Montmorillonite (Mt) has high cation exchange capacity and thus has been studied extensively for its cation exchange interactions with other cations. However, molecular simulations for the forces governing the cation exchange on Mt surfaces or in the interlayer spaces were limited. In this study, Mt with K+ and Na+ in the interlayer spaces was tested for its cation exchange with Cd2+ in solution and the forces driving the cation exchange reaction were simulated by molecular simulations. The experimental results showed that Na+ in Na + Mt was completely exchanged by Cd2+, while only 50% of K+ in K + Mt was exchanged by Cd2+. A larger
Cadmium (Cd) has been extensively used in the fields of mining, electroplating, electrolyzing, painting, alloying, plastic generation, and textile processing [
Montmorillonite (Mt) is a 2 : 1 phyllosilicate mineral and has high cation exchange capacity (CEC) that can absorb and retain metal cations in its interlayer space to achieve electrical neutrality. Interlayer space hydrated cations are mainly stabilized by electrostatic interactions, and they could be exchanged by other cations in solution. The species of interlayer space hydrated cations would affect interlayer space, surface area, adsorption, swelling, dispersion, and crystal stability of Mt [
Molecular modeling recently has been recognized as an efficient method for understanding the microstructure of interlayer spaces [
In this study, we investigated cation exchange reaction of Cd2+ for K+ or Na+ in the interlayer space of Mt experimentally in conjunction with XRD analyses and MD simulation, in order to provide the driving force for interactions between the Mt layers and the interlayer space cation as affected by the cation radii and the degrees of hydration.
The montmorillonite used was SWy-2 obtained from the Clay Mineral Repositories in Purdue University (West Lafayette, IN) and was used without further purification. It has a chemical formula of (Ca0.12 Na0.32 K0.05)[Al3.01 Fe(III)0.41 Mg0.54][Si7.98 Al0.02]O20(OH)4, a CEC of
The initial Cd2+ concentrations varied from 0.2 to 30 mmol/L for the adsorption isotherm study and were fixed at 2 mmol/L for the kinetic study and pH dependency study. The mass of SWy-2 used was 0.2 g while the volume of solution used was 10 mL for all studies except the kinetic study, in which 20 mL of solution was used. The solid and solution were combined in each 50 mL centrifuge tube and shaken for 2 h at 150 rpm and room temperature for all studies except the kinetic study, in which the shaking time was 1, 3, 5, 30, 60, 120, 600, and 1200 min. After the mixtures were centrifuged at 10000 rpm for 20 min, the supernatants were filtered through 0.22
Free swelling was determined using 1 g of Mt and 100 mL of water in 100 mL graduated cylinder; the mixture was allowed to fully swell with periodic agitation and eventually to set out. The increase of volume after swelling was determined from the difference between the final swelling volume and the initial dry volume of the solid and expressed as mL/g.
Powder XRD analyses were performed on a Rigaku D/max-IIIa diffractometer (Tokyo, Japan) with a Ni-filtered CuK
X-ray fluorescence (XRF) measurements were carried out using a portable XRF spectrometer (Oxford Instruments) with a molybdenum anode, at 25 kV and 0.1 mA. A Si-PIN detector from AMPTEK was employed and characterized by an energy resolution of about 200 eV at 5.9 keV.
FTIR spectra of samples were collected on a Nicolet-560 spectrometer (Thermal Nicolet Co., USA) from 400 to 4000 cm−1 with a nominal resolution of 4 cm−1. For each spectrum 16 runs were collected and averaged. The SWy-2 specimens were prepared by adding approximately 1% of the sample powder to dry KBr powder.
Thermogravimetric (TG) analyses were carried out on TGA Q-500 (TA Instruments, New Castle, USA) from room temperature to 800°C, at a heating rate of 10°C/min under a nitrogen flow of 60 mL/min. TG curves were used to determine the percentage of mass loss. Differential scanning calorimetry (DSC) was performed using a differential scanning calorimeter (TA Instruments Q100) fitted with a cooling system using liquid nitrogen. It was calibrated with an indium standard. Samples of 6 mg of Mt were accurately weighed into aluminum pans, sealed, and then heated from 30 to 800°C at 10°C/min under a nitrogen flow of 60 mL/min.
Molecular simulation was performed under the module “CASTEP” of Materials Studio 6.0 software to investigate the sorption sites of K+, Na+, and Cd2+ on SWy-2. The primitive unit cell of SWy-2 was optimized with the generalized gradient approximation (GGA) for the exchange-correlation potential (PW91) that is appropriate for the relatively weak interactions present in the models studied. The resulting primitive unit cell was characterized by the parameters
The cation exchangeability of Mt is affected by interaction force of interlayer space cation and Mt layer and force of binding water. To investigate the impact of species of interlayer space cation on cation exchangeability, the experiment was designed first to compare properties of K + Mt and Na + Mt, as Na+ and K+ belong to the same main group and have the same electrovalence, but different ionic radii. To keep all conditions the same, except the type of interlayer cations, the Na + Mt was first exchanged with K+ to prepared K + Mt. After cation exchange by K+, the Na + Mt was completely converted to K + Mt (Table
Chemical composition of Mts and their chemical formula.
Chemical composition (%) | Na + Mt | Na + Mt after Cd2+ exchange | K + Mt | K + Mt after Cd2+ exchange |
---|---|---|---|---|
SiO2 | 68.00 | 62.56 | 65.38 | 66.33 |
Al2O3 | 15.43 | 14.60 | 15.16 | 15.23 |
Fe2O3 | 5.64 | 6.75 | 6.62 | 6.55 |
MgO | 3.35 | 2.86 | 3.51 | 2.87 |
CaO | 2.36 | 2.95 | 3.35 | 2.12 |
Na2O | 1.70 | 0.00 | 0.24 | 0.00 |
K2O | 0.00 | 0.00 | 4.73 | 2.04 |
CdO | 0.00 | 9.48 | 0.00 | 3.66 |
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Chemical formula | ( |
( |
( |
( |
The thickness of dehydrated Mt is 9.8 Å, and the interlayer space will be expanded when Na+ and K+ hydrated cations enter the interlayer space. As hydration ability of different cations varies, interlayer spaces of Mts with different cations in the interlayer space differ apparently. K+ concentrations of varying multiples of CEC in solution will result in different amounts of K+ uptake in Mt, which will result in a change in
X-ray diffraction patterns of K-MMT. The
Hydration of cations on interlayer space and outer surface leads to volume expansion of Mt, called free swell. The free swelling of Mt decreased as the amount of K+ uptake increased (Table
Free swelling (mL/g) of raw Na + Mt and Mt after being in contact with KCl of varying amounts for 120 min.
Na + Mt | 0.25 |
0.33 |
0.5 |
0.7 |
1 |
2 |
5 |
10 |
---|---|---|---|---|---|---|---|---|
41.5 | 36 | 30 | 27 | 21.5 | 18 | 14 | 12 | 11.5 |
Interaction forces between Mt layer and interlayer space cation are electrostatic attraction, intermolecular forces, van der Waals forces, and so forth. Owing to combination of these interactions, metal cations stay stable in the interlayer space place. The experiment used Cd2+ to exchange interlayer space cations of Na+ or K+ from Na + Mt or K + Mt. By discussing influence of time and concentration of Cd2+ on exchanging Na+ or K+ from Na + Mt or K + Mt, exchanging rate and volume of two cationic Mts can also be calculated. The difficulty of exchanging cation in Mt interlayer space is relevant with interaction force of interlayer cation and layer, which means stronger interaction force leading to more difficulty in exchange [
The cation exchange process of Cd2+ for Na+ or K+ in the interlayer space of Mt is relatively fast (Figure
Effect of time/min (a) and Cd2+ equilibrium concentration/mmol/L (b) on Cd2+ exchanged quantities.
The results of XRD analyses indicated the progressive completion of Cd2+ exchange for Na in Na + Mt and partial exchange of Cd2+ for K+ in K + Mt. This could be used to determine element composition and content of exchanged Na + Mt and K + Mt and then to calculate the amount of cations exchanged. XRF results showed that the content of Na+ for Na + Mt after being exchanged by Cd2+ is 0 (Table
The effect of initial solution concentration on exchange capacity had a similar result; that is, the interlayer cation Na+ in Na + Mt can be completely exchanged by Cd2+ ions, the amount of exchange increases as Cd2+ concentration increased, and exchange balance was achieved when the input Cd2+ was 3 times the CEC. The discrepancy in exchange amounts of Mts with different interlayer space cations is due to diverse interaction forces of layers and interlayer cations [
It can be indicated from interlayer space of Mt (Figure
X-ray diffraction patterns of Na-MMT cations exchanged with Cd2+ (
X-ray diffraction patterns of K-MMT cations exchanged with Cd2+ (
The FTIR spectra of Mt in contact with different amounts of Cd2+ were illustrated in Figure
FTIR analyses of K-MMT, Na-MMT, and Cd-MMT.
For K + Mt, as the amount of Cd2+ uptake increased, the band within 1630~1640 cm−1 increased gradually, while band at 1447 cm−1 disappeared. Bands at these two positions are attributed to bending vibration of hydroxyl in adsorbed water, which means changes in interaction force between Mt layer and interlayer cation would lead to variation in position and intensity of these bands. However, the band at 1035 cm−1 representing bending vibration of Si-O-Si in Mt lattice and band at 3625 cm−1 representing bending of Al-O-H in Mt structural water both decrease as intercalation amount of Cd2+ increases, meaning interaction force of K+ and Mt layer was stronger, leading to a minor change in Mt structure that affected force in layer. For Na + Mt, as Cd2+ exchange preceded, except for disappearance of absorption band at 1447 cm−1 and increase in transmittance at 1648 cm−1, bands at other positions showed no apparent changes. It means that Na + Mt had a weaker interaction force between the Mt layer and interlayer cations in comparison to K + Mt. This would lead to an easy exchange of Cd2+ for Na+.
The mass change within 25~175°C was due to removal of adsorbed surface and interlayer space water of Mt [
TG and DTA curves of K-MMT (a, b), Na-MMT (c, d), and Cd-MMT.
Based on the aforementioned experimental results, CASTEP module in Material Studio 6.0 was used to calculate energies of interactions between Mt layers and different cations in the interlayer space. The optimal configuration was speculated when stable cations coexist with Mt layer. The energy of interactions was −78425, −76537, and −75480 eV for Cd−, Na−, and K + Mt, respectively, which is positively related to the interlayer space of these three Mts and negatively related to the ionic radii of Cd2+, Na+, and K+. Our results agree well with general observations that the smaller the cation, the more hydrated the cation. Apart from that, the interlayer space is also determined by hydration of cation; the more the water molecules surrounded a cation, the greater the interlayer space is [
The interlayer space of K + Mt, Na + Mt, and Cd + Mt was 10.55, 12.5, and 14.81 Å, respectively. MD simulation showed that minimum distances from K+, Na+, and Cd2+ to oxygen in Mt silicate-oxygen tetrahedron would be 2.303, 2.443, and 4.648 Å, respectively (Figure
Molecular dynamic simulation of the distance and angle of K+ (a, b), Na+ (c, d), and Cd2+ (e, f) with [SiO4].
One Na+ cation is surrounded by two water molecules, while distances from the central cation to the two water molecules were different. Distances from K+ or Cd2+ to oxygen in water molecules were all longer than 2.5 Å, impossible to form a chemical bond. In addition, the position of Cd2+ to water differs from that of Na+ and K+, as Na+ and K+ are surrounded by two water molecules, monolayer arranged, while Cd2+ is surrounded by four water molecules and bilayer arranged. This would lead to expansion of the interlayer space to 14.81 Å. The interaction force of K+ and Mt layer is the strongest, and that of K+ and water molecular is the weakest; interaction force of Na+ and water is the weakest, resulting in a higher free swelling of Na + Mt.
Cd2+ was tested to exchange with different interlayer cations in Mts. By comparing exchange capacity, interlayer space, and energy, we discovered great diversity in degrees of cation exchange of different Mts. Na + Mt can be thoroughly exchanged by Cd2+. Conversely, only 60% of K in Mt can be exchanged by Cd2+ at an input amount equivalent to 3 times the CEC. The result of XRD indicated that exchange of Na+ by Cd2+ in Mt was uniform and complete, and molecule dynamics simulation proved that electrostatic force of K+ is larger than that of Na+, showing an easier cation exchange of Cd2+ for Na+. A better swelling was found for Na + Mt compared to K + Mt, suggesting a better hydration of Na+ in the interlayer space of Mt. The result of molecule dynamic simulation showed energies of three Mts following Cd + Mt < Na + Mt < K + Mt. Interaction force with Mt layer followed K+ > Na+ > Cd2+. Therefore, Mts with diverse interaction cations have different properties and should be used in different fields.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This present work was supported by the International S&T Cooperation (Grant no. 2014DFA91000).