The mechanism, rates and consequences of basaltic glass dissolution: I. An experimental study of the dissolution rates of basaltic glass as a function of aqueous Al, Si and oxalic acid concentration at 25°C and pH = 3 and 11

Geochimica et Cosmochimica Acta, Vol. 65, No. 21, pp. 3671–3681, 2001 Copyright © 2001 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/01 $20.00 ⫹ .00

Pergamon

PII S0016-7037(01)00664-0

The mechanism, rates and consequences of basaltic glass dissolution: I. An experimental study of the dissolution rates of basaltic glass as a function of aqueous Al, Si and oxalic acid concentration at 25°C and pH ⴝ 3 and 11. ERIC H. OELKERS1,* and SIGURDUR R. GISLASON2 1

Ge´ochimie: Transferts et Me´canismes, CNRS/URM 5563–Universite´ Paul Sabatier, 38 rue des Trente-six Ponts, 31400 Toulouse, France 2 Science Institute, University of Iceland, Dunhagi 3, 107 Reykjavik, Iceland (Received October 19, 2000; accepted in revised form March 19, 2001)

Abstract—Steady state basaltic glass dissolution rates were measured as a function of aqueous aluminum, silica, and oxalic acid concentration at 25° C and pH 3 and 11. All rates were measured in mixed flow reactors, performed in solutions that were strongly undersaturated with respect to hydrous basaltic glass, and exhibited stoichiometric Si versus Al release. Rates are independent of aqueous silica activity, but decrease with increasing aqueous aluminum activity at both acidic and basic conditions. Increasing oxalic acid concentration increased basaltic glass dissolution rates at pH 3, but had little affect at pH 11. All measured rates can be described within experimental uncertainty using

冉 冊

rⴝk

aH3 ⫹ aAl ⫹3

0.35

where r signifies the surface area normalized basaltic glass steady state dissolution rate, k refers to a rate constant equal to 10⫺11.65 (mol of Si)/cm2/s, and ai represents the activity of the subscripted aqueous species. The observation that all rates obtained in the present study can be described by a single regression equation supports strongly the likelihood that basaltic glass dissolution is controlled by a single mechanism at both acidic and basic pH and in both the presence and absence of organic acids. Taking account of the dissolution mechanisms of similarly structured and compositioned minerals, and previously published studies of basaltic glass dissolution behavior, basaltic glass dissolution likely proceeds via 1) the relatively rapid and essentially complete removal of univalent and divalent cations from the near surface; 2) aluminum releasing exchange reactions between three aqueous H⫹ and Al in the basaltic glass structure; followed by 3) the relatively slow detachment of partially liberated silica. The breaking of Al-O bonds does not destroy the glass framework; it only partially liberates the silica tetrahedral chains by removing adjoining Al atoms. Basaltic glass dissolution rates are proportional to the concentration of partially detached framework Si tetrahedra near the surface, which is linked through the law of mass action for the Al/proton exchange reaction to aqueous aluminum activity. Copyright © 2001 Elsevier Science Ltd creases gently with increasing pH at basic conditions. The minimum rate is found at ⬃pH ⫽ 6 at low temperatures, but this minimum moves to lower pH with increasing temperature. Berger et al. (1994) measured basaltic glass dissolution rates as a function of aqueous Si concentration at temperatures from 150 to 300°C. Daux et al. (1997) reported that basaltic glass dissolution rates at 90°C and pH⬃8 approach zero as the chemical affinity of the overall Al, Fe, Si-oxide network hydrolysis reaction approaches zero. The overall goal of this manuscript series is to 1) generate a consistent mechanism and set of predictive equations describing both long and short term basaltic glass dissolution rates as a function of temperature and solution composition including pH, organic acid concentration, and saturation state of the basaltic glass; and 2) apply these equations to the improved understanding of the extent and consequences of basaltic glass dissolution in natural systems. Towards this goal, basaltic glass steady state dissolution rates were measured in the present study as a function of aqueous Si, Al, and organic acid concentration at pH⫽3 and 11. These data are used to establish the basaltic glass dissolution mechanism, and to generate a transition state theory based ‘far from equilibrium’ rate equation. The

1. INTRODUCTION

Because of its widespread occurrence on the ocean floor and in volcanic terrains, its emission during volcanic eruptions, and its relatively rapid dissolution rate, basaltic glass plays a major role in the global flux and cycling of numerous metals and nutrients. Moreover, basaltic glass is viewed as a natural analog for various radioactive waste forms (e.g., Byers et al., 1985; Lutze et al., 1985). Consequently, significant efforts have been focused on understanding the dissolution rates and alteration mechanisms of this solid (Furnes, 1975; Crovisier et al., 1983, 1985, 1987, 1989a, 1989b; Malow and Lutze, 1984; Grambow, 1985; Grambow et al., 1985; Gı´slason and Eugster 1987a, 1987b; Berger et al., 1987, 1988, 1994; Guy and Schott, 1989; Zhou and Fyfe, 1989; Ghiara et al., 1993; Gı´slason et al., 1993; Staudigel and Hart, 1983; Daux et al., 1997; Oelkers et al., 1999). Guy and Schott (1989) observed that the pH dependence of basaltic glass dissolution rates at 50 to 200°C mimics that of aluminum (hydr)oxide mineral solubility; it exhibits a sharp increase with decreasing pH at acidic conditions, but it in*Author to whom correspondence should be addressed (oelkers@ lmtg.ups-tlse.fr). 3671

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E. H. Oelkers and S. R. Gislason

purpose of this communication is to report the results of this combined experimental/theoretical study. 2. THEORETICAL BACKGROUND

The standard state adopted in this study is that of unit activity for pure minerals and H2O at any temperature and pressure. For aqueous species other than H2O, the standard state is unit activity of the species in a hypothetical 1 molal solution referenced to infinite dilution at any temperature and pressure. All aqueous activities and chemical affinities in the present study were generated using EQ3NR (Wolery, 1983). Equilibrium constants used in these calculations were taken from SUPCRT92 (Johnson et al., 1992) for all species and minerals other than for those aqueous metal organic complexes given in Table 1 of Oelkers and Schott (1998) and for hydrated basaltic glass (see Daux et al., 1997). The equilibrium constant for hydrated basaltic glass hydrolysis, consistent with the reaction SiAl0.36O2(OH)1.08 ⫹ 1.08H⫹ ⫽ SiO2 ⫹ 0.36 Al3⫹ ⫹ 1.08 H2O

(1)

was estimated from the stoichiometrically weighted sum of the amorphous silica and gibbsite hydrolysis reactions (Bourcier et al., 1990). The logarithm of the equilibrium constant of reaction 1 at 25°C obtained from this estimate is 0.079. The initial dissolution of fresh basaltic glass is characterized by the rapid preferential removal of alkali and alkaline-earth metals, commonly referred to as modifying elements from the glass surface through metal/proton exchange reactions (Thomassin and Touray, 1979; Berger et al., 1987; Guy and Schott, 1989; Crovisier et al., 1990). These exchange reactions have been confirmed by hydrogen depth profiling and XPS analyses reported by Schott (1990). These reactions lead to a leached surface layer that is strongly depleted in these modifying elements, consisting of a partially detached framework strongly enriched in the glass forming elements, Si, Al, and Fe(III). The size of this leached surface layer grows with time until the diffusion rate of the glass modifying elements through the leached surface layer is equal to dissolution rate of the leached layer itself. The long term dissolution is then stoichiometric (Gı´slason and Eugster, 1987a; Crovisier et al., 1992). The quantity of time required to attain stoichiometric basaltic glass dissolution is rapid; Guy and Schott (1989) and Crovisier et al. (1990) reported that basaltic glass dissolution reached stoichiometric dissolution within several hours at neutral to alkaline conditions. Similar results were observed at 25°C at both acid and basic pH by Gı´slason and Oelkers (2001b). The overall long term basaltic glass dissolution rate is equal to either the rate of destruction of the Si/Al/Fe(III) rich leached layer or the diffusion rate of glass modifying cations from within the glass. It follows from the discussion described above that the overall long term steady state dissolution rate of basaltic glass is equal to that of the destruction of its Si/Al/Fe(III) rich leached layer. This molar ratio of the metals comprising this leached layer is consistent with that of the fresh basaltic glass such that it has the ratio 1:0.36:0.02 for Si:Al:Fe(III), respectively. These metals are generally found in tetrahedral coordination. The role of Fe(III) on basaltic glass dissolution is not considered in the present study because of its low concentration

in the leached layer and difficulties controlling Fe(III) concentration in solutions containing aqueous Fe(II) originating from glass dissolution. Insight into the dissolution mechanism of this tetrahedrally coordinated leached layer can be obtained by considering the dissolution mechanism of the feldspar minerals, whose frameworks are comprised of Si and Al tetrahedra; the Si to Al ratio of the feldspar framework varies from 3 for the alkali feldspars to 1 for anorthite. According to Oelkers et al. (1994), Gautier et al. (1994), and Oelkers and Schott (1995) the fastest pathway for the destruction of the alkali-feldspar framework, and thus its dissolution mechanism consists of two steps: 1) the removal of a small fraction of the aluminum from the near surface by Al-proton exchange reactions followed by 2) the removal of those Si adjoining the exchanged aluminum sites. The origin of this mechanism is the relatively fast rate of breaking tetrahedral Al-O bonds versus tetrahedral Si-O bonds; Al can be removed from this framework via Al-proton exchange reactions leading to the partial liberation of adjoining Si tetrehedra. As partially detached Si tetrehedra are more reactive than more fully attached Si tetrehedra (Gautier et al., 2001), this two step pathway is faster than the direct removal of Si tetrehedra before the removal of adjoining Al atoms. This mechanism is also supported by the relative dissolution rates of the feldspars as a function of their Al content; the higher the Al content of a feldspar, the higher its dissolution rate (cf. Casey et al., 1991). Similarly, natural glass dissolution rates appear to increase with their Al content; for example basaltic glass dissolution tend to be two or more orders of magnitude faster than that of rhyolite glass (Techer, 1999). Al-proton exchange reaction on the alkali-feldspar surface can be expressed as Al⬍ ⫹ 3H⫹ ⫽ H3⬍ ⫹ Al3⫹

(2)

where the symbol ⬍ indicates that the metal or proton is attached to the solid surface. The law of mass action for reaction (2) can be expressed as K2 ⫽

a Al3⫹X H3⬍

(3)

a H3 ⫹X Al⬍

where ai stands for the activity of the subscripted aqueous species, Xi represents the mole fraction of the indicated species at the mineral surface, and K2 designates the equilibrium constant for reaction (2). Assuming that aluminum sites contain either the original metal or protons requires 1 ⫽ X H3⬍ ⫹ X Al⬍

(4)

which can be combined with Eqn. 3 to yield X H3⬍ ⫽ K 2

冉 冊冒冉 冉 冊冊 a H3 ⫹ a Al3⫹

1⫹K 2

a H3 ⫹ a Al3⫹

.

(5)

In accord with transition state theory, the far from equilibrium dissolution rate is controlled by the fastest pathway available for destroying the slowest breaking bond essential for the mineral structure. The rate of breaking this bond can in many cases be considered proportional to the concentration of a surface species containing this bond. This ‘rate controlling’ surface species is commonly referred to as a precursor complex (see Wieland et al., 1988; Oelkers et al., 1994); the relation

Basaltic glass dissolution rates

3673

between its concentration and the far from equilibrium dissolution rate can be expressed as r ⫹ ⫽ k ⫹X P• ,

(6)

where r⫹ designates the surface area normalized far from equilibrium dissolution rate and k⫹ refers to a rate constant consistent with the P• precursor complex. This precursor complex is assumed to be in equilibrium with the dissolving glass and therefore the variation of its concentration with aqueous solution composition can be deduced from the law of mass action for the reaction forming this complex. Assuming that partially detached Si tetrahedra formed by reaction (2) constitute the rate controlling precursor complex, Eqn. 5 and (6) can be combined to give r⫹ ⫽ k

冉冉 冊冒冉 冉 冊冊冊 a H3 ⫹ a Al3⫹

1⫹K 2

a H3 ⴙ a Al3⫹

1/n

(7)

where k ⬅ kⴙK1/n 2 , and n corresponds to a stoichiometric coefficient equal to the number of moles of Si rich precursor complex formed by each mole of aluminum atoms removed via exchange reaction (2). Taking account of both its structure and variation of dissolution rates as a function of solution composition, Oelkers et al. (1994) and Gautier et al. (1994) concluded that for the case of alkali-feldspars, three Si rich precursor complexes are formed from the removal of each aluminum and thus n ⫽ 3. 3⫹ 3 ⫹/a The form of Eqn. 7 is such that when K2(aH Al ) is substantially less than 1, there is a significant Al concentration at the near surface, the relative number of surface precursor complexes is low, and dissolution rate will depend on both pH and aqueous Al3⫹activity consistent with r⫹ ⫽ k

冉 冊 a H3 ⫹ a Al3⫹

1/n

(8)

3⫹ 3 ⫹/a In contrast, when K2(aH Al ) is substantially greater than 1, essentially all Al is removed from the near surface, the surface precursor complex concentration maximizes, and dissolution rates become independent of both pH and aqueous Al3⫹ activity consistent with

r⫹ ⫽ k .

(9)

It is only at these conditions where a true dissolution plateau is found (see Lasaga et al., 1994; Oelkers, 1996). For the case of the alkali-feldspars, rates have been measured as a function of aqueous aluminum and/or chemical affinity over the temperature range 25 to 150°C from pH 2 to 9, and aqueous Al3⫹ activities ranging from 10⫺4 to 10⫺25 (Chou and Wollast, 1984, 1985; Knauss and Wolery, 1986; Oelkers et al., 1994; Gautier et al., 1994; Oelkers and Schott, 1998; Oelkers, 2002). All measured rates are consistent with Eqn. 8; a true dissolution plateau has yet to be attained. The observations summarized above suggest that the basaltic glass dissolution mechanism is consistent with that illustrated in Figure 1. Basaltic glass dissolution begins by the exchange of monovalent and divalent cations via metal-proton exchange reactions. These exchange reactions lead to formation of a leached framework strongly enriched in the glass forming elements, Si, Al, and Fe(III). This framework is destroyed by a

Fig. 1. Schematic illustration of the basaltic glass dissolution mechanism consisting of a) the removal of univarient and divalent cations from the glass surface via proton exchange reactions, b) the partial removal of Al from the glass framework via proton exchange reactions, and c) the final liberation of Si (see text).

two step process, a partial removal of aluminum atoms by Al-proton exchange reactions followed by the liberation of partially detached Si tetrahedrals. By analogy with the similarly structured alkali-feldspar aluminosilicate framework, it seems likely that its dissolution rates are consistent with Eqn. 8. The logarithmic analog of Eqn. 8 can be written log r ⫹ ⫽ log k ⫹ 1/n log

冉 冊 a H3 ⴙ a Al⫹3

(10)

Eqn. 10 suggests that constant pH far from equilibrium basaltic glass dissolution rates will be independent of aqueous Si activity, but decrease with increasing aqueous Al activity (e.g., increasing aqueous Al concentration or decreasing aqueous Al complex formation). The degree to which basaltic glass dissolution rates are consistent with the mechanism illustrated in Figure 1 and Eqn. 8 and 10 are assessed below through the interpretation of measured dissolution rates as a function of aqueous Si, Al, and oxalate ion concentration at pH 3 and 11.

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E. H. Oelkers and S. R. Gislason

Table 1. Composition of the Basaltic Glass Used in the Experiments and Comparison with MORB Basalts. All Compositions, Other than those in Column 2, are Given in Weight Percent. Stapafell Icelanda Oxide weight % SiO2 TiO2 Al2O3 Fe2O3 FeO Fe (tot) MnO MgO CaO Na2O K2O P2O5

48.12 1.564 14.62 1.11 9.82 0.191 9.08 11.84 1.97 0.29 0.195

Si Ti Al Fe(III) Fe(II) Fe (tot) Mn Mg Ca Na K P

Stapafell Icelanda Weight %

Stapafell Icelandb Weight %

Oceanic crustc Weight %

MORBd Weight %

22.493 0.938 7.738 0.776 7.633 8.410 0.148 5.476 8.462 1.461 0.241 0.085

22.994 0.679 7.889 1.772 7.037 7.923 0.130 5.134 9.696 1.386 0.120 0.078

23.091 0.839 8.150 1.888 5.908 7.796 0.232 4.583 8.934 1.929 0.249 0.087

23.300 0.851 8.100 7.970 0.145 4.830 8.210 1.930 0.085 0.071

a) Sample from the Stapafell mountain used in the experiments b) Chemical analyses of rock from the Stapafell mountain (Niels Oskarsson personal communication). c) Ronov and Yaroshevsky (1976). d) GERM (2000)

Although the mechanism described above is written in terms of those aqueous species that dominate acidic solutions, the mechanism and equations described above may be equally applicable at basic conditions. Within this mechanism, dissolution is presumed to proceed via a series of exchange reactions followed by the detachment of partially detached Si tetrehedra. Each of the exchange reactions is sufficiently faster than the final irreversible detachment of Si that they reach equilibrium at the mineral surface. Although the exact mechanism through which each of the exchange reactions proceeds may change with solution pH, equilibrium between a metal in solution and this metal attached to the surface can still be described using a single chemical reaction such as reaction (2). Note that by combining reaction (2) with a reaction describing the formation of the Al(OH)⫺ 4 aqueous complex given by Al3⫹ ⫹ 3H2O ⫹ OH⫺ ⫽ Al(OH)4⫺ ⫹ 3H⫹ ,

(11)

Al⬍ ⫹ 3H2O ⫹ H⫺ ⫽ H3⬍ ⫹ Al(OH)4⫺

(12)

yields

which is the equivalent of Eqn. 2 written in terms of those aqueous species which dominate at high pH. Because reaction (11) involves only aqueous species it will be at equilibrium. Consequently, the law of mass action for reaction (2) and reaction (12) will yield identical surface species distributions. This is analogous to the solubility of gibbsite, which can be accurately calculated at all pH from the law of mass action of the reaction Gibbsite ⫹ 3H⫹ ⫽ Al3⫹ ⫹ 3H2O

(13)

so long as one takes account of aqueous species distribution. The degree to which the mechanism and equations described above can describe basaltic glass dissolution rates at both high and low pH are determined below. 3. MATERIAL PREPARATION AND EXPERIMENTAL METHODS

Basaltic glass was obtained from the volcanic ash of Stapafell mountain southwestern Iceland. This sample was

mildly ground and dried sieved to obtain the 40 to 120 ␮m size fraction. This fraction was cleaned ultrasonically using first deionized water, then acetone to remove fine particles. The resulting basaltic glass powder was subsequently dried overnight at 110°C. The specific surface area of the cleaned powder was 2300 ⫾ 10% cm2/gm as determined by both krypton and nitrogen adsorption using the 3 point B.E.T. method. The chemical composition of the basaltic glass, determined by Rontgen Florencence at McGill University, is given in Table 1. The data presented in this table indicates this basaltic glass is similar to that of the mean oceanic crust reported by (Ronov and Yaroshevsky, 1976) and of mean ocean ridge basalt MORB (GERM, 2000), and has a chemical formula normalized to one Si consistent with Si Ti0.02 Al0.36 Fe(III)0.02 Fe(II)0.17 Mg0.28 Ca0.26 Na0.08 K0.008 O3.45. All dissolution experiments were performed in mixed flow reactors identical to that used by Oelkers (2001) for forsterite dissolution and by Oelkers and Schott (2001) for enstatite dissolution. Basaltic glass powder dissolution occurred in 250 mL Azlon plastic beakers continuously stirred with floating Teflon stirring bars. These reactors were immersed in a water bath held at a constant temperature ⫾1°C. Fluid was injected into this reactor using a Gilson peristaltic pump, which allows fluid flow rates from 0.01 to 10 gm/min. The solution left the reactor through a 0.45 ␮m filter. No additional filtering was performed on outlet fluid sampled obtained from either reactor before chemical analysis. The inlet fluid during all experiments was stored in compressible, sealed polyethylene containers. Each experimental series consisted of a sequence of experiments performed on a single basaltic glass powder. At the onset of each experimental series, the powder and a quantity of initial inlet fluid were introduced to the reactor. The inlet fluid was passed through the reactor at a constant flow rate until the outlet solution attained a steady state Si concentration. Steady state outlet concentrations were obtained after an elapsed time ranging from 2 h to 2 d, depending on the flow rate. After this steady state was verified with a minimum of three constant Si concentration outlet fluid samples taken obtained over several residence times (defined as the volume of the reactor divided by

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3675

the reactive fluid flow rate), inlet fluid composition, and/or fluid flow rate were changed to the next desired experimental condition. Dissolution experiments were carried out in fluids comprised of demineralized H2O, Merck reagent grade HCl, NaOH, C2H2O4, and AlCl3, and H4SiO4 obtained by the dissolution of amorphous silica for one week at 90° C. pH 11 inlet solutions were bubbled with N2 before their use in an attempt to minimize dissolved CO2 concentration. Aluminum compositions of the inlet and outlet fluids were determined using atomic adsorption spectroscopy (Perkin Elmer Zeeman 5000); silica compositions were measured using the Molbyldate Blue method (Koroleff, 1976). The reproducibility of chemical analyses were ⫾4% for Si and Al concentrations greater than 0.5 and 0.01 ppm, respectively, but on the order of ⫾10% at lower concentrations. Aqueous concentrations of other metals were not analyzed. Outlet fluid pH was measured at 25° C immediately after sampling. Outlet solutions for experiments run at pH ⫽ 3 were undersaturated with respect to all possible secondary phases except for BG99 –3–16 and BG99 –3–17 which were approximately saturated with respect to quartz. Some

Fig. 2. The difference between inlet and outlet solution Al concentration (⌬[Al]) as a function of the corresponding difference between inlet and outlet solution Si concentration (⌬[Si]). The symbols represent measured solution compositions and the solid line corresponds to the Al/Si ratio of the dissolving basaltic glass.

Table 2. Summary of Basaltic Glass Steady State Dissolution Rates Obtained in the Present Study at a Temperature of 25°C near pH 3. Experiment Number

Surface Area (cm2)

Fluid Flow Rate (gm/min)

Outlet pH

Oxalate Conc.a

Inlet Si Conc.a

Outlet Si Conc.a

Inlet Al Conc.a

Outlet Al Conc.a

Dissolution Rateb

⌬Al/⌬Sic

3⫹ LOG(aH /aA13⫹)d

Bg98–6–18 Bg98–6–25 Bg98–6–31 Bg98–8–3 Bg98–8–14 Bg98–8–20 Bg98–1–6 Bg98–1–13 Bg98–1–19 Bg98–7–05 Bg98–7–20 Bg98–7–34 Bg98–7–50 Bg98–7–55 Bg99–2–03 Bg99–2–05 Bg99–2–08 Bg99–2–11 Bg99–2–12 Bg99–2–13 Bg99–2–15 Bg99–2–17 Bg99–3–02 Bg99–3–04 Bg99–3–05 Bg99–3–09 Bg99–3–11 Bg99–3–13 Bg99–3–14 Bg99–3–15 Bg99–3–16 Bg99–3–17 Bg99–4–2 Bg99–4–4 Bg99–4–10

7305 7305 7305 7305 7305 7305 7122 7122 7122 6229 6229 6229 6229 6229 8050 8050 8050 8050 8050 8050 8050 8050 7975 7975 7975 7975 7975 7975 7975 7975 7975 7975 7900 7900 7900

2.66 2.59 2.58 2.61 2.53 2.64 3.53 3.24 3.21 2.53 2.47 2.48 2.44 2.45 0.46 0.47 0.18 0.18 0.35 4.79 0.74 0.16 0.85 0.65 0.86 0.63 0.73 0.37 0.31 0.59 0.50 0.29 0.49 0.49 0.49

3.05 3.08 3.01 3.00 3.04 3.03 3.09 3.10 3.08 3.08 3.18 3.03 3.16 3.00 2.99 2.96 3.11 3.03 3.00 2.98 3.11 3.04 3.05 3.03 2.98 3.00 2.95 2.97 3.08 2.98 3.04 3.00 3.10 2.91 3.03

0 0 0 0 0 0 0 50 0 0 10 0 100 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 4.91 0 0 0 0 0 7.9 7.9 17.5 17.5 0 0 0

1.09 2.06 0.36 2.24 1.48 2.04 1.67 3.76 2.2 1.67 3.31 1.78 4.26 1.86 6.98 2 4.7 6.27 5.87 0.85 3.32 13.6 10.2 7.32 3.57 4.27 2.46 4.27 18.9 16.1 22.6 26.8 6.51 16.9 12.8

37 0 185 0 18.5 0 0 0 0 0 0 0 0 0 0 100 100 50 0 0 0 0 0 0 10 20 75 75 0 0 0 0 0 0 0

39.34 1.3 194 1.42 20.09 0.62 0.75 0.53 1.72 0.87 1.16 0.58 0.91 0.74 6.51 101.2 99.3 53.2 3.52 0.54 2.11 5.69 2.00 2.41 11.9 21.6 74.8 77.2 4.07 2.93 1.24 3.01 3.2 6.51 3.98

66.20 121.73 21.22 133.18 85.40 122.97 137.84 284.65 165.11 113.14 218.67 118.31 278.24 121.68 67.05 19.63 17.22 22.98 42.66 84.28 50.93 45.33 93.97 99.59 63.79 56.44 37.68 33.37 71.47 100.27 53.61 55.49 66.61 172.57 130.67

2.15 0.63 25.00 0.63 1.07 0.30 0.45 0.14 0.78 0.52 0.35 0.33 0.21 0.40 0.93 0.60 ⫺0.15 0.51 0.60 0.64 0.64 0.42 0.38 0.33 0.53 0.37 ⫺0.08 0.52 0.37 0.36 0.24 0.32 0.49 0.39 0.31

⫺5.38 ⫺3.99 ⫺5.96 ⫺3.79 ⫺5.06 ⫺3.52 ⫺3.79 ⫺2.24 ⫺4.12 ⫺3.82 ⫺3.49 ⫺3.49 ⫺1.77 ⫺3.51 ⫺4.42 ⫺5.53 ⫺5.97 ⫺5.46 ⫺4.19 ⫺3.31 ⫺4.29 ⫺4.52 ⫺4.09 ⫺4.11 ⫺4.66 ⫺4.97 ⫺5.36 ⫺5.44 ⫺4.49 ⫺4.05 ⫺3.85 ⫺4.12 ⫺4.45 ⫺4.18 ⫺4.33

a) mol/kg ⫻ 105 b) mol/cm2/sec ⫻1015 c) The difference between the inlet and outlet Al concentration divided by the corresponding Si concentration difference. d) Computed from the aqueous concentrations provided in this table using the EQ3 computer code (Wolery, 1983).

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Fig. 3. Variation of steady state basaltic dissolution rates obtained in oxalic acid free solutions at pH⫽3 and 11 with aqueous silicon concentration ([Si]). The symbols correspond to experimental data reported in Tables 2 and 3 and the error bars correspond to a ⫾0.1 log unit uncertainty, which is consistent with the ⬃⫾20% uncertainty estimated for these data.

experiments performed at pH 11 in Al-rich inlet solutions were supersaturated with respect to gibbsite; calculations performed with the aid of SUPCRT92 (Johnson et al., 1992) indicate that 3 ⫹/ gibbsite is supersaturated for all solutions for which log (aH 3⫹ aAl ⬍ 7.8. The measured aqueous Al/Si concentration ratios in the outlet solutions (see below) suggest that, at most, negligible quantities gibbsite precipitated during any of the experiments. Due to the likely persistence of dissolved CO2 in the pH 11 inlet solutions, all reactive fluids at this pH were likely supersaturated with respect to calcite, magnesite, dolomite, and other carbonate minerals. 4. RESULTS AND DISCUSSION

The difference between inlet and outlet Al concentration for all steady state experiments is depicted as a function of the corresponding change in Si concentration in Figure 2. The solid line in this figure corresponds to the Al/Si ratio of the dissolving basaltic glass. For the most part, steady state basaltic dissolution was close to stoichiometric with respect to Al/Si ratio. Much of the scatter in this figure stems from experiments performed in Al or Si rich inlet solutions for which the uncertainty between inlet and outlet Al or Si concentrations is relatively large.

Fig. 4. Variation of steady state basaltic dissolution rates obtained in oxalic acid free solutions at pH⫽3 and 11 with aqueous aluminum concentration ([Al]). The symbols correspond to experimental data reported in Tables 2 and 3 and the error bars correspond to a ⫾0.1 log unit uncertainty, which is consistent with the ⬃⫾20% uncertainty estimated for these data.

Steady state dissolution rates based on Si release (r) were computed from measured steady state solution compositions using r⫽

⌬m SiF s

(14)

where ⌬mSi stands for the aqueous Si concentration difference between the inlet and outlet fluid, F represents the fluid flow rate, and s denotes the total basaltic glass surface area present in the reactor. Resulting dissolution rates, together with measured inlet and outlet total oxalate, aluminum, and silica concentrations, and solution pH for all experiments are listed in Tables 2 and 3. The surface areas used to calculate these rates were those measured from nitrogen adsorption using the B.E.T. method on the fresh (unreacted) glass powder. Rates reported in this study are, therefore, given in units of moles of Si released per cm2 of initial basaltic glass surface area per second. All experiments were performed in solutions that were strongly undersaturated with respect to hydrated basaltic glass; the chemical affinities for hydrated basaltic glass dissolution, consistent with reaction (1), were in excess of 12 kJ/mol for all

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3677

Table 3. Summary of Basaltic Glass Steady State Dissolution Rates Obtained in the Present Study at a Temperature of 25°C near pH 11. Experiment Number

Surface Area (cm2)

Fluid Flow Rate (gm/ min)

Outlet pH

Oxalate Conc.a

Inlet Si Conc.a

Outlet Si Con c.a

Inlet Al Conc.a

Outlet Al Conc.a

Dissolution Rateb

⌬Al/⌬Sic

3⫹ LOG(aH /aAl3⫹)d

BG5–8 BG5–15 BG5–18 BG7–5 BG7–8 BG7–12 BG7–16 BG7–22 BG8–10 BG8–13 BG8–19 BG8–25 BG8–26 BG8–31 BG8–34 BG8–35 BG9–14 BG9–19 BG9–21 BG9–25

15225 15225 15225 28200 28200 28200 28200 28200 18600 18600 18600 18600 18600 18600 18600 18600 27300 27300 27300 27300

0.95 0.46 0.17 1.03 0.49 0.47 0.49 0.22 0.50 0.25 0.23 0.21 0.18 0.20 0.22 0.22 0.14 0.24 0.44 0.44

10.97 10.97 10.96 10.98 10.98 11.00 10.92 10.97 11.01 11.00 11.03 10.99 11.00 11.02 11.00 10.94 11.06 11.01 10.99 11.04

0 0 0 0 0 0 0 0 10 0 100 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 11.44 0 0 0 0 0 8.39 0 0 0 0 0 0

0.96 1.49 3.04 1.46 0.99 2.45 0.80 16.10 1.71 2.87 2.48 3.26 3.26 11.30 2.95 0.93 5.54 0.80 0.54 1.88

0 0 0 0 10 0 30 0 0 0 0 0 0 0 0 50 0 50 50 0

0.30 0.44 1.12 0.51 10.40 0.84 31.20 1.92 0.64 0.92 0.95 1.24 1.12 0.97 1.04 48.20 1.80 50.84 50.30 0.52

9.92 7.52 5.49 8.89 2.84 6.78 2.33 6.17 7.58 6.40 5.20 6.16 5.35 5.11 5.87 1.85 4.87 1.16 1.45 5.07

0.31 0.30 0.37 0.35 0.40 0.34 1.50 0.41 0.37 0.32 0.38 0.38 0.35 0.33 0.35 ⫺1.94 0.32 1.04 0.56 0.27

⫺6.80 ⫺6.96 ⫺7.34 ⫺7.06 ⫺8.37 ⫺7.33 ⫺8.66 ⫺7.60 ⫺7.25 ⫺7.37 ⫺7.48 ⫺7.47 ⫺7.36 ⫺7.46 ⫺7.43 ⫺8.91 ⫺7.85 ⫺9.15 ⫺9.08 ⫺7.24

a) mol/kg ⫻ 105 b) mol/cm2/sec ⫻ 1015 c) The difference between the inlet and outlet Al concentration divided by the corresponding Si concentration difference. d) Computed from the aqueous concentrations provided in this table using the EQ3 computer code (Wolery, 1983).

experiments. At these conditions the measured dissolution rates (r) are equal to the far from equilibrium dissolution rates (r⫹). To assess the potential affects of surface area evolution during the experiments on computed dissolution rates, experimental conditions (solution composition and fluid flow rate) were repeated during some of the experimental series. The repeated runs produced rates that were within ⬃20% of those originally measured. For example in experimental series BG8, experiments BG8 –13, BG8 –25, BG8 –26, and BG8 –34 were performed on a single powder at essentially identical conditions. Measured steady state rates were 6.4, 6.16, 5.35, and 5.87 ⫻ 10⫺15 mol/cm2/s, respectively, for these four experiments. Measured steady state basaltic glass dissolution rates in oxalate-free solutions at both pH 3 and 11 are illustrated as a function of aqueous silicon and aluminum concentration, respectively, in Figures 3 and 4. These rates exhibit a scattered behavior as a function of aqueous silica concentration in Figure 3. In contrast, these data appear to decrease systematically with increasing aqueous Al concentration as illustrated in Figure 4 at both acidic and basic pH. Measured steady state basaltic glass dissolution rates obtained in Si and Al free inlet solutions at both pH 3 and 11 are illustrated as a function of aqueous oxalic acid concentration in Figure 5. Rates measured in the presence of oxalic acid appear to be faster than those obtained in oxalate-free solutions at pH 3; rates appear to be independent of oxalic acid concentration at pH 11. The degree to which these observations may be due to either the affect of aqueous oxalic acid on aqueous aluminum activity or it adsorption on the basaltic glass surface is discussed below. The variation of the logarithm of all steady state basaltic glass dissolution rates obtained in the present study at pH 3 and 11 with the logarithm of the corresponding reactive fluid ac-

⫹3 3 ⫹/a tivity ratio (aH Al ) are illustrated in Figure 6. At both pH, the logarithm of all rates including those measured in the presence of aqueous oxalic acid, are found to plot as a single function of ⫹3 3 ⫹/a log (aH Al ). The solid curves in Figure 6 represents a least squares fit of the experimental data consistent with Eqn. 10. The coefficient of determination (R2) of these regressions are 0.71 and 0.95, respectively, for pH 3 and pH 11. The slopes of the regression curves are equal to ⫺0.29 with 95% confidence limits of ⫾ 0.06 at pH ⫽ 3 and ⫺0.35 with 95% confidence limits of ⫾ 0.04 at pH ⫽ 11. The intercept of these two regression curves are ⫺11.89 with 95% confidence limits of ⫾ 0.28 for pH 3 and ⫺11.61 with 95% confidence limits of ⫾ 0.32 for pH 11. The slopes and intercepts of the two regression curves in Figure 6 are thus identical within experimental uncertainty. Results of a single regression of all of the data presented in Figure 6 are illustrated in Figure 7. The resulting regression equation is

r ⫽ 10⫺11.64⫾0.13 (mol Si/cm2/sec) s

冉 冊 a H3 ⫹ aAl⫹3

0.35⫾0.02

. (15)

A close correspondence between the regression curve and all the experimentally obtained data points is evident in Figure 7 demonstrating that this single equation describes all measured rates. This observation suggest strongly that the basaltic glass dissolution follows the mechanism described in Figure 1 at both acid and basic conditions and in both the presence and absence of aqueous organic acids. It should also be noted that the slope of this regression curve is consistent with n⫽3 in Eqn. 7, 8, and 10, which is equal to that found for the alkali feldspars by Oelkers et al. (1994) and Gautier et al. (1994). The close correspondence between the symbols and regres-

3678

E. H. Oelkers and S. R. Gislason

Fig. 5. Variation of steady state basaltic dissolution rates obtained from Al and Si-free inlet solutions at pH⫽3 and 11 with aqueous oxalic acid concentration. The symbols correspond to experimental data reported in Tables 2 and 3 and the error bars correspond to a ⫾0.1 log unit uncertainty, which is consistent with the ⬃⫾20% uncertainty estimated for these data.

sion curve in Figure 7 indicates that the affect on rates of aqueous oxalate stems from its affect on aqueous aluminum activity. The fact that aqueous oxalate increases rates in acidic solutions but not in basic solutions can be attributed to the aqueous complexing behavior of this anion. The oxalate anion forms strong aqueous complexes with aqueous aluminum at acid pH, thus its presence significantly decreases aAl⫹3 at this pH. In contrast, there is little aqueous aluminum-oxalate complexation at basic pH, and thus the presence of oxalate negligibly affects aAl⫹3 at these conditions. As it decreases aAl⫹3 at acid pH but negligibly affects aAl⫹3 at basic pH, the presence of oxalate will only increase basaltic glass dissolution rates at acidic conditions. Owing to the success of the transition state theory based rate equation presented in the present study (Eqn. 7 or 9) in describing the affect of oxalate ion on basaltic glass dissolution rates, it seems likely that the affect of other aqueous Al complexing anions, such as other organic acid anions or

Fig. 6. Variation of the logarithm of all steady state basaltic dissolution rates obtained in the present study at pH ⫽ 3 and 11 with log 3⫹ (aH /aAl⫹3). The filled squares and white stars correspond to experimental data obtained in oxalate free and oxalate bearing inlet solutions, respectively, reported in Tables 2 and 3 and the error bars correspond to a ⫾0.1 log unit uncertainty, which is consistent with the ⬃⫾20% uncertainty estimated for these data. The linear curve represents a least squares fit of the data; the equation and coefficient of determination (R2) of this curve are given in the figure.

fluoride, on basaltic glass dissolution rates can be successfully estimated using Eqn. 7 together with aqueous aluminum activities determined from solute speciation calculations such as EQ3NR (Wolery, 1983). The pH dependence of 25°C steady state far from equilibrium basaltic glass dissolution rates at various oxalate and acetate ion concentrations are illustrated in Figure 8, and reported in Table 4. Predicted dissolution rates have a similar behavior to those predicted for albite dissolution by Oelkers and Schott (1998). The affect of the presence of acetate has only a negligible affect on basaltic glass dissolution rates. Its maximum affect is at pH 3 to 6, where the presence of 10⫺3 mol/kg acetate ion increases basaltic glass dissolution rates by less than a factor of 2. In contrast, oxalate anion affects strongly the rates of basaltic glass dissolution at pH 3 to 6; at these conditions, the presence of 10⫺3 mol/kg oxalate ion increases basaltic glass dissolution rates by a factor of 50. There is no

Basaltic glass dissolution rates

3679

Fig. 7. Regression plot of all basaltic glass dissolution rate data obtained in the present study. The filled squares correspond to experimental data reported in Tables 2 and 3 and the error bars correspond to a ⫾0.1 log unit uncertainty, which is consistent with the ⬃⫾20% uncertainty estimated for these data. The linear curve represents a least squares fit of the data; the equation and coefficient of determination (R2) of this curve are given in the figure (see text).

affect on basaltic glass dissolution rates of acetate or oxalate ion at pH⬍2 or pH⬎7. The pH dependence of organic acid basaltic glass dissolution rate enhancement has important consequences in natural systems. The pH of most river waters is between 7 and 8 (Gı´slason et al., 1996; Berner and Berner, 1996); the pH of seawater is 8.2 (Riley and Chester, 1971; GERM, 2000). Unpolluted rainwater has a pH of 5.6, though acid rain can have pH as low as 3 and even 2 in the vicinity of erupting volcanoes (Gı´slason et al., 1996; Berner and Berner, 1996). The pH of waters in organic rich soils typically ranges from 4 to 7 (Berner and Berner, 1996, Gı´slason et al., 1999). Results obtained in the present study suggest that there will be little affect of the presence of organic acids on basaltic glass dissolution rates at the pH of typical river or ocean waters. In contrast, on and in the soil column, where the pH is lower, organic acid basaltic glass dissolution rate enhancement is greatest. It follows that the secretion of organic acids by terrestrial plants will be most efficient in releasing nutrients such as Ca, Mg, K, Fe, and P from basaltic glass at the pH’s typical of terrestrial soil solutions. The more organic acid a given plant and its associated micro-organism is able to secrete, the better it is equipped to compete for basaltic glass derived nutrients. This finding is in concert with microscopic and macroscopic measured enhancement of natural chemical weathering rate of basalt in bare versus vegetated areas. (Gı´slason et al. 1996; Berner and Cochran 1998; Moulton and Berner 1998; Stefansson and Gı´slason 2001). 5. EXPERIMENTAL AND COMPUTATIONAL UNCERTAINTIES

Uncertainties associated with the rate constants generated above arise from a variety of sources, including the measurement of aqueous solution concentrations, fluid flow rates, and basaltic glass powder surface area. The uncertainties in the

Fig. 8. Calculated values of the logarithm of steady state basaltic glass dissolution rates as a function of pH and a total aluminum concentration of 10⫺6 mol/kg in the presence of the indicated molal concentration of a) oxalate ion and b) acetate ion. The curves were generated using Eqn. 7 together with values of aqueous Al⫹3 activities generated using the EQ3NR computer code (Wolery; 1983).

measured values of the total aqueous silica and aluminum concentration are on the order of 4% or less for solutions containing ⬎0.5 ppm Si and 10 ppb Al; at lower concentrations the uncertainties are on the order of 10%. Uncertainty in the measured pH of these solutions are on the order of ⫾0.04 pH units. Uncertainties in fluid flow rate measurements are not more than 2%. In contrast, the uncertainties associated with the B.E.T. surface area measurement of the basaltic powder is ⫾10%. In addition the mineral surface area changed somewhat over the duration of each experiment. To assess the temporal affects of changing basaltic glass surface areas on the resulting dissolution rates, one of the final fluid flow rates for several of the mineral samples of a single fluid composition was set approximately equal to the first. The difference in the resulting fluid concentrations were on the order of 15% or less. A reasonable estimate of the relative uncertainty is the sum of the uncertainties associated with 1) the measurement of fluid compositions; 2) fluid flow rates; and 3) potential changes in basaltic glass reactive surface area during the experiments. Considering the estimates of each of these contributions, the relative uncertainties among basaltic glass dissolution rates reported in the present study are on the order of ⫾20%. The

3680

E. H. Oelkers and S. R. Gislason

Table 4. Computed Logarithms of Surface Area Normalized Far from Equilibrium Basaltic Glass Dissolution Rates for Solutions Containing 10⫺6 mol/kg Total Dissolved Al in the Presence and Absence of Dissolved Oxalate and Acetate. Dissolution Rates are Given in mol/cm2/sec. pH

organic Free

10⫺4 mol/kg oxalate

10⫺3 mol/kg oxalate

10⫺4 mol/kg acetate

10⫺3 mol/kg acetate

1 2 3 4 5 6 7 8 9 10 11

⫺10.21 ⫺11.45 ⫺12.55 ⫺13.60 ⫺14.57 ⫺15.16 ⫺15.07 ⫺14.75 ⫺14.40 ⫺14.14 ⫺13.70

⫺10.21 ⫺11.43 ⫺12.35 ⫺12.99 ⫺13.81 ⫺14.80 ⫺15.07 ⫺14.75 ⫺14.40 ⫺14.14 ⫺13.70

⫺10.21 ⫺11.28 ⫺11.77 ⫺12.11 ⫺12.85 ⫺13.86 ⫺14.86 ⫺14.75 ⫺14.40 ⫺14.14 ⫺13.70

⫺10.21 ⫺11.45 ⫺12.55 ⫺13.59 ⫺14.56 ⫺15.16 ⫺15.07 ⫺14.75 ⫺14.40 ⫺14.14 ⫺13.70

⫺10.21 ⫺11.45 ⫺12.55 ⫺13.53 ⫺14.43 ⫺15.14 ⫺15.07 ⫺14.75 ⫺14.40 ⫺14.14 ⫺13.70

overall uncertainties are somewhat higher, as they will also include the uncertainties associated with the B.E.T. surface area measurement of the basaltic glass powder. Taking this latter contribution into account, the overall uncertainties of the rates reported in this study are on the order of ⫾30%. 6. CONCLUSIONS

The experimental results and their interpretations presented above strongly support the concept that basaltic glass dissolution proceeds by a single mechanism at both low and high pH and in the presence and absence of aqueous organic acid anions. This mechanism likely consists of 1) the relatively rapid and essentially complete removal of univalent and divalent cations from the near surface; 2) aluminum releasing exchange reactions between three aqueous H⫹ and Al in the basaltic glass structure; followed by 3) the relatively slow detachment of partially liberated silica. Basaltic glass dissolution rates are proportional to the concentration of partially detached tetrahedral Si near the glass surface. The variation with solution composition of this rate controlling species, and thus basaltic glass dissolution rates, can be deduced directly from the law of mass action for the aluminum for proton exchange reaction creating this species. The resulting rate expression, parameterized using rate constants generated from laboratory experiments, can be used to predict far from equilibrium dissolution rates as a function of solution composition. It is thus a powerful tool for predicting the consequences of basaltic glass-water interaction in many of natural environments. The dependence of rate parameters with temperature, the temporal evolution of basaltic glass dissolution stoichiometry, the dependence of rates on the chemical affinity of the basaltic glass hydrolysis reaction, and the application of these rates to natural systems will be discussed in detail in subsequent papers in this series (Gı´slason and Oelkers, 2001a, 2001b) Acknowledgments—We would like to thank Jacques Schott, Jean-Louis Dandurand, K. Vala Ragnarsdottir, Carlos Jove´ , Jean-Marie Gauthier, Oleg Pokrovsky, Matthildur B. Stefa´ nsdo´ ttir, Eydis S. Eiriksdottir, Audur Andre´ sdo´ ttir, Andri Stefa´ nsson, Ingvi Gunnarsson, Bjo¨ rn Gudmundsson and Stacey Callahan for helpful discussions during the course of this study. Jocelyne Escalier, Jean-Claude Harrichouri and Jean-Marie Gauthier provided expert technical assistance. Don Voight at Penn State University measured the surface area of the glasses. Brent Grambow provided insightful constructive comments on an earlier version of this manuscript. Financial support from Centre National de

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