Acid-Base Properties of Aqueous Suspensions of Homoionic Sepiolite and Palygorskite

Acid-base properties of Na-sepiolite and Na-palygorskite were studied by potentiometric titrations at 298 K and two ionic strength I = 0.1 and I = 0.002. Intrinsic constants of deprotonation were calculated by two different methods: a) Stumm method, by extrapolating to zero the function that relates surface charge with the logarithms of apparent acidity constants and b) with MINTEQ program by minimizing the differences between surface H concentration data and the values obtained from deprotonation constants proposed according to the Diffuse-Double-Layer Model (DDLM). Hydroxyl groups located at the broken edges of these fibrous clay minerals (SOH) and permanent charge sites (X) were considered as reactive sites. The determined values of the acid-base constants for Na-sepiolite and Na-palygorskite were intermediate between those for SiO2 and γ-Al2O3, which is in agreement with minerals that contain moderately strong-acidity and weak-acidity surface groups. The SOH groups showed an initial increase (after SOH2 deprotonation), forming a plateau with a slight decrease at high pH values (8-9) due to the formation of SO sites. X sites adsorbed H, Na or Mg ions.


Introduction
Sepiolite and palygorskite are classified as phyllosilicates, because according to the definition of this mineral group [1], they contain a continuous two-dimensional tetrahedral sheet.However, they differ from the other layer silicates in lacking continuous octahedral sheets (Figure 1).Consequently, ribbons containing MgO 6 (AlO 6 ) octahedral groups and rectangular channels run parallel across to the octahedral sheet [4], the width of the ribbons and the channels differing a little in sepiolite and in palygorskite.In pa-  [2]; (b) Sepiolite structure [3]. Figure adapted from [4], 1988, and [5].lygorskite, Mg 2+ occupies only 29% to 76% of the octahedral sites while in sepiolite, it fills 90% to 100% of these sites [6].The structure of these clay minerals includes three types of active adsorption sites: basal O atoms on the tetrahedral sheets of the ribbons, H 2 O molecules coordinated to Mg 2+ at the edges of structural ribbons and SiOH groups associated in large number with terminal Si tetrahedral at the external surfaces.Other structural surface groups such as Al 2 -OH, Al-OH, and Mg-OH have been detected in palygorskite [7].It is likely that some other hydroxyl groups may appear on the external surface of the clay minerals where broken bonds give rise to incomplete tetrahedral or octahedral structural units.In addition, as a result of some isomorphous replacement in the tetrahedral sheet, such as Al 3+ for Si 4+ , negatively charged adsorption sites are observed.This structural charge must be balanced by cations at or near the mineral surface.The spatial distribution of these exchangeable "counterions" in the vicinity of the surface greatly affects the colloidal behavior of clay minerals.
Sepiolite and palygorskite have been used for sorbent purposes since 1930.
They were used to adsorb grease, oil, water, and other undesirable substances spilled on the floor of factories.Adsorbent granules are also used as pesticide carriers and bleaching agents for mineral and vegetable oils, butter, and wine.
The acid-base properties of the surface may have a relevant role on these adsorption processes.The interaction of clay mineral surfaces with H + and OH − must be known when surface models are used to explain the adsorption mechanism of different chemical species.A great deal of effort has been expended in developing models to describe the clay interfacial region.Some researchers have assessed the adsorption of H + and different metallic ions on hydrous oxides, soils, soil fractions and clay minerals [8]- [19].In these works, a wide variety of thermodynamic models describing the adsorption of chemical species to different surfaces have been developed, including the constant capacitance model (CCM), the diffuse-double-layer model (DDLM), the triple-layer model (TLM), and the multisite complexation model (MUSIC).The first group (CCM, DDLM and TLM), also termed two-pk models, treats the surfaces on a single-site basis.
These models have been successfully used in describing acid-base behavior in smectitic clay minerals.However, only a few works deal with fibrous clay minerals as sepiolite and/or palygorskite whose edge surfaces also present reactive Mg-OH groups.The aim of this study was to explain acid-base properties of Na-sepiolite and Na-palygorskite by modeling adsorption processes based on the existence of different structural sites using the DDLM.The DDLM model was chosen because of the simplicity of the application considering the weakness of clay mineral samples.

Clay Minerals
A sepiolite from Vallecas (Madrid) and a palygorskite from Torrejón el Rubio (Cáceres), Spain, were used as adsorbents in this study.The < 2 µm 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 AgNO 3 became negative.Then, the air-dried Na-clay mineral was gently ground in a porcelain mortar.Specific surface area was determined by N 2 adsorption at the temperature of liquid N 2 , 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 NH 4 -acetate [21].Representative major and trace elements for the samples were chemically determined by acid dissolution for total elemental analysis [22].

Surface Charge Determinations
Proton surface-charge determinations were performed by potentiometric titration of the suspensions with 0.1 M HNO 3 and 0.1 M NaOH solutions at 298 K, under N 2 atmosphere for displacing the dissolved CO 2 .
The suspensions were prepared in 0.1 M or 0.001M NaNO 3 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 oneminute 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 Mg 2+ 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.

Chemical Equilibria
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 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 ),

Calculation Procedures
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 Total surface sites were considered as the maximum H + or OH − adsorption of surface groups.
[ ] ( SOH HNO added to suspension HNO added to blank 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) SO ~ in the basic zone 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 2 X Mg k (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.

Surface and Exchange Properties of Clay Minerals
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-N 2 surface area of Na-sepiolite was found to be 251.5 m 2 g −1 (at −197˚C) and 478 m 2 g −1 with EGME vapor adsorption (at 25˚C), assuming a molecular cross-sectional area of 52 Å, indicating a possible greater penetration of pores and channels by EGME vapor compared to N 2 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.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 0,025 Ca 0,088 K 0,10 Mg by Al 3+ and Fe 3+ in the tetrahedral sheet was observed.

Protonation-Deprotonation Equilibria
Data providing proton adsorption on homoionic forms of sepiolite and paly- 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 Nasepiolite and Na-palygorskite is in agreement with the behavior of permanent charge phyllosilicates.At I = 0.002 some Mg 2+ structural sites are released by dissolution, so these sites are able to capture H + .

Deprotonation Equilibria Calculated According to Stumm Method
Figure 3 shows Stumm's method of calculation for the intrinsic deprotonation constants and the values obtained for k a1int and k a2int .
The comparison between the deprotonation constants (k a1int and k a2int ) of SiO 2 greater than γ-Al 2 O 3 constants highlighted the known fact that, Si-OH groups in SiO 2 are more acid than Al-OH groups in γ-Al 2 O 3 (see Table 2).The calculated values for k a1int constants of Na-sepiolite and Na-palygorskite were intermediate between those for SiO 2 and γ-Al 2 O 3 according with minerals that con-   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.

Deprotonation Equilibria Analyzed through the Surface Speciation Using MINTEQ Program with DDLM
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 .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 Mg 2+ but at very low concentration (X 2 Mg = 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.

Conclusions
Intrinsic constants of deprotonation were calculated by two different ways: 1) Stumm method, by extrapolating to zero the function that relates Q with the logarithms of apparent acidity constants.ions.

X-
Ray diffraction (XRD) was performed using CuKα (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).
permanent charge site constants.Eq 5 takes into account the possible re-adsorption of free Mg 2+ released during the treatment from octahedral sheet.
moderately strong-acidity and weak-acidity surface groups, although the values were more similar to those of γ-Al 2 O 3 .Sepiolite and palygorskite have abundant Mg 2+ 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: SiO 2 (znpc = 3.5), MgO (znpc = 12.0), and γ-Al 2 O 3 (znpc = 8.5).

Figure 4 (
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.

Figure 4 .
Figure 4. Computed adsorption of H + and OH − and distribution of surface species for Na-sepiolite at I = 0.1.

Figure 5 .
Figure 5. Computed adsorption of H + and OH − and distribution of surface species for Na-palygorskite at I = 0.1.

2 )
Using MINTEQ program by minimizing the differences between surface [H + ] data and the values calculated from deprotonation constants proposed according to DDLM.The calculated values for k aint constants of these clay minerals were intermediate between those for SiO 2 and γ-Al 2 O 3 being in agreement with minerals that contain moderately strong-acidity and weak-acidity surface groups, although the values were more similar to those of γ-Al 2 O 3 .The comparison of the intrinsic constants calculated by the Stumm method and those calculated by MINTEQ program showed little coincidence between them.Experimental values were in agreement with data from MINTEQ program.In surface species distribution, SOH 2 + surface sites decreased as pH increased, SOH groups (usually considered the reactive sites to cation complexation) initially increased forming a plateau with a slight decrease (pH 8 -9) due to the formation of SO − sites.X − sites (permanent negative charge) resulting from isomorphic substitutions or vacancies in the mineral structure adsorbed H + , Na + or Mg 2+

Table 1 .
Some characteristics of the fibrous clay minerals.