-This work was performed with principal support from the Subsurface Science Program, U.S. Department of Energy. The manuscript was initially prepared while W.J.U. was a chercheur associe at the Centre de Geochimie de la Surface (CNRS) in Strasbourg, France. We thank PC. Bennett, R.C. Surdam, G.R. Holdren, K.A. Traexler and an anonymous reviewer for their comments and suggestions for the improvement of this manuscript. Editorial handling: G. R. Holdren Jr. REFERENCES Amrhein C. and Suarez D. L. ( 1988) The use of a surface complexation model to describe the kinetics of ligand-promoted dissolution of anorthite. Geochim. Cosmochim. Acta 52,2185-2793. Amrhein C.,and Suarez D.,L. (1992) Some factors affecting the dissolution kinetics of anorthite at 25°C. Geochim. Cosmochim. Acta 56, 1815-1826. Baumann V. H. ( 1960) Reaktion der Kieselsaure mit o-Diphenolen. Beitr. zur Silikose-Forschung. 4, 43-50. Baumann V. H. (1963) Reaktion der Kieselsaure mit polymeren o-Diphenolen in Wasser. Kolloid-Z. Z. Polym. 188, 23-28. Bennett P. C. (1991) Quartz dissolution in organic-rich aqueous systems. Geochim. Cosmochim. Acta 55, 1781- 1797. Bennett P. C. and Siegel D. I. (1987) Increased solubility of quartz in water due to complexation by dissolved organic compounds. Nature 326, 684-687. Bennett P. C., Melter M. E., Siegel D. I., and Hassett J. P. (1988) The dissolution of quartz in dilute aqueous solutions of organic acids at 25°C. Geochim. Cosmochim. Acta 52. 1521-1530. Bemer R. A., Lasaga A. C., and Garrels R. M. ( 1983) The carbonatesilicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years. Amer. J. Sci. 283, 641683. Blum A. and Lasaga A. (1988) Role of surface speciation in the low temperature dissolution of minerals. Nature 331, 43 l-433. Brady P. V. ( 1991) The effect of silicate weathering on global temperature and atmospheric CO,. J. Geophys. Rex 96, 1810118106. Brady P. V., and Walther J. V. ( 1989) Controls on silicate dissolution rates in neutral and basic pH solutions at 25°C. Geochim. Cosmochim. Acta 53, 2823-2830. Burch T. E., Nagy K. L., and Lasaga A. C. ( 1993) Free energy dependence of albite dissolution kinetics at 80°C pH 8.8. Chem. Geol. 105,137-162. Carroll-Webb S. A. and Walther J. V. (1988) A surface-complex reaction-model for the pH-dependence of corundum and kaolinite dissolution rates. Geochim. Cosmochim. Acta 52, 2609-2623. Casey W. H., Westrich H. R., and Arnold G. W. (1988) Surface chemistry of labradorite feldspar reacted with aqueous solutions at pH = 2,3, and 12. Geochim. Cosmochim. Acta 52,2795-2807. Casey W. H., Westrich H. R., and Holdren G. R. ( 1991) Dissolution rates of plagioclase at pH = 2 and 3. Amer. Mineral. 76, 211217. Chou L. and Wollast R. ( 1984) Study of the weathering of albite at room temperature and pressure with a fluidized bed reactor. Geochim. Cosmochim. Acta 48, 2205-2217. Chou L. and Wollast R. ( 1985) Steady-state kinetics and dissolution mechanisms of albite. Amer. J. Sci. 285, 963-993. Dove P. M. and Crerar D. A. (1990) Kinetics of quartz dissolution in electrolyte solutions using a hydrothermal mixed-flow reactor. Geochim. Cosmochim. Acta 54, 955-969. Fein J. B. and Hestrin J. E. ( 1994) Experimental studies of oxalate complexation at 80°C: Gibbsite, amorphous silica, and quartz solubilities in oxalate-bearing solutions. Geochim. Cosmochim. Acta 58,4817-4829. Franklin S. P., Hajash A., Jr., Dewers T. A., and Tieh T. T (1994) The role of carboxylic acids in albite and quartz dissolution: An experimental study under diagenetic conditions. Geochim. Cosmochim. Acta 58,4259-4279. Furrer G.and Stumm W. (1986) The coordination chemistry of weathering: I. Dissolution kinetics of 6-A120, and BeO. Geochim. Cosmochim. Acta 50, 1847-1860.
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Garrels R. M. and Mackenzie F. T. ( 197 1) Evolution of Sedimentary Rocks. W. W. Norton and Co. Goldich S. S. (1938) A study in rock weathering. J. Gem/. 46, 1758. Graustein W. C. ( 1977) Calcium oxalate; occurrence in soils and effect on nutrient and geochemical cycles. Science 198, 12521254. Harrison W. J. and Thyne G. D. (1992) Predictions of diagenetic reactions in the presence of organic acids. Geochim. Cosmochim. Acta 56, 56.5-586. Helgeson H. C., Murphy W. M., and Aagaard P. ( 1984) Thermodynamic and kinetic constraints on reaction rates among minerals in aqueous solutions. II. Rate constants, effective surface area, and the hydrolysis of feldspar. Geochim. Cosmochim. Acta 48,24052432. Holdren G. R., Jr., and Speyer P. M. ( 1987) Reaction rate-surface area relationships during the early stages of weathering: Il. Data on eight additional feldspars. Geochim. Cosmochim. Acta 51, 2311-2318. Huang W. H. and Kiang W. C. ( 1972) Laboratory dissolution of plagioclase in water and organic acids at room temperature. Amer. Mineral. 57, 1849 - 1859. Kummert R. and Stumm W. ( 1980) The surface complexation of organic acids on hydrous y-Alz03. J. Colloid Interface Sci. 75, 373-385. MacGowan D. B. and Surdam R. C. (1988) Difunctional carboxylic acid anions in oilfield waters. Org. Geochem. 12, 245-259. Manley E. P. and Evans L. J. (1986) Dissolution of feldspars by low molecular weight aliphatic and aromatic acids. Soil Sci. 141, 106- 112. Marley N. A., Bennett P., Janecky D. R., and Gaffney J. S. ( 1989) Spectroscopic evidence for organic diacid complexation with dissolved silica in aqueous systems: I. Oxalic acid. Org. Geochem. 14, 525-528. Mast M. A. and Drever J. 1. (1987) The effect of oxalate on the dissolution rates of oligoclase and tremolite. Geochim. Cosmochim. Acta 51, 2559-2568. Nagy K. L. and Lasaga A. C. (1992) Dissolution and precipitation
kinetics of gibbsite at 80 C and pH 3. The dependence on solution saturation state. Geochim. Cosmochim. Acta 56, 3093-3111. Oxburgh R., Drever J. I., and Sun Y.-T. (1994) Mechanism of plagioclase dissolution in acid solution at 25°C. Geochim. Cosmochim. Acta 58,661-669. Parasuraman C. S. ( 1995) Mechanism of potential ennoblement on passive metals due to biofilms in seawater. Doctoral dissertation, Univ. Delaware. Stillings L. L. and Brantley S. L. (1995) Feldspar dissolution at 25°C and pH 3: Reaction stoichiometry and the effect of cations. Geochim. Cosmochim. Acta 59, 1483- 1496. Stumm W., Kummert R., and Sigg L. (1980) A ligand exchange model for the adsorption of inorganic and organic ligands at hydrous oxide surfaces. Croatica Chem. Acta 53, 291-312. Tandura S. N., Voronkov M. G., and Alekseev N. V. ( 1986) Molecular and electronic structure of penta- and hexacoordinate silicon compounds. Topics in Current Chemistry 131, 99- 186. Vandevivere P., Welch S. A., Ullman W. J., and Kirchman D. L. ( 1994) Enhanced dissolution of silicate minerals by bacteria at near-neutral pH. Microbial Ecol. 27, 241-25 1. Welch S. A. ( 1991) The effect of organic acids on mineral dissolution rates and stoichiometry. M.S. thesis, Univ. Delaware. Welch S. A. and Ullman W. J. (1992) Dissolution of feldspars in oxalic acid solutions. In Proceedings of the 7th Water Rock Interaction Meeting, Park City, Utah, USA, 13- 19 July 1992 (eds. Y. K. Kharaka and A. S. Maest), pp. 127-130. Balkema. Welch S. A. and Ullman W. J. ( 1993) The effect of organic acids on plagioclase dissolution rates and stoichiometry. Ge&him. Cosmochim. Acta 57, 2725-2736. Welch S. A. and Vandevivere P. (1995) Effect of microbial and other naturally occurring polymers on mineral dissolution. Geomicrobiol. J. 12, 227-238. Wogelius R. A. and Walther J. V. (1991) Olivine dissolution at 25°C: Effects of oH, CO?, and organic acids. Geochim. Cosmochim. Acta 55, ‘943-954. Zutic V. and Stumm W. ( 1984) Effect of organic acids and fluoride on the dissolution kinetics of hydrous alumina. A model study using the rotating disc electrode. Geochim. Cosmochim. Acta 48, 1493-1503.
PII SOO16-7037(%)00134-2
Feldspar dissolution in acidic and organic solutions: Compositional and pH dependence of dissolution rate S. A. WELCH and W. J. ULLMAN College of Marine Studies, University of Delaware, Lewes, DE 19958, USA
Abstract-The steady-state dissolution rates of plagioclase feldspars into inorganic acid solutions in a flow-through reactor increased with Al content of the mineral from 1.4 * 10-l’ mol Si/m’/s for albite to 5.6. 10e9 mol Si/m*/s for bytownite. A similar trend was observed for minerals dissolved in neutral solutions although the rates were lower. The results of these experiments are used to develop a simple empirical equation to describe the dissolution of tectosilicates (quartz + feldspars): RH = k,az where RH is the dissolution rate of tectosilicates in acid solution, a H+ is the activity of H + ion, and kH and nH are
dependent
on
the
aluminum
fraction
in
the
1
tectosilicate
framework
1
Al = Al* and nH = -0.052 + 4.23 * . This model, with its strong Al + Si Al + Si [ [ dependence on Al fraction, suggests that tectosilicate dissolution in acid solution results primarily from attack at Al sites at the mineral surface. In acidic oxalate solutions the steady-state dissolution rates were, in some cases, up to a factor of 10 higher than dissolution rates in inorganic solutions at the same pH and appeared to have a similar dependence on pH and mineral composition, at least away from the extremes in aluminum fraction (quartz and bytownite). On the basis of the results of the experiments with acidic oxalate and previous experiments showing a linear dependence of feldspar dissolution rate on organic ligand concentration, an empirical expression for the ligand-promoted component of tectosilicate dissolution rates as measured by silica release (RL) is proposed: RL = (oh [L] - k&z;;+ + lRb(Si) where the first term describes the effect of competitive proton and ligand attack at Al sites at the mineral surface leading to silica release to solution and Hi(Si) reflects the smaller rate of attack at Si sites (oh is a factor depending on the ligand, [L]is the ligand concentration, kH and a ,++ are as given above, and n describes the pH dependence of ligand- and proton-promoted dissolution and is taken to be equal to nH away from the extremes of aluminum fraction). The strong dependence of dissolution rate in acidic organic solutions on aluminum fraction indicates that both protons and ligands attack the mineral surface at the same, presumably Al, sites. = -11.24
+ 25.98 *
1. INTRODUCTION
Solution chemistry, such as pH, organic content, saturation state, and ionic strength also affects dissolution rates. At the pH of most natural environments, 5-8, the rates of silicate and aluminum-silicate dissolution in the laboratory are low and nearly independent of pH (Chou and Wollast, 1985; Brady and Walther, 1989; Wogelius and Walther, 1991; Welch and Ullman, 1993). However, dissolution rates increase away from neutral pH. Chemical weathering rates in natural environments affected by anthropogenic acids, such as acid rain or acid mine drainage, or in areas affected by natural sources of acids, such as from vulcanism, respiration, or in microbial micro-environments (where pH of less than 3 have been measured; Parasuraman, 1995 ) , should be strongly dependent on pH. Soluble organic acids, which are common constituents of groundwater, surface water, sedimentary porewater, and oilfield formation water, increase the dissolution rate and solubility of feldspars and other minerals in the laboratory and in nature (Bennett and Siegel, 1987; Mast and Drever, 1987; Bennett et al., 1988; MacGowan and Surdam, 1988; Welch, 1991; Bennett, 1991; Wogelius and Walther, 1991; Welch and Ullman, 1993). As with inorganic acids, weathering rates of plagioclase feldspars should depend on the relative abundance of Al and Si sites at the mineral surface as the Al site is more susceptible to attack by ligands than the
Quartz and feldspars are the most abundant
rock-forming minerals at the Earth’s surface. Chemical weathering of these minerals is important for controlling groundwater geochemistry and quality and can affect the porosity and permeability in aquifers; therefore chemical weathering can impact the rates of groundwater flow and transport of contaminants. Over longer timescales the dissolution of Ca,Mg-rich silicate minerals, such as the Ca,Al-rich plagioclase, bytownite, regulates atmospheric CO2 concentration, and can therefore affect global climate (Bemer et al., 1983; Brady, 1991). The rates of chemical weathering of rock-forming minerals are controlled by both the compositional and textural characteristics of the minerals and the composition of the solutions into which they dissolve. Observations of naturally weathered feldspars (Goldich, 1938) and laboratory experiments (Casey et al., 1991; Stillings and Brantley, 1995) confirm the importance of crystal chemical characteristics and show that Ca,Al-rich plagioclases dissolve faster than Na,Si-rich plagioclases in similar weathering solutions. Textural characteristics, such as dislocation density, outcrops, and twinning and crystallographic order also may affect reaction rates as dissolution is thought to occur preferentially at high energy sites at mineral surfaces (Ho&en and Speyer, 1987; Casey et al., 1988). 2939
S. A. Welch and W. J. Ullman
2940
Si site ( Amrhein and Suarez, 1988; Blum and Lasaga, 199 I 1. Insoluble organic polymers may also inhibit dissolution by irreversible adsorption at the mineral surface. reducing the number or activity of reactive sites (Welch and Vandevivere, 1995). The saturation state of the solution with respect to the primary and secondary mineral phases may also affect weathering rates. At the initiation of the dissolution process in dilute systems, solutions should be undersaturated with respect to the primary dissolving phase and possible secondary precipitates and mineral dissolution rate should be independent of solution composition. However, as reaction proceeds and dissolution products accumulate (or if product ions are already present in the initial solution), dissolution rate decreases with the thermodynamic driving force for dissolution (Nagy and Lasaga, 1992; Burch et al., 1993 ). Other ions may reduce the dissolution rates of feldspars by competing with H+ ions for cation exchange sites at the mineral surface, one of the initial steps in the proton-promoted dissolution process or, as in the case of quartz, enhance rates by altering the structure of the silicate surface to permit higher accessibility to the water molecules needed for hydrolysis (Dove and Crerar, 1990; Stillings and Brantley, I995 ) . This paper describes the results of a series of dissolution experiments in mixed-bed flow-through reactors in acidic aqueous solution. Experiments were performed on five members of the tectosilicate family ranging in composition from quartz to bytownite to determine how mineral dissolution rates depend on pH and on mineral composition. The results of these experiment are used to develop an empirical expression describing the observed rates. The empirical model is a modification of previous models for proton-promoted dissolution (Blum and Lasaga, 1988; Amrhein and Suarez. 1988), but includes the dependence on mineral composition. Additional experiments using a reaction medium containing oxalate ion, a naturally occurring organic ligand (Graustein, 1977) known to form stable complexes with aluminum in solution and also known to catalyze silicate and aluminumsilicate mineral dissolution (Bennett et al., 1988; Welch, 1991; Harrison and Thyne, 1992; Welch and Ullman. 1993; Franklin et al., 1994; Fein and Hestrin, 1994)) were performed to determine the effect of mineral composition and pH on ligand-promoted dissolution rates. An additional empirical model based on these experiments and previous work on the dependence of dissolution rate on ligand concentration is proposed to account for the synergistic effect of protonand ligand-promoted mechanisms on tectosilicate dissolution rates, although not all required parameters can be estimated on the basis of the present experiments. Catechol, a ligand that forms strong bidentate complexes with both aluminum and silica in solution and, presumably, at Al and Si sites at the crystal surface and that catalyzes mineral dissolution reactions (Zutic and Stumm, 1984; Stumm et al., 1980; Kummert and Stumm, 1980; Baumann, 1960,1963; Tandura et al., 1986), was also used in some experiments. 2. MATERIALS 2.1. Experimental
AND METHODS
Apparatus
Reactors similar to those used by Chou and Wollast (1984) were constructed for these experiments. These
mixed-bed, flow-through reactors simulate the conditions of a partially open (to both mass and energy transfer) geochemical system. The solution in this type of reactor is kept well away from equilibrium with respect to the dissolving phase by a constant replenishment of solution from a dilute reservoir. The reactor volume is approximately 10 mL. One gram of feldspar or quartz sand was added to each reactor at the beginning of the experiment. The reactor vessel and tubing was either cleaned with 10% HCI or replaced between subsequent experiments. The recycled flow rate was held at 4 mL/ min to mix particles in the reactor. The sampling flow rate (Q) was held constant at 6 mL/h. The reactions were run at room temperature (22 5 0.X). Experiments were completed in under ten days in order to minimize the change in surface area that accompanies longer experiments (Stillings and Brantley, 1995 ). Dissolution rate was determined from the rate of silicate release from the mineral surface after the concentration of dissolved silica (Cs,.,,) in the effluent stream became constant (3-S days). These “steady-state” rates are the focus of this paper as they represent the rates of destruction of the silicate or aluminum-silicate mineral framework. Although our short experiments (days to a week) yield reaction rates up to an order of magnitude higher than longer experiment (weeks to months) they should reliably estimate the dependence of dissolution rate on important characteristics of the solution and solid in nature. Silica release rate is taken as an indicator of the mineral dissolution rate: Dissolution
Rate =
Qc G-cc - Cs,-mp) Surface Area
’
where Csl~,“Pis the concentration of silica in the input solution, if any. Molar silica release rates could be converted to molar mineral dissolution rates using the stoichiometry of the dissolving phase. We have chosen to use silica release rates rather than mineral dissolution rates in our discussion, as these and previous dissolution experiments using our reactor at room temperature are only rarely stoichiometric (Welch and Ullman, 1992, 1993). Therefore, the stoichiometric correction is not useful for predicting the rates of release of other component ions from the dissolving phase. 2.2. Experimental Solids Bulk feldspar mineral samples (Table I ) were purchased from Wards Natural Science Establishment (Rochester, NY, USA). Only samples without extensive weathering rinds or significant concentrations of other mineral phases were used. Thin sections were made from representative samples of each mineral for petrographic evaluation All of the feldspar samples were coarse grained and showed twinning. The bytownite rock sample contained traces of metal oxides, garnets, and very-fine-grained illite which occurred in cracks and along grain boundaries. The igneous labradorite A sample had some small exsolved phases within the larger feldspar crystals. Clays and iron oxides were present as accessory minerals. The metamorphic labradorite B sample had been identified as andesine by the supplier. Many of the feldspar crystals from this rock sample were zoned. Labrador&e B contained garnet, muscovite, and illite as accessory minerals. The two labradorites had nearly identical composition but were collected from different locations, and have different paragenetic histories, and presumably different crystallographic and textural characteristics as well. The albite sample contained muscovite and quartz. Quartz sand was purchased from Fisher Scientific. The
Dissolution kinetics of feldspar
2941
Table 1. Composition, aluminum fraction, specific surface area and sampling location of minerals used in
sand did not appear to contain any impurities and was used as delivered. Prior to use in these experiments each feldspar sample was crushed to a uniform grain size and cleaned of impurities and fines. The feldspars were initially crushed in a jaw crusher to a coarse sand to pebble size. Large impurities were removed by hand picking. Samples were then ground to a fine sand size using a disc grinder and sieved to separate size fractions ( <125pm, 12.5-250pm, >250pm). The 125-250pm size mineral grains used in our reactors were ultrasonically cleaned in deionized water to remove fine particles until a clear supematant was observed (approximately twenty times). Samples were dried overnight at 90°C. A magnet was used to remove magnetic minerals and iron filings introduced by the crushing procedure. Samples were further purified using a Frantz Isodynamic Magnetic Separator. The composition of the feldspars was determined by X-ray fluorescence spectrometry at X-ray Assay Laboratories (Toronto, ON, Canada). The surface areas of the starting materials (Table 1) were measured using single point B.E.T. with a krypton gas adsorbate by Quantachrome Corporation (Syosset, NY). Scanning Electron Micrographs (SEM) were taken of representative subsamples of the starting materials and of some of the minerals after reaction (Welch, 1991; Welch and Ullman, 1993). 2.3. Experimental
Solutions
Stock solutions of reagent grade oxalic acid and catechol in water ( 100 mM) were diluted for the mineral dissolution experiments and the pH of these initial reaction solutions and distilled water was adjusted using dilute hydrochloric acid, nitric acid, sodium hydroxide, or ammonium hydroxide. Based on preliminary experiments neither chloride nor nitrate ions were expected to have an effect on dissolution. Cl-, however, was found to interfere with some analytical procedures; therefore, nitric acid was used in preference to hydrochloric acid in most experiments. Ammonium hydroxide was used in preference to other bases since the ammonium ion is not a major constituent in the mineral phases under study and therefore would not directly affect the thermodynamic solubility of the mineral. Ionic strength of the input solutions ranged from approximately 0 to 2 mM. Dissolution experiments were performed in solutions ranging from pH = 3 to 10 although the principal focus of this paper is on the acidic experiments. To minimize photodegradation, the catechol solutions and experiments were kept in the dark. The reactor effluents were collected daily and filtered with a 0.4 pm Nuclepore filter before analysis. The pH was determined potentiometrically using an Orion Research Model 701A Digital Ionalyzer equipped with a Corning semi-micro combination electrode and did not differ from that of the initial solution by more than 0.2 pH units. Dissolved silica concentrations were determined using the silicomolybdenum blue method on a Technicon AutoAnalyzer II. Standard addition experiments showed that there was no significant matrix effect on the determination of silica using this method in up to 1
mM organic solutions. Aluminium, calcium, and sodium were also determined for some experiments using atomic absorption spectroscopy to verify the saturation state of the solution and determine the nonstoichiometry of the dissolution process. 3. RESULTS
Nearly all the dissolution experiments had an initial timedependent and often nonreproducible dissolution period which lasted from a few hours to a day, followed by slower and constant “steady-state” dissolution (Welch, 1991; Welch and Ullman, 1993). Even though Si release reached “steady-state,” dissolution was not stoichiometric (Welch, 1991; Welch and Ullman, 1993). Sodium and calcium were preferentially leached compared to Si. The ratio of the release of the framework ions, Al/Si, depended on solution pH and organic ligand content. In inorganic-acid solution, the Al/Si release ratios were greater than or equal to the Al/ Si in the mineral and decreased to below detection (
S. A. Welch and W. J. Ullman
s”-10 -
~0.36
-10
~~......_.._.__~___~___..~~~
\ n
,r 4
-11 ,
0.0
I
I
,
/
0.1
0.2
0.3
0.4
-12 h
0.5
Al fraction FIG. 1. Steady-state dissolution rates for quartz, albite, labradorite A and B, and bytownite dissolved in 1 mM oxalate ( * * A * . ) and 1 mM nitrate (- 0 -) at pH 3. Aluminum fraction equals Al/ (Si+Al) of the initial solid material (see Table 1).
was much less striking than in the acidic solutions. Dissolution rates in neutral catechol solutions were 2 to 8 times greater than dissolution rates in water and were much less dependent on mineral composition than the rates in oxalic acid solutions. The dependence of mineral dissolution rates in inorganic and 1 mM oxalate solutions is shown in Fig. 3. For all of the feldspars the steady-state dissolution rates increased with
2
r _/-- _--ii:,
I
??
0.1
0.2
0.3
0.4
8 10
2
4
6
8 10
FIG. 3. Log steady-state dissolution rates vs. pH for five tectosilicate minerals dissolved in inorganic (a through e) or 1 mM oxalate solutions (f through j). The exponent n in each figure is the slope of the lines shown in the acidic range (pH 3 to 7) of the curve and is equivalent to nH or nb in Table 2 and in the text.
CL:’
decreasing pH in the neutral to acid range. The dissolution rate for the labradorite B sample was nearly constant in the neutral pH range (pH 6 to 8) and then increased with decreasing pH (Fig. 3~). Quartz dissolution rate was independent of pH in the absence of oxalate (Fig. 3e and 3h). Although the presence of oxalate in solution increases the tectosilicate dissolution rates compared to inorganic solutions at the same pH, the degree of ligand-promoted enhancement of dissolution depends on pH and mineral composition. For example, the dissolution rate of bytownite in oxalate solution at pH 3 was approximately twice the rate in the
_12J-____ 0.0
6
PH
??
_----
4
Ob
Al fraction FIG. 2. Steady-state dissolution rates for quartz, albite, labradorite A and B, and bytownite dissolved in 1 mh4 solutions of oxalate (- *A* *), catechol (--W-), and in water (- 0 -) at approximately pH 6.
Dissolution kinetics of feldspar inorganic solution (Figs. 1, 3a and 3f). However, at pH 6, the dissolution rate of bytownite in oxalate was approximately 10 times the rate in inorganic solution (Figs. 2 and 3a, f). Oxalic acid had a lesser effect on the Na,Si-rich feldspars: increases in dissolution rates ranged from a factor of 1.1 to 5. 4. DISCUSSION The results of our experiments indicate that tectosilicate dissolution rates are affected by both mineral and solution composition and that the effects of pH and oxalate concentration in solution are not uniform over the range of mineral compositions. 4.1. Effect of Mineral
Composition
on Dissolution Rate
The observed rates of feldspar dissolution at neutral pH are generally consistent with those summarized in Helgeson et al. (1984). Their work, which is based on compilations of studies on alkali feldspars, shows that the hydrolysis rate of feldspars is nearly constant (lo-” ’ o.3 moles Si/m*/s) from pH = 2.9 to 8 at 25°C. Although our dissolution rates at neutral pH are approximately one order of magnitude higher than those reported in Helgeson et al. (1984) and show a systematic trend with Al content, all of our values at neutral pH fall within an order of magnitude of each other (rate =lO-lo to lo-” moles Si/m’/s). The increase in dissolution rates with increasing Al content that we observed in acidic solutions is also consistent with other studies. Casey et al. ( 1991) analyzed results of batch dissolution experiments with plagioclase feldspars at pH = 2 and 3. At pH = 2 the dissolution rate of feldspars ranged from 3 * lo-” moles Si/m’/s for albite to 4.7 - 10m9 moles Si/m’/s for bytownite. These are approximately the same values that we obtained in our experiments at pH = 3 (1.4. IO-” moles Si/m*/s for albite to 5.6. lo-’ moles Si/ m*/s for bytownite). Stillings and Brantley (1995) also found that silica release rate at pH = 3 increased approximately linearly with increasing Al content from 2.2. lo-” moles Si/m*/s for albite to 1.3. lo-” moles Si/m’/s for bytownite. Even though the trends with composition are similar, Stillings and Brantley’s (1995) rates are one to two orders of magnitude lower than ours for similar experimental conditions, presumably because their experiments ran longer than ours. They also measured an increase in surface area which results in a lower dissolution rate/area. Huang and Kiang (1972) and Manley and Evans ( 1986) also showed that Ca,Al-rich members of the plagioclase series dissolve more readily than Na,Si-rich members in organic acid solutions in batch reactors. It is not clear why there is such a large difference between the dissolution rates of the two chemically similar labradorites in our experiments in the same solutions. Differences in experimental design, (i.e., differences in flow and/or mixing rates) can produce different apparent reaction rates (Welch and Ullman, 1993; Amrhein and Suarez, 1992), although this is unlikely to be a factor in this study since experiments were done under identical flow conditions. Casey et al. ( 1991) also reported differences in dissolution rates of
2943
Na,Si-rich plagioclase from factors of 2 to 5 in replicate experiments with the same mineral or with minerals of similar composition but from different localities. Differences in reaction rates for more Ca,Al-rich plagioclases were much larger. If dissolution rate is related only to the effective surface area and mineral composition, then these two samples, which have nearly identical composition and surface area (Table 1) , should have shown very similar behavior in all solutions. Crystallographic or textural characteristics must also be important. The two labradorites were formed at different temperatures and should differ in crystallographic order and, therefore, the degree of coordination of the structural Si atoms to Al. If the rate of silica release is controlled by the proton-promoted and ligand-promoted attack at adjacent Al sites (see discussion below), these crystallographic differences could lead to the observed differences in dissolution rate. The present data, however, are insufficient to test this hypothesis. An alternative or additional explanation of these differences in dissolution rate is the difference in the number or extent of macroscopic surface features. Thin sections also showed differences in the exsolved phases and extent of zonation and twinning, all of which could lead to the observed differences in dissolution rate. 4.2. Effect of pH and Mineral Composition on Dissolution Rate The steady-state dissolution rates of the feldspars show a strong dependence on solution pH reflecting the importance of proton-promoted dissolution mechanisms. Dissolution rate decreases as pH increases in the acidic range, is fairly constant in near neutral solutions (pH 6 to 8), and then increases with pH in the basic range (Fig. fc,f,g ). This trend was observed for all of the feldspar experiments in this study and is consistent with previous studies for albite (Chou and Wollast, 1984, 1985) and other feldspars (Brady and Walther, 1989), olivine (Blum and Lasaga, 1988; Wogelius and Walther, 1991) , kaolinite and corundum (Carroll-Webb and Walther, 1988), and aluminum oxides (Furrer and Stumm, 1986). In contrast to these results, Mast and Drever ( 1987) and Helgeson et al. ( 1984) found no dissolution rate dependence on pH. The discrepancy between these two and the larger group of observations may be due to the reduced efficiency of proton-promoted dissolution in the alkali-rich feldspars studied by the latter group. The conclusions of Helgeson et al. (1984), however, were based on a compilation of a variety of studies using different methodologies. Recent work on mineral dissolution shows that differences in experimental procedures can have a significant impact on measured dissolution rates (Casey et al., 1991; Amrhein and Suarez, 1992; Welch and Ullman; 1993) which makes it difficult to rely on conclusions based on such compilations. Blum and Lasaga ( 1988, 1991) have shown that the rate of dissolution for a silicate mineral in acidic solution is related to the adsorption of excess protons on the mineral surface and is, as a result, an exponential function of hydrogen ion activity. For a given mineral and a given acid: RH = kHa2+
(1)
S. A. Welch and W. J. Ullman
2944
of surface silica sites with Al content of the mineral or that there is an increase in protonation of aluminum sites that subsequently leads to the release of surface silicate molecules to solution. This latter conclusion is more consistent with our data as Si release from quartz is essentially pH independent in the acid region (Table 2; Fig. 3e), indicating that protonation of Si sites on the mineral surface of framework silicates is not a principal mechanism leading to silica release. Brady and Walther (1989) have similarly concluded that protonation of Al surface sites is the principal mechanism of feldspar dissolution in acidic solutions and that protonation of Si sites is less important. According to their hypothesis, and consistent with our results, silica sites are destabilized by proton adsorption and possible detachment of neighboring Al tetrahedra. The model described in Eqns. 1 and 2 can be easily modified to include the rate dependence on mineral composition as both log kH and nH are strong functions of the square of the aluminum fraction [Al/( Al+%)] of the mineral (Casey et al., 1991; Fig. 4). From the rates of dissolution determined in the experiments with inorganic acids, log ku for the proton-promoted surface reaction can be estimated from a regression of the data in Fig. 4a:
-5
-6 -7 A 8
-8 -9 -10 -11 -12 II
log k, = - 11.24 + 25.98 *
(3)
The pH dependence ( nH) of dissolution rate can be estimated from the data in Fig. 4b:
I/I
0.0
nn = -0.052
+ 4.23 *
0.00 0.05 0.10 0.15 0.20 0.25
FIG. 4. (a) Logarithm of dissolution rate at pH = 0, for the inorganic experiments (kH; - 0 -) and for the oxalate experiments (/CL,; . *A. *); and (b) pH dependence of dissolution rate for the inorganic experiments (n”; - t -_) and oxalate experiAl ments (~2~; . *A. 4) vs. left of the minerals. [ Al + Si I
log RH = log kH - n,pH
z 1
(4)
The constant terms in these two empirical equations can be taken as estimates of the rate of hydrolysis for the Alfree member of the tectosilicate mineral family, quartz. The term containing aluminum fraction reflects the preferential impact of protons on aluminum sites, as discussed above. Combining Eqns. 2, 3, and 4, we can write an empirical dissolution rate equation for estimating the dissolution rate of tectosilicates in acidic to neutral solutions if the solution pH and mineral composition are known. log RH =
or equivalently
-+[ Al + Si
-11.24
+ 25.98 *
[P&i])
(2)
where RH is the mineral dissolution rate in inorganic acid solutions (as measured by silica release rate in this study), kH is the (hypothetical) reaction rate at pH = 0, pH = -log a”+ (where uH+ is the activity of H’ ion in solution), and rru is a parameter describing the variation of rate with proton activity shown for each mineral in Fig. 3.
In our experiments using inorganic acids, rzu increased systematically with the aluminum content of the minerals from 0.001 for quartz to 0.86 for bytownite (Fig. 4b). Oxburgh et al. (1994) found a similar relationship for plagioclase feldspars. If the pH relationship reflects surface protonation as suggested by Blum and Lasaga (1988), this trend indicates either that there is an increase in the protonation
-
-0.052
+ 4.23 * [&I’)
*PH
(5)
where RH is in units of moles Si/m2/s. Figure 5 shows the relationship between measured and predicted rates (using Eqn. 5) for a wide range of experiments on feldspar and
quartz dissolution in inorganic acid solutions from our work and from other flow-through and batch reactor studies for which data is available. Our model overestimates rates determined in many other mineral dissolution studies by up to an order of magnitude. This overestimation may be due to differences in the length of our experiments and those of other investigators and the differences in reactors used. A better fit to the bulk of the data reported by others could
Dissolution kinetics of feldspar
2945
Table 2. Rate constants and slope @H dependence on dissolution rate in acid solutions) for minerals dissolved in inorganic and oxalic acid solutions(EquationsI, 2, 7 and 8). Mineral
OxalicAcid
InorganicAcid log 4,
“n
be achieved by reducing the value of the constant term in the equation describing log ku ( - 11.24 in Eqns. 3 and 5) by approximately 0.5 to 1 unit to account for the higher rates of reaction that are found in our reactor compared to those of other investigators. The form of the empirical equation (Eqn. 5)) however, appears to remain the same and yields a reasonable prediction of the results of other experiments after this change. 4.3. Effect of Oxalate, pH and Mineral Composition on Dissolution Rates Organic ligands, such as oxalate ion, can also enhance the rates of silicate mineral dissolution by the destabilization of
loa k,L
L nu
the mineral surface due to the adsorption and reaction with metal sites at the mineral surface (Kummert and Stumm, 1980). Our experiments indicate that the impact of oxalate, a ligand that forms strong complexes with Al in solution, on mineral dissolution rates varies with both mineral composition and pH (Figs. 1 and 2). The impact of ligands on dissolution rate, however, is dependent on the nature of the ligand as well. In contrast to oxalate, which has a greater impact on dissolution rates of the Ca,Al-rich members of the tectosilicate family, catechol, a ligand that forms strong complexes with both Al and Si, enhances dissolution rates approximately uniformly over the whole range of tectosilicate composition. The systematic variation with pH on the impact of oxalate ion on the rates of mineral dissolution indicates that both organic ligands (oxalate in the case covered here) and protons can attack surface sites and catalyze dissolution and that these reactions proceed in parallel (Welch and Ullman, 1993; Amrhein and Suarez, 1988). Under these conditions, the Amrhein and Suarez (1988) model suggests that the overall rate of dissolution as measured in our experiments in organic acid solution, Rh, can be treated as the sum of the proton-promoted rate as given by Eqn. 1 and the ligandpromoted rate, RL: Rb = RH + RL
-12
-11
-10
-9
-8
-7
Log Measured Rate (mole Si/m*/s) FIG. 5. A comparison of measured rate vs. predicted rate using Eqn. 5. Data are from our work (- 0 - ; this study; Welch, 1991; Welch and Ullman, 1993). The other flow-through data (0) are
from Oxbnrgh et al. ( 1994), Chou and Wollast ( 1984, 1985), and Brady and Walther (1989). Batch dissolution data (V) are from Holdren and Speyer ( 1987) and Casey et al. ( 1991). The dashed line shown (---) is the line describing a one order of magnitude reduction in the rate of dissolution predicted by our experiments. The results of most reported experiments are within an order of magnitude of our predictions (see text).
(6)
In principal, therefore, the impact of the ligand-promoted dissolution alone can be determined by calculating the difference between the measured rate of dissolution in the organiccontaining acidic solutions and in solutions of the same pH but containing no ligands. Rb and RH have very similar functional dependence on pH due to the dominance of the proton-promoted mechanism of dissolution in mixed acid/ligand solutions (Figs. 1 and 2). Equations 1 and 2 can therefore be used to model mineral dissolution rates in the acidic oxalate solutions as well. In these experiments, nh (where the superscripts and subscripts indicate that dissolution has both a proton and oxalate dependence) ranges from 0.59 for bytownite to 0.22 for quartz (Table 2; Fig. 3). The more Na,Si-rich feldspars do not show a significant change in surface proton activity when organic ligands are present (as reflected in the similarity of nH and nkj. However, n for bytownite decreases from 0.86 ( =nH) in inorganic solutions to 0.59 (=nkJ in oxalate solu-
S. A. Welch and W. J. Unman
2946
tions, implying that proton activity on the bytownite surface can be affected by variables other than the proton activity in solution. In our experiments, surface proton activity also changes for quartz dissolved in oxalate (nb = 0.22) compared to dissolution in inorganic solutions ( nH - 0). Bennett et al. (1988) had nearly the opposite results for quartz dissolved in batch reactors. Oxalate enhanced quartz dissolution at neutral pH, however, quartz dissolution rate was lower in acidic oxalate solutions (pH 3) than in acidic inorganic solutions. They also observed an increase in quartz dissolution rates with increasing solution pH from 3 to 7. It is not clear why there is a discrepancy between our results and those of Bennett et al. ( 1988). Our experiments lasted a short time, lOO- 150 hours, and we may not attain steadystate rates in that time even though Si release from minerals is fairly constant. Other factors which were not controlled in our experiments, such as ionic strength and temperature, could also have affected dissolution rates. Table 2 shows that in all but the extremes of tectosilicate compositions (bytownite and quartz) the pH dependence on dissolution rate (n) is indistinguishable between the organic and the inorganic experiments (n = nH = n”,, For these cases the total dissolution rate can be written as Rh = kba’A+ = k,a’;-
+ R’-
(7)
or solving for RL: RL = (kb - k,)ais
(8)
In a previous papers concerning bytownite dissolution in oxalate and gluconate solutions, we have shown that k”, is a linear function of ligand concentration (Welch and Ullman, 1992; Vandevivere et al., 1994). Thus, a more complete kinetic formulation might be given by RL = (K~[L]
-
k,)as*
(9)
where [L] is the concentration of ligand in solution. This model and the results of the experiments on which it is based suggest that there is a competition between the ligand-promoted and proton-promoted dissolution mechanism of tectosilicate dissolution. The competition may be the result of simultaneous ligand and proton attack at the same mineral surface sites, possibly involving a protonated ligand (Welch and Ullman, 1992). Consistent with the hypothesis that proton attack occurs primarily at Al sites at the mineral surface, ligand or protonated-ligand attack is assumed to occur at the Al sites. Si release from the mineral framework is then largely the result of the processes occurring at the adjacent Al sites. This conceptual model is consistent with the evidence that dissolution rates, as determined by Si release, are a function of the Al concentration of the mineral. The results of the quartz dissolution experiments indicate that a direct attack at Si sites by the organic ligands leading to Si release to solution is also possible but may be too small to be reliably detectable in our experiments. For oxalate ion, RL,(Si) ~10-‘“.23 moles/m2/s
(10)
where the superscripts and subscripts indicate that this term may be dependent on proton and ligand concentrations as
well. A complete expression of silica release from tectosilicate dissolution should include this term as well: RL = (K~[L]
-k,)ah+
+ Rb(Si)
(11)
It is possible lR”,(Si) is substantially smaller than indicated in Eqn. 10, in part, because competition between Al and Si sites for protons and ligands may favor Al (as it does in solution), and thus minimize the direct attack at Si surface sites in aluminous tectosilicates. Although it is possible on the basis of the experiments reported here to estimate a value for kh for oxalate, substantially more experiments are needed over a range of oxalate concentrations and mineral compositions to estimate this value with confidence. Three possible explanations exist for the apparent difference in the pH dependence in the rates with and without organic ligands in the bytownite experiments. All three explanations suggest that the rate of silica release in acidic organic solutions should have been higher than observed. The first possibility is that oxalate irreversibly adsorbs to the surface under the experimental conditions, thus reducing the number of sites available for proton and ligand attack. Oxalate can also react with Ca from the mineral to form stable calcium oxalate phases (Graustein, 1977). Under these conditions there may be transport inhibition and/or a reduction in available oxalate concentration and therefore an apparent reduction in oxalate-promoted dissolution rate. Although we cannot document dissolution inhibition in the presence of oxalate, we have observed dissolution inhibition in the presence of organic polymers (Welch and Vandevivere, 1995) and, in some of our experiments using oxalate, we have observed a decrease in Ca concentration in the reactor effluent compared to similar experiment without oxalate (Welch, 1991). The second possibility is that secondary aluminum silicates may have precipitated, removing Si from solution and yielding only minimum estimates of the primary dissolution rate. Concentrations of dissolved Al and Si in the acidic organic experiments are approximately 0.5 mM, close to the equilibrium saturation levels for several aluminosilicate clay minerals in the absence of complexing ligands. No secondary precipitates were observed in photomicrographs of the reaction products of these experiments (Welch, 1991; Welch and Ullman, 1993) although the SEM method may not be sensitive enough to detect precipitates unless they are widespread across the mineral surfaces. The third possibility is that there is additional transport limitation at high surface reaction rates. Blum and Lasaga (1988) noted that transport limitation at low pH and high rates of dissolution leads to a deviation from the linear dependence of dissolution rate on pH. It may be that our inclusion of high rate measurements in the data used to calculate the pH dependence has lead to the artifactual determination of nb and kh for this mineral. Thus, the calculated values of n under these circumstances would reflect minimum estimates of the true dependence of the primary rate of dissolution on pH. If the rate at pH 6 is assumed to be correct and the pH dependence of rate is assumed to be the same in the organic acid experiments as in the inorganic experiment, then log tin would be = -4.5 and the form of the expression for Rh would be the same as for the other minerals. With
Dissolution kinetics of feldspar
the present data, there is no way to verify these estimates of kb and nb for bytownite. Our model, like that of Amrhein and Suarez ( 1988), describes the dissolution rate of feldspars in acidic oxalate solutions as the linear sum of the effects of proton-promoted dissolution and ligand-promoted dissolution. In contrast to the Amrhein and Suarez ( 1988) model which assumes that ligand-promoted dissolution occurs only at Al sites and proton-promoted dissolution occurs at Si sites, our model implies that both ligands and protons attack principally at Al sites. Amrhein and Suarez (1988) relied heavily in their modeling on the conclusions of Helgeson et al. (1984) who concluded, in contrast to our observations, that inorganic weathering rates of all feldspars are essentially constant over a wide range of pH and feldspar composition. Our modified model, which includes the possible interactions between ligands and protons at a single site at the mineral surface (Eqn. lo), appears to be a better description of our experimental results. Some additional support for the contention that ligands, such as oxalate, attack silicate mineral surfaces primarily at the same Al sites where protons attack comes from the comparison of the impact of catechol solutions on dissolution with that of oxalate. Catechol, which strongly complexes both Al and Si in solution and presumably at surfaces, uniformly increases tectosilicate dissolution across the whole range of compositions. In contrast, oxalate, which strongly complexes Al but only weakly complexes Si in solution and presumably at surface, has a significantly greater impact on the dissolution of the Ca,Al-rich endmembers of the tectosilicate family than on the Si-rich endmembers. 5. CONCLUSIONS Tectosilicate dissolution rates into inorganic and organic solutions depend on many mineralogical and solution variables. Dissolution rates are generally consistent with the Goldich weathering sequence: Ca,Al-rich feldspars weather faster than Na,Si-rich feldspars which weather faster than quartz. The pH dependence on dissolution rate, nH, increases with increasing Al content of the mineral, indicating that the Al site is preferentially attacked by protons. The dissolution rates of all feldspars increased as solution acidity increased in the acid range, although proton concentration had no effect on quartz in these experiments. Organic acid anions increase the dissolution rates of all the minerals compared to inorganic acid solutions, but have a much larger effect on more aluminum-rich minerals due to the apparent preferential adsorption at Al-sites and resulting ligand-promoted dissolution. Organic acids also enhance quartz dissolution rates, indicating that there is some small Si-organic interaction at the mineral surface as well. The Al-organic interaction is probably more important over the wide range of aluminum silicates. The dissolution models proposed here suggest that the detachment of the structural framework ion, Si, can be due to proton attack, ligand attack, or simultaneous protons and ligands attack at the same Al sites. Our results indicate that these contributions may be of varying importance depending on mineral composition, the type and concentration of ligands, and the total acidity.
2941
Acknowledgments-This work was performed with principal support from the Subsurface Science Program, U.S. Department of Energy. The manuscript was initially prepared while W.J.U. was a chercheur associe at the Centre de Geochimie de la Surface (CNRS) in Strasbourg, France. We thank PC. Bennett, R.C. Surdam, G.R. Holdren, K.A. Traexler and an anonymous reviewer for their comments and suggestions for the improvement of this manuscript. Editorial handling: G. R. Holdren Jr. REFERENCES Amrhein C. and Suarez D. L. ( 1988) The use of a surface complexation model to describe the kinetics of ligand-promoted dissolution of anorthite. Geochim. Cosmochim. Acta 52,2185-2793. Amrhein C.,and Suarez D.,L. (1992) Some factors affecting the dissolution kinetics of anorthite at 25°C. Geochim. Cosmochim. Acta 56, 1815-1826. Baumann V. H. ( 1960) Reaktion der Kieselsaure mit o-Diphenolen. Beitr. zur Silikose-Forschung. 4, 43-50. Baumann V. H. (1963) Reaktion der Kieselsaure mit polymeren o-Diphenolen in Wasser. Kolloid-Z. Z. Polym. 188, 23-28. Bennett P. C. (1991) Quartz dissolution in organic-rich aqueous systems. Geochim. Cosmochim. Acta 55, 1781- 1797. Bennett P. C. and Siegel D. I. (1987) Increased solubility of quartz in water due to complexation by dissolved organic compounds. Nature 326, 684-687. Bennett P. C., Melter M. E., Siegel D. I., and Hassett J. P. (1988) The dissolution of quartz in dilute aqueous solutions of organic acids at 25°C. Geochim. Cosmochim. Acta 52. 1521-1530. Bemer R. A., Lasaga A. C., and Garrels R. M. ( 1983) The carbonatesilicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years. Amer. J. Sci. 283, 641683. Blum A. and Lasaga A. (1988) Role of surface speciation in the low temperature dissolution of minerals. Nature 331, 43 l-433. Brady P. V. ( 1991) The effect of silicate weathering on global temperature and atmospheric CO,. J. Geophys. Rex 96, 1810118106. Brady P. V., and Walther J. V. ( 1989) Controls on silicate dissolution rates in neutral and basic pH solutions at 25°C. Geochim. Cosmochim. Acta 53, 2823-2830. Burch T. E., Nagy K. L., and Lasaga A. C. ( 1993) Free energy dependence of albite dissolution kinetics at 80°C pH 8.8. Chem. Geol. 105,137-162. Carroll-Webb S. A. and Walther J. V. (1988) A surface-complex reaction-model for the pH-dependence of corundum and kaolinite dissolution rates. Geochim. Cosmochim. Acta 52, 2609-2623. Casey W. H., Westrich H. R., and Arnold G. W. (1988) Surface chemistry of labradorite feldspar reacted with aqueous solutions at pH = 2,3, and 12. Geochim. Cosmochim. Acta 52,2795-2807. Casey W. H., Westrich H. R., and Holdren G. R. ( 1991) Dissolution rates of plagioclase at pH = 2 and 3. Amer. Mineral. 76, 211217. Chou L. and Wollast R. ( 1984) Study of the weathering of albite at room temperature and pressure with a fluidized bed reactor. Geochim. Cosmochim. Acta 48, 2205-2217. Chou L. and Wollast R. ( 1985) Steady-state kinetics and dissolution mechanisms of albite. Amer. J. Sci. 285, 963-993. Dove P. M. and Crerar D. A. (1990) Kinetics of quartz dissolution in electrolyte solutions using a hydrothermal mixed-flow reactor. Geochim. Cosmochim. Acta 54, 955-969. Fein J. B. and Hestrin J. E. ( 1994) Experimental studies of oxalate complexation at 80°C: Gibbsite, amorphous silica, and quartz solubilities in oxalate-bearing solutions. Geochim. Cosmochim. Acta 58,4817-4829. Franklin S. P., Hajash A., Jr., Dewers T. A., and Tieh T. T (1994) The role of carboxylic acids in albite and quartz dissolution: An experimental study under diagenetic conditions. Geochim. Cosmochim. Acta 58,4259-4279. Furrer G.and Stumm W. (1986) The coordination chemistry of weathering: I. Dissolution kinetics of 6-A120, and BeO. Geochim. Cosmochim. Acta 50, 1847-1860.
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