A sepiolite from Vallecas (Madrid) and a palygorskite from Torrejón el Rubio (Cáceres), Spain, were used as adsorbents in this study. The < 2 mm fraction was saturated with Na⁺ by repeated washing with 5 × 10⁻¹ M NaCl solution. Excess salt was removed by several washings with bidistilled water until Cl⁻ detection using 0.5 M AgNO₃ became negative. Then, the air-dried Na-clay mineral was gently ground in a porcelain mortar.

Specific surface area was determined by N₂ adsorption at the temperature of liquid N₂, after degassing the samples. The classical BET equation was used for external surface area calculation. Total surface area was determined by ethylene glycol monoethyl ether (EGME) method [20] .

Cation exchange capacity (CEC) of the samples was obtained by saturation with a buffered-pH 7 1 M NH₄-acetate [21] .

X-Ray diffraction (XRD) was performed using CuKa (1.5046 Å) radiation (35 kV, 15 mA) on a Rigaku Geiger Flex, DMax III-C horizontal goniometer equipped with a microcomputer system with graphite monochromator.

IR spectra were obtained on a Nicolet FTIR, Nexus 470 spectrophotometer using an Avatar Diffuse Reflectance accessory for the solid dispersion as KBr (1.35 mg of sample:295 mg of KBr).

Representative major and trace elements for the samples were chemically determined by acid dissolution for total elemental analysis [22] .

Proton surface-charge determinations were performed by potentiometric titration of the suspensions with 0.1 M HNO₃ and 0.1 M NaOH solutions at 298 K, under N₂ atmosphere for displacing the dissolved CO₂.

The suspensions were prepared in 0.1 M or 0.001M NaNO₃ solutions. In the first case the ionic strength remained constant during titrations, but in the second case it varied in the range 0.001 to 0.003 due to acid or base additions. Then, in this study, the lower ionic strength will be considered to be I = 0.002. The suspensions were shaken for 5 minutes, and pH was measured at one- minute intervals after the addition of acid (or base) to reach a new titration point. The initial pH was 8.2 for Na-sepiolite, and 9.0 for Na-palygorskite. When potentiometric titration was ended the suspensions were centrifuged, and free Mg²⁺ concentration was determined in the supernatant solutions by Atomic Absorption Spectrometry using a GBC 932 B computerized spectrometer. Potentiometric titrations in the absence of the clay minerals (blanks) were also performed. A Cole-Parmer pHmeter was used. In all the experiments, the concentration of adsorbent in the solution was 10 g L⁻¹. All experiments were carried out in duplicate.

Hydroxyl groups located at the broken edges of these fibrous clay minerals and permanent charge sites will be called SOH groups and X⁻ sites respectively. They are assumed to participate in pH-dependent protonation-deprotonation reactions and cation adsorption reactions as follows [23] :

where k_XH, k_XNa and are X⁻ permanent charge site constants. Eq 5 takes into account the possible re-adsorption of free Mg²⁺ released during the treatment from octahedral sheet.

In eqs 1 and 2, k_a1app and k_a2app represent “conditional or apparent acidity” equilibrium constants. The conditional or apparent equilibrium constants can be experimentally measured and will be equal to the thermodynamic constants (K) in an ideal behavior situation. Corresponding to the equilibria in both equations are expressions of the law of mass action in system subject to an electric field, which can be written

k_a1int and k_a2int represent intrinsic acidity equilibrium constants,

[ ] is molar concentration (mol L⁻¹),

y is the surface potential,

F is Faraday´s constant.

R is universal gas constant,

T is temperature (K).

Surface charge data were analyzed in two different ways:

1) Stumm method was used to obtain the two unique intrinsic acidity constants (Equations (6) and (7)), which determine acid-base characteristics of the surface.

This method extrapolates to zero the function that relates surface charge (Q) with the logarithms of “conditional or apparent acidity” constants k_a1app and k_a2app [24] .

Note that in Equations (1) and (2) [H⁺] = 10^?pH, and in the acid zone,

and in the basic zone,

Total surface sites were considered as the maximum H⁺ or OH⁻ adsorption of surface groups.

On the other hand, the charge due to interaction between H⁺ and OH⁻ ions and the surface groups is accessible from the titration curves based on charge balance as follows:

where C_B and C_A are the concentrations (mol L⁻¹) of added strong base and added strong acid, respectively.

With reference to Equations (10) and (11)

Finally, a plot of the negative logarithm of k_a1app and k_a2app versus Q will yield the negative logarithm of the intrinsic equilibrium constant (k_a1int and k_a2int), upon linear extrapolation to zero surface charge.

2) MINTEQ program [25] allowed the calculation of k_a1app, k_a2app, k_XH, k_XNa and (Equations (1)-(5)) by minimizing the differences between experimental surface [H⁺] and the data set obtained by the program from deprotonation constants proposed.

To account for these experimental results the diffuse-double-layer model (DDLM, plane geometry, without counter-ion accumulation) with the option “fixed surface charge”, which permit representation of X⁻ sites surface reactions (Equations (3)-(5)) was used.

Some surface and exchange properties of the selected homoionic samples are reported in Table 1. BET specific areas were smaller than EGME areas since the first method is used to determine only the external surface area. The BET-N₂ surface area of Na-sepiolite was found to be 251.5 m² g⁻¹ (at −197˚C) and 478 m² g⁻¹ with EGME vapor adsorption (at 25˚C), assuming a molecular cross-sec- tional area of 52 Å, indicating a possible greater penetration of pores and channels by EGME vapor compared to N₂ under these conditions. The same behavior was observed in Na-palygorskite (Table 1). Channel surfaces are also reported (see Table 1). CECs and specific surface area values obtained are in agreement with the reported values in the literature for reference minerals [6] .

XRD patterns showed a highly pure sepiolite sample and a well-crystallized palygorskite sample containing some quartz as impurity. Sepiolite gave a strong XRD line at 12.12 Ǻ with moderate reflections at, 4.49, 4.29, 4.02, 3.74, 3.34, and 3.18, and diffuse reflections at 7.5, and 5.04 Å. For palygorskite, a strong reflection occurred at 10.52 Å, and moderate reflections at 6.36 to 6.44, 5.38 to 5.42, 4.46 to 4.49, 4.24 to 4.26, 3.61 to 3.69, and 3.23 Å.

IR absorption spectra of both clay minerals permitted the vibration band assignments. Sepiolite showed stretching vibrations of structural OH in the edge silanol groups and in the octahedral sheet (3685 cm⁻¹), OH-stretching bands of coordinated H₂O (3619; 3561 cm⁻¹), adsorbed H₂O stretching vibration (3371; 3231 cm⁻¹), OH-bending vibration of coordinated H₂O (1662; 1625 cm⁻¹), stretching vibration of Si-O in tetrahedral sheet (1211 cm⁻¹), stretching vibration of

Si-O-Si groups (bridges between alternating Al-Mg-silicate ribbons) (900 - 1000 cm⁻¹) and Si-O-bending vibrations (478; 466 cm⁻¹). In the same region (465 - 470 cm⁻¹) would be overlapped Si-O-Mg vibration. For palygorskite stretching vibration of structural OH in edge (Mg, Al)-OH groups (3611; 3576; 3541 cm⁻¹), OH-stretching bands of adsorbed H₂O (3379; 3274 cm⁻¹), OH-bending vibration of coordinated H₂O (1658 cm⁻¹), stretching vibration of Si-O in tetrahedral sheet (1188; 1122; 1091; 1029; 982 cm⁻¹), stretching vibration of Si-O-Si groups (bridges between alternating Al-Mg-silicate ribbons) (1188 cm⁻¹), Si-O-Al bending vibration (513 cm⁻¹), Si-O bending vibration (482 cm⁻¹), Si-O-Mg bending vibration (450 cm⁻¹) were observed. These assignments are in agreement with the reported values in the literature for reference minerals [6] . Details of XRD patterns and IR spectra can be found in [26] .

The approximately formula of the unit cell of the minerals was determined for sepiolite [Na_0,025Ca_0,088K_0,10Mg_8,16Si_11,27Al_0,64Fe_0,12O₃₀(OH)₄(H₂O)₄] and palygorskite [Na_0,22Mg_2,53 Al_1,78Si_7,88Al_0,22Fe_0,37O₂₀(OH)₄(H₂O)₄)]. Some substitution of Si⁴⁺ by Al³⁺ and Fe³⁺ in the tetrahedral sheet was observed.

Data providing proton adsorption on homoionic forms of sepiolite and palygorskite at I = 0.1 and I = 0.002 (titration points) are shown in Figure 2. However, it should be noted that these potentiometric titration curves take account of dissolution processes involving structural Mg²⁺ ions. Acid amount associated with Mg²⁺ dissolution was discounted from initial acid amount (2H⁺ by Mg²⁺ released). To gain insight into the nature of proton adsorption and in order to confirm the relative position of the curve, i.e. to determine if protons adsorp-

tion increases or decreases with the increase of I; at different points of the titration curve on extreme pH values, some drops of NaNO₃ saturated solution were added. These additions produced a decrease in pH values at pH 3 and pH 10. Then, it can be assumed that the titration curves run parallel one to the other, without crossing points, in both clay minerals. In this case, the behavior of Na- sepiolite and Na-palygorskite is in agreement with the behavior of permanent charge phyllosilicates. At I = 0.002 some Mg²⁺ structural sites are released by dissolution, so these sites are able to capture H⁺.

Figure 3 shows Stumm’s method of calculation for the intrinsic deprotonation constants and the values obtained for k_a1int and k_a2int.

Intrinsic protonation-deprotonation (Stumm and DDLM) constants and ½ (pk_a1int + pk_a2int) values are listed in Table 2.

The comparison between the deprotonation constants (k_a1int and k_a2int) of SiO₂ greater than g-Al₂O₃ constants highlighted the known fact that, Si-OH groups in SiO₂ are more acid than Al-OH groups in g-Al₂O₃ (see Table 2). The calculated values for k_a1int constants of Na-sepiolite and Na-palygorskite were intermediate between those for SiO₂ and g-Al₂O₃ according with minerals that con-

tain moderately strong-acidity and weak-acidity surface groups, although the values were more similar to those of g-Al₂O₃. Sepiolite and palygorskite have abundant Mg²⁺ in the octahedral layer, and therefore since Mg-OH edge groups are of lower acid character than Al-OH groups, the lower k_a2int constant values are consistent with the presence of Mg-OH groups.

For pure oxides, the magnitude ½(pk_a1int + pk_a2int) indicates the point of zero net proton charge (znpc), but for fibrous clay minerals like sepiolite and palygorskite it shows both the acidity of the surface groups and the pH value towards which aqueous clay suspensions tends. The calculated ½(pk_a1int + pk_a2int) values for both clay minerals reflect the composition of sepiolite and palygorskite showing the influence of the different constituents: SiO₂ (znpc = 3.5), MgO (znpc = 12.0), and g-Al₂O₃ (znpc = 8.5).

In addition, sepiolite and palygorskite have permanent negative charge. This charge is found diffused throughout the structure causing a negative electric field and this fact could be another cause for the observed lower acidity on the edge groups.

The comparison of the intrinsic constants calculated by Stumm method and those obtained by MINTEQ (Table 2) shows little agreement between them. This distinctive feature can be related with the limitations of both theoretical methods. In Stumm method, the constants k_a1int and k_a2nt involve all the acid-base reactions while in DDLM the constants are unfold in a set of various constants k_a1, k_a2, and k_XH.

Figure 4 (Na-sepiolite) and Figure 5 (Na-palygorskite) show the fitting between the H⁺ adsorption experimental points and the theoretical curve calculated using Equations (1)-(4), considering the DDLM. Surface species distribution (at I = 0.1) as a function of pH is displayed too.

The surface sites SOH₂⁺ decrease as pH increases for both clay minerals. The surface sites SO⁻ increase as pH increases (this behavior can be observed from 8 - 9 pH units). The SOH groups initially increase (after SOH₂⁺ deprotonation) then, a plateau with a slight decrease at pH higher than 8 or 9 is observed. The decrease is due to the formation of SO⁻ sites.

The permanent charge sites are usually found as XNa at I = 0.1 as can be seen in Figure 5. At low ionic strength (I = 0.002), for both clay minerals (Figures are not show), the sites are sodium-free and occupied by Mg²⁺ but at very low concentration (X₂Mg = 1.6 × 10⁻⁶).

According to the DDLM, H⁺, OH⁻ and Na⁺ ions are considered adsorbed in the surface plane, as inner-sphere complexes. This is accepted by the traditional theory for the first two ones (usually known as potential determining ions), but not for Na⁺. The small Na⁺ ion remains as an outer-sphere complex (surrounded by a solvation shell) due to its great hydration energy.

Lately, for the analysis of surface structure of clay minerals, and H⁺, OH⁻, and some cations adsorption new computer treatments (Monte Carlo simulation and Molecular Dynamics [30] ) have been applied. The aim is to reproduce the thermodynamic equilibrium properties of a system in which the particles interact through potential energy functions. One of the most interesting aspects that have been studied in this context is the site type in which Na⁺ is adsorbed on the surface. Molecular Dynamic Model treatment [31] would indicate that Na⁺ is adsorbed on the edges as inner sphere complexes.