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C: Surfaces, Interfaces, Porous Materials, and Catalysis

To Distinguish Electrostatic, Coordination Bond, Nonclassical Polarization and Dispersion Forces on Cation-Clay Interactions Dian Liu, Wei Du, Xinmin Liu, Rui Tian, and Hang Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08133 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 4, 2019

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The Journal of Physical Chemistry

To Distinguish Electrostatic, Coordination Bond, Nonclassical Polarization and Dispersion Forces on Cation-Clay Interactions Dian Liu, Wei Du, Xinmin Liu, Rui Tian* and Hang Li*

College of Resources and Environment & Chongqing Key Laboratory of Soil Multi-Scale Interfacial Process, Southwest University, Chongqing 400715, China

AUTHOR INFORMATION Correspondences: Rui Tian E-mail: [email protected] Hang Li E-mail: [email protected]. Phone: 086-023-68250674. Fax:086-023-68250444. ORCID Rui Tian: 0000-0002-8849-7706 Hang Li: 0000-0002-8486-6631 Notes The authors declare no competing financial interest.

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Recent researches have suggested inorganic ions give rise to complex interfacial adsorption effects, but people do not fully understand the mechanisms at present. In this study, the interface adsorption energies of H+ (without extranuclear electron), Li+ and Cs+ (with extranuclear electrons but possessing a large difference in ionic radius) on montmorillonite surface were estimated to elucidate the contribution of electrostatic, coordination bond, nonclassical polarization and dispersion forces to interface adsorption energies. The results showed that under given cationic concentrations, the equilibrium adsorption energies followed the sequence of Cs+ > H+ > Li+. Moreover, the adsorption energies of H+ (with minimum ion-radius) was close to Cs+ (with largest ion-radius) but much larger than that of Li+ under relative low cationic concentrations, whereas the adsorption energies of Cs+, H+ and Li+ approached each other under the highest cationic concentration of 0.1 mol L-1, although their ionic sizes are in great difference. With these results, we conclude: for Li+, the observed adsorption energy could be fully explained by the classic electrostatic force; for H+, the non-electrostatic adsorption energy was from the coordinate bond between H+ and O atom at surface, and the coordinate bond adsorption energy of H+ was electric field-dependent; for Cs+, under relative low electrolyte concentrations, the non-electrostatic adsorption energy was from the non-classic polarizability of Cs+, and under the high electrolyte concentration of 0.1 mol L-1, the non-electrostatic adsorption energy was from the dispersion force of Cs+ and NO3- through ion pair adsorption.


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1. Introduction It has long been found that the adsorption strength of alkali metal ions on charged clay minerals followed the sequence of Li+ < Na+ < K+ < Rb+ < Cs+.1,2 This effect is commonly referred to as specific ion or Hofmeister effect, which is important in adsorption processes of inorganic ions onto particle, such as metal oxides, clay minerals, polymeric materials, as well as proteins surfaces.3-5 For a long time, people believed that specific ion effects were caused by ion hydration, and thought that the larger the ion was, the smaller the hydration radius and thus the stronger the electrostatic adsorption force would be. Since the univalent metal ion radius follows Li+ < Na+ < K+ < Rb+ < Cs+, and the ion hydration radius follows Li+ > Na+ > K+ > Rb+ > Cs+, the adsorption strength of the ions eventually follows Li+ < Na+ < K+ < Rb+ < Cs+. Nevertheless, new evidences in recent ten or more years have shown that the origin of the specific ion effects was far from being as simple as ion hydration, and possibly important new scientific foundations may embed in the specific ion effects.6 Parsons et al. pointed out that the scientific connotation of specific ion effects was far more profound than that of the concept of simple as ion hydration.7 In paper published in Nature Chemistry, Jungwirth and Cremer even indicated that the correctness of ion hydration to explain ion specific effects had never been confirmed.8 In paper published in Science, Tobias and Hemminger indicated that specific ion effects will continue to challenge all relevant theories.9 The most common explanations for specific ion effects involve ion hydration,10-12 ion size,13-15 quantum fluctuation forces (dispersion effect),16 and electrostatic and induction forces.17 In fact, all these effects may be intertwined in a complex system and together determine the Hofmeister effects.18,19 The recent work of Liu et al.20 illustrated that non-classical polarization of ions would be an important reason for the Hofmeister effects of adsorbed ions on clay mineral surface. The related studies by Li et al. have pointed out that the ionic non-classic polarization could explain the observed specific ion effects occurring at clay surface, and they found that 3 / 23 ACS Paragon Plus Environment


the adsorbed cations can be strongly and non-classically polarized by the electric field of 108-109 V m-1 setting up by the surface charges of clay, and the observed polarizabilities were hundreds of times larger than the classical values of polarizability,21-27 Du et al. further estimated the excess adsorption energy of cation arising from the non-classical polarization, and found this energy could reach to an intensity of more than twice the electrostatic energy of the cation. 28 It is no doubt that the interfacial adsorption effects of inorganic ions on clay mineral surfaces is complex, while the mechanism of these complex effects are so far not fully comprehended. H+ is a very special cation with the following three features. Firstly, H+ does not possess electrons, thus no polarization could occur. Secondly, compared with other monovalent cations, the radius and the hydration radius of H+ are the smallest, thus it can produce the strongest electrostatic adsorption energy. Thirdly, the vacant orbital of H+ could accept lone-pair electron to form coordinate bond. Li+ is a cation with the least electrons and bears the smallest ionic radius (0.076 nm), and thus exhibits the weakest polarizability in alkali cations (0.029 Å3). As a result, the induction and dispersion forces could be neglected in Li+-clay interactions,22 and only the classic Coulomb force of the strongly hydrated cation should be taken into account. Comparatively, Cs+ is the largest (0.167 nm in radius) and bears the strongest polarizability (2.56 Å3). Therefore, the comparison study of the adsorption energy among the three special cations may provide an approach to estimate the respective contribution of the four fundamental interaction forces, electrostatic, coordination bond, non-classical polarization and dispersion forces, on cation-clay interaction. The aim of this study is to distinguish the four different forces on cation-clay interactions, through estimation the excess adsorption energies of the three cations at montmorillonite surface respectively.

2. Materials and methods 2.1. Preparation of material and sample 4 / 23 ACS Paragon Plus Environment

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Montmorillonite was employed as the experimental material, which was purchased from Wu Hua Tian Bao Mineral Resources Co., Ltd., Inner Mongolia, China. Based on the combined determination method,29 the specific surface area and cation exchange capacity (CEC) of the clay mineral were determined as 725 m2 g-1 and 1150 mmol(-) kg-1; the hydrodynamic diameter of the montmorillonite particles ranged from 100 to 550 nm with the maximum in the size distribution at 230 nm; the counter-ion of the original sample was Ca2+. The original sample was analyzed by X-Ray Diffraction (XD-3, Beijing Purkinje General Instrument Co., Ltd., Beijing, China), and the main peaks represented the layer spacing were 15.743 Å, which were in the range of the montmorillonite diffraction peaks (10-15.4 Å). For obtaining the H+- or Cs+-saturated sample, every sample was weighed into a 200 mL triangle bottle and washed successively by dispersion, centrifugation, and decantation with three portions of 0.1 mol/L HNO3 or CsNO3 solution respectively, then three portions of ultrapure water. Following the final decantation of supernatant liquid, the clay mineral was dried at 298 K, crushed and sieved through a 0.25-mm mesh for the cation exchange experiments. 2.2 Ion adsorption kinetics Miscible displacement technique was employed to the adsorption kinetics studies. 30 The experimental installation was shown in Fig. 1. Here Li+ adsorption onto H+ saturated clay (Li+/H+ exchange) were taken as an example to illustrate the experimental procedure. Approximate 0.5 g H+ saturated sample was placed on the exchange column. The spreading area of the sample was approximately 15 cm2 and the thickness of the sample was 0.02 – 0.03 cm. Then the exchange solutions (LiNO3) with given concentration (0.0001 mol L-1, 0.01 mol L-1 and 0.1 mol L-1 respectively) flowed across the H+ saturated sample layer with a constant flow of 1 mL min-1. The exchanged effluent was collected by tubes settled on the automatic collector (DBS-100, Shang-Hai QPHX Instrument Co., Ltd., Shanghai, China) every 10 5 / 23 ACS Paragon Plus Environment


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minutes. The concentrations of Li+ and Cs+ in effluents were determined by the flame photometer (AP1401, Shang-Hai AP Analysis instrument Co., Ltd.) while the concentration of H+ was estimated by iterative calculations with the ion activity measured by the pH meter.31 The amount of adsorbed cations was calculated by the concentration difference before and after each exchange experiments.

Fig. 1. The experimental installation for cation exchange.

2.3 Measurement of zeta potential The zeta potentials of H+-montmorillonite under different Cs+ concentrations were measured with the ZEV3600 Zeta potential analyzer, and Cs+ concentration (CsNO3) was set as 0, 0.01 and 0.1 mol L-1. 2.4 Data process for ion adsorption kinetics Previous studies had shown that the essence of ion exchange adsorption is the diffusion of ions in the external electric field formed by the surface charge of clay particles.32 In light of this, a mechanistic model of ion exchange adsorption kinetics, indicating the existence of first-order and zero-order kinetics characteristics, was established.

33-35

When ions were

strongly adsorbed on the surface (such as chemical bonds or strong electrostatic forces), the ion adsorption process would follow the zero-order rate equation: dN X (t )  kX ( 0 ) N X (t )0 dt

(1)

in which: k X( 0 ) 

 2 DX 4l 2

l

Sf 0 (X)  e 0



 X ( x ) Z X F ( x ) RT

dx 

 2 DX 2l

Sf 0 (X) e




 X ( 0 ) Z X F ( 0 ) 2 RT

(2)

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where NX(t) is the cation adsorption amount; kX(0) is the zero-order rate constant for the cation; NX(t) is the cation adsorption amount or equilibrium adsorption amount when t is approaching  (t)；DX is the ionic diffusion coefficient; l is the average thickness of the fixed liquid adjacent to the particle surface; S is the specific surface area of the sample; f0(X) is the ionic concentration in bulk solution; R and T are the gas constant and absolute temperature, respectively; γX(x) is the effective charge coefficient taken the effective charge of polarized ions at interface x into consideration; F is Faraday’s constant; ZX is the original valence of the cation; ψ(x) is the potential at position x. If the strong-force adsorption of the cation (there is only normal weak electrostatic effect) was not experimentally measured, a first-order adsorption rate would be observed, which can be expressed as:34,35  dN X (t ) N X (t )   k X(1) 1   dt  N X t    

(3)

in which k X(1) 

 2 DX 4l 2

l

Sf 0 (X)  e



 X ( x ) Z X F ( x ) RT

0

dx 

 2 DX 2l

Sf 0 (X) e



 X ( 0 ) Z X F ( 0 ) 2 RT

(4)

and

N X t     Sf 0 (X)  e l



 X ( x ) Z X F ( x ) RT

0

dx  2 Slf 0 X e



 X ( 0 ) Z X F ( 0 ) 2 RT

(5)

The above kinetic model earns the label of the "mechanism model" because the physical constants in which possess definite physical meanings. In addition, Eqs. (2) and (4) explicitly show how physical parameters such as ion diffusion coefficient, ion diffusion distance, particle specific surface areas, particle surface potential, ion valence, ion non-classical polarizability, and the medium temperature influence the ion adsorption rate.


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In theory, considering the strong-force absorption occurs firstly and followed by weak-force adsorption, if the strong-force adsorption present, the zero-order rate process will appear firstly and then change to the first-order rate process with weak-force adsorption as shown in Fig. 2. Remained charge number y (cmol(-)/kg)

●K+, ○ Na+ kX(1)

A

kX(0)

Zero-order

dNX(t)/dt

First-order

y=98.6

NX(t)

NX(t→∞)

Fig. 2. Schematic diagrams of cationic adsorption kinetics curves as strong force adsorption presence.

When ion adsorption only involves weak-force adsorption (for example, the electric field is strongly shielded by cations), the ion adsorption process will merely exhibit a first-order rate characteristic as shown in Fig. 3. ●K+, ○ Na+ kX(1)

A First-order

dNX(t)/dt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Remained charge number y (cmol(-)/kg)

M


y=98.68-21.8

NX(t)

NX(t→∞)

Fig. 3. Schematic diagrams of cationic adsorption kinetics curves as strong force adsorption absence.

3. Results and discussion 3.1 Experimental results of X+(Li+or H+) adsorption kinetics in X+-Cs+ exchange In X+-Cs+ exchange, the adsorption amount of X+ (Li+ or H+) changed with time was shown in Fig.S1 (see Supporting Information). Considering dNX(t)/dt ≈ [NX(tm+1) − NX(tm)]/(tm+1 − tm) and NX(t) ≈ NX(tm) + 0.5[NX(tm+1) − NX(tm)], in which NX(tm+1) and NX(tm) 8 / 23 ACS Paragon Plus Environment

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represented adsorption amounts of ions at tm+1 and tm respectively, and m = 0, 1, 2, 3,……. 29,32,36.

Based on the data shown in Fig.S1 (see Supporting Information), the relationship of

dNX(t)/dt vs. NX(t) could be obtained and shown in Fig. 4. 20

2500

25000

-t ) (mmol kg-1 min-1)

m+1 m

0.1 mol L-1 Li+

0.01 mol L-1 Li+

0.0001mol L -1 Li+

103[NX+1-NX] / (t

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15

2000

20000

1500

15000

1000

10000

10

5

500

y1=-1.0694x+16.856 R =0.9887 4

8

5000

12

16

30

0

20

30

60

90

120

150

1200

0.0001 mol L-1 H+

0

1000

20

800

16000

600

12000

y0=24.271

10

400

y1=-0.2078x+41.067

5 0

200

R2=0.9572 25

50

75

R2=0.9931

100 125 150 175 200

0

200

400

24000

0.01 mol L-1 H+

25

15

y1=-26.931x+24169

R2=0.9935

2

0

y1=-20.659x+2492.2

600

800

1000

1200

0.1 mol L-1 H+

20000

8000

y1=-3.0397x+969.64 2

R =0.973 50

100

4000 150

200

250

300

350

400

0

y1=-19.99x+21563 R2=0.9918 200

400

600

800

1000

1200

NX(tm+1/2) (mmol kg-1)

Fig. 4. The relationship of dNX(t)/dt vs. NX(t) (X = Li or H) under electrolyte concentrations of 0.0001, 0.01 and 0.1 mol L-1 respectively in X+-Cs+ exchange.

From Fig. 4, we can see that Li+ adsorption kinetics in Li+-Cs+ exchange only exhibited the weak-force first-order adsorption at all the set electrolyte concentrations. On the other hand, the H+ adsorption only exhibited the weak-force first-order adsorption when the H+ concentration was as high as 0.01 mol L-1. However, when the H+ concentration was as low as 0.0001 mol L-1, the H+ adsorption showed the strong-force zero-order adsorption. In addition, for a given ion, the higher the ion concentration was, the faster the adsorption rate would be, and the ion adsorption rate could be directly characterized by the corresponding rate coefficient. For example, when the Li+ concentrations were 0.0001, 0.01 and 0.1 mol L-1, the adsorption rate coefficients were 16.856, 2492.2 and 24169 mmol kg-1 min-1, respectively. Here is another example, when the H+ concentrations were 0.0001, 0.01, and 0.1 mol L-1, the adsorption rate coefficients were 41.067, 969.64 and 21563 mmol kg-1 min-1, respectively. 9 / 23 ACS Paragon Plus Environment


However, for different ions, since the equilibrium adsorption amounts were different under a given electrolyte concentration, the adsorption rate coefficient could not be used to characterize the difference in adsorption rates of different ions. According to Eq (3), the NX(t) value at dNX(t)/dt = 0 is the equilibrium adsorption amount NX(t→∞) of the ion. Fig. 4 shows that, with the increase of ion concentrations, the NX(t→∞) increased. For instance, when Li+ concentrations increased from 0.0001, 0.01, to 0.1 mol L-1, the equilibrium adsorption amounts of NLi(t→∞) increased from 15.76, 120.6, to 897.4 mmol kg-1, respectively; similarly, when H+ concentrations increased from 0.0001, 0.01, and 0.1 mol L-1, the equilibrium adsorption amounts of NH(t→∞) increased from 197.6, 319.0, 1079 mmol kg-1, respectively. Those results indicated that, as the ion concentrations of Li+ and H+ increased from 0.0001, 0.01, to 0.1 mol L-1, the NH(t→∞)/NLi(t→∞) decreased from 12.54, 2.645 to 1.202. The results also show that at the all concentrations of ions, the equilibrium adsorption amounts of Li+ and H+ were lower than the CEC of montmorillonite, which indicated that the adsorption equilibrium observed in the experiment was actually a metastable equilibrium rather than a true equilibrium. However, as H+ concentration was 0.1 mol L-1, the NH(t→∞) approached to the CEC. 2.2 Experimental results of X+(Li+or Cs+) adsorption kinetics in X+-H+ exchange In X+-H+ exchange, the adsorption amount of X+ (Li+ or Cs+) changed with time was shown in Fig.S2 (see Supporting Information). Similarly, the relationship between dNX(t)/dt and NX(t) for X = Li+ and Cs+ can be obtained from the data in Fig.S2 (see Supporting Information), and the results are shown in Fig. 5. From Fig. 5, the following points could also be seen. Firstly, the Li+ adsorption kinetics in X+-H+ exchange also only exhibited the weak-force first-order adsorption at all the set electrolyte concentrations. Secondly, the Cs+ adsorption only exhibited the weak-force first-order adsorption when the concentration of Cs+ reached 0.01 mol L-1 or higher. However, when the Cs+ concentration was as low as 0.0001 10 / 23 ACS Paragon Plus Environment

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mol L-1, the Cs+ adsorption showed the strong-force zero-order adsorption. This was also consistent with the experimental results of H+ adsorption in H+-Cs+ exchange. Thirdly, for a given ion of Li+ or Cs+, the higher the ion concentration was, the faster the adsorption rate would be. For example, when the Li+ concentrations were 0.0001, 0.01 and 0.1 mol L-1, the adsorption rate coefficients were 14.78, 409.2 and 25330 mmol kg-1 min-1, respectively; and when the Cs+ concentration were 0.0001, 0.01, and 0.1 mol L-1, the adsorption rate coefficients were 18.307, 2094.6 and 28217 mmol kg-1 min-1, respectively. These results suggested that the rate coefficient of Cs+ adsorption in Cs+-H+ exchange was higher than that of H+ adsorption in H+-Cs+ exchange, but the case was just on the contrary when the ion concentration was as low as 0.0001 mol L-1. 20

2500

25000

0.01 mol L-1 Li+

15

0.1 mol L-1 Li+

2000

20000

1500

15000

1000

10000

10

5

5

10

30 25

0.0001mol L-1 Cs+

20 15 10 5 0

500

y=-0.8652x+14.78 R2=0.9701

y1=-16.964x+2409.2

5000

2

R =0.9883 15 103[Ni+1-Ni]/(t -ti)/mmol·kg-1·min-1 i+1

0

m+1 m

-t ) (mmol kg-1 min-1)

0.0001 mol L-1 Li+

103[NX+1-NX] / (t

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

25

0

50

100

150

200

2500

19

R2=0.9939 200

160

200

240

280

y1=-0.0739x+20.126

10 20 30 40 50 60 70 80 90 100 110 120

1200

15000

1000 R2=0.9837

1000

20000

1500

Ni(tm+1/2)/mmol·kg-1

y0=18.3067

800

0.1 mol L-1 Cs+

16

120

600

25000

2000

17

400

30000

0.01 mol L-1 Cs+

18

15

0

y1=-23.825x+25330

500 0

10000

y1=1.7779x+2094.6 2

5000

R =0.9903 200

400

600

800

1000

1200

0

y1=-14.958x+28217 R2=0.9942 400

800

1200

1600

2000

2400

NX(tm+1/2) (mmol kg-1)

Fig. 5. The relationship of dNX(t)/dt vs. NX(t) (X = Li or Cs) under electrolyte concentrations of 0.0001, 0.01 and 0.1 mol L-1 respectively in X+-Cs+ exchange.

Fig. 5 also showed that, for Li+ and Cs+ adsorption in Li+-H+ and Cs+-H+ exchange, the higher the ion concentration was, the greater the equilibrium adsorption amounts would be observed. Under the Li+ concentrations of 0.0001, 0.01 and 0.1 mol L-1, the equilibrium adsorption amounts were 17.08, 142.0, and 1063.2 mmol kg-1, respectively. Compared with 11 / 23 ACS Paragon Plus Environment


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the equilibrium adsorption amounts of Li+ in Li+-H+ exchange, the equilibrium adsorption amounts of Li+ in Li+-Cs+ exchange was smaller under the same concentration, which implied that Li+ exchanging Cs+ is more difficult than Li+ exchanging H+. Under the Cs+ concentration of 0.0001, 0.01 and 0.1 mol L-1, the equilibrium adsorption amounts of Cs+ in Cs+-H+ were 272.3, 1178.1 and 1886.4 mmol kg-1, respectively, these results showed that the equilibrium adsorption amounts of Cs+ were higher than that of H+ in H+-Cs+ exchange (it was 197.6, 319.0, or 1079 mmol kg-1, respectively), implying Cs+ could be stronger adsorbed at montmorillonite surface than that of H+. Moreover, the equilibrium adsorption amounts of Cs+ was much higher than the CEC of montmorillonite as the concentration of Cs+ increased to 0.1 mol L-1, which implied that there might be non-electrostatic force adsorption of Cs+ on the montmorillonite surface. 2.3 Adsorption energy of Li+, H+ and Cs+ on the surface of montmorillonite From the experimental results of the adsorption kinetics of Li+, Cs+, and H+ in Li+-Cs+, Li+-H+, Cs+-H+, and H+-Cs+ exchange, it can be seen that there were great differences in the adsorption rates and equilibrium adsorption amounts. Actually, according to Eq. (5), the difference in equilibrium adsorption amounts results from the difference in the adsorption energy of ions on the surface. Based on Eq. (5), we have: N X t     2Slf 0 X e



wX ( 0 ) 2 RT

（6）

where wx(0) = γx(0)Zxψ(0) is the adsorption energy of ion x at the surface of montmorillonite. If ions interactions in bulk solution were considered, Eq. (6) should be written as:

N X t     2 Slf 0X  0 ( X )e



wX ( 0 ) 2 RT

（7）

in which γ0(X) is the activity coefficient of ion X in bulk solution. On the other hand, the diffusion layer near the interface could not be completely 12 / 23 ACS Paragon Plus Environment

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stretched in the experiment, so the actual solution volume in the sample should be used instead of the volume of the diffuse layer S·l. The solution content of the sample during the exchange experiments was measured to be 0.79 L kg-1. Thus Eq. (7) changes into: wX (0)  2 RT ln

N X t    1.58 f 0 X 

（8）

The adsorption energy of ions at montmorillonite surface could be calculated according to Eq. (8), and the results were shown in Table 1. Table 1 The comparison of cation adsorption energies exchange systems

cation species /X+

Li+ X+-Cs+ exchange

H+

Li+ X+-H+ exchange

Cs+

electrolyte concentration /mol L -1

equilibrium adsorption amounts of cation /mmol kg-1

adsorption energy of cation at surface wx(0) /kJ mol-1

0.0001 0.01 0.1 0.0001 0.01 0.1 0.0001 0.01 0.1 0.0001 0.01 0.1

15.8 120.6 897.4 197.6 319.0 1078.7 17.1 142.0 1063.2 272.3 1178 1886

-22.8 -10.5 -9.9 -35.4 -15.4 -10.9 -23.2 -11.4 -10.8 -36.9 -21.8 -13.6

From Table 1, it can be seen that: the ion adsorption energy for any given concentrations followed the sequence: Cs+ > H+ > Li+; moreover, the adsorption energy of H+ approached to that of Cs+ under the lowest electrolyte concentration of 0.0001 mol L-1 while the adsorption energies of Cs+, H+ and Li+ approached to each other under the highest electrolyte concentration of 0.1 mol L-1. Obviously, those results cannot be explained by the ionic radius, nor by the ion hydration radius. First, from the perspective of the ionic radius, the radius of H+ was the smallest, followed by Li+, and radius of Cs+ was the largest. Therefore, the interfacial adsorption energy sequence should be Cs+ < Li+ < H+. But the experimental result was just the opposite. Secondly, if the adsorption energy was dominated by ion hydration radius, the 13 / 23 ACS Paragon Plus Environment


sequence of adsorption energy should be H+ > Cs+ > Li+ since the sequence of ion hydration radius was Li+ > Cs+ > H+, which did not meet the experimental results. Thirdly, if the ionic size and the hydration volume were adopted to interpret the results, the differences of adsorption energy would only be significantly different at high concentrations, because the volume effect of ions at low concentrations was negligible. However, the differences of adsorption energy between Cs+ and Li+ as well as between H+ and Li+ changed with cationic concentrations, shown in Fig. 6, indicated that the higher the cationic concentration was, the smaller the difference of cation adsorption energy would be. Ion radius and hydration radius therefore could not explain the experimental results of cation adsorption energy at montmorillonite surface. The data in Table 1 indicates that, the adsorption force of Cs+ on montmorillonite surface was so strong that it was even higher than that of H+. In another report, the normal and shear forces between mica surfaces across aqueous cesium salt solutions was investigated by a surface force balance and it was discovered that there was no evidence of hydration repulsion between the mica surfaces compared with other alkali metal ions.37 This result implies that, indeed Cs+ could be much more strongly adsorbed at mica surface than other alkali metal ions (montmorillonite and mica have the same molecular structure). We know that, both montmorillonite and mica would be hydrophobic because of their symmetric molecular structure, their strong hydrophilic nature is produced from their surface negative charges. The observation of surface hydration absence at mica surface for Cs+ adsorption under rather low Cs+ concentrations indicates that, the surface negative charges had been neutralized by Cs+. Obviously, the classic electrostatic and dispersion forces cannot explain such strong adsorption force for Cs+ on montmorillonite/mica surface.


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Electrolyte concentration ( M)

D-value of ions dsorption energy (kJ mol-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 -2 -4

0.0001

0.0010

0.0100

0.1000

wCH (0)  wCLi (0)

wCCs (0)  wCLi (0)

-6 -8 -10 -12 -14

Fig. 6. The differences of adsorption energy between Cs+ and Li+ between H+ and Li+ changed with cationic concentrations

Considering the polarizability of Li+ was too low and it was difficult for Li+ to form a covalent bond on the surface, classic electrostatic force dominated the adsorption of Li+. According to the Gouy-Chapman theory and the approach suggested by Li et al.38, the surface potential of montmorillonite can be calculated as -0.286, -0.168, and -0.109V at 0.0001, 0.01 and 0.1 mol L-1 LiNO3 concentrations, and then the electrostatic adsorption energies of Li+ at the concentrations were calculated to be -27.6, -16.2 and -10.6 kJ mol-1, respectively, at 0.0001, 0.01 and 0.1 mol L-1. Obviously, if both Cs+ and H+ were merely electrically adsorbed on the surface, they bore the same electrostatic adsorption energy with Li+. The interfacial adsorption energies of Li+ (Table 1) measured under the concentrations of 0.0001 and 0.01 mol L-1 were smaller than the theoretical calculation values while that measured under 0.1 mol L-1 Li+ concentration met with the calculated values. As H+ or Cs+ on the surface was only partially replaced by Li+ in Li+-H+ or Li+-Cs+ exchange under concentrations of 0.0001 and 0.01 mol L-1, and the equilibrium adsorption amounts of Li+ at both concentrations were less than the cation exchange capacity of montmorillonite. While the concentration of Li+ was up to 0. 1 mol L-1, the equilibrium adsorption amounts of Li+ was close to the CEC (especially in Li+-H+ exchange), thus all the counter ions in the diffusion layer were almost Li+, in which the adsorption energy estimated by Gouy-Chapman theory was consistent with the 15 / 23 ACS Paragon Plus Environment


experimental data. This experimental results in both Li+-H+ or Li+-Cs+ exchanges proved that indeed only electrostatic adsorption occurred between Li+ and the surface of montmorillonite. Since the adsorption energy of Li+ in Table 1 represented the electrostatic adsorption energy of ions, the differences of adsorption energy between H+ and Li+ or between Cs+ and Li+ (Table 1) would be the non-electrostatic adsorption energy of these two cations (see Fig. 6). Therefore, Fig. 6 showed the relationship between the non-electrostatic adsorption energy and the ion concentration of H+ and Cs+ at the interface. From Fig. 6 we can see the following points: (1) the non-electrostatic adsorption energies of H+ and Cs+ increased sharply with decreasing concentrations, indicating that the non-electrostatic adsorption energy of these two cations must be electric field-dependent. Because the lower the concentration of counter ions, the stronger the electric field were near the interface, and the adsorption energy of H+ and Cs+ must be larger. (2) This electric field-dependent non-electrostatic adsorption energy of H+ could be seemingly explained by the coordination adsorption theory. By reasons of an empty orbit of H+ and the highly active lone pair electrons of O atom by the electric field at surface under low electrolyte concentrations, a strong coordination bond between H+ and O atoms will be possibly produced. If this speculation was right, the coordinate bond adsorption energy will decrease with increasing electrolyte concentration, because the electric field near surface decreased with increasing electrolyte concentration, and the coordinate adsorption energy could possibly disappear under high electrolyte concentrations. Indeed, the experimental result of the coordinate adsorption energies for H+ decreased with the increase of electrolyte concentration, and the adsorption energies of H+ approached to the value of Li+ under the concentration of 0.1 mol L-1, for this electrolyte concentration the coordinate bonding of H+ on surface disappeared and the classic electrostatic adsorption energy dominate H+ adsorption. Therefore, the coordinate bond adsorption energy of H+ at interface was electric field-dependent, and it would decrease with decreasing electric field. 16 / 23 ACS Paragon Plus Environment

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(3) Obviously, however, this mechanism of coordinate bonding could not be applied to explain the observed adsorption energy of Cs+. Nevertheless, recent studies have found that Cs+ can generate strong excess adsorption energy due to strong nonclassical polarization in strong electric fields formed at low ion concentrations.28,34 So nonclassical polarization seemingly could explain the excess adsorption energy of Cs+ under low concentration. However, the adsorption energy of Cs+ still obviously higher than that of Li+ under 0.1 mol L-1 concentration, suggesting a new non-classical adsorption energy in addition to the non-classic polarization possibly presented under 0.1 mol L-1 Cs+ concentration. We know that Cs+ bears the largest classic polarizability among the three cations, the electric field was therefore strongly screened under the high electrolyte concentration of 0.1 mol L-1; as a result, the classically dispersion force for Cs+ at surface possibly became important.7,39 Meanwhile, NO3- also bear large polarizability and can produce strong dispersion force under high electrolyte concentrations. Thus, the negatively charged NO3- and positively charged Cs+ could be adsorbed at surface through dispersion force simultaneously. In other words, NO3and Cs+ could form ion pairs and could be adsorbed on the surface through dispersion force. The correctness of this speculation can be proved by the following experimental facts. If ion pairs adsorption occurred by the dispersion force, the equilibrium adsorption amounts of Cs+ on the surface would exceed the amounts of surface negative charges (or CEC), and at the same time the net surface charges would still negative, otherwise the excess adsorption of Cs+ could lead to the net surface charge sign reversal. Firstly, the experimental results have shown that at high electrolyte concentration (0.1 mol L-1), the equilibrium adsorption amounts of Cs+ on the surface indeed exceeded the amounts of surface negative charge (CEC). At the same time, the zeta potential changed with the increase of Cs+ concentration measured (shown in Fig. 7). It can be seen that the excess adsorption of Cs+ did not lead to the net surface charge reversal. Therefore, under relative low electrolyte concentrations, the electric fields were 17 / 23 ACS Paragon Plus Environment


strong, the non-electrostatic adsorption energy for Cs+ would be from the non-classic polarizability of Cs+, and then, under the electrolyte concentrations higher than 0.1 mol L-1, the electric field was strongly screened, the non-electrostatic adsorption energy for Cs+ would be from the dispersion force of Cs+ and NO3- through ion pair adsorption. CCs (mol L-1) 0

Zeta potential (mv)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.0001

0.0010

0.0100

0.1000

-5 -10 -15 -20 -25

Fig. 7 Variation of Zeta potential with Cs+ concentration

4. Conclusions The above discussion could lead to the following conclusions: firstly, the observed adsorption energy of Li+ could be fully explained with the classic electrostatic force, while the adsorption energies for H+ and Cs+ could not; secondly the non-electrostatic adsorption energies of H+ was from the coordinate bond between H+ and O atom at surface, and the coordinate bond adsorption energy of H+ was found electric field-dependent; thirdly, under relative low electrolyte concentrations, the non-electrostatic adsorption energy for Cs+ was from the non-classic polarizability of Cs+, and under the high electrolyte concentration of 0.1 mol L-1, the non-electrostatic adsorption energy for Cs+ was from the dispersion force of Cs+ and NO3- through ion pair adsorption. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 41501241 and Grant No.41530855) and the Fundamental Research Funds for the Central Colleges (SWU116049). 18 / 23 ACS Paragon Plus Environment

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Table of Contents (TOC) Image

Keywords: Electric field near particle surface; Ion exchange adsorption; Kinetics; Interface adsorption energy


To Distinguish Electrostatic, Coordination Bond ... - ACS Publications

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