effect of molybdate adsorption on some surface properties of nano-ball allophane

CLay Science 11,405-416(2001)

EFFECT OF MOLYBDATE ADSORPTION ON SOME SURFACE

PROPERTIES OF NANO-BALL ALLOPHANE

ELSADIG AGABNA ELHADI, NAOTO MATSUE and TERUO HENMI

Faculty of Agriculture, Ehime University, 3-5-7 Tarumi, Matsuyama 790-8566, Japan

(Received April 20, 2001. Accepted August 29, 2001)

ABSTRACT

Cation exchange capacity (CEC) of nano-ball allophane measured at sameequilibrium pH increased with molybdate adsorption at initial molybdate concen-trations of 0.1 and 1.6mM. The increase in CEC was attributed either to deprotonationof silanol group near the adsorption site and/or negative charge carried by themolybdate. Great differences in the amounts of molybdate adsorption between thetwo initial molybdate concentrations (0.1mM and 1.6mM) was not reflected in thechange in CEC. This may be due to polymerization of molybdate at higher con-centrations (more than 0.2mM), therefore, increase in CEC was ascribed to ad-sorption of monomeric molybdate species. Increase in CEC was greater for allophanesamples with lower Si/Al ratios than for allophane sample with higher Si/Al ratio,in agreement with the trend of adsorption. Ab inito molecular orbital calculationsindicated that Bonsted acidity of silanol group of allophane near adsorption siteincreased with the molybdate adsorption. The increase in acidity together with freeMo-OH or Mo-O- groups of molybdate adsorbed in monodentate form contributesthe increase in CEC with the molybdate adsorption.

Key words: Allophane, Molybdate, Adsorption, Surface properties

INTRODUCTION

The poorly ordered or short-range ordered aluminum silicate, nano-ball allophane, isknown to have variable charge characteristics. The charge characteristics are different fromthose of other variable charge minerals such as gibbsite and goethite: it can have both

positive and negative charges simultaneously (Parfitt, 1980), because the main negative andpositive charge sites are separated in the structure of nano-ball allophane. Allophane iscomposed of gibbsite sheet with SiO4 tetrahedra attached to it, and has hollow sphericalmorphology locating the SiO4 tetrahedra inside of the sphere (Parfitt and Henmi, 1980).The positive charge comes from aluminol group (Al-OH) located at the pore region of thehollow sphere, nano-ball, whereas the negative charge comes from silanol group (Si-OH)inside of the nano-ball (Fig. 1).

Adsorption of anions on clay minerals via ligand exchange reaction is likely to causea change in measured surface charge to more negative value. This change in surface chargedistinguishes the ligand exchange reaction from simple anion exchange reactions (McBride,2000). The anion and cation exchange capacities (AEC and CEC) of allophane are alsoknown to change after adsorption of some anions. Increase in CEC and decrease in AECwere observed with phosphate (Johan et al., 1999) and oxalate and citrate (Hanudin et al.,

406 E.A. Elhadi et al.

B A

C

D

FIG. 1. Morphology and structure of nano-ball allophane. A: morphology in section, B: atomic

arrangement near the pore, C and D: atomic arrangement in the cross section near the pore.

2000) adsorption, whereas both CEC and AEC decreased after adsorption of borate (Son,1999). The increase in CEC and decrease in AEC after phosphate adsorption wasattributed to the inductive effect of adsorbed phosphate, which accelerated deprotonationreaction of inherent functional groups of allophane, Si-OH and Al-OH2+ (Johan et al.,1999).

Molybdate adsorption has been investigated for a variety of Al and Fe oxides, clayminerals and soils (Sabine Goldberg and Forster, 1998), but detailed study on the effect

of molybdate adsorption on the charge characteristics is not common. The reaction ofmolybdate on nano-ball allophane was found to be electrostatic reaction, simple anionexchange, followed by ligand exchange reaction between Mo-O- and aluminol groups atlower concentrations up to 0.1 mM where molybdate exists only as monomeric form

(Elhadi et al., 2000). The specifically adsorbed molybdate is expected to have some effecton charge characteristics of nano-ball allophane. The purpose of this paper was thereforeto know the change in charge characteristics of nano-ball allophane with molybdate

Effect of Molybdate Adsorption on Allophane 407

adsorption, and to elucidate the mechanism of the change using molecular orbital analysis

and detailed information of the chemical structure of the nano-ball allophane.

MATERIALS AND METHODS

Allophane samples were separated from inner part of pumice grains collected fromdifferent places in Japan. The sample KyP was collected from Kurayoshi, Tottori pre-fecture, KiP from Kitakami, Iwate prefecture, and KnP was collected from Kakino,Kumamoto prefecture. Fine clay fraction (<0.2 pm) was separated from the inner part ofthe pumice grains after removing the outer part in order to eliminate any possible con-tamination of the sample with impurities such as volcanic glass, opaline silica and imogolite.The separation was carried out by ultrasonification at 28 kHz and then dispersion at pH 4for sample with low Si/Al ratio (KyP and KiP) and at pH 10 for KnP sample with highSi/Al ratio (Henmi and Wada, 1976). The collected samples were flocculated using NaCl,washed with water to remove excess salt and then subjected to freeze-drying. Collectedfreeze-dried samples were analyzed using X-ray diffraction (XRD), infrared spectroscopy

(IR) and thermal analysis; the results obtained suggest that these samples do not containimpurities described above. The Si/Al atomic ratio was 1.34:2, 1.42:2 and 1.98 :2 for KyP,KiP and KnP, respectively.

Molybdate adsorption on the allophane samples was carried out by equilibrating 50 mgfreeze-dried allophane sample with 100 mL of mixed solution of Na2MoO4, NaC1 andwater. The initial molybdate concentration of the mixed solution was 0, 0.1 and 1.6 mM,while total Na concentration was kept at 10 mM by adjusting the amount of NaCl added.The pH of the mixed solution was adjusted from 3 to 10 by using either HC1 or NaOH.The mixture was shaken for 24 h, and then centrifuged at 5000 rpm for 20 min. The pH ofthe supernatant was measured, and the concentration of Mo and Na in the supernatantwas determined by atomic absorption spectrophotometer. The concentration of Cl in thesupernatant was measured by mercury (II) thiocynate method (Huang and Johns, 1966).After decantation, weight of the centrifuge tube plus residue was measured to know thevolume of entrained solution. The adsorbed and entrained Na and Cl were extracted with1 M NH4NO3, and the amounts of adsorbed Na and Cl on the allophane samples (CECand AEC) were calculated.

RESULTS AND DISCUSSION

Effect of molybdate adsorption on charge characteristics of allophaneFigures 2, 3 and 4 show pH dependence of CEC and AEC of KyP, KiP and KnP,

respectively, in 10 mM NaC1 before and after molybdate adsorption. With increasingequilibrium pH, CEC tends to increase whereas AEC tends to decrease for all the cases. Atand below pH 8, the allophane samples have both positive and negative charges. This isattributed to the difference in location of positive charge (Al-OH2+) and the negativecharge (Si-O-) in the structure of nano-ball allophane (Fig. 1). Furthermore, at sameequilibrium pH, KnP sample (high Si/Al ratio) gives higher CEC values as compared toKyP and KiP (Figs. 2, 3 and 4). This is attributed to the difference in chemical structurebetween the samples. The KnP with high Si/Al ratio has higher content of accessory,

polymeric SiO4 tetrahedra, which cause increase in the Ka value of Si-OH group in


FIG. 2. Charge characteristics of KyP sample before and after molybdate ad-

sorption.

allophane (Matsue and Henmi, 1993; Henmi et al., 1997). The fundamental structure ofallophane as proposed by Matsue and Henmi (1993) and Henmi et al. (1997) has Si/Alratio of 0.5, imogolite structure, and additional accessory SiO4 tetrahedra increase theratio. Thus, KnP sample can be described as SiO4 adsorption product of low Si/A1 ratioallophane such as KyP and KiP, and the adsorbed SiO4 tetrahedra caused an increasein the amount of the negative charge. Lower AEC values at the same equilibrium pHobserved for KnP in comparison with KyP and KiP is mainly attributed to its loweraluminol groups content per unit mass: the amount of aluminol groups per unit massdecreases with increasing Si/Al ratio of allophane (Son et al., 1998). Another reason issteric hindrance effect by the polymeric SiO4 tetrahedra on the aluminol groups at the poreregion.


FIG. 3. Charge characteristics of KiP sample before and after molybdate ad-

sorption.

In general, the CEC of allophane sample tend to increase after the molybdate ad-sorption. As shown in Figs. 2, 3 and 4, the CEC of all allophane samples tends to increaseand the AEC tends to decrease after adsorption of molybdate at initial concentrations of0.1 mM and 1.6 mM. The increase in CEC after adsorption would be attributed either tothe charge carried by molybdate or from the deprotonation of silanol group near molybdateadsorption site. On the other hand, decrease in AEC is probably due to neutralization of

positive charge (A1—OH2+) by molybdate anion at lower pH. Although there was a greatdifference in the amount of molybdate adsorbed between at 0.1 mM and at 1.6 mM

(Elhadi et al., 2000), this difference was not reflected in the change in CEC (Figs. 2, 3 and4). This could be related to the fact that at higher concentration Mo existed and adsorbedon allophane as polymeric forms (Elhadi et al., 2000). The change in CEC could, therefore,


FIG. 4. Charge characteristics of KnP sample before and after molybdate ad-

sorption.

can be attributed mainly to the monomeric form of molybdate.From Figs. 2, 3 and 4, net change in CEC (ACEC) with the 0.1 mM molybdate

adsorption was calculated for the three nano-ball allophane samples at initial pHs of 3, 4and 6. The ACEC is the difference in CEC values of allophane before and after themolybdate adsorption at a same pH (equilibrium pH after the adsorption). Figure 5 showsrelationship between molybdate adsorption and ACEC for KyP, KiP and KnP samples.The amounts of molybdate adsorbed on KyP were 20.0, 13.5 and 7.0 cmol kg-1 at pH 3,

pH 4 and pH 6, respectively, and corresponding ACEC were 13.5, 11.7 and 5.4 cmol kg-1.For . KiP sample, similar results were obtained. On the other hand for KnP sample, theamount of molybdate adsorbed was less than that for KyP and KiP samples (14.0, 4.0 and3.2 cmol kg-1 respectively for pH 3, 4 and 6), and corresponding ACEC values were alsosmaller (10.0, 4.7 and 4.5 cmol kg-1) . The change in CEC after adsorption, ACEC, was


KyP

KiP

KnP

FIG. 5. Relationship between amount of molybdate adsorption and increase in

CEC. Initial molybdate concentration was 0.1 mM.

less than the amount of molybdate adsorbed for all the cases, but clear positive rela-tionships were observed between them (Fig. 5).

Proposed mechanism for charge developmentAt the pore region of nano-ball allophane three kinds of aluminol groups, Al-OH,

A1-OH2 and Al-OH2+, exist at lower pH. The reaction between HMoO4- and Al-OH2+can be described as:

412 E. A. Elhadi et al.

(1)

In this equation, the top Al-OH and the bottom Si-OH represent those near molybdate

adsorption site. Reaction 1 releases H2O and causes no change in the solution pH .

However, the observed constancy in the pH with molybdate adsorption doesn't necessarily

mean that neither H+ nor OH- was released with adsorption. Because at pH around 4

molybdate exist as both neutral and anions species, the selective adsorption of anion

species causes shift in chemical equilibrium between neutral and anionic species to release

H+ in solution. Therefore, other reaction with Al-OH group may take place simulta-

neously.

(2)

Reactions 1 and 2 describe adsorption of molybdate as monodentate form. However ,bidentate reaction is also possible at pH above 5 where Mo exist mainly as MoO4 2- . The

scheme of reaction is described below.

(3)

Reaction 3 indicates that 2 OH-are released upon adsorption of molybdate. Slightincrease in the pH at initial pH 6 was observed after adsorption. However, allophane-Mocomplex from equations 1, 2 and 3 has no charge on its surface.

To explain the observed increase in the CEC after adsorption, following reactionequations are proposed on which deprotonation of silanol group near adsorption siteis assumed. Johan et al.(1999) suggested that adsorption of phosphate will acceleratedeprotonation reaction of near silanol group. Thus, equations 1, 2 and 3 can be rewrittenrespectively as:

(4)


(5)

(6)

Reactions 4, 5 and 6 describe the deprotonation of silanol group near molybdate ad-sorption site, which leads to the development of new negative charge on the allophane-Mocomplex. This may explain the apparent increase in the CEC after the adsorption (Figs. 2,3 and 4). Molecular orbital calculations (next section) indicated that these reactions are

possible. Other possibility for development of new negative charge on allophane-Mocomplex is the deprotonation of Mo-OH. The molecular orbital calculations suggest thedeprotonation of Mo-OH is more foreseeable than Si-OH. However, Johan (1999) suggestedthe deprotonation of near silanol group upon phosphate adsorption is more possible thanP-OH. The reactions for the deprotonation of Mo—OH are written as below.

(7)

(8)

Reactions 4 and 7 give H+ as one of the products, and this may explain the observeddecrease in the pH at pH 3 for all samples at molybdate concentration higher than 0.2mM (Elhadi et al., 2000). As described before, at molybdate concentration higher than 0.1mM polymerization of Mo starts. However, from the change in CEC after adsorption, partof Mo will be in the monomeric from. At pH 6 slight increases in the pH was observed.This means reaction 2 (monodentate), 3 and 6 (bidentate) are more likely to take place.Because at pH 6 molybdate exist mainly as MoO42-, bidentate reaction is more possible.

Ab initio calculation of surface acidityThe experimental results showed no change in pH after adsorption of molybdate in the

monomeric form. In addition, no charge develops from reactions 1, 2 and 3. The possibleway, therefore, to explain the apparent increase in CEC is to assume that adsorption ofMo has inductive effect on the silanol group, which leads to dissociation of proton fromsilanol group near the adsorption site. The deprotonation of silanol group provides newnegative charge site on the allophane surface. This may explain the apparent increase in


CEC after the molybdate adsorption. Other possibilities are the deprotonation of Mo-OH,which also leads to an increase in the negative charge of the surface after adsorption.Deprotonation of either silanol group or Mo-OH will not only leads to increase innegative charge but also the surface acidity of allophane-Mo complex.

Siggel and Thomas (1986) defined gas phase acidity of a molecule (A) as the differencebetween the total energy of the anion, Ea, plus that of a proton at infinity, which is zero,and the total energy of the molecule, Em.

A= Ea-Em (9)

Bronsted acidity has been defined as the ability of a molecule to donate proton. It may

be expressed as either the equilibrium constant, Ka, or the free-energy change, ƒ¢G•‹, for

the reaction (Siggel et al., 1988).

RH→ R-+ H+ (10)

In gas phase acidity calculation it has been common to consider only the enthalpy

change (AH•‹), which is calculated as the difference in enthalpy between the energy minimum

of RH and R-. In comparison of the relative acidity of two similar compounds, the

entropy of hydrogen ions can be canceled. According to Bartmess and McIver (1979) the

entropy of proton is the same for all acids. Thus, for two molecules,

RIH→ RI-+ H

RIIH→ RII-+ H,

lower obtained value for AH•‹ indicates stronger Bronsted acidity. In this study, difference

in total energy at the energy minimum between RH and R-, AE•‹, was used instead of

AH•‹ (Siggel et al., 1988). Molecular orbital method used for obtaining AE•‹ was ab initio

RHF/3-21G basis set in Gaussian 98W. Cluster model of nano-ball allophane were built

up with 2 Al octahedra and one Si tetrahedron with the following bond distances:

Al-O=0.1912 nm, Si-O=0.165 nm, O-H=0.095 nm.

Figure 6 shows model clusters used for the ab initio molecular orbital calculations with

and without molybdate adsorption. Model A simulates cluster of original allophane,

whereas models B and C simulates allophane-Mo complexes with adsorbed molybdate

in monodentate (model B) and in bidentate form (model C). In the models, the hydrogens

of Si-OH with asterisk will dissociate, and ƒ¢E•‹ value of the dissociation reaction like

equation 10 was calculated. For model A, allophane without molybdate adsorption, obtained

AE•‹ value was +0.713 a.u. (atomic unit), whereas AE•‹ for models B and C were 0.012 and

0.022 a.u. lower than that of model A. This indicates that Bronsted acidity of silanol group

near molybdate adsorption site increase with the adsorption, and degree of the increase

in Bronsted acidity is greater for bidentate adsorption (model C) than for monodentate

adsorption (model B). In monodentate adsorption (model B), however, there is a free

Mo-OH which releases proton. The ƒ¢E•‹ value of dissociation reaction of this proton was

calculated as +0.565 a.u., indicating stronger acidity of this Mo-OH than that of Si-OH.

Therefore, if molybdate adsorbed on allophane in monodentate form, much of Mo-OH in

the molybdate will dissociate (equation 7), and if MoO4 2-adsorbed in monodentate form,

free Mo-O-will remain deprotonated.


A B C

FIG. 6. Model clusters used for ab initio molecular orbital calculations. A: allophane, B: allophane-

molybdate complex in monodentate form, C: allophane-molybdate complex in bidentate form.

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effect of molybdate adsorption on some surface properties of nano-ball allophane

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