Wat. Res. Vol. 35, No. 17, pp. 4111–4125, 2001r 2001 Elsevier Science Ltd. All rights reserved

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MODELING DESCRIPTION AND SPECTROSCOPIC

EVIDENCE OF SURFACE ACID–BASE PROPERTIES OF

NATURAL ILLITES

WENXIN LIU*

SKLEAC, Research Center for Eco-Environmental Sciences, Academy Sinica, Beijing 100085, China

(First received 3 August 2000; accepted in revised form 21 March 2001)

AbstractFThe acid–base properties of natural illites from different areas were studied by potentiometrictitrations. The acidimetric supernatant was regarded as the system blank to calculate the surface siteconcentration due to consideration of substrate dissolution during the prolonged acidic titration. Thefollowing surface complexation model could give a good interpretation of the surface acid–base reactionsof the aqueous illites:

> SOH2 > SO@þHþ; pK inta ¼ 3:73B4:06;

Al3þþH4SiO4þ2H2O2AlðOHÞ2ðOSiðOHÞ3Þ þ 3Hþ; pKf2¼ 9:85B10:71;

> SOHþAl3þþH4SiO42> SOAlðOSiðOHÞ3Þþþ2Hþ;pKSC¼ 3:31B3:40:

Both experimental and optimized data revealed some similarities in the acid–base behaviors of the differentillites. Based on matching results with the standard XRD patterns, the changes in intensities of thediagnostic 0 0 l diffraction lines indicated that the formation of surface Al–Si complexes or precipitatesappeared preferable to the bulk deposition of Al(OH)3 solid. Similar features were also reflected in themicroscopic Raman spectra, where the vibrational peaks of Si–O and Al–O bonds diminished followingacid attack, then rose again after hydroxide back titration. The varied ratio of signal intensity betweenIVAl and VIAl species present in 27Al MAS NMR spectra, together with the nearly stable BET surfaceareas after acidic titration, showed that the basal planes in the layer structure of natural illite contributedto the dissociation of structural components as well as the edge faces. The combined spectroscopicevidence plus the acid–base titration results suggested that the interactions of illite surface sites with silicicacid and aluminum species, released from illite substrate, should be considered in the modelingdescription. r 2001 Elsevier Science Ltd. All rights reserved

Key wordsFnatural illites, acid–base titration, surface complexation model, spectroscopic evidence

INTRODUCTION

Dissolution, complexation and precipitation arefundamental processes for governing weathering

and (neo)formation of clay minerals, and alsodramatically affect the reactivity and mobility ofvarious contaminants through the geosphere (Nagy,1995). Acid treatment is a common means to modify

the surface/catalytic properties of clay minerals viaactivation of surface sites, and therefore has mis-cellaneous applications in industry (Komadel et al.,

1990, 1996; Tkac et al., 1994). Since the aforemen-tioned reactions generally occur in the region of thesolid/aqueous interface, numerous studies have been

performed in the relevant aspects, e.g., mechanisticinterpretation, model refinement and simulationstrategy (Katz and Hayes, 1995; Hiemstra and Van

Riemsdijk, 1996; Borkovec, 1997; Lutzenkirchen,

1998). The corresponding techniques for identifyingthe reaction products have been greatly developed aswell (McGuire et al., 1999).

Key components, present during liberation andformation of natural aluminosilicates in aquaticenvironments, usually include various hydrolyzedaluminum species, silicic acid, as well as their

productFhydrous aluminosilicates (HAS). SolubleAl–Si complexes may be intermediates to activatedcomplexes generated on the surface of minerals, and

control the formation of metastable and new, stableminerals (Browne and Driscoll, 1992). Therefore, thepresence of silicic acid can evidently influence the

distribution of aluminum species in acidic environ-ments, in terms of low-order soluble Al–Si complexes(Browne and Driscoll, 1992), multinuclear HAS

(Farmer and Lumsdon, 1994), and colloidal pre-cipitates (Fitzgerald et al., 1992).*E-mail: wenxin@mail.rcees.ac.cn

4111

As to the formation mechanism of soluble HAS

species, investigators have proposed several plausiblepathways: (1) Al species could combine with (ortho)silicic acids via condensation reactions between thealuminol and silanol groups; (2) silicic acid could

inhibit the nucleation of solid Al(OH)3 at growthsites on Al(OH)3 lattices (Exley and Birchall, 1993);(3) substitution of silicic acid for the water molecule

mass in the inner-coordination sphere of the hexa-aqua aluminum species (Browne and Driscoll, 1992);and (4) aluminates, converted from hexa-aqua Al by

addition of hydroxide, were inserted into oligomericor low molecular weight silicates to generate poly-meric aluminosilicates (Fitzgerald et al., 1997). In

addition, the structural and interfacial properties ofsynthetic amorphous aluminosilicates in electrolytesolution seemed to be similar to those of (hydrous)feldspathoids and poorly ordered aluminosilicate

catalysts with Si–O–Al@ groups and hydroxylatedsurface groups (Avena and De Pauli, 1996). By using29Si/27Al NMR as well as differential thermal analysis

and thermogravimetry (DTA–TG), a recent reporthas shown that the coexisting anions, such as sulfate,chloride and nitrate, influenced the interaction

between silicic acid and aluminum ions, andsuggested a montmorillonite-like structure for thesynthetic amorphous aluminosilicates (Miyazaki and

Yokoyama, 1999). These results are in agreementwith the fact that reconstitution of acid-leached clayminerals by treating with basic solutions results in amica-like structure (Nagy, 1995).

Since the natural particles, present as a matrix or acomponent, usually participate in various surfacereactions in aquatic medium, their effects on species

distribution should be taken into account in therelevant process. Bouallou et al. (1998) demonstratedthat the presence of highly charged Al13 polymeric

cations within pH 5.5B6.0 could trigger a series ofreactions: (1) inclusion of IVAl into the silica frame-work; (2) production of surface-coordinated octahe-dral VIAl; and (3) polymerization of some

heterogeneous complexes, leading to the formationof a metastable or amorphous Al–Si dispersed solidphase.

To date, potentiometric titration is still a mainapproach to study the surface acid–base chemistry ofclay minerals. Some papers have shown that dissolu-

tion of solid substrate, as well as the subsequentsurface complexation or precipitation, exerted aneffect upon the surface acid–base behaviors of clay

minerals, especially on the quantitative evaluation ofproton consumption using the back titration data(Lu and Smith, 1996). Consequently, the criticalinteractions between soluble aluminum species and

silicic acid, originated from substrate dissolution,should not be neglected.Natural illite with petrologic significance is a major

constituent of clay minerals in the Earth’s crust, andhas been the subject of a variety of investigations (Duet al., 1997; Sinitsyn et al., 2000; Kulik et al., 2000).

However, due to the distinctions in sample impurity,

pretreatment, experimental condition and modelassumption, reported data and conclusions on sur-face reactions may differ from case to case. It isunclear whether or not the minor impurities fre-

quently occurred in natural samples sufficientlyinfluence the surface charge nature of the multi-phase natural illites (Sinitsyn et al., 2000). Thus,

current knowledge is still deficient with respect to thecomplexity in crystal structure and chemical compo-sition of these naturally occurring particles in the

form of isomorphic replacement, mixed-layereddisorder. Furthermore, direct observations on thecorresponding alterations of illite surface character-

istics caused by acid–base titration as well asidentification of the produced surface complexes orprecipitates is also desirable. Use of combinedspectroscopic techniques may provide consistent

and complementary evidence not only for yieldinginsights into microscopic aspects of surface com-plexation reactions, but also for resolving some

ambiguities in spectral signal assignments, whichare usually encountered when using a single method.In the present paper, the main objectives are to

further extend the understanding of the surface acid–base chemistry of illitic materials, by comparison ofnatural samples collected from different sources, and

to validate the presumed reactions, which wereincorporated into the surface complexation model(SCM) for describing the acid–base behaviors, by thedirect spectroscopic evidence at the molecular level.

EXPERIMENTAL

Materials

The natural illites were obtained from: (1) WeiChang(WC) in northern China; and (2) LiuPanShui (LPS) insouthern China. Both initial samples were ground andsieved to less than 53 mm fractions. For other chemicals inour experiments, diluted HNO3 was standardized againsttris(hydroxymethyl) aminomethane (THAM). NaOH solu-tion was made by dissolving pure solid sodium hydroxide inboiled de-ionized water, and standardized using an acidsolution of known concentration. No further specificpretreatment, such as removal of carbonates, organic matterand iron oxides coatings (Sinitsyn et al., 2000), wereconducted in the present study. Thereafter, the followingsolid samples were individually prepared:

Hydrated samples. A 20 g l@1 stock suspension withoutbackground electrolyte was equilibrated in a refrigerator(41C) for at least 2 weeks prior to the titration experiments.Then, for each illite, an aliquot of 10 g l@1 suspension(diluted from the stock solution with de-ionized water) wascentrifuged at 20,000 rpm, 101C for 30min. Afterwards, theseparated solid phase was dried at 601C for 12 h and groundto a powder.

Titrated samples. The back titration procedure includedtwo sequential steps, i.e., acidimetric and alkalimetrictitrations. The sample suspension was firstly acidified toaround pH 3, then back titrated with hydroxide to thealkaline region (Du et al., 1997; Sinitsyn et al., 2000). Thepotentiometric titration was implemented at 25.070.51C(oil thermostat) by an automatic system for precise

Wenxin Liu4112

electromotive force (e.m.f., potential in mV), equipped witha standard glass electrode connected with a double junctionAg/AgCl reference electrode. For each illite, 50ml of10 g l@1 suspension was prepared in a fixed 0.1mol l@1

NaNO3 ionic strength. N2 was purged continuously through10% NaOH, 10% H2SO4, de-ionized water and 0.1mol l

@1

NaNO3, then into the suspension to eliminate CO2 until theelectrode potential was stable.(a) Acidimetric samples. Each 10 g l@1 suspension in

0.1M NaNO3 medium was forward titrated in 0.1-mlincrements, using a standard 0.1M HNO3 solution untilthe system pH was below 3. Then, centrifugation anddesiccation were identical to the foregoing hydratedsamples. The supernatants, after acidic titration, wereregarded as the acidimetric supernatants for solubilitydeterminations. The equilibrium concentration of H+ wasderived from the output of the glass electrode, based on theNernst equation.(b) Alkalimetric samples. Based on the prior acidimetric

titration, a successive back titration was performed bydropwise addition in increments of 0.1ml, of a 0.05Mstandard NaOH solution until the pH exceeded 10.Note: this end-point was different from the one adopted inpreparing the alkalimetric samples for spectroscopic in-vestigation (i.e., up to about pH 5.5–6.0, and the final statuswas maintained for 24 h). Subsequently, the solid powderswere prepared according to the same procedure mentionedabove. For acidimetric and alkalimetric samples, no furtherwashing or rinsing step was utilized to avoid possibledisturbance.The time required for equilibrium would usually vary

from system to system, depending on the system conditions.During the whole potentiometric procedure, the definedequilibrium tolerance at solid–water interface after eachaddition of the titrant was that the drift of the measuredpotential should be lower than 1mVh@1 at 251C, andconsecutive purging with pure N2 and magnetic stirringwere employed.Main elemental compositions of the initial solids

(o53 mm) and the acidimetric supernatants were determinedusing inductively coupled plasma-optical emission spectro-metry (ICP-OES, ARL Model 3580). Moreover, thecharacteristic layer spacing in crystal structure and theexternal surface area were measured by X-ray powderdiffraction (Siemens D5000 diffractometer, CuKa radiation)and by the N2/BET method (Micrometrics, ASAP Model2000), respectively. The micrographs of illite samples wereexamined by scanning electron microscope (SEM) (Philips,XL 30).

Spectroscopic measurements

FT-IR. 1mg solid sample was mixed with 100mg KBr(spectroscopic grade, oven-dried at 1101C overnight). Usingthe diffuse reflectance infrared Fourier transform (DRIFT)technique, all spectra were recorded on a Perkin-Elmer FT-IR 2000X spectrometer with a spectral resolution of4 cm@1. 200 scans acquired at a rate of 0.2 cm@1 s@1

were averaged in the wavenumber range 370–4000 cm@1.

Microscopic Raman. A Renishaw 1000 microprobesystem with a CCD detector was used to detect theRaman spectra excited, at room temperature, by aSpectra-Physics Model 127 He–Ne laser (633 nm). Thesample was fixed on the stage of a microscope equipped witha 50� objective lens (Olympus BHSM). Twenty-fiveacquisitions, with a detection time of 8–10 s, were summedat maximum magnification to improve the signal/noiseratio. All spectra, between 100 and 2000 cm@1 at aresolution of 2 cm@1, were calibrated using the 520 cm@1

line of a silicon wafer. For each sample, the spectra obtainedfrom different detection area of the powders werereproducible. Since the spectrometer eliminates the

Rayleigh line close to 150 cm@1, the interpretation ofbands below 200 cm@1 was disregarded due to poorreliability. All spectral manipulation was performed by theGRAMS software package.

X-ray powder diffraction. The illite samples were analyzedon a powder diffractometer operated in the reflection modewith CuKa radiation. Diffraction traces were reproduciblein the region of 51 and 8512y at a scanning rate of 0.021s@1.The precision of d-spacing measurement was 0.01 A.

Solid state nuclear magnetic resonance (NMR). Theresonance signals of solid state NMR were obtained atroom temperature by a CMX360 spectrometer (8.45T,Chemagnetics). All samples were separately packed into7mm magic angle spinning (MAS) rotors sealed with endcaps. The resonant frequencies of 29Si and 27Al were 71.508and 93.795MHz, respectively. The p=2 pulse width andinterval for 29Si were 4.5ms and 2 s, respectively, thecorresponding values being 2.4 ms and 2 s for 27Al. Theacquisition time was 68 ms with 25ms delay for 29Si (900–3000 accumulations), and 41ms with 20 ms delay for 27Al(128–1024 accumulations). The spinning rates were 6.0 and7.0 kHz for 29Si and 27Al NMR spectra, respectively.Tetramethylsilane (TMS) was selected as the externalreference for the chemical shift values of 29Si, and 1.0MAl(H2O)6Cl3 solution for that of

27Al. In addition, since thepreliminary test for cross-polarization (CP) with 1H showedno apparent improvement on the signal resolution for 29SiNMR, the CP technique was not adopted for the actualmeasurements.

MODEL HYPOTHESIS AND DATA PROCESSING

Equivalent points (Ve) and standard potential of glasselectrode (E 0)

The corresponding equivalent points in the alkali-metric back titration (Veb1 and Veb2 in Fig. 1) were

determined from the linear regression analysis by theGran plots. Consequently, the E0 value (in mV) ofthe selected titration system could be derived to

change the potential output of the glass electrode into@log[H+].

Zero point of titration (ZPT)

Based on the Gran plots mentioned above, thedripped hydroxide ions were consecutively shared bysurplus H+ (before Veb1), various surface acceptors

(between Veb1 and Veb2) as well as the suspension pH(after Veb2) (Du et al., 1997). In this study, Veb1 inFig. 1 was regarded as the ZPT, due to only acid–base neutralization presented in solution phase

before this point. The total amount of reactedprotons at each titration point, in term of TOTP,was therefore evaluated by:

TOTP ¼ @ Vb@Veb1ð Þ�CbV0 þ Vat þ Vb

ðmol l@1Þ ð1Þ

where Vat is the total volume of HNO3 added, Vb andCb are the volume and concentration of NaOH,

respectively, and V0 denotes the initial volume of thesuspension. After ZPT calibration, the hydroxideback titration data were utilized as input values in the

Surface acid–base nature of natural illites 4113

stoichiometric matrix for optimization by the FI-

TEQL program (Westall, 1982).

Model hypothesis

A general description of interfacial reactions

should include ion-exchange and surface complexa-tion occurred at weakly acidic surface sites (>XH)and amphoteric surface sites (>SOH), respectively.

However, a recent report has indicated that, a highand constant ionic strength (I) makes the constantcapacitance model (CCM) as a physically realistic

choice of an interfacial model for surface acid–base

characters (Lutzenkirchen, 1998). The manipulationof multiple ionic strength data with one comprehen-sive parameter set as well as some inconsistencies are

thus avoided (Lutzenkirchen, 1999a, b). This succinctmethod with fewer adjustable parameters allows datatreatment on the concentration scale, and in principleinvolves strictly conditional parameters. In our case,

the dominant background electrolyte (0.1mol l@1

NaNO3) actually kept the ionic strength of titrationsystems stable, the ion-exchange reactions could be

neglected. More recently, a comprehensive triple

Fig. 1. In situ Gran plots of hydroxide back titration. (a) The sample suspension; (b) the acidimetricsupernatant. Here, the solid and open squares denote the acidic side of titration data of WC and LPSsamples, respectively, the solid and open up-triangles are the alkaline side of titration data of WC and LPS

supernatants, respectively.

Wenxin Liu4114

layer model (TLM) based on Gibbs linear free energy

minimization other than mass balance was applied tothe description of a standard reference illitic mineralafter purification (Sinitsyn et al., 2000). Although thesensitivity analysis was performed, the possibility of

potential uncertainties would increase, attributable tomore adjustable or pseudo-fixed parameters in-volved. As a result, the constant capacitance model

(CCM) was employed in the current paper.

Surface site type and density

According to crystallographic analysis, the illite

surface sites located in 2 : 1 layer structures should beconsidered as heterogeneous with different bindingaffinities of protons and other ions. The typicalmorphology of illite may lead to a variety of potential

surface site types, including weakly acidic basalplanar surfaces formed by the planes of tetrahedralsiloxane sheet, amphoteric silanol and aluminol

surface sites on the edge surfaces, frayed edges andinterlayer sites (Kulik et al., 2000). It has beenemphasized that the total number of surface sites

should be determined experimentally or a valueshould be assigned on the basis of other studies(Davis et al., 1998). However, unlike pure (hydr)o-

xides or standard reference clay minerals where thesurface site density can be calculated based on theideal morphological structure (Pivovarov, 1997), anaccurate estimation of the crystallographic presenta-

tion for the naturally occurring illites and the ratio(or relative proportion) between different site groupsis virtually impossible, due to various interstratifica-

tion manners of constituent layers as well ascompositional impurity and structural defects onsurfaces caused by local environments of origins. In

addition, the resulting SCM with fewer optimizedparameters, which are restricted or preset a priori,may reduce the uncertainties in the model andincrease the predictive potential (Sinitsyn et al.,

2000). In view of simplicity and practicality, the sitedistribution on illite surfaces is therefore regarded ashomogeneous in this study.

In contrast to the values presumed according to theclassical particle morphology or optimized by thefitting programs in some papers, the surface site

concentration (SSC) was estimated as follows andfixed during optimization:

SSC¼Veb2@Veb1ð Þsample�Cb@Veb2@Veb1ð Þblank�Cb

V0

ðmol l@1Þ ð2Þ

At each titration point, the quantity of protons

reacted with all surface sites (PR) was computedusing

PR ¼ TOTP@10@pH þ 10@ðpKw@pHÞh i

�

ðV0 þ Vat þ VbÞ ðmolÞð3Þ

here, pH and Kw stand for the@log[H+] and ionic

product of water in 0.1mol l@1 medium, respec-tively.

RESULTS AND DISCUSSION

Surface proton-reaction model

Titration blank. In this paper, the proton reactions

in the system blank were interpreted by those ofSi(OH)4 and Al

3+ in aqueous solution (see Table 1),since natural illite is mainly constituted by hydrous

aluminosilicates.The acidimetric supernatant as the system blank is

more accurate, due to consideration of structural

elements released from matrix. Therefore, the actualmeasurements of total Si and Al in the acidimetricsupernatant by ICP-OES were imported into the

chemical equilibrium stoichiometric tableau ofFITEQL. Because the hydrolyzed aluminum speciesand the coexisting silicic acids, arising from acidicdissociation of the illite matrix, will generate a variety

of HAS complexes as mentioned before, theirinfluence on illite surface properties should be takeninto account as well. For simplicity, only the low-

order monomeric form was considered:

Al3þþH4SiO42AlðOSiðOHÞ3Þ2þþHþ pKf1

In this study, 2.7470.023 was adopted for pKf1,

which was estimated based on potentiometric titra-tion (Sjoberg, 1998).Meanwhile, since the used FITEQL cannot fit the

bulk deposition observed in the acidic supernatantduring the hydroxide titration, a supposed reactionto simulate the interactions between aluminumspecies and silicic acid during the hydroxide titration

was further introduced,

Al3þþH4SiO4þ2H2O2AlðOHÞ2ðOSiðOHÞ3Þ

þ3Hþ pKf2

where Kf2 represents the formation constant for the

produced complex.

Sample suspensions. The surface acid–base prop-erties of the different illites are quite similar, as shownby the titration curves in Fig. 2.

In addition to the relevant procedures consideredin the acidimetric supernatants (titration blank), theinteractions between HAS species and illite surface

sites in particle suspension were further proposed forthe model description to derive the surface acidityconstants (see Table 1). After input of other fixedparameters, namely, surface site concentrations

(SSC) and BET surface areas (ABET), the optimiza-tion and adequacy testing were accomplished bytuning the electrical capacitance (C) value in the

FITEQL, until the best fit was attained (Westall,1982). The concerned details are listed in Table 1 anddepicted by Fig. 2.

Surface acid–base nature of natural illites 4115

In this model, the homogeneous surface groupswere only subject to deprotonation. The supporting

evidence includes: (1) the specific distribution patternin the Gran plot (Fig. 1) implies no apparent surfacereaction before Veb1; (2) the negative PR values (see

Fig. 2) show that deprotonation is predominantwithin the studied pH range (pH 3–10); and (3) aprevious study of Zeta potential determination hasindicated that the surface of WC illite carried

negative charge even in the acidic surroundings (Duet al., 1997). Since the model supposes that depro-tonation dominated the illite surface behavior, the

contribution of the >SiOH sites appears to be moresignificant than that of the >AlOH sites. It is notsurprising if we consider high proportion of SiO2existed in the chemical compositions (the weightpercentages of SiO2 and Al2O3 were 72.5 and 14.9 forWC, and 64.7 and 15.6 for LPS samples, respec-

tively). The influence of acidimetric titration on thespecific external surface areas of the illite samples wasnot strong (see Table 1). To some extent, the lattercase also implies that, in the layer structure of natural

illites, the surface sites on basal tetrahedral sheetsmay contribute to the reactivity of illite as well as thesurface sites on edge faces, which is confirmed by the

ensuing spectroscopic information.

From Fig. 2, the surface acid–base properties ofaqueous illites from different locations could be well

delineated by the CCM involved surface complexa-tion or precipitation of HAS. The formation constant(KSC) of the assumed surface HAS species is

relatively stable. After comparison, the approximatepK int

a values for the two illites exhibited somesimilarities in their acid–base properties. The corre-sponding results are in accordance with those in a

former study as well (Du et al., 1997).As indicated by Lu and Smith (1996), the titration

behavior of complex minerals may not be simply

represented by an oxide mixture with compositionalequivalence. In fact, surface complexation modelparameters acquired during a particular fitting

procedure are not necessarily unique. In many cases,the situation of real samples in their pristine statusmay deviate, to different extent, from the typical one

reflected by the standard reference materials. In asense, the results of such studies are difficult orinvalid to compare simply and directly, owing tointrinsic differences in sample preparation, experi-

mental techniques, data interpretation and thecontrasting mineralogical purity of the samples(Sinitsyn et al., 2000). Obviously, discriminations in

experimental protocol as well as the complexity

Fig. 2. Model illustrations of the potentiometric back titrations for different illite suspensions and theconcerned experimental data. The solid and open squares denote 10 g l@1 suspension of WC and LPS in0.1mol l@1 NaNO3; the solid and open circles are acidimetric supernatants of WC and LPS, respectively.The dotted lines are simulation results of the acidimetric supernatant using Si(OH)4 and Al

3+ protonreactions as well as complex formation between Al and Si(OH)4 in aqueous solutions. The solid anddashed lines represent model description of the titration data of WC and LPS illites, respectively. Theembedded figure shows the titration curves for two illite samples (all 10 g l@1, 0.1mol l@1 NaNO3) andtheir corresponding supernatants. The solid and open up-triangles are acidimetric titration of WC andLPS illites, respectively. The solid and open squares denote alkalimetric titration of WC and LPS illites,respectively. The solid and open circles stand for alkalimetic titration of WC and LPS acidimetric

supernatants, respectively.

Wenxin Liu4116

Table 1. Reaction constants of Al3+ and Si(OH)4 in aqueous solution, and model description of proton behavior on illite surfaces

Reaction Constants (Smith and Martell, 1976)

Al3++H2O2AlOH2++H+ log K=@5.52 (251C, I=0.1)Al3++2H2O2Al(OH)2

++2H+ log K=@10.40 (251C, I=0.1)Al3++3H2O2Al(OH)3+3H

+ log K=@16.30 (251C, I=0.1)Al3++4H2O2Al(OH)4

@+4H+ log K=@23.00 (251C, I=0.0)Al3++H4SiO42Al(OSi(OH)3)

2++H+ log Kf1=@2.7470.023 (Sjoberg, 1998)Al3++H4SiO4+2H2O2Al(OH)2(OSi(OH)3)+3H

+ log Kf2, optimized by FITEQL programSi(OH)42SiO(OH)3

@+H+ log K=@9.46 (251C, I=0.5)Si(OH)3

@2SiO(OH)22@+2H+ log K=@22.02 (251C, I=0.5)

Model reactions Constant

Surface complexation Model >SOH2SO@+H+ pKaint

Al3++H4SiO42Al(OSi(OH)3)2++H+ pKf1

Al3++H4SiO4+2H2O2Al(OH)2(OSi(OH)3)+3H+ pKf2

>SOH+Al3++H4SiO42>SOAl(OSi(OH)3)++2H+ pKSC

Model parameters WC LPS

Cs (g l@1) 10 10

I 0.1mol l@1 NaNO3 0.1mol l@1 NaNO3ABET

a (m2 g@1) 25.15 49.19ABET

b (m2 g@1) 26.19 53.40SDc (nm@2) 3.2 2.5

SSC (mmol l@1) 1.38 2.20C (Fm@2) 1.2 0.8pKa

int 3.7370.021 4.0670.014pKSC 3.3170.034 3.4070.042pKf1 2.7470.023 2.7470.023pKf2d

d 10.7170.066 9.8570.085VY 9.52 14.42

aABET is the specific surface area before acidimetric titration.bABET is the specific surface area after acidimetric titration and used in the modeling calculation.cSD denotes the surface site density calculated using the equation: SD=(SSC�NA)/(CS�ABET� 1018), (sites nm@2), where NA is the Avogadro constant (6.022� 1023mol@1), CS represents the solid concentration

(g l@1).dThe VY values of pKf2 for WC and LPS samples are 5.92 and 13.97, respectively, optimized by FITEQL based on the actual measurements of acidic supernatants. VY is the overall variance of the fitting procedure.

Surfaceacid–basenatureofnaturalillites

4117

inherent in the structure and chemistry of 2 : 1 layerclay minerals may partly elucidate the varied range ofsurface electrochemistry reported. Undoubtedly, it is

significant for the future study to standardize theprotocols adopted in the experimental investigationof the surface charge, considering the complexity of

this group of clay minerals.In the following, some spectroscopic techniques

are employed to offer some direct evidence for

the formation of surface Al–Si complexes or pre-cipitates during acid–base titration. Combination ofmultiple spectroscopic techniques can providethe comprehensive information for validating the

corresponding model assumptions of surfacereaction equilibria aforementioned and for resolvingsome ambiguities in spectral signal assignment,

which are often encountered when using a singlemethod.

Scanning electron microscope image (SEM)

The SEM images of illite particles revealed plate-like crystals stacked in the layer structure (see Fig. 3)which reflects the typical features of sheet silicates.

FT-IR

The assignment of infrared vibrations is summar-

ized in Table 2. Typical spectra for the illite samplesafter acid–base treatments are illustrated in Fig. 4.The impact of acidic titration on the bulk

configuration of illite was not so significant in theexperiments, since the liberated extent of structuralelements, i.e., Si and Al, was less than 1% with

respect to the studied solid matrix (the dissolutionresults by ICP-OES not shown due to limitation ofspace). This situation differs from the results of otherstudies, where intensity reduction and band shift of

infrared vibrations for the specific functional groupsaccounted for the extent of dissolution, wereremarkable, possibly due to the use of relatively

acute reaction conditions (Komadel et al., 1990,1996). However, a new IR absorption band lying inthe range of 1370 and 1385 cm@1, appeared for both

illite samples after acid–base potentiometric titration.This band is attributed to the v3 vibrations of NO3

@

groups (Nakamoto, 1997), which originated from

background electrolyte (0.1M NaNO3) and fromaddition of 0.1M HNO3 during the acidimetrictitration. There are two possible sources for this

Fig. 3. SEM images of the illite samples: (a) WC; (b) LPS (o53 mm).

Wenxin Liu4118

nitrate absorption band. One is the residual fractionof NO3

@ after solid–solution separation, and for the

hydrated samples, the missing absorption band isprobably caused by no addition of support electro-lyte in the course of preparation. The other is theformation of surface Al–Si nitrate complex, where

the nitrate groups may locate in the outer sphere asthe counter ions for charge balance, e.g., in the formof >SOAl(OSi(OH)3)NO3. Another report also

demonstrated that the presence of some anions, suchas nitrate, could affect the local structure of Al and Siin synthetic amorphous aluminosilicates, and alter

the surface properties, for example, surface chargeand adsorption capacity (Miyazaki and Yokoyama,1999).

Raman spectroscopy

The assignment of Raman bands is listed in Table2. Figure 5 depicts the microscopic Raman spectra ofthe illite samples prior to, and after each stage of the

acid–base titration. The most prominent variation ofthe illite surfaces is illustrated by the change inscattered intensity of symmetric Si–O bending vibra-tions near 465 cm@1. The acidimetric titration

induced not only diminishment of the peak height,but also significantly broadening of the peak widthwith respect to that of the hydrated illite. Then this

signal, to a certain degree, rose again as a result ofhydroxide back titration. As for the bands corre-sponding to Al–O vibrations in the region of 650–

750 cm@1 (Nakamoto, 1997), similar results couldalso be found in our study.

Consequently, the Raman spectroscopic informa-tion suggests that some products containing Al–O–Sibond structure were generated on the illite surfacesduring the hydroxide back titration. This phenom-

enon results from the interactions between soluble Alspecies and silicic acid, originated from acid-leachingreaction of the illite substrate during the potentio-

metric titration.

XRD profiles

The related alterations by acid–base titration aredescribed, on an identical scale, by linear counts

versus 2y angle in Fig. 6. The characteristic d(0 0 1)spacing values for WC and LPS illites are 9.99 and9.90 A, respectively.

According to Fig. 6, the heights of several 0 0 ldiffraction lines, such as basal reflection d(0 0 1) at912y, d(0 0 2) at 1812y and d(0 0 3) at 26.512y, whichare usually regarded as the diagnostic reflections fornatural illites (Sroden, 1984), sharply decreasedrelative to those of hydrated samples during theacidic titration procedure. This situation is in accord

with that in other reports (Komadel et al., 1990),though stronger experimental conditions were ap-plied in their cases, such as concentrated acid and

heating. To some extent, the intensities of certaindiffraction peaks resumed after hydroxide backtitration up to around pH 5.5–6, especially for the

Table 2. Vibration and resonant assignment of IR, Raman and NMR spectra

IR peak (cm@1) Assignment References

3600–3630 Inner OH stretching motions, e.g., Mg2OH, Al2OH Farmer (1974)3300–3350 Vibrations of organic matter Farmer (1974)1610–1650 HOH bending (water molecule) vibrations Farmer (1974)3260 and 1430 Vibrations of NH4

+ Russell and Fraser (1994)1370–1385 v3 vibrations of NO3

@ groups Nakamoto (1997)798 and 1090 Quartz (free Si–O) vibrations Komadel et al. (1990)1010–1030 Si–O stretching in-plane (Si–O–Si) vibrations Farmer (1974), Komadel et al. (1996)915–950 Al2OH in-plane vibrations Farmer (1974)825–840 Al–Mg–OH (or tetrahedral Al–O) out-of-plane vibrations Farmer (1974)750 Al–O–Si in-plane vibrations Farmer (1974)600–700 OH bending vibrations Farmer (1974)400–550 Si–O (or Si–O–Al) bending in-plane vibrations Farmer (1974), Komadel et al. (1996)

Raman peak(cm@1)

Assignment References

1000–1100 Asymmetric vibrations of Si–O–Si Nakamoto (1997)650–750 Al–O vibrations Nakamoto (1997)450–550 Symmetric vibrations of Si–O–Si Nakamoto (1997)750–830 v1 (A1) vibrations of SiO4 unit Nakamoto (1997)300–400 v2 (E) vibrations of SiO4 unit Nakamoto (1997)800–1000 v3 (F2) vibrations of SiO4 unit Nakamoto (1997)450–600 v4 (F2) vibrations of SiO4 unit Nakamoto (1997)

Nuclei Assignment Reference

29Si Quartz Q30Al (smectite) Q30Al Q31Al Q32Aldiso (ppm) @108.5 @94.2 @89.5 @86.5 @83 Tkac et al. (1994), Lausen et al. (1999)

27Al VIAl VAl IVAl Sidebands Lippmaa et al. (1986),Goodman and Chudek (1994)

diso (ppm) 2.8–3.6, @10 30–31 61, 69–71 79, @73

Surface acid–base nature of natural illites 4119

reflection line at 26.512y with a d-spacing of 3.34–3.37 A.

Based on comparison with standard diffractionpatterns in the built-in database of the instrumentsystem, the searching and matching results showedthat a dispersed solid phase was created during the

alkalimetric titration. This probably corresponds tothe formation of surface (hydrous) Al–Si complexesor precipitates containing Si, Al and O structural

components, preferable to bulk aluminum hydroxide,since there is no appropriate pattern for solidAl(OH)3 (e.g., bayerite or nordstrandite) present in

the XRD plots. In a sense, these results conform with

those in the microscopic Raman spectra and verifythe mechanistic explanation in which the presence of

silicic acid can restrain or block the nucleation ofsolid Al(OH)3.

Solid-state MAS NMR29Si NMR spectra. Figure 7 portrays the 29Si MASNMR spectra of the illite samples. Generally, themain structural units in layered silicates are presented

in terms of Q3mAl (m=0–3), as listed in Table 2.Here the superscript is the number of bridgingoxygen atoms coordinated to a Si, and m represents

Fig. 4. FT-IR spectra of different illites after acid–base titration: (a) WC; (b) LPS.

Wenxin Liu4120

the extent of tetrahedral Al substitution in the next-nearest coordination sphere.The poor resolution of the @80 to @100 ppm

resonance reflected various substitution styles of IVAl

for Si, that was, lack of Si–Al ordering in thetetrahedral sheet and a range of Si–O–Si(Al) bondangles. Based on curve-fitting assignments for the

broad peak, the presence of multiple bands centeredat @92 ppm, was assigned to an overlapping signalcaused by several Q3mAl structural units, e.g.,

@86.5 ppm (Q31Al), @89.5 ppm (Q30Al), andsmectite Q30Al at @94.2 ppm ascribed to smectiticinterstratification during diagenesis (Lausen et al.,1999). The distribution pattern and relative contribu-

tion of different Q3mAl structural units, wereconsistent with the characteristics revealed by thecorresponding XRD plots as well (Fig. 6), in which

the illite samples exhibited different degrees ofcrystallinity and different mixed-layer modes of thecomponent layers (i.e., interstratification of illite(I)

Fig. 5. Microscopic Raman spectra for the two illites: (a) WC; (b) LPS. For simplicity and comparison, allspectra have been shown in splitting mode (i.e., displayed as an equally divided vertical stack of separateplots) and performed using auto-Y-scale manipulation. The intensity scale of the acidimetric samples has

been magnified.

Surface acid–base nature of natural illites 4121

and smectite(S) layers), as shown by the generalpeak-shape parameters (e.g., intensity, sharpness and

symmetry) of the diffraction lines at 91 and 3512y,respectively. In addition, a quartz impurity wasassociated with the resonant signal at @108.5 ppm

(Tkac et al., 1994), and could be reinforced by the IRabsorption at 1090 and 798 cm@1 (see Fig. 4).In principle, modification of the textural confor-

mation of the silicate matrix can be detected by 29Si

MAS NMR (Lausen et al., 1999). Unfortunately, therelevant spectral change produced by the acid–basetitration procedures was not so distinguishable in our29Si MAS NMR spectra, except for the WC illite

after back titration, where the resonant intensity ofQ3mAl structural units decreased significantly. There

are two possible explanations for this case. First, thelosses of Si and Al relative to the studied massframework, followed the acidic titration, are not

great enough to be detected by 29Si MAS NMR.Second, due to the low spectral resolution or theoverlap of neighboring broad resonant signals, theinterpretation in terms of exact local configurations is

particularly difficult.

27Al NMR spectra. For sheet aluminosilicates, the

aluminum complexes are commonly composed of

Fig. 6. XRD plots of the illite samples before and after acid–base titration. (a) WC illite; (b) LPS illite.

Wenxin Liu4122

fourfold tetrahedral aluminum (IVAl) and six-

coordinated octahedral aluminum (VIAl). Thedetailed assignment for 27Al resonant signals istabulated in Table 2, and the corresponding spectra

for the different illites are shown in Fig. 8.In general, 2 : 1 layer clay minerals are assumed

to dissolve preferentially inward from the edges of

the crystal structure (Nagy, 1995). Therefore, theconcerned NMR signal intensity of VIAl shouldbe lessened more readily than that of IVAl, since

the latter is located in the relatively stable tetra-hedral unit (i.e., in the three-dimensional frame-work of siloxane sheet) due to isomorphic substitu-tion (Tkac et al., 1994). In some cases, however,

the basal planes inside the layer structure are alsohighly reactive, which is ascribable to shallow stepedges, structural defects and impurities (Johnsson

et al., 1992). Komadel et al. (1990) also indicatedthat after a part of octahedral sheets had beendissolved during acid attack, an amorphous form of

silicic acid could be produced from the silicontetrahedra.In this study, the resonant peak of IVAl split into

two distinct components at higher magnetic field, asin another publication (Goodman and Chudek,1994). For WC illite after acid treatment, the ratioof the integral values between IVAl and VIAl signals,

decreased from 0.73 to 0.58 compared to that of thehydrated sample. This phenomenon suggests that notonly octahedral Al on the edge faces, but also

tetrahedral Al on the basal planes, through iso-morphic replacement for Si, is active during theacidimetric titration, consistent with the results

reported in other studies (Johnsson et al., 1992;

Komadel et al., 1996). Moreover, the nearly stableBET surface areas, based on comparison between thevalues of hydrated and acidimetric illites (see Table

1), also implies that the basal planes inside the illitecrystal structure, where aluminum existed in thesiloxane layer, and for charge compensation, part of

the bridging oxygen atoms were replaced by bridgingOH groups (i.e., in the form of >Al–OH–Si(Al)o),make a contribution to the reactivity of the natural

illites in this study.The varied trend of 27Al MAS NMR signal

intensity of the alkalimetric samples, is uncertain(see Fig. 8). In the case of WC illite, the relative

contribution of the four-coordinated Al componentafter hydroxide back titration, was greater than thatin the acidimetric sample (the intensity ratio of IVAl

to VIAl rose from 0.58 up to 0.69), inferring thepresence of this aluminum species in the reactedproducts. There appears to be some associations

between the increase in peak height of IVAl species inthe tetrahedral layers, and the reduction in intensityof Q3mAl units in the 29Si MAS NMR spectra for the

alkalimetric WC illite sample. In other words, somealuminum atoms are trapped in the three-dimen-sional framework built from the reorganized tetra-hedral sheets (i.e., substitute part of silicon atoms),

then the replacement may lead to the decrease insignal intensity of 29Si resonance (see Fig. 7c).Nevertheless, this feature was not observed for the

LPS sample, and the reason for the inappreciablechange of its resonant signal is not clear, possibly dueto the weak signal response.

Fig. 7. 29Si MAS NMR spectra: (a) hydrated WC; (b) acidimetric WC; (c) alkalimetric WC; (d) hydratedLPS; (e) acidimetric LPS; (f) alkalimetric LPS.

Surface acid–base nature of natural illites 4123

Similarly, by the aid of 27Al and 29Si MAS NMRfor Al–Si hydrosols, Fitzgerald et al. (1992) suggested

that these colloidal precipitates mainly contained thetetrahedral structural units of Q3mAl as well, such asQ30Al and Q31Al. Moreover, the spectral features of27Al and 29Si MAS NMR for the synthetic amor-

phous aluminosilicates (Miyazaki and Yokoyama,1999), were analogous to those of the surface Al–Siphase formed in this study. As mentioned above,

both the enhanced intensities of Al–O and Si–Obonds after hydroxide back titration in the micro-scopic Raman spectra and the matching XRD

patterns provided by the standard database confirmthe presence of Al–Si complexes or precipitates atillite surface sites, preferable to bulk Al(OH)3. This is

probably caused by the interactions between solublesilicic acids, hydrolyzed Al species and illite surfaces.Consequently, based on macroscopic description

using thermodynamic model and the combined

spectroscopic evidence given in this paper, acidicrelease of substrate components together with thesubsequent formation of surface Al–Si complexes,

may exert a significant influence on the surface acid–base behaviors of the natural illites.

CONCLUSIONS

In the present paper, the surface acid–base

characteristics of different aqueous illites werestudied by potentiometric titration and spectroscopicinvestigation. On the basis of experimental records

and corresponding analyses, the following conclu-sions can be drawn:

(1) Proton reactions in the acidimetric supernatantof illite suspensions can be described by those ofAl3+ and Si(OH)4 in aqueous solution, as well as thecomplex formation of hydroxyaluminosilicate (HAS)

species. It is more reasonable to regard the acidi-metric supernatant as the system blank of the naturalillite suspension when evaluating the surface site

concentration (SSC) due to a consideration of acidicdissolution of structural components during theprolonged titration.

(2) A surface proton-reaction model, assumed thatsurface complexation or precipitation of HAS speciesoccurred at homogeneous surface sites, is proposed

to capture the surface acid–base characteristics of thenatural illite samples from different origins. Withinthe scope of the CCM, the model can givesatisfactory descriptions of the experimental data.

Some similarities are shown in the surface acid–baseproperties ascribable to the similar values of pK int

a ofthe different illites. In addition, the formation

constant for the surface (hydroxy) aluminosilicatecomplexes or precipitates is relatively stable.(3) Based on titration results and the combined

spectroscopic information, the interactions of illitesurface sites with silicic acid and aluminum species,originated from dissolution of illite solids, should be

considered in the model description of surface acid–base chemistry, due to the makeup of surface Si–Alcomplexes or precipitates.

Fig. 8. 27Al MAS NMR spectra: (a) hydrated WC; (b) acidimetric WC; (c) alkalimetric WC; (d) hydratedLPS; (e) acidimetric LPS; (f) alkalimetric LPS.

Wenxin Liu4124

AcknowledgementsFThis study was under the auspices ofLulea University of Technology (LTU), Sweden. Theauthor would express sincere gratitude to Prof. W. Forslingand Doc. Z. X. Sun of LTU for their beneficial instructionand suggestion, and be also grateful to Drs. O. N.Antzutkin, M. Lindberg, L. M. Wu, and Q. Du, and toMrs. M. Renheimer of LTU for their kind assistance onexperimental operation.

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