J[A1] . Cent. South Univ. Technol. (2008) 15: 193-197

DOI: 10.1007/s11771-008-0037-4

Chemical composition and surface charge properties of montmorillonite

LIU Xiao-wen(刘晓文), HU Min(胡 敏), HU Yue-hua(胡岳华)

(School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China)

Abstract: The effects of the cell parameter and chemical composition on the surface charge properties of five kinds of different colour montmorillonites were studied. The results indicate that the surface isoelectric point(IEP) of the montmorillonite shows positive correlation with the mass fractions of Fe₂O₃ and K₂O, but it has little relation to the mass fractions of other chemical compositions. At around pH=6.8, the surface zeta potential of the montmorillonite shows the negative relationship with the mass fractions of Fe₂O₃ and MgO, but it does not linearly correlate to the mass fractions of other chemical compositions. Cell parameter(b₀) of the montmorillonite expresses negative linear relationship with mass fractions of K₂O and Na₂O, so does c₀sin β with mass fractions of SiO₂ and Fe₂O₃. And there is no specific relationship between b₀ and IEP of different montmorillonites, but there is positive correlation between c₀sinβ and IEP of different montmorillonite samples.

Key words: surface charge property; zeta potential; cell parameter; montmorillonite

1 Introduction

Montmorillonite is of a 2?1 layered structure, a single layer of aluminum octahedral sheet sandwiched between two layers of silicon tetrahedral sheets^[1-2]. It is well known that montmorillonite particles carry two kinds of electric charges: a variable (pH dependent) charge resulting from proton adsorption/desorption reactions on the surface of hydroxyl groups and a structural negative charge resulting from isomorphous substitutions within the clay structure. As a consequence of this negative potential, montmorillonite has negative electrophoretic mobility and very important cation adsorption and cation exchange properties^[3]. The surface charge properties of montmorillonite have been investigated by many researchers^[4-9]. Montmorillonite has a large cation exchange capacity^[5]. The substitution of Fe²⁺ and Mg²⁺ atoms for Al³⁺ in the octahedral layer creates a positive charge deficit, giving the overall struc- ture a net negative charge. The isomorphic substitution is the origin of the permanent charges that exist on the surface of montmorillonite. The OH and O atoms on the broken edges of montmorillonite hydrolyze and form Lewis acid or Lewis base functional groups that are the sources of the pH-dependent charge. Therefore, based on the molecular structure of montmorillonite, cations can be adsorbed by either electrostatic attraction on the basal plane or formation of covalent bonds with the functional groups on the broken edges^[10-12].

Zeta potential represents the electric potential in the shear plane between a particle and the surrounding liquid when the charged particle moves in an electric field^[13]. So in this work, the relationship between the chemical composition and zeta potential, the isoelectric point(IEP) and cell parameters of montmorillonite was investigated.

2 Experimental

2.1 Materials

Five kinds of different colour montmorillonite samples were from Xinyang, Henan. They were crushed and ground by an agate mortar to less than 5 μm for X-ray diffraction analysis and zeta potential measure- ment. The particle sizes of the samples were measured using a Delsa440sx zeta instrument. Analytical grade HCl and NaOH were used for pH adjustment. The high- purity water used in the experiments was distilled water.

2.2 Methods

2.2.1 X-ray diffraction

To determine mineral phases of the samples, a powder diffractometer (Shimadzu) in the reflection mode was used with graphite-filtered Cu K_α as radiation source (40 kV, 300 mA). The goniometer runs from 3? to 80? at a 2θ speed of 10 (?)/min.

2.2.2 Zeta potentials

The zeta potential and particle size were measured by standard procedures on a Beckman Coulter zeta potential instrument (Delsa440SX) simultaneously. The

montmorillonite was added into a 40 mL beaker. The suspension was put into the SK7200LH ultrasonic cleaning machine for 2 min and transferred to the testing vessel, and then the zeta potential measurement was made. pH value was measured to an accuracy of ±0.02(REX Model PHS-3C pH meter).

3 Results and discussion

3.1 XRD patterns

The XRD patterns of the samples are shown in Fig.1. X-ray diffraction analysis indicates that five kinds of different colour montmorillonite samples are essentially pure. The basal spacing d₍₀₀₁₎ values of these samples vary from 1.522 46 to 1.565 65 nm, suggesting that the samples are calcium-montmorillonites.

Fig.1 XRD patterns of different montmorillonite samples

The (060) plane is very important for montmorillonite^[14]. And d₍₀₆₀₎ can be used to distinguish the type of smectite. When d₍₀₆₀₎=0.149 0-0.151 0 nm, it belongs to dioctahedral montmorillonite; when d₍₀₆₀₎= 0.153 0-0.155 0 nm, it belongs to trioctahedral smectite. The difference of d₍₀₆₀₎ is caused by substitution of  Mg²⁺ for Al³⁺, and the more the Al³⁺ substituted by Mg²⁺ is, the larger the d₍₀₆₀₎ is. The d₍₀₆₀₎ values of samples vary

from 0.149 95 to 0.150 04 nm, so they belong to dioctahedral montmorillonite. Compared with JCPDS (Joint Committee Powder Diffraction Standard), the samples belong to calcium-montmorillonite.

3.2 Zeta potentials

The zeta potentials of the montmorillonite as a function of pH value are shown in Fig.2. The results suggest that the zeta potentials of different montmorillonites have similar trend. But there are different zeta potentials among the five kinds of different colour montmorillonites when measured under the same conditions. It can be attributed to that the zeta potential is dependent on the pH value of solution, mineral phase, crystal imperfections, chemical compositions, crystal forms and crystal surface structure^[15]. The IEP of the montmorillonite varies from 0.28 to 1.05 by extrapolation (Table 1).

Fig.2 Zeta potential—pH chart of different montmorillonite samples

3.3 Chemical compositions

The particle sizes, chemical compositions of the five kinds of different colour montmorillonite samples are shown in Table 2.

Table 1 IEP of different montmorillonite samples

Table 2 Particle sizes and chemical compositions of montmorillonite samples

Two selected plots of the chemical compositions vs IEP for the five samples are shown in Fig.3. Mass fraction of Fe₂O₃ of the montmorillonite shows positive correlation with IEP. Its correlation coefficient is 0.622; and mass fraction of K₂O shows positive correlation with IEP. Its correlation coefficient is 0.799. Other chemical compositions such as the mass fractions of Al₂O₃, SiO₂, CaO, MgO and TiO₂ in Table 2 show little correlation with IEP.

Fig.3 Plots of chemical composition vs isoelectric point(IEP) of different montmorillonite samples: (a) Mass fraction of Fe₂O₃; (b) Mass fraction of K₂O

The crystal structure of montmorillonite is similar to that of illite. If the pH value of the isoelectric point of the edge surface of the montmorillonite is thought to be 6.8, the surface zeta potential of the montmorillonite may mainly represent the electric potential in the shear plane between the surrounding liquid and the base plane (001) surface at pH=6.8^[16]. Fig.4 shows chemical compositions of the different montmorillonite samples vs the zeta potentials at the point of solution pH=6.8. The surface zeta potential shows negative correlation with mass fraction of Fe₂O₃ in the montmorillonite; its correlation coefficient is -0.684; the surface zeta potential also shows negative correlation with mass fraction of MgO in the montmorillonite, its correlation coefficient is -0.796. The results suggest that ferrous and magnesic ions are readily released into the solution as electrolytes. Higher negative zeta potential is related to a higher level of exchangeable or soluble ferrous and magnesic ions in clays.

Fig.4 Plots of chemical composition vs zeta potential of different montmorillonite samples at pH value of 6.8: (a) Mass fraction of Fe₂O₃; (b) Mass fraction of MgO

3.4 Cell parameters

The cell parameters of the montmorillonite are calculated and shown in Table 3. It is found that b₀ is close to its theoretical value (0.894 nm)^[17].

Table 3 Cell parameters of montmorillonite samples

Fig.5 shows the chemical compositions vs cell parameters of different montmorillonite samples. Mass fractions of K₂O and Na₂O show the negative linear correlation with b₀ of montmorillonite, so does mass fractions of SiO₂ and Fe₂O₃ with c₀sinβ.

Fig.6 shows the IEP vs cell parameters of different

Fig.5 Plots of chemical compositions vs cell parameters of different montmorillonite samples: (a) Mass fraction of K₂O; (b) Mass fraction of Na₂O; (c) Mass fraction of SiO₂; (d) Mass fraction of Fe₂O₃

Fig.6 Plots of IEP vs cell parameters of different montmorillonite samples

montmorillonite samples. There is no specific relationship between b₀ and IEP of different montmorillonite samples, whereas there exists positive correlation between IEP and c₀sinβ. It may be caused by the ions in the interlayer.

4 Conclusions

1) IEP of the montmorillonite varies from 0.28 to 1.05 by extrapolation, and IEP shows positive correlation with the mass fractions of Fe₂O₃ and K₂O. At around pH=6.8, the mass fractions of Fe₂O₃ and MgO of the montmorillonite show the negative relationship with the surface zeta potential.

2) The mass fractions of K₂O and Na₂O of the montmorillonite express negative correlation with b₀, so does mass fractions of SiO₂ and Fe₂O₃ with c₀sinβ value.

3) There is no specific relationship between b₀ and IEP, but there is positive correlation between c₀sinβ and IEP. So much further work is needed to fully understand the mineral-solution interface reaction.

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(Edited by CHEN Wei-ping)

Foundation item: Project(2005CB623701) supported by the Major State Basic Research Development Program of China

Received date: 2007-07-10; Accepted date: 2007-10-09

Corresponding author: HU Yue-hua, Professor; Tel: +86-731-8879815; E-mail: hyh@mail.csu.edu.cn

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