Evidence of the Existence of Three Types of Species at the Quartz

Feb 20, 2002 - These data allow proposing a two-step deprotonation model of the quartz surface where two surface oxygen atoms are bonded to one silico...
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J. Phys. Chem. B 2002, 106, 2937-2945

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Evidence of the Existence of Three Types of Species at the Quartz-Aqueous Solution Interface at pH 0-10: XPS Surface Group Quantification and Surface Complexation Modeling Y. Duval,† J. A. Mielczarski,*,† O. S. Pokrovsky,‡ E. Mielczarski,† and J. J. Ehrhardt§ Laboratoire “EnVironnement et Mine´ ralurgie”, UMR 7569 CNRS, INPL-ENSG, B.P. 40, 54501 VandoeuVre-le` s-Nancy, France, Laboratoire de Ge´ ochimie, UPS-CRNS (UMR5563), 314000 Toulouse, France, and Laboratoire de Chimie Physique et Microbiologie pour l’EnVironnement, UMR 9992-CNRS, 54600 Villers-le` s-Nancy, France ReceiVed: July 23, 2001; In Final Form: NoVember 26, 2001

The surface composition of a quartz surface reacted with various aqueous solutions of pH 0-10 was qualitatively and quantitatively evaluated using X-ray photoelectron spectroscopy (XPS). The positions and intensities of the recorded Si 2p and O 1s lines change depending on solution conditions. The O 1s line, where the position varies more significantly, was analyzed in detail showing three components corresponding to three surface species: >SiOH2+, >SiOH0, and >SiO- (where > represents the bulk quartz). The changes in the Si 2p spectra support these findings. The atomic ratio between the surface oxygen and silicon atoms was found to be 1.8. These data allow proposing a two-step deprotonation model of the quartz surface where two surface oxygen atoms are bonded to one silicon surface atom and where the most deprotonated surface sites show the SiO- configuration. Physisorbed water in the amount of around 10% of the monolayer was also found on all samples under spectroscopic investigation. The density of the >SiO- group increases significantly with an increase of pH, whereas the surface concentration of the >SiOH2+ group is the highest at pH 0. The maximum of the neutral >SiOH0 group is observed at pH 6. These results indicate immediately the validity of a 2-pK model of protonation of quartz/electrolyte interface versus a 1-pK model. The 2-pK surface capacitance model of the electric double layer having pK1 ) - 1.0 and pK2 ) 4.0 was derived from the XPS data. The new surface stability constants allow a quantitative description of quartz surface charge and dissolution kinetics in neutral to alkaline solutions and explicitly account for an increase of quartz dissolution rate at pH < 2 due to significant increase of the concentration of the >SiOH2+ surface species. These results provide, for the first time, direct atomic level spectroscopic evidence of the validity of the chemical surface speciation approach, notably the existence of the charged >SiOH2+ and >SiO- species at the quartz surface and their relative densities in acidic and alkaline solutions.

Introduction The natural abundance of quartz and its important technological applications stimulated, both in the past and present times, a collection of vast amounts of experimental information and modeling results aimed at characterization of quartz surface chemistry and reactivity in aqueous solutions. In particular, numerous studies have been devoted to measuring quartz and amorphous silica surface charge and ζ potentials,1-4 dissolution kinetics,5-12 and infrared spectroscopic observations of surface groups.13-18 This allowed extraction of a rigorous picture of the SiO2/H2O interface and formulation of various mechanistic/ thermodynamic models describing quartz reactivity and charging behavior in aqueous solutions19 as well as ab initio calculations of silica interactions with water.20,21 Moreover, because of its relatively simple surface chemistry, quartz is often used as a model compound for testing various physicochemical models for metal oxide/solution interfaces and mineral surfaces (see, for example, refs 22-25). Many applications of quartz and * Corresponding author. E-mail: [email protected]. Fax: (33) 3 83 59 62 55. † INPL-ENSG. ‡ UPS-CRNS (UMR5563). § UMR 9992-CNRS.

silicates depend on their unique surface properties, which rely on the nature and density of surface specific groups. At present there are two principal models applied to describe ionization at oxide-water interfaces. The most common is a 2-pK model that assumes two consecutive protonations:26

>MeOH0 + H+ ) >MeOH2+

(1)

>MeOH0 - H+ ) >MeO-

(2)

Here the intrinsic stability constants are given by

K1 )

{>MeOH2+} {>MeOH0}aH+

K2 )

{>MeO-}aH+ {>MeOH } 0

(

-Ψ0F 2.3RT

(

)

exp

exp

)

-Ψ0F 2.3RT

where {>i} stands for surface species concentration, ai is the activity in solution, Ψ is the electrostatic potential of the surface plane, F is the Faraday constant, R represents the gas constant, and T is the temperature.

10.1021/jp012818s CCC: $22.00 © 2002 American Chemical Society Published on Web 02/20/2002

2938 J. Phys. Chem. B, Vol. 106, No. 11, 2002 There are three primary types of surface species responsible for quartz surface electric charge, dissolution, and adsorption/ desorption properties, and two of them are produced from neutral silanol species >SiOH0, which undergo protonation and deprotonation in acidic and basic solutions. It is not correct to assume these reactions as consecutive ones where both hydrogen ions are bounded to the same single oxygen atom. In this case the difference between the consecutive pK values has to be larger than 10, which is not observed experimentally. Therefore, other surface reactions were proposed27 where two oxygen atoms are bonded to each metal atom:

>[Me(OH2)2]+ ) >Me(OH2)OH + H+ >Me(OH2)OH ) >[Me(OH)2]- + H+ In this case it is considered, as a basic unit of a 2-pK model, two neighboring oxygen atoms coordinated to a different metal center. This assumption explains the small splitting of the two pK values derived from experimental results. As a simpler explanation, a single protonation step, a 1-pK model, can be considered for which a surface reaction can be described:

>[MeOH2] +1/2 ) >[MeOH] -1/2 + H+ As was concluded from theoretical consideration,27 in most cases the 1-pK model provides a sufficiently accurate description of the proton titration of surface groups, whereas the 2-pK model can describe all titration data. The proton titration measurements, similarly as electrokinetic ones, give macroscopic information. They cannot deliver direct information about the real composition and structure of surface groups. Of these three surface species listed for the classical 2-pK model, only the >SiOH0 has been directly observed on quartz or silica surface using spectroscopic (mostly infrared-based) techniques. The other two surface species have been conventionally postulated, as for other simple oxides, to describe the surface ionization and resulting charging behavior. The physical meaning of the >SiOH2+ and >SiO- species has not been yet well established because of lack of any spectroscopic observations of these species.7 In this paper we attempted, for the first time, to relate the XPS spectroscopic observation of the surface atomic composition of quartz after its contact with aqueous solutions of different pHs to the corresponding change in quartz surface speciation and to propose surface complexation model. It should be underlined that spectral evidence for the presence of the three surface groups was found even though the spectroscopic XPS study was performed under high-vacuum conditions and held at these conditions for a few hours. The existence of the >SiOH2+ and >SiO- surface species inferred from this approach allows us to propose the 2-pK model and explain the quartz surface charge and dissolution rate variations with pH both in acid and alkaline solutions. Background Surface Complexation Modeling of Quartz. The acid-base properties of quartz and silica surfaces have been extensively investigated. The intrinsic stability constants of reactions 1 and 2 are fairly well-known. They were inferred from surface charge data obtained by surface titration of solid powders in various electrolyte solutions. It is important to note, however, that most of the available information concerns amorphous silica/solution interfaces and there are only a few titrations2,3,28,29 and electro-

Duval et al. kinetic studies3,4 for quartz. The pK values for quartz proposed in the literature22,30 are in the following ranges: pK1 ) 3.0 ( 1.0 and pK2 ) 7.0 ( 1.0 for reactions 1 and 2, respectively. These values are valid for constant capacitance model (CCM) of the electric double layer (EDL) with a capacitance of 1 F/m2. They allow one to calculate the isoelectric point for quartz, i.e., pH of the solution in which the electrophoretic mobility or streaming potential (ζ potential) of quartz equals zero. The range of pHiep values reported in the literature, from 0.5 to 3.5,26 is in agreement with value 2.0 predicted from the model. As a first approximation, we use this model in our surface speciation calculation for quartz as the CCM is the simplest EDL model. This model was used successfully for a number of various types of solids in aqueous solutions.32 The density of quartz sites depends on the type of crystal plane developed. We used the value 8 sites/nm2, which is the average for 001 and 101 planes and is widely used in surface complexation modeling.22,23 The MINTEQA2 computer program33 was used to calculate the equilibrium species and surface speciation equilibriums in the system SiO2(s)-H2O-HNO3-KOH system. This program combines surface equilibriums, homogeneous solution equilibriums, and mass balance calculation. The set of equations obtained is solved by iteration using the Newton-Raphson method. The activity coefficients of free aqueous ions were calculated by means of the Davies equation. Previous Spectroscopic Studies of Quartz and Silica Surfaces. For over last 50 years, a significant amount of information on the surface chemical groups of quartz and silicawater interface has been collected using IR spectroscopy. Unfortunately, the majority of these spectroscopic data describe transformation surface of OH groups during heating of silica or quartz sample to very high temperature, up to 1000 °C. Several steps of surface dehydration are clearly identified; however, there is no detail information available on the surface group presence in room temperature. The total concentration of OH groups on fully hydroxylated surface of silica does not depend on their origins and specific surface areas being a physicochemical constant: 4.9 OH groups/nm2 as determined by the deuterium-exchange method,17,34 which is also in agreement with tritium exchange results (4.6 sites/nm2 34). NMR studies of amorphous silica powders indicate that approximately 47% of surface hydroxyl groups are geminal (i.e., H-bonded Si(OH)2 groups) and the remaining 53% are single silanol.36 The deprotonation reaction2 has been investigated by Marshall et al.,37 who deduced, in good agreement with SCM, a pK2 ) 7.2 from infrared spectroscopic studies noting OH stretching vibrations induced in the presence of organic compounds (acetone, phenols) of different acidities. To what extend these findings for amorphous silica may serve as a guide to understanding the surface chemistry of quartz is still a matter of debate. For example, in contrast to observations on silica, Gallei and Parks15 reported the existence of two sharp bands (at 3649 and 3627 cm-1) on the quartz surface dried at 110 °C. They assigned these bands to two different OH sites on the surface. However, Koretsky et al.,18 using DRIFT spectroscopy, were not able to detect any surface groups besides terminal silanol groups at a single type of surface site (band at 3745 cm-1). The only spectroscopic evidence for the ionization of silanol groups on quartz surfaces has been recently obtained by Du et al.,38 who reported, using infrared-visible sum-frequency generation, that the water molecules interact with a quartz surface via two opposing forces: hydrogen bonding with silanol groups at neutral to acid pH and electrostatic interaction with charged >SiO- species at pH g 10. However, no quantitative

XPS Surface Group Quantification and Modeling

J. Phys. Chem. B, Vol. 106, No. 11, 2002 2939

TABLE 1: Detected Impurities in Quartz impurity

amt (ppm)

impurity

amt (ppm)

Na+ Li+

SiO- groups is present. At higher pH, after crossing some negative surface charge, the free >SiO- groups attract counterions from solution (K+) and their negativity is reduced. This discussion indicates that the major features predicted from surface speciation model for quartz contacted with aqueous solution can be derived from the recorded XPS spectra. These spectra are recorded in high vacuum at temperature 25 °C. At similar conditions, by the use of the mass spectrometric thermal analysis in conjunction with the temperature-programmed desorption, less than a monolayer of physisorbed water was determined on amorphous silica surface.34 Close inspection of the experimental O 1s spectra obtained for quartz at different pH (Figure 1) does not suggest the presence of a monolayer of physisorbed water as the top layer in ultrahigh vacuum. If the adsorbed water is present in monolayer quantity, we should be able to see an O 1s component at the position about 533.5 eV for all samples under investigation. This suggests that the amount of physisorbed water on quartz surface groups is only a small part of monolayer and the majority of the water molecules are probably adsorbed on charged sites as the >SiOH2+ and >SiO-, which is in agreement with recent results of Du38 et al. and Rudzinski et al..24 The low level of physisorbed water allows determining the density of the surfacecharged groups OH2+ and O- with appropriate precision. Quantitative Approach to the Surface Composition of Quartz Contacted with Aqueous Solutions at Different pH’s. For appropriate quantitative evaluation of the surface species, especially the charged ones, the background subtraction and peak deconvolution have to be performed on the O 1s experimental

Duval et al.

Figure 2. XPS O 1s lines for each pH after subtraction of line A shown in Figure 1.

TABLE 3: Relative Intensities of the Recorded XPS Lines 0 2 4 6 8 10

Si 2p

O 1s

C 1s

K 2p

1.00 1.00 1.00 1.00 1.00 1.00

1.85 1.84 1.86 1.86 1.95 1.85

0.26 0.46 0.34 0.30 0.52 0.53

nd nd nd nd nd 0.01

spectra. At first, the bulk oxygen Si-O-Si component and the oxygen component from the surface OH0 groups, at least those having the same position in the XPS spectra for all quartz samples, should be subtracted. To perform this subtraction the appropriate references are required. Unfortunately, there are no available precise references of the bulk quartz oxygen, as well as of the surface SiOH0 groups. The following procedure was developed to allow a proper subtraction of the strong oxygen signals from bulk and the oxygen of preponderant surface OH0 groups. The experimental O 1s lines for all investigated pH are plotted in Figure 1. The intensities (areas) of the lines vary a little because of some differences in carbon contamination levels (Table 3). With the assumption that the total amount of the available surface sites is constant, the intensities of the oxygen lines were corrected to the same value by the use of the spectrum at pH 0 as the reference. Then the common area for all experimental lines located between the peaks representing results for pH 0 and 8 (Figure 1, line A) was fitted by a Gaussian/ Lorentian peak shape. The peak A that is in the envelope of all experimental lines represents the bulk quartz oxygen and the significant part of the surface oxygen bonded as the surface OH0 groups, corresponding to about 70% of the intensity of each line. This peak was subtracted from the O 1s line for each pH, and the resultant spectra after the subtraction are presented in Figure 2. These lines show a semiisosbestic point that indicates a transformation of at least one type of the surface species to another. The oxygen lines after subtraction (Figure 2) were deconvoluted to three components related to the >SiOH2+, >SiOH0, and >SiO- surface groups having positions at around 533, 532, and 531 eV, respectively. The fitting accuracy was better than 0.998. The exact positions of the three components and their relative intensity ratios are presented in Figures 3 and 4a. It can be observed that the position of the SiOH0 surface groups (the parts which were not subtracted as the common for all samples) shifts gradually to lower BE with an increase of pH. Moreover, a local minimum at pH 8 could be noticed. These observations can be explained by assuming the interaction of the SiO- groups with the SiOH0 groups through the formation of hydrogen bonding. The increase of the amount of the SiO-

XPS Surface Group Quantification and Modeling

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Figure 3. Binding energy of three surface species and their shift depending on solution conditions (Gaussian/Lorentian band shape). Figure 5. Schematic representation of quartz surface structure: (a) atomically smooth surface; (b) real surface of quartz after contact with aqueous solution.

Figure 4. Relative surface density of three surface species obtained; (a) directly from deconvolution of data presented in Figure 2; (b) after correction of the amount of the >SiOH0 species (see text).

groups with an increase of pH (Figure 4a) causes more of the neutral SiOH0 groups to be influenced by the negative SiOgroups. They are exposed to a more electronegative environment resulting in the observed shift of the SiOH0 position. The lowest BE observed at pH 8 agrees with the highest concentration of the SiO- groups. At pH 10 and higher, the potassium ions are adsorbed on the >SiO- sites as counterions lowering the negative charge of the >SiO- groups and in consequence also that of the SiOH0 groups. Similar lowering of BE is observed (Figure 3) for the >SiO- component at pH 8, which supports the proposed explanation. The band assigned to the >SiOH2+ groups shows almost a constant position at 533.2 eV (Figure 3). The intensity of this

band decreases sharply with pH increasing from 0 to 4 (Figure 4a) and then reaches a value around of 8% for higher pH. This observation suggests that the band component at 533.2 eV represents two surface species, i.e., the OH2+ surface groups and adsorbed water. The band position above 533 eV is characteristic for adsorbed water.45 Hence, we can conclude that the quartz under high-vacuum conditions and at room temperature is still covered by nearly 10% of the adsorbed water monolayer. The amount of the > SiOH2+ groups decreases from around 30% at pH 0 to almost zero at pH 5. The amount of the >SiO- groups increases gradually from 8% at pH 0 to 50% at pH 10. The amounts of the >SiOH0 groups presented in Figure 4a are not absolute values because the significant and not precisely known part of the OH0 signal (common for all quartz samples) was subtracted together with the bulk Si-O-Si oxygen intensity. Therefore, to obtain the real surface coverage by the >SiOH0 groups at different pH, a constant value should be added to each value of the >SiOH0 presented in Figure 4a. We assumed the subtraction of the SiOH2+, >SiOH0, and >SiO-. This indicates immediately that the 2-pK surface dissociation model has to be applied to describe the quartz-water interface. Another important observation is that the ratio between the surface oxygen and silicon atoms is close to 2.0. As a result of these spectroscopic studies, a two-step deprotonation model of quartz surface can be proposed (Figure 9) where two surface oxygen

Figure 9. Two-step deprotonation model of quartz surface proposed on the basis of this spectroscopic work. Figure 7. Relative surface density of three surface species obtained by deconvolution of data presented in Figure 1.

It has to be underlined that the experimentally determined amount of the surface SiOH2+ and SiO- groups is nearly equal at pH 2 (Figure 4), which is in a perfect agreement with a value pHPZC reported for quartz.23,26 To verify how the results presented in Figures 3 and 4 are sensitive to modification of deconvolution parameters, the curve fitting was also carried out with a pure Gaussian line shape. In this case, an additional fourth component at about 534 eV has to be added to achieve a good fitting (above 0.998). The obtained results are presented in Figures 6 and 7. They are very similar to those obtained with mixed Gaussian-Lorentian band shape. The fourth component at about 534 eV is assigned to adsorbed water, which represents nearly 10% of monolayer coverage. This is in agreement with the reported strong adsorption of water on >SiO- sites as was proven spectroscopically38 and from calorimetric measurements.24 The second element that can provide information about surface composition of quartz is silicon. The Si 2p lines obtained at different pH’s show a much lower shift with a change of solution pH than that recorded for O 1s lines. Nevertheless, the tendency is identical for silicon and oxygen lines (Figures 8 and 2). The highest BE of the Si line is observed for a sample treated in solution at pH 0, and the lowest BE, for a sample prepared at pH 8. This observation indicates clearly the presence of different surface Si sites, which are influenced by the surface OH2+, OH0, and O- groups. It is important to note that atomic ratio of the total surface oxygen (after subtraction of the bulk Si-O-Si oxygen) to the total surface silicon (after subtraction of the bulk silicon) is almost constant and about 1.8 for all quartz samples prepared in solution at 0-10 pH. This finding supports the surface structure presented in Figure 5b.

atoms are bonded to one silicon surface atom and where the most deprotonated surface sites show the SiO- configuration. This model of surface protonation is a modification of the proposition based on theoretical consideration with the assumption of the two oxygen atoms coordinating metal center.27 A relationship between the predicted, based on the commonly used SCM (pK1 ) 3.0, pK2 ) 7.0), and the measured in this work concentration of the surface species for quartz is plotted in Figure 10. The correlation is positive; i.e., there is a decrease of the >SiOH0 concentration with increasing of pH, which is accompanied by the increase in the >SiO- surface concentrations. In contrast, the concentration of the >SiOH2+ surface species is strongly underestimated as the SCM predicts their amount e0.7% at pH 0. Adsorption of potassium counterions on quartz surface in alkaline solutions can be described within a triple layer model

Figure 10. Correlation between predicted and measured in this work concentration of surface species for quartz. SCM calculation was performed by assuming pK1 ) 3.0 and pK2 ) 7.0. Numbers at points indicate pH values.

XPS Surface Group Quantification and Modeling

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Figure 13. Comparison of streaming potential (ζ potential) data for quartz4 and surface charge difference obtained from XPS spectra (∆σ ) {>SiOH2+} - {>SiO-}) as a function of pH. Figure 11. Speciation at the quartz-solution interface as calculated from the experimental data obtained from this work for the surface complexation model by assuming constant capacitance of the electric double layer and pK1 ) -1.0 and pK2 ) 4.0.

Figure 14. Rate of quartz dissolution in neutral and basic solutions (6 < pH < 12) as a function of deprotonated surface species concentration determined in this work. The rate dissolution data are from ref 5. The surface species concentrations were calculated using CCM with new surface stability constants pK1 ) -1.0 and pK2 ) 4.0. Numbers at points indicate pH values. Figure 12. Correlation between predicted and measured in this work concentration of surface species for quartz. SCM calculation was performed by assuming pK1 ) -1.0 and pK2 ) 4.0. Numbers at points indicate pH values.

of the electric double layer.30 According to this model, nearly 10-15% of surface sites at pH 10 are occupied by potassium ions in the form of >SiO-K0 species decreasing the concentration of the negative >SiO- groups. This is in perfect agreement with the presented XPS observations (Table 3). In view of poor quantitative agreement between the measured quartz surface group densities and those predicted from classic SCM, we attempted to generate an alternative set of surface stability constants for quartz to obtain a better agreement with the XPS experimental data. The resulted new pK values for quartz, assumings constant capacitance of the EDL C ) 1 F/m2, are pK1 ) -1.0 and pK2 ) 4.0 (Figure 11). The new set of surface constants within the CCM provides a perfect description of XPS results for three surface species for full range of pH from 0 to 10 as is illustrated in Figure 12. This SCM implies a surface stability constant for the silanol deprotonation reaction, pK2 ) 4, that is very different from the corresponding constant for solution silica monomer dissociation, pK(aq) ) 9.8.26 This is in contrast to amorphous silica that exhibits similar surface and solution deprotonation reaction constants.22 It can be explained by different arrangement of the >SiO- and >SiOH0 groups on quartz and amorphous silica. For silica, the low packing density of the O(H) groups and the location of reactive surface groups result in the formation of surface groups with an arrangement similar to those existing in solution, such as H4SiO4(aq), whereas the dense and crystalline structure of quartz implies much shorter Si-O(H) distances and, consequently,

different silica energies for the formation of the surface and solution species. The derived from XPS data the new pK values were tested against the available information for quartz surface properties and its reactivity in an aqueous solution. Unfortunately, unlike for amorphous silica, there is almost nil surface charge measurement for highly a crystalline quartz surface similar to that used in this study. We compared in Figure 13 streaming potential (ζ potential) values for crystalline quartz measured by Cases4 with the calculated values of surface charge ∆σ ) {>SiOH2+} {>SiO-} from the XPS study as a function of pH. Both ζ potential and ∆σ depend on pH in a very similar way. Other evidence for validity of the calculated pK values can be obtained from the analysis of surface-controlled dissolution kinetics of quartz. It has been widely argued5,19,20,36 that the dissolution rate of quartz at pH g 7 is positively correlated with its surface charge or the concentration of the >SiO- surface species. Using the data of Brady and Walther5 as an example, we plotted in Figure 14 the quartz dissolution rate at 6 < pH < 12 versus surface concentration of the >SiO- species. The latter values were calculated using the new pK values. In accord with previous studies,46 a linear relationship is observed with a slope close to 4. The reaction order of 4 with respect to surface density of the >SiO- groups is in accord with surface coordination model of Stumm47,48 for the dissolution of simple oxides; the reaction order with respect to rate controlling species is being related to the valence of the central cation. One of the important results of this study is the quantitative determination of positively charged >SiOH2+ species at pH below 4. The new pK set shows that at pH 0 up to 16% of the

2944 J. Phys. Chem. B, Vol. 106, No. 11, 2002

Figure 15. Rate of quartz dissolution in acidic solutions (pH < 4) plotted as a function of >SiOH2+ surface concentration. The dissolution rate data are from ref 12, and the concentration of >SiOH2+ surface species is calculated using CCM with pK1 ) -1.0 and pK2 ) 4.0. Numbers at points indicate pH values.

surface can be occupied by >SiOH2+ species. This is the major difference between the usual view of surface chemistry of quartz and that derived from these XPS measurements. Positive surface species on amorphous silica49,50 and natural quartz29 were already detected by electrophoretic technique at pH < 2. Other evidence of the presence of positive surface species comes from studies of quartz dissolution kinetics5,10-12,29 that unambiguously demonstrate the increase of the dissolution rate with decrease of pH below 2. Wollast and Chou12 noted that, below pH 3, dissolution rates might be proportional to the concentration of the positively charged surface species, >SiOH2+, rates increasing with decreasing pH as the concentration of the latter species increased. Within the surface coordination theory,47 a proton-promoted dissolution rate is directly related to the concentration of protonated surface groups; i.e.,

R ) k{>SiOH2+} where k is the kinetic constant. We presented in Figure 15 a relationship between the dissolution rate for quartz at pH < 4 12 and the concentration of the >SiOH2+ groups as calculated using the new pK set. A linear relationship is observed with a slope close to 1, which implies that, in acid solutions, the protonation of one Si-O bond is large enough to form a surface precursor complex for dissolution reaction. In the search for thermodynamic relationships in ion adsorption at oxide/electrolyte systems, it was recently found51 that a pK1 is proportional to PZC. Taking the PZC value of quartz derived from Figure 13, the expected value of the pK1 is -1.5. This value is in an excellent agreement with the pK1 ) -1.0 determined in this work and very far away from that reported in the literature of pK1 ) 3.0 ( 1.0. This interesting theoretical consideration51 also underlined other parameters such as the presence of an inert electrolyte and temperature-related phenomena to have significant effects on the protonation processes of oxides in aqueous solution. These subjects are considered for future work that could provide new interesting insights into surface group formation on quartz and generally on oxides. Conclusions The XPS studies gave direct and on atomic scale information about the surface groups present on quartz contacted with aqueous solution at different pH. Though the spectroscopic

Duval et al. studies are performed under high-vacuum conditions, the recorded spectra present clear evidence of the formation of three type surface species: > SiOH2+; >SiOH0; >SiO-. This indicates that a 2-pK model has to be applied to describe surface phenomena at the quartz-solution interface. The two-step deprotonation model of quartz surface is proposed where two surface oxygen atoms are bonded to one silicon surface atom and where the most deprotonated surface site shows the >SiOconfiguration. Physisorbed water in the amount of nearly 10% of monolayer coverage was also found on all spectroscopically investigated samples. The relative concentrations of the surface species were determined in relationship to the solution conditions. On the basis of the experimental results, a new pK set for the quartz-solution interface is proposed for a wide pH range from 0 to 10. This allows, for the first time, to explain the increase of quartz dissolution rate with decreasing of pH below 2 due to positive >SiOH2+ surface charged groups. The new calculated pK set is also consistent with existing experimental data on negative surface charge >SiO- species at pH > 6 and the related dissolution rate. Acknowledgment. Part of this work was supported by the European Community (Project BRPR6CT98-0619). The technical assistance of Jacques Lambert is appreciated. References and Notes (1) Bolt, G. H. J. Phys. Chem. 1957, 61, 1166. (2) Ahmed, S. M. A. Can. J. Chem. 1966, 44, 1663. (3) Li, H.; de Bruyn, P. Surf. Sci. 1986, 5, 203. (4) Cases, J. M. Les phe´nome`nes physico-chimiques a` l’interface. Ph.D. Thesis, INPL, Nancy, France, 1967. (5) Brady, P. V.; Walther, J. V. Chem. Geol. 1990, 82, 253. (6) House, W. A.; Orr, D. R. J. Chem. Soc., Faraday Trans. 1992, 88, 233. (7) Dove, P. Am. J. Sci. 1994, 294, 665. (8) Dove, P.; Nix, C. J. Geochim. Cosmochim. Acta 1997, 61, 3329. (9) Dove, P. Geochim. Cosmochim. Acta 1999, 63, 3715. (10) Knauss, K. G.; Wolery, T. J. Geochim. Cosmochim. Acta 1988, 52, 43. (11) Bennet, P. C. Geochim. Cosmochim. Acta 1991, 55, 1781. (12) Wollast, R.; Chou, L. In Physical and Chemical Weathering in Geochemical Cycles; Lerman, A., Meybeck, M., Eds.; NATO ASI Series No. 251; Reidel: Dordrecht, The Netherlands, 1988; p 11. (13) Anderson, J. H.; Wickersheim, K. A. Surf. Sci. 1964, 2, 252. (14) Morrow, B. A.; Cody, I. A. J. Phys. Chem. 1976, 80, 1995. (15) Gallei, E.; Parks, G. A. J. Colloid Interface Sci. 1972, 38, 650. (16) Morrow, B. A.; Cody, I. A. J. Phys. Chem. 1975, 79, 761. (17) Zhuravlev, L. T. Langmuir 1987, 3, 316-318. (18) Koretsky, C. M.; Sverjensky, D. A.; Salisbury, J. W.; D’Aria, D. M. Geochim. Cosmochim. Acta 1997, 61, 2193. (19) Xiao, Y.; Lasaga, A. C. Geochim. Cosmochim. Acta 1996, 60, 2283. (20) Koudriachova, M. V.; Beckers, J. V. L.; de Leeuw, S. W. Comput. Mater. Sci. 2001, 20, 381. (21) Ugliengo, P.; Saunders: V.; Garrone, E. J. Phys. Chem. 1990, 94, 2260. (22) Hiemstra, T.; Van Riemsdijk, W. H. J. Colloid Interface Sci. 1989, 133, 105. (23) Hiemstra, T.; Van Riemsdijk, W. H. J. Colloid Interface Sci. 1990, 136, 132. (24) Rudzinski, W.; Charmas, R.; Piasecki, W.; Prelot, B.; Thomas, F.; Villieras, F.; Cases, J. M. Langmuir 1999, 15, 5977. (25) Rudzinski, W.; Charmas, R.; Partyka, S.; Thomas, F.; Bottero, J. Y. Langmuir 1992, 8, 1154. (26) Iller, P. K. The chemistry of Silica; J. Wiley & Sons: New York, 1979. (27) Borkovec, M. Langmuir 1997, 13, 2608. (28) Riese, A. C. Adsorption of radium and thorium onto quartz and kaolinite: A comparison of solution/surface equilibriums models. Ph.D. Dissertation, Colorado School of Mines, 1982. (29) Cadore, E. Me´canismes de dissolution du quartz dans les solutions naturelles. Etude expe´rimentale et mode´lisation. Ph.D. Dissertation, Universite´ Paul Sabatier, Toulouse France, 1995. (30) Sahai, N.; Sverjensky, D. A. Geochim. Cosmochim. Acta 1997, 61, 2801.

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