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Electric fields within clay materials: How to affect the adsorption of metal ions
Accepted Manuscript Electric Fields within Clay Materials: How to Affect the Adsorption of Metal Ions Xiong Li, Hang Li, Gang Yang PII: DOI: Reference:
S0021-9797(17)30440-X http://dx.doi.org/10.1016/j.jcis.2017.04.040 YJCIS 22252
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
7 February 2017 5 April 2017 9 April 2017
Please cite this article as: X. Li, H. Li, G. Yang, Electric Fields within Clay Materials: How to Affect the Adsorption of Metal Ions, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis.2017.04.040
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Electric Fields within Clay Materials: How to Affect the Adsorption of Metal Ions Xiong Li, Hang Li, Gang Yang* College of Resources and Environment & Chongqing Key Laboratory of Soil Multi-scale Interfacial Process, Southwest University, Chongqing 400715, China * To whom correspondence should be addressed: E-mail: [email protected]; Phone: 086-023-68251504; Fax: 086-023-68250444.
1
Abstract: Electric fields exist ubiquitously in chemical and biological systems while how to affect the interfacial adsorption processes remain elusive. Here, molecular dynamics simulations are used to understand at a molecular level the adsorption of metal ions at the interface of aqueous solutions and clay materials that are generally endowed with strong electric fields. In absence of electric fields, even Cs +, one of the strongest adsorbed metal ions, is facile to detach from solvated clay surfaces. Electric fields are critical to construct stable inner-sphere complexes and enhance pronouncedly the adsorption strengths of both inner- and outer-sphere complexes. Heavy metal ions such as Pb2+ that are exclusively outer-sphere adsorbed are driven inner-sphere by electric fields, causing the adsorption strengths to surpass Cs + and explaining partially the serious pollution to clay systems. Adsorption quantities of inner-sphere alkali ions increase with electric fields and in the case of Pb2+, are closely correlated with the intensities of electric fields. Previous models fail to account for Hofmeister effects occurring at solvated clay surfaces, and forceful supports are given to polarization effects that correctly interpret Hofmeister series (Cs+ > Na+) and respond towards the change of electric fields. Keywords: electric fields; Hofmeister effects; molecular dynamics; polarization effects; potential of mean force
2
Introduction Clay materials that are generally layer-type aluminosilicates represent an essential constituent of the earth’s crust, and adsorption of metal ions at clay surfaces is associated closely with a wide spectrum of important processes such as preservation of nutrients and water, management of heavy metal ions and radioactive wastes as well as regulation of redox reactions [1, 2]. Isomorphous substitutions are pervasive within almost all types of clay materials resulting in a plethora of surface charges and strong electric fields (108 109 V/m), while a molecular-level understanding of how electric fields affect the adsorption of metal ions remains enigmatic. Negative charges created by the deprotonation of surface silanol groups (Si-OH →
Si-O-) were often used as prototypes to model the electric fields within clay
materials [3-6], and Hofmeister effects were explicitly indicated during the adsorption of different metal ions: The potentials of mean force (PMF) showed that Li+ corresponds to a larger activation energy than Cs+ for the desorption from solvated clay surfaces, probably due to that the smaller ionic size is beneficial to enhance hydrophilicity and further adsorption strength [3]. Argyris et al. [4] claimed that when adsorbed at clay surfaces, Na+ rather than Cs+ ions are less mobile. In consequence, the underlying Hofmeister series were suggested to follow as Li+ > Cs+ and Na+ > Cs+, which were corroborated by other computational studies [5, 6]. The silanol model (Si-OH
→
Si-O-) underlines the importance of electrostatic interactions, while it is
known to us that electrostatic force cannot, at least alone, account for Hofmeister phenomena at clay surfaces [7, 8]. Recently, our group [9, 10] has demonstrated that the activation barriers for the aggregation of clay particles differ substantially in the various alkali cation solutions and the resulting Hofmeister series decline in the order Cs+ > Rb+ > K+ > Na+ > Li+, which were substantialized by the ion-exchange kinetics 3
and equilibrium experiments [11, 12] as well as the modified electric double layer (EDL) theory taking into account steric, polarization and valence effects [13]. Resembling Hofmeister effects were observed for a wide range of chemical and biological processes such as protein stability and enzymatic catalysis [8, 14-16]. In this work, molecular dynamics (MD) simulation were conducted to provide a molecular-level understanding of how electric fields resulting from isomorphous substitutions within clay materials affect the adsorption behaviors of metal ions, with special focuses on: a) responses of the adsorption of metal ions towards the addition and elevation of electric fields and b) mechanisms for Hofmeister effects during the adsorption of metal ions at the interface of clay materials and aqueous solutions.
Computational details In line with previous works [17-19], an orthogonalized unit cell of kaolinite was expanded along the x, y and z directions forming a supercell (9 × 9 × 4), and the inward tetrahedral SiO4 and octahedral AlO6 surfaces were separated by a vacuum layer (40.0 Å thick) that was filled with 5097 water molecules to maintain a density of 1.0 g/cm3. Isomorphous substitutions are pervasive in almost all types of clay materials such as montmorillonite and kaolinite [20-23], and here, 0, 2, 4, 8, and 16 Al3+/Si4+ replacements were constructed to establish a wide range of electric fields corresponding to 0, 0.01, 0.04, 0.07, and 0.14 Cm-2 surface charge densities (), respectively. For a specific electric field, 0.18, 0.36, and 0.72 mol/L Cs+ ions were considered by replacing certain numbers of water molecules with Cs +-Cl- ion pairs in addition to Cs+ ions balancing the negative charges created by Al3+/Si4+ substitutions. A total of 15 MD simulations were conducted for CsCl solutions, and the initial configuration of 0.72 mol/L CsCl solutions in contact with kaolinite surfaces ( = 0 4
Cm-2) was shown in Figure 1. Systems of PbCl2 and NaCl solutions interacting interfacially with kaolinite models that carry the various charge densities were prepared similarly. MD simulations were run using the Gromacs-4.6.5 software package [24]. Clay materials and ions were described by the CLAYFF force field (Table S1) while water solvent was accounted for by the flexible SPC model [25, 26], which have been sufficiently verified to model the interfacial processes as presently investigated [4, 17-19, 27, 28]. Periodic boundary conditions (PBC) were applied, and 12.0 Å cutoff radii were defined for Ewald electrostatic summation and van der Waals (vdW) interactions.
Long-rang
electrostatic
interactions
were
handled
by
the
Particle-Mesh-Ewald (PME) method. The equations of motion were integrated by the leapfrog algorithm with 2.0 fs time step [29]. The simulation temperature (T = 300 K) and pressure (p = 1 bar) were controlled respectively by V-rescale and Parrinello-Rahman barostats [30, 31]. 10.0 ns MD simulations were conducted for each system, and unless otherwise noted, analyses were based on the latter 5.0 ns MD trajectories, since all systems have reached the equilibrium states since approximately 2.0 ns, see the root-mean-square deviation (RMSD) profiles in Figures S1-S9. The free energy profiles for the adsorption of metal ions at the interface of clay materials and aqueous solutions were estimated by PMF via umbrella sampling [32, 33].
Results and discussion Cs+ ranks among those metal ions that have the lowest hydration energies and in line with previous results [17, 34-36], is inclined to be inner-sphere adsorbed at the interface of clay materials and aqueous solutions (Figures 1, S10 and S11). Figure 2 derived from the atomic density profiles (Figure S12) further indicates that for any 5
electric field resulting from isomorphous substitutions ( = 0 0.14 Cm-2), the inner-sphere Cs+ ions are always the major adsorption species, and increase of electric fields enhances pronouncedly the adsorption quantities (Nad). The inner-sphere adsorption quantities show a nearly direct proportion with the intensities of electric fields, while the outer-sphere Cs+ ions are considerably less affected and decline slightly at relatively dense salt concentrations; e.g., at 0.72 mol/L, the inner-/outer-sphere Cs+ ions are counted to be 23.8/10.6, 28.2/7.8, 31.4/6.7, 36.8/6.7 and 46.0/5.7 for = 0, 0.01, 0.04, 0.07 and 0.14 Cm-2, respectively. At relatively dilute concentrations (e.g., 0.18 mol/L), electric fields affect mainly the allocation of inner- and outer-sphere Cs+ species, and the promotion effects by electric fields, especially regarding to the inner-sphere metal ions, are more pronounced at relatively dense concentrations; e.g., for = 0.14 vs. 0 Cm-2, the inner-sphere Cs+ ions amount to 46.0 vs. 23.8 at 0.72 mol/L in contrast to 16.0 vs. 12.7 at 0.18 mol/L. For clay materials with no isomorphous substitutions ( = 0 Cm-2), two local energy minima are presented in the PMF profiles for Cs+ adsorption onto kaolinite surfaces corresponding respectively to inner-sphere (2.9 Å) and outer-sphere (4.7 Å) species [17, 34-36], see Figure 3. However, the energy barrier is so low (19.7 kJ mol-1) that desorption of inner-sphere Cs+ ions can occur facilely at ambient conditions. Considering that Cs+ is one of the strongest metal ions to interact with solvated clay surfaces, other metal ions are postulated to also desorb readily and enter into aqueous solutions. The PMF profiles show that the desorption of inner-sphere Cs+ ions becomes apparently more difficult due to the presence of electric fields, and the corresponding energy barriers increase in a direct proportion with the electric field intensities, amounting to 19.7, 42.7, 52.3, 61.3 and 99.1 kJ mol-1 for = 0, 0.01, 0.04, 0.07 and 0.14 Cm-2, respectively. In consequence, metal ions have been firmed at 6
clay surfaces by the electric fields resulting from isomorphous substitutions, which guarantee the preservation of nutrients and water molecules. The stability enhancements by electric fields can also observed from the time-evolution trajectories (Figure S12 S14) showing that the inner-sphere Cs+ ions under stronger electric fields are more focused at the adsorption sites during MD simulations. This is substantialized by the results of root mean square fluctuations (RMSFs) (Table S2): 14, 19, 20, 26 and 37 Cs+ ions are ascribed to strong inner-sphere adsorption (RMSF 1.2 Å) for = 0.00, 0.01, 0.04, 0.07 and 0.14 Cm-2 (0.72 mol/L), respectively. With the gradual elevation of electric fields, the PMF peaks corresponding to outer-sphere Cs+ ions decay mildly while those corresponding to inner-sphere Cs+ ions are reinforced speedily with their positions shifting somewhat towards clay surfaces. The fundamental enhancements of inner-sphere Cs+ stabilities due to increase of electric fields are corroborated by the remarkable reduction of energy barriers for the transformation from outer- to inner-sphere species. In line with previous reports [17-19], Pb2+ forms exclusively the outer-sphere adsorption species at the interface of aqueous solutions and clay materials with no isomorphous substitutions ( = 0 Cm-2), due to the considerable hydration energies that are applicable to other heavy metal ions. The presence of electric fields at clay surfaces is beneficial to counteract hydration energies and the inner-sphere Pb2+ ions begin to emerge; in addition, the adsorption of inner-sphere Pb2+ ions is promoted significantly by elevating electric fields (Figures 2 and S15 S18); e.g., the inner-sphere Pb2+ ions are counted to be 0, 2.0, 4.0, 7.8, 15.5 for = 0, 0.01, 0.04, 0.07, and 0.14 Cm-2 (0.72 mol/L), respectively. Intriguingly, the inner-sphere Pb2+ ions seem relevant only with the electric fields and are not affected by the change of salt concentrations, corroborating that it is electric fields that create and govern the 7
adsorption of inner-sphere Pb2+ ions. Owing to competition with inner-sphere species, the numbers of outer-sphere Pb2+ ions increase first and then reduce as the electric fields are gradually elevated, and for a specific electric field, more outer-sphere Pb2+ ions occur at higher salt concentrations, as a result of the saturation of inner-sphere Pb2+ ions (Figure 2). In absence of electric fields ( = 0 Cm-2), the PMF profiles of Pb2+ are characterized by the single minimum at 5.1 Å corresponding to the outer-sphere adsorption and the outer-sphere Pb2+ ions require an energy barrier of 27.8 kJ mol-1 for the transfer to bulk solutions. The electric fields improve the stabilities by shifting towards clay surfaces and elevating energy barriers; e.g., at 4.9 Å and 56.7 kJ mol-1 for = 0.14 Cm-2. Accordingly, even under strong electric fields, the outer-sphere Pb2+ ions are not likely to result in serious pollutions to clay systems that we are currently suffering. Meanwhile, a new sharp peak emerges at 3.0 Å in the PMF profiles (Figure 3) corresponding to the inner-sphere Pb2+ ions, and the energy barriers for the desorption processes show a persistent increase with the intensities of electric fields: 79.8, 84.0, 96.9 and 115.8 kJ mol-1 respectively for = 0.01, 0.04, 0.07 and 0.14 Cm-2, which is in line with the low RMSF values for all these Pb2+ ions ( 1.2 Å, see Table S2). In consequence, electric fields profoundly alter the adsorption of Pb2+ ions and reverse the adsorption strengths of Pb2+ and Cs+: Under relatively weak electric fields (e.g., = 0.01 Cm-2), the adsorption strengths of inner-sphere Pb2+ ions are already almost twice as those of Cs+ ions (79.8 vs. 42.7 kJ mol-1), and the stronger electric fields that occur ubiquitously within clay materials enhance substantially the adsorption of Pb2+ ions, with regard to both quantity and strength. As a result, although with appreciable hydration energies, heavy metal ions (e.g., Pb2+) instead of alkali metal ions (e.g., Cs+) are preferential to bind with solvated clay surfaces that cause serious pollutions to clay systems. 8
Only the outer-sphere complexes are constructed for the adsorption of Na+ ions at solvated clay surfaces without isomorphous substitutions ( = 0 Cm-2) [17, 34, 35, 37, 38], while isomorphous substitutions are pervasive within almost all types of clay materials and hence we are concerned with Hofmeister series in presence of electric fields resulting from isomorphous substitutions. As in the case of Pb2+ adsorption, Na+ ions are driven significantly by electric fields towards clay surfaces forming inner-sphere complexes, see Figure 4 and Figure S19 S21 [7, 8]. The numbers of inner-sphere Na+ ions show a substantial increase with electric fields and at 0.72 mol/L, are equal to 2, 4.2, 6.8 and 11.7 respectively for = 0.01, 0.04, 0.07 and 0.14 Cm-2, which are, however, apparently less than those of inner-sphere Cs+ ions. The energy barriers for the desorption of inner-sphere Na+ ions, according to the PMF profiles (Figure 4), are calculated to be 10.5, 14.0, 16.8 and 25.1 kJ mol-1 respectively for = 0.01, 0.04, 0.07 and 0.14 Cm-2, showing very limited increases with electric fields. The adsorption of different metal ions at the interface of clay materials and aqueous solutions exhibits clear Hofmeister effects that follow as Cs+ > Na+, in good agreement with the experimentally ion-exchange equilibrium, ion-exchange kinetics and aggregation results [9-12]; Moreover, Hofmeister series (Cs+ > Na+) remains consistent with the change of electric fields, and no reversal has been detected. The energy barrier differences for the desorption of inner-sphere Cs+ and Na+ ions are 32.2, 38.3, 44.5 and 74.0 kJ mol-1 respectively for = 0.01, 0.04, 0.07 and 0.14 Cm-2 and show a pronounced increase with electric fields. Thus, strong supports are given to the standpoint that polarization effects are responsible for Hofmeister effects occurring at the interface of clay materials and aqueous solutions (Figure 5): As compared to Na+, Cs+ has obviously softer electronic configurations and is more polarized by electric fields, correctly predicting that Hofmeister series follows as Cs+ > Na+; in addition, 9
Cs+ responds more actively to the change of electric fields and hence Hofmeister effects should be significantly magnified due to the elevation of electric fields. Figure 5 also illustrates two other models: a) The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory has for a long time been regarded as the cornerstone of colloidal science, where ions are assumed to be point charges. According to the DLVO theory, the adsorption strengths of all alkali cations onto clay surfaces are identical to each other; b) The silanol model (i.e., deprotonation of surface silanol groups, Si-OH
→
Si-O-) has often been used as prototypes of electric fields within clay materials [3-6]. Electrostatic interactions play a dominant role therein and Hofmeister series follows as Na+ > Cs+. However, electrostatic interactions have been known to fail to account for the Hofmeister phenomena at clay surfaces that usually carry abundant charges [7, 8]. Results obtained thus far are assumed to be beneficial to understand charged systems that are ubiquitous in chemistry and biology, such as proteins, DNA and metal oxides.
Conclusions Isomorphous substitutions are pervasive within almost all types of clay materials resulting in strong electric fields, and in this work, molecular dynamics simulations have been used to unveil at a molecular level how electric fields affect the adsorption of metal ions. In absence of electric fields, metal ions that were generally believed to show strong affinities to solvated clay materials (e.g., Cs+) are very facile to detach from the surfaces, while their stabilities can be pronouncedly enhanced by the presence of electric fields. Both adsorption quantities and strengths increase in direct proportion to the intensities of electric fields. Electric fields induce the formation of inner-sphere adsorption complexes for heavy metal ions (e.g., Pb2+) and their 10
stabilities are more promoted due to increase of electric fields than those of alkali ions, which in part explain the serious pollution of soil systems by heavy metal ions. Adsorption of different metal ions at the interface of clay materials and aqueous solutions exhibits clear Hofmeister effects that follow as Cs+ > Na+, and Hofmeister effects are magnified significantly due to the increase of electric fields. Strong supports are thus given to the standpoint that polarization effects are responsible for Hofmeister effects. The present results can be safely extended to other charged systems such as proteins, DNA and metal oxides.
Acknowledgements This work was sponsored by the National Natural Science Foundation of China (21473137) and Fourth Excellent Talents Program of Higher Education in Chongqing (2014-03).
Additional Information Competing financial interests: The authors declare no competing financial interests.
References (1) Bowers, G. M.; Hoyt, D. W.; Burton, S. D.; Ferguson, B. O.; Varga, T.; Kirkpatrick, R. J. In Situ 13C and 23Na Magic Angle Spinning NMR Investigation of Supercritical CO2 Incorporation in Smectite-Natural Organic Matter Composites. J. Phys. Chem. C 2014, 118, 3564-3573. (2) Gupta, S. S.; Bhattacharyya, K. G. Adsorption of Heavy Metals on Kaolinite and Montmorillonite: A Review. Phys. Chem. Chem. Phys. 2012, 14, 6698-6723. (3) Hocine, S.; Hartkamp, Remco.; Siboulet, B.; Duvail, M.; Coasne, B.; Turq, P.; Dufreche, J. F. How Ion Condensation Occurs at a Charged Surface: A Molecular Dynamics Investigation of the Stern Layer for Water-silica Interfaces. J. Phys. Chem. C 2016, 120, 963-973. 11
(4) Argyris, D.; Cole, D. R.; Striolo, A. Ion-Specific Effects under Confinement: The Role of Interfacial Water. ACS Nano 2010, 4, 2035-2042. (5) Dewan, S.; Carnevale, V.; Bankura, A.; Bafrooei, A. E.; Fiorin, G.; Klein, M. L.; Borguet, E. Structure of Water at Charged Interface: A Molecular Dynamics Study. Langmuir 2014, 30, 8056-8065. (6) Hartkamp, R.; Siboulet, B.; Dufreche, J. F.; Coasne, B. Ion-Specific Adsorption and Electroosmosis in Charged Amorphous Porous Silica. Phys. Chem. Chem. Phys. 2015, 17, 24683-24695. (7) Jungwirth, P.; Tobias, D. J. Specific Ion Effects at the Air/Water Interface. Chem. Rev. 2006, 106, 1259-1281. (8) Nostro, P. L.; Ninham, B. W. Hofmeister Phenomena: An Update on Ion Specificity in Biology. Chem. Rev. 2012, 112, 2286-2322. (9) Tian, R.; Yang, G.; Li, H.; Gao, X. D.; Liu, X. M.; Zhu, H. L.; Tang, Y. Activation Energies of Colloidal Particle Aggregation: Towards a Quantitative Characterization of Specific Ion Effects. Phys. Chem. Chem. Phys. 2014, 16, 8828-8836. (10) Tian, R.; Yang, G.; Tang, Y.; Liu, X. M.; Li, R.; Zhu, H. L.; Li, H. Origin of Hofmeister Effects for Complex Systems. PLOS One 2015, 10, e0128602. (11) Du, W.; Li, R.; Liu, X. M.; Tian, R.; Li, H. Specific Ion Effects on Ion Exchange Kinetics in Charged Clay. Colloids Surf. A 2016, 509, 427-432. (12) Liu, X. M.; Li, H.; Du, W.; Tian, R.; Li, R.; Jiang, X. J. Hofmeister Effects on Cation Exchange Equilibrium: Quantification of Ion Exchange Selectivity. J. Phys. Chem. C 2013, 117, 6245-6251. (13) Liu, X. M.; Yang, G.; Li, H.; Tian, R.; Li, R.; Jiang, X. J.; Ni, J. P.; Xie, D. T. Observation of Significant Steric, Valence and Polarization Effects and Their Interplay: A Modified Theory for Electric Double Layers. RSC Adv. 2014, 4, 1189-1192. (14) Jungwirth, P.; Cremer, P. S. Beyond Hofmeister. Nature Chem. 2014, 6, 261-263. (15) Bauduin, P.; Renoncourt, A.; Touraud, D.; Kunz, W.; Ninham, B. W. Hofmeister Effect on Enzymatic Catalysis and Colloidal Structures. Curr. Opin. Colloid Interface Sci. 2004, 9, 43-47. (16) Salis, A.; Ninham, B. W. Models and Mechanisms of Hofmeister Effects in Electrolyte 12
Solutions, and Colloid and Protein Systems Revisited. Chem. Soc. Rev. 2014, 43, 7358-7377. (17) Vasconcelos, I. F.; Bunker, B. A. Molecular Dynamics Modeling of Ion Adsorption to the Basal Surfaces of Kaolinite. J. Phys. Chem. C 2007, 111, 6753-6762. (18) Li, X.; Li, H.; Yang, G. Promoting the Adsorption of Metal Ions on Kaolinite by Defect Sites: A Molecular Dynamics Study. Sci. Rep. 2015, 5, 14377. (19) Li, X.; Li, H.; Yang, G. Configuration, Anion-Specific Effects, Diffusion, and Impact on Counterions for Adsorption of Salt Anions at the Interfaces of Clay Minerals. J. Phys. Chem. C 2016, 120, 14621-14630. (20) Kumar, N.; Zhao, CL.; Klaassen, A.; van den Ende, D.; Mugele, F.; Siretanu, I. Characterization of the Surface Charge Distribution on Kaolinite Particles Using High Resolution Atomic Force Microscopy. Geochim. Cosmochim. Acta 2016, 175, 100-112. (21) Mcbride, M. B. Copper(II) Interactions with Kaolinite: Factors Controlling Adsorption. Clays Clay Miner. 1978, 2, 101-106. (22) Youell R. F. Isomorphous Replacement in the Kaolin Group of Minerals. Nature 1958, 181, 557-558. (23) Michalkova, A.; Robinson, T. L.; Leszczynski, J. Adsorption of Thymine and Uracil on 1:1 Clay Mineral Surfaces: Comprehensive ab initio Study on Influence of Sodium Cation and Water. Phys. Chem. Chem. Phys. 2011, 13, 7862-7881. (24) van der Spoel, D.; Lindahl, E.; Hess, B.; Groenhof, G.; Mark, A. E.; Berendsen, H. J. C. GROMACS: Fast, Flexible, and Free. J. Comput. Chem. 2005, 26, 1701-1718. (25) Cygan, R. T.; Liang, J. J.; Kalinichev, A. G. Molecular Models of Hydroxide, Oxyhydroxide, and Clay Phases and the Development of a General Force Field. J. Phys. Chem. B 2004, 108, 1255-1266. (26) Mizan, T. I.; Savage, P. E.; Ziff, R. M. Comparison of Rigid and Flexible Simple Point Charge Water Models at Supercritical Conditions. J. Comput. Chem. 1996, 17, 1757-1770. (27) Zhu, R.; Molinari, M.; Shapley, T. V.; Parker, S. C. Modeling the Interaction of Nanoparticles with Mineral Surfaces: Adsorbed C60 on Pyrophyllite. J. Phys. Chem. A 2013, 117, 6602-6611. (28) Dequidt, A.; Devemy, J.; Malfreyt, P. Confined KCl Solution between Two Mica Surfaces: Equilibrium and Frictional Properties. J. Phys. Chem. C 2015, 119, 22080-22085. 13
(29) Hockney, R. W.; Goel, S. P.; Eastwood, J. W. Quiet High-Resolution Computer Models of a Plasma. J. Comput. Phys. 1974, 14, 148-158. (30) Bussi, G.; Donadio, D.; Parrinello, M. Canonical Sampling through Velocity Rescaling. J Chem. Phys. 2007, 126, 014101. (31) Nose, S.; Klein, M. L. Constant Pressure Molecular Dynamics for Molecular Systems. Mol. Phys. 1983, 50, 1055-1076. (32) Torrie, G. M.; Valleau, J. P. Non-Physical Sampling Distributions in Monte-Carlo Free-Energy Estimation-Umbrella Sampling. J. Comput. Phys. 1977, 23, 187-199. (33) Kumar, S.; Rosenberg, J. M.; Bouzida, D.; Swendsen, R. H.; Kollman, P. A. The Weighted Histogram Analysis Method for Free-Energy Calculations on Biomolecules. Ι. The Method. J. Comput. Chem. 1992, 13, 1011-1021. (34) Churakov, S. V. Mobility of Na and Cs on Montmorillonite Surface under Partially Saturated Conditions. Environ. Sci. Technol. 2013, 47, 9816-9823. (35) Rotenberg, B.; Marry, V.; Malikova, N.; Turq, P. Molecular Simulation of Aqueous Solutions at Clay Surfaces. J. Phys.: Condens. Matter 2010, 22, 284114. (36) Loganathan, N.; Yazaydin, A. O.; Bowers, G. M.; Kalinichev, A. G.; Kirkpatrick, R. J. Structure, Energetics, and Dynamics of Cs+ and H2O in Hectorite: Molecular Dynamics Simulations with an Unconstrained Substrate Surface. J. Phys. Chem. C 2016, 120, 10298-10310. (37) Ikeda, T. First-Principles-Based Simulation of Interlayer Water and Alkali Metal Ions in Weathered Biotite. J. Chem. Phys. 2016, 145, 124703. (38) Bowers, G. M.; Singer, J. W.; Bish, D. L.; Kirkpatrick, R. J. Alkali Metal and H 2O Dynamics at the Smectite/Water Interface. J. Phys. Chem. C 2011, 115, 23395-23407.
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Figure captions Figure 1. Initial and equilibrium configurations of 0.72 mol/L CsCl solutions in contact with the basal surfaces of kaolinite carrying different charge densities (indicated in the legends). With regard to the initial configuration (0 Cm-2), dashed red lines are used to schematically divide aqueous solutions into three regions. Cs+ and Cl- ions are presented as orange and green balls, respectively.
Figure 2. Numbers of adsorbed Cs+ and Pb2+ ions (Nad) for CsCl and PbCl2 solutions in equilibrium with the tetrahedral SiO4 surface of kaolinite with different charge densities. Figure 3. Potentials of mean force (PMF) for Cs+ and Pb2+ adsorption at the tetrahedral SiO4 surface of kaolinite with different charge densities. Figure 4. Numbers of adsorbed Na+ ions (Nad) and corresponding potentials of mean force (PMF) for the interaction of NaCl solutions and kaolinite with different charge densities. Figure 5. Schematic comparisons of Na+ vs. Cs+ adsorption driven by (a) point charge; (b) silanol model where the negative charge is produced by the deprotonation of surface silanol groups in clay minerals (Si-OH → Si-O-) and (c) electric field resulting from isomorphous substitutions within clay minerals, respectively.
15
Initial (0 Cm-2)
0 Cm-2
tetrahedral surface
bulk solution octahedral surface
0.01 Cm-2
0.04 Cm-2
0.07 Cm-2
0.14 Cm-2
Figure 1
16
0.18 mol/L
0.36 mol/L
0.72 mol/L
0.36 mol/L
0.72 mol/L
Nad (Cs+)
40
Inner Outer
30
20
10
0 20
0.18 mol/L
10
5
0. 01 0. 04 0. 07 0. 14 0. 00 0. 01 0. 04 0. 07 0. 14 0. 00 0. 01 0. 04 0. 07 0. 14
0 0. 00
Nad (Pb2+)
15
Inner Outer
Charge density (Cm-2) Figure 2
17
PMF (kJ·mol-1)
0 -20
Cs+
-40
0.00 Cm -2 0.01 Cm -2 0.04 Cm -2 0.07 Cm -2 0.14 Cm
-2
-60 -80
-100
PMF (kJ·mol-1)
0 -20
Pb2+
-40
-2
0.00 Cm -2 0.01 Cm -2 0.04 Cm -2 0.07 Cm -2 0.14 Cm
-60 -80
-100 -120 0
2
4
6
8
10
12
14
16
18
Distance to the surface (Å) Figure 3
18
15
0.36 mol/L
0.18 mol/L
0.72 mol/L
Inner Outer
Nad
10
5
0.
14
07 0.
04 0.
01 0.
14 0.
0.
07
04 0.
01 0.
0.
14
07 0.
04 0.
0.
01
0
Charge density (Cm-2)
-5
-1
PMF (kJ·mol )
0
-10
-2
0.01 Cm -2 0.04 Cm -2 0.07 Cm -2 0.14 Cm
-15 -20 -25 0
2
4
6
8
10
12
14
16
18
Distance to the surface (Å) Figure 4
19
Figure 5
20
Electric Fields within Clay Materials: How to Affect the Adsorption of Metal Ions Xiong Li, Hang Li, Gang Yang* College of Resources and Environment & Chongqing Key Laboratory of Soil Multi-scale Interfacial Process, Southwest University, Chongqing 400715, China
21
S0021-9797(17)30440-X http://dx.doi.org/10.1016/j.jcis.2017.04.040 YJCIS 22252
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
7 February 2017 5 April 2017 9 April 2017
Please cite this article as: X. Li, H. Li, G. Yang, Electric Fields within Clay Materials: How to Affect the Adsorption of Metal Ions, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis.2017.04.040
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Electric Fields within Clay Materials: How to Affect the Adsorption of Metal Ions Xiong Li, Hang Li, Gang Yang* College of Resources and Environment & Chongqing Key Laboratory of Soil Multi-scale Interfacial Process, Southwest University, Chongqing 400715, China * To whom correspondence should be addressed: E-mail: [email protected]; Phone: 086-023-68251504; Fax: 086-023-68250444.
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Abstract: Electric fields exist ubiquitously in chemical and biological systems while how to affect the interfacial adsorption processes remain elusive. Here, molecular dynamics simulations are used to understand at a molecular level the adsorption of metal ions at the interface of aqueous solutions and clay materials that are generally endowed with strong electric fields. In absence of electric fields, even Cs +, one of the strongest adsorbed metal ions, is facile to detach from solvated clay surfaces. Electric fields are critical to construct stable inner-sphere complexes and enhance pronouncedly the adsorption strengths of both inner- and outer-sphere complexes. Heavy metal ions such as Pb2+ that are exclusively outer-sphere adsorbed are driven inner-sphere by electric fields, causing the adsorption strengths to surpass Cs + and explaining partially the serious pollution to clay systems. Adsorption quantities of inner-sphere alkali ions increase with electric fields and in the case of Pb2+, are closely correlated with the intensities of electric fields. Previous models fail to account for Hofmeister effects occurring at solvated clay surfaces, and forceful supports are given to polarization effects that correctly interpret Hofmeister series (Cs+ > Na+) and respond towards the change of electric fields. Keywords: electric fields; Hofmeister effects; molecular dynamics; polarization effects; potential of mean force
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Introduction Clay materials that are generally layer-type aluminosilicates represent an essential constituent of the earth’s crust, and adsorption of metal ions at clay surfaces is associated closely with a wide spectrum of important processes such as preservation of nutrients and water, management of heavy metal ions and radioactive wastes as well as regulation of redox reactions [1, 2]. Isomorphous substitutions are pervasive within almost all types of clay materials resulting in a plethora of surface charges and strong electric fields (108 109 V/m), while a molecular-level understanding of how electric fields affect the adsorption of metal ions remains enigmatic. Negative charges created by the deprotonation of surface silanol groups (Si-OH →
Si-O-) were often used as prototypes to model the electric fields within clay
materials [3-6], and Hofmeister effects were explicitly indicated during the adsorption of different metal ions: The potentials of mean force (PMF) showed that Li+ corresponds to a larger activation energy than Cs+ for the desorption from solvated clay surfaces, probably due to that the smaller ionic size is beneficial to enhance hydrophilicity and further adsorption strength [3]. Argyris et al. [4] claimed that when adsorbed at clay surfaces, Na+ rather than Cs+ ions are less mobile. In consequence, the underlying Hofmeister series were suggested to follow as Li+ > Cs+ and Na+ > Cs+, which were corroborated by other computational studies [5, 6]. The silanol model (Si-OH
→
Si-O-) underlines the importance of electrostatic interactions, while it is
known to us that electrostatic force cannot, at least alone, account for Hofmeister phenomena at clay surfaces [7, 8]. Recently, our group [9, 10] has demonstrated that the activation barriers for the aggregation of clay particles differ substantially in the various alkali cation solutions and the resulting Hofmeister series decline in the order Cs+ > Rb+ > K+ > Na+ > Li+, which were substantialized by the ion-exchange kinetics 3
and equilibrium experiments [11, 12] as well as the modified electric double layer (EDL) theory taking into account steric, polarization and valence effects [13]. Resembling Hofmeister effects were observed for a wide range of chemical and biological processes such as protein stability and enzymatic catalysis [8, 14-16]. In this work, molecular dynamics (MD) simulation were conducted to provide a molecular-level understanding of how electric fields resulting from isomorphous substitutions within clay materials affect the adsorption behaviors of metal ions, with special focuses on: a) responses of the adsorption of metal ions towards the addition and elevation of electric fields and b) mechanisms for Hofmeister effects during the adsorption of metal ions at the interface of clay materials and aqueous solutions.
Computational details In line with previous works [17-19], an orthogonalized unit cell of kaolinite was expanded along the x, y and z directions forming a supercell (9 × 9 × 4), and the inward tetrahedral SiO4 and octahedral AlO6 surfaces were separated by a vacuum layer (40.0 Å thick) that was filled with 5097 water molecules to maintain a density of 1.0 g/cm3. Isomorphous substitutions are pervasive in almost all types of clay materials such as montmorillonite and kaolinite [20-23], and here, 0, 2, 4, 8, and 16 Al3+/Si4+ replacements were constructed to establish a wide range of electric fields corresponding to 0, 0.01, 0.04, 0.07, and 0.14 Cm-2 surface charge densities (), respectively. For a specific electric field, 0.18, 0.36, and 0.72 mol/L Cs+ ions were considered by replacing certain numbers of water molecules with Cs +-Cl- ion pairs in addition to Cs+ ions balancing the negative charges created by Al3+/Si4+ substitutions. A total of 15 MD simulations were conducted for CsCl solutions, and the initial configuration of 0.72 mol/L CsCl solutions in contact with kaolinite surfaces ( = 0 4
Cm-2) was shown in Figure 1. Systems of PbCl2 and NaCl solutions interacting interfacially with kaolinite models that carry the various charge densities were prepared similarly. MD simulations were run using the Gromacs-4.6.5 software package [24]. Clay materials and ions were described by the CLAYFF force field (Table S1) while water solvent was accounted for by the flexible SPC model [25, 26], which have been sufficiently verified to model the interfacial processes as presently investigated [4, 17-19, 27, 28]. Periodic boundary conditions (PBC) were applied, and 12.0 Å cutoff radii were defined for Ewald electrostatic summation and van der Waals (vdW) interactions.
Long-rang
electrostatic
interactions
were
handled
by
the
Particle-Mesh-Ewald (PME) method. The equations of motion were integrated by the leapfrog algorithm with 2.0 fs time step [29]. The simulation temperature (T = 300 K) and pressure (p = 1 bar) were controlled respectively by V-rescale and Parrinello-Rahman barostats [30, 31]. 10.0 ns MD simulations were conducted for each system, and unless otherwise noted, analyses were based on the latter 5.0 ns MD trajectories, since all systems have reached the equilibrium states since approximately 2.0 ns, see the root-mean-square deviation (RMSD) profiles in Figures S1-S9. The free energy profiles for the adsorption of metal ions at the interface of clay materials and aqueous solutions were estimated by PMF via umbrella sampling [32, 33].
Results and discussion Cs+ ranks among those metal ions that have the lowest hydration energies and in line with previous results [17, 34-36], is inclined to be inner-sphere adsorbed at the interface of clay materials and aqueous solutions (Figures 1, S10 and S11). Figure 2 derived from the atomic density profiles (Figure S12) further indicates that for any 5
electric field resulting from isomorphous substitutions ( = 0 0.14 Cm-2), the inner-sphere Cs+ ions are always the major adsorption species, and increase of electric fields enhances pronouncedly the adsorption quantities (Nad). The inner-sphere adsorption quantities show a nearly direct proportion with the intensities of electric fields, while the outer-sphere Cs+ ions are considerably less affected and decline slightly at relatively dense salt concentrations; e.g., at 0.72 mol/L, the inner-/outer-sphere Cs+ ions are counted to be 23.8/10.6, 28.2/7.8, 31.4/6.7, 36.8/6.7 and 46.0/5.7 for = 0, 0.01, 0.04, 0.07 and 0.14 Cm-2, respectively. At relatively dilute concentrations (e.g., 0.18 mol/L), electric fields affect mainly the allocation of inner- and outer-sphere Cs+ species, and the promotion effects by electric fields, especially regarding to the inner-sphere metal ions, are more pronounced at relatively dense concentrations; e.g., for = 0.14 vs. 0 Cm-2, the inner-sphere Cs+ ions amount to 46.0 vs. 23.8 at 0.72 mol/L in contrast to 16.0 vs. 12.7 at 0.18 mol/L. For clay materials with no isomorphous substitutions ( = 0 Cm-2), two local energy minima are presented in the PMF profiles for Cs+ adsorption onto kaolinite surfaces corresponding respectively to inner-sphere (2.9 Å) and outer-sphere (4.7 Å) species [17, 34-36], see Figure 3. However, the energy barrier is so low (19.7 kJ mol-1) that desorption of inner-sphere Cs+ ions can occur facilely at ambient conditions. Considering that Cs+ is one of the strongest metal ions to interact with solvated clay surfaces, other metal ions are postulated to also desorb readily and enter into aqueous solutions. The PMF profiles show that the desorption of inner-sphere Cs+ ions becomes apparently more difficult due to the presence of electric fields, and the corresponding energy barriers increase in a direct proportion with the electric field intensities, amounting to 19.7, 42.7, 52.3, 61.3 and 99.1 kJ mol-1 for = 0, 0.01, 0.04, 0.07 and 0.14 Cm-2, respectively. In consequence, metal ions have been firmed at 6
clay surfaces by the electric fields resulting from isomorphous substitutions, which guarantee the preservation of nutrients and water molecules. The stability enhancements by electric fields can also observed from the time-evolution trajectories (Figure S12 S14) showing that the inner-sphere Cs+ ions under stronger electric fields are more focused at the adsorption sites during MD simulations. This is substantialized by the results of root mean square fluctuations (RMSFs) (Table S2): 14, 19, 20, 26 and 37 Cs+ ions are ascribed to strong inner-sphere adsorption (RMSF 1.2 Å) for = 0.00, 0.01, 0.04, 0.07 and 0.14 Cm-2 (0.72 mol/L), respectively. With the gradual elevation of electric fields, the PMF peaks corresponding to outer-sphere Cs+ ions decay mildly while those corresponding to inner-sphere Cs+ ions are reinforced speedily with their positions shifting somewhat towards clay surfaces. The fundamental enhancements of inner-sphere Cs+ stabilities due to increase of electric fields are corroborated by the remarkable reduction of energy barriers for the transformation from outer- to inner-sphere species. In line with previous reports [17-19], Pb2+ forms exclusively the outer-sphere adsorption species at the interface of aqueous solutions and clay materials with no isomorphous substitutions ( = 0 Cm-2), due to the considerable hydration energies that are applicable to other heavy metal ions. The presence of electric fields at clay surfaces is beneficial to counteract hydration energies and the inner-sphere Pb2+ ions begin to emerge; in addition, the adsorption of inner-sphere Pb2+ ions is promoted significantly by elevating electric fields (Figures 2 and S15 S18); e.g., the inner-sphere Pb2+ ions are counted to be 0, 2.0, 4.0, 7.8, 15.5 for = 0, 0.01, 0.04, 0.07, and 0.14 Cm-2 (0.72 mol/L), respectively. Intriguingly, the inner-sphere Pb2+ ions seem relevant only with the electric fields and are not affected by the change of salt concentrations, corroborating that it is electric fields that create and govern the 7
adsorption of inner-sphere Pb2+ ions. Owing to competition with inner-sphere species, the numbers of outer-sphere Pb2+ ions increase first and then reduce as the electric fields are gradually elevated, and for a specific electric field, more outer-sphere Pb2+ ions occur at higher salt concentrations, as a result of the saturation of inner-sphere Pb2+ ions (Figure 2). In absence of electric fields ( = 0 Cm-2), the PMF profiles of Pb2+ are characterized by the single minimum at 5.1 Å corresponding to the outer-sphere adsorption and the outer-sphere Pb2+ ions require an energy barrier of 27.8 kJ mol-1 for the transfer to bulk solutions. The electric fields improve the stabilities by shifting towards clay surfaces and elevating energy barriers; e.g., at 4.9 Å and 56.7 kJ mol-1 for = 0.14 Cm-2. Accordingly, even under strong electric fields, the outer-sphere Pb2+ ions are not likely to result in serious pollutions to clay systems that we are currently suffering. Meanwhile, a new sharp peak emerges at 3.0 Å in the PMF profiles (Figure 3) corresponding to the inner-sphere Pb2+ ions, and the energy barriers for the desorption processes show a persistent increase with the intensities of electric fields: 79.8, 84.0, 96.9 and 115.8 kJ mol-1 respectively for = 0.01, 0.04, 0.07 and 0.14 Cm-2, which is in line with the low RMSF values for all these Pb2+ ions ( 1.2 Å, see Table S2). In consequence, electric fields profoundly alter the adsorption of Pb2+ ions and reverse the adsorption strengths of Pb2+ and Cs+: Under relatively weak electric fields (e.g., = 0.01 Cm-2), the adsorption strengths of inner-sphere Pb2+ ions are already almost twice as those of Cs+ ions (79.8 vs. 42.7 kJ mol-1), and the stronger electric fields that occur ubiquitously within clay materials enhance substantially the adsorption of Pb2+ ions, with regard to both quantity and strength. As a result, although with appreciable hydration energies, heavy metal ions (e.g., Pb2+) instead of alkali metal ions (e.g., Cs+) are preferential to bind with solvated clay surfaces that cause serious pollutions to clay systems. 8
Only the outer-sphere complexes are constructed for the adsorption of Na+ ions at solvated clay surfaces without isomorphous substitutions ( = 0 Cm-2) [17, 34, 35, 37, 38], while isomorphous substitutions are pervasive within almost all types of clay materials and hence we are concerned with Hofmeister series in presence of electric fields resulting from isomorphous substitutions. As in the case of Pb2+ adsorption, Na+ ions are driven significantly by electric fields towards clay surfaces forming inner-sphere complexes, see Figure 4 and Figure S19 S21 [7, 8]. The numbers of inner-sphere Na+ ions show a substantial increase with electric fields and at 0.72 mol/L, are equal to 2, 4.2, 6.8 and 11.7 respectively for = 0.01, 0.04, 0.07 and 0.14 Cm-2, which are, however, apparently less than those of inner-sphere Cs+ ions. The energy barriers for the desorption of inner-sphere Na+ ions, according to the PMF profiles (Figure 4), are calculated to be 10.5, 14.0, 16.8 and 25.1 kJ mol-1 respectively for = 0.01, 0.04, 0.07 and 0.14 Cm-2, showing very limited increases with electric fields. The adsorption of different metal ions at the interface of clay materials and aqueous solutions exhibits clear Hofmeister effects that follow as Cs+ > Na+, in good agreement with the experimentally ion-exchange equilibrium, ion-exchange kinetics and aggregation results [9-12]; Moreover, Hofmeister series (Cs+ > Na+) remains consistent with the change of electric fields, and no reversal has been detected. The energy barrier differences for the desorption of inner-sphere Cs+ and Na+ ions are 32.2, 38.3, 44.5 and 74.0 kJ mol-1 respectively for = 0.01, 0.04, 0.07 and 0.14 Cm-2 and show a pronounced increase with electric fields. Thus, strong supports are given to the standpoint that polarization effects are responsible for Hofmeister effects occurring at the interface of clay materials and aqueous solutions (Figure 5): As compared to Na+, Cs+ has obviously softer electronic configurations and is more polarized by electric fields, correctly predicting that Hofmeister series follows as Cs+ > Na+; in addition, 9
Cs+ responds more actively to the change of electric fields and hence Hofmeister effects should be significantly magnified due to the elevation of electric fields. Figure 5 also illustrates two other models: a) The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory has for a long time been regarded as the cornerstone of colloidal science, where ions are assumed to be point charges. According to the DLVO theory, the adsorption strengths of all alkali cations onto clay surfaces are identical to each other; b) The silanol model (i.e., deprotonation of surface silanol groups, Si-OH
→
Si-O-) has often been used as prototypes of electric fields within clay materials [3-6]. Electrostatic interactions play a dominant role therein and Hofmeister series follows as Na+ > Cs+. However, electrostatic interactions have been known to fail to account for the Hofmeister phenomena at clay surfaces that usually carry abundant charges [7, 8]. Results obtained thus far are assumed to be beneficial to understand charged systems that are ubiquitous in chemistry and biology, such as proteins, DNA and metal oxides.
Conclusions Isomorphous substitutions are pervasive within almost all types of clay materials resulting in strong electric fields, and in this work, molecular dynamics simulations have been used to unveil at a molecular level how electric fields affect the adsorption of metal ions. In absence of electric fields, metal ions that were generally believed to show strong affinities to solvated clay materials (e.g., Cs+) are very facile to detach from the surfaces, while their stabilities can be pronouncedly enhanced by the presence of electric fields. Both adsorption quantities and strengths increase in direct proportion to the intensities of electric fields. Electric fields induce the formation of inner-sphere adsorption complexes for heavy metal ions (e.g., Pb2+) and their 10
stabilities are more promoted due to increase of electric fields than those of alkali ions, which in part explain the serious pollution of soil systems by heavy metal ions. Adsorption of different metal ions at the interface of clay materials and aqueous solutions exhibits clear Hofmeister effects that follow as Cs+ > Na+, and Hofmeister effects are magnified significantly due to the increase of electric fields. Strong supports are thus given to the standpoint that polarization effects are responsible for Hofmeister effects. The present results can be safely extended to other charged systems such as proteins, DNA and metal oxides.
Acknowledgements This work was sponsored by the National Natural Science Foundation of China (21473137) and Fourth Excellent Talents Program of Higher Education in Chongqing (2014-03).
Additional Information Competing financial interests: The authors declare no competing financial interests.
References (1) Bowers, G. M.; Hoyt, D. W.; Burton, S. D.; Ferguson, B. O.; Varga, T.; Kirkpatrick, R. J. In Situ 13C and 23Na Magic Angle Spinning NMR Investigation of Supercritical CO2 Incorporation in Smectite-Natural Organic Matter Composites. J. Phys. Chem. C 2014, 118, 3564-3573. (2) Gupta, S. S.; Bhattacharyya, K. G. Adsorption of Heavy Metals on Kaolinite and Montmorillonite: A Review. Phys. Chem. Chem. Phys. 2012, 14, 6698-6723. (3) Hocine, S.; Hartkamp, Remco.; Siboulet, B.; Duvail, M.; Coasne, B.; Turq, P.; Dufreche, J. F. How Ion Condensation Occurs at a Charged Surface: A Molecular Dynamics Investigation of the Stern Layer for Water-silica Interfaces. J. Phys. Chem. C 2016, 120, 963-973. 11
(4) Argyris, D.; Cole, D. R.; Striolo, A. Ion-Specific Effects under Confinement: The Role of Interfacial Water. ACS Nano 2010, 4, 2035-2042. (5) Dewan, S.; Carnevale, V.; Bankura, A.; Bafrooei, A. E.; Fiorin, G.; Klein, M. L.; Borguet, E. Structure of Water at Charged Interface: A Molecular Dynamics Study. Langmuir 2014, 30, 8056-8065. (6) Hartkamp, R.; Siboulet, B.; Dufreche, J. F.; Coasne, B. Ion-Specific Adsorption and Electroosmosis in Charged Amorphous Porous Silica. Phys. Chem. Chem. Phys. 2015, 17, 24683-24695. (7) Jungwirth, P.; Tobias, D. J. Specific Ion Effects at the Air/Water Interface. Chem. Rev. 2006, 106, 1259-1281. (8) Nostro, P. L.; Ninham, B. W. Hofmeister Phenomena: An Update on Ion Specificity in Biology. Chem. Rev. 2012, 112, 2286-2322. (9) Tian, R.; Yang, G.; Li, H.; Gao, X. D.; Liu, X. M.; Zhu, H. L.; Tang, Y. Activation Energies of Colloidal Particle Aggregation: Towards a Quantitative Characterization of Specific Ion Effects. Phys. Chem. Chem. Phys. 2014, 16, 8828-8836. (10) Tian, R.; Yang, G.; Tang, Y.; Liu, X. M.; Li, R.; Zhu, H. L.; Li, H. Origin of Hofmeister Effects for Complex Systems. PLOS One 2015, 10, e0128602. (11) Du, W.; Li, R.; Liu, X. M.; Tian, R.; Li, H. Specific Ion Effects on Ion Exchange Kinetics in Charged Clay. Colloids Surf. A 2016, 509, 427-432. (12) Liu, X. M.; Li, H.; Du, W.; Tian, R.; Li, R.; Jiang, X. J. Hofmeister Effects on Cation Exchange Equilibrium: Quantification of Ion Exchange Selectivity. J. Phys. Chem. C 2013, 117, 6245-6251. (13) Liu, X. M.; Yang, G.; Li, H.; Tian, R.; Li, R.; Jiang, X. J.; Ni, J. P.; Xie, D. T. Observation of Significant Steric, Valence and Polarization Effects and Their Interplay: A Modified Theory for Electric Double Layers. RSC Adv. 2014, 4, 1189-1192. (14) Jungwirth, P.; Cremer, P. S. Beyond Hofmeister. Nature Chem. 2014, 6, 261-263. (15) Bauduin, P.; Renoncourt, A.; Touraud, D.; Kunz, W.; Ninham, B. W. Hofmeister Effect on Enzymatic Catalysis and Colloidal Structures. Curr. Opin. Colloid Interface Sci. 2004, 9, 43-47. (16) Salis, A.; Ninham, B. W. Models and Mechanisms of Hofmeister Effects in Electrolyte 12
Solutions, and Colloid and Protein Systems Revisited. Chem. Soc. Rev. 2014, 43, 7358-7377. (17) Vasconcelos, I. F.; Bunker, B. A. Molecular Dynamics Modeling of Ion Adsorption to the Basal Surfaces of Kaolinite. J. Phys. Chem. C 2007, 111, 6753-6762. (18) Li, X.; Li, H.; Yang, G. Promoting the Adsorption of Metal Ions on Kaolinite by Defect Sites: A Molecular Dynamics Study. Sci. Rep. 2015, 5, 14377. (19) Li, X.; Li, H.; Yang, G. Configuration, Anion-Specific Effects, Diffusion, and Impact on Counterions for Adsorption of Salt Anions at the Interfaces of Clay Minerals. J. Phys. Chem. C 2016, 120, 14621-14630. (20) Kumar, N.; Zhao, CL.; Klaassen, A.; van den Ende, D.; Mugele, F.; Siretanu, I. Characterization of the Surface Charge Distribution on Kaolinite Particles Using High Resolution Atomic Force Microscopy. Geochim. Cosmochim. Acta 2016, 175, 100-112. (21) Mcbride, M. B. Copper(II) Interactions with Kaolinite: Factors Controlling Adsorption. Clays Clay Miner. 1978, 2, 101-106. (22) Youell R. F. Isomorphous Replacement in the Kaolin Group of Minerals. Nature 1958, 181, 557-558. (23) Michalkova, A.; Robinson, T. L.; Leszczynski, J. Adsorption of Thymine and Uracil on 1:1 Clay Mineral Surfaces: Comprehensive ab initio Study on Influence of Sodium Cation and Water. Phys. Chem. Chem. Phys. 2011, 13, 7862-7881. (24) van der Spoel, D.; Lindahl, E.; Hess, B.; Groenhof, G.; Mark, A. E.; Berendsen, H. J. C. GROMACS: Fast, Flexible, and Free. J. Comput. Chem. 2005, 26, 1701-1718. (25) Cygan, R. T.; Liang, J. J.; Kalinichev, A. G. Molecular Models of Hydroxide, Oxyhydroxide, and Clay Phases and the Development of a General Force Field. J. Phys. Chem. B 2004, 108, 1255-1266. (26) Mizan, T. I.; Savage, P. E.; Ziff, R. M. Comparison of Rigid and Flexible Simple Point Charge Water Models at Supercritical Conditions. J. Comput. Chem. 1996, 17, 1757-1770. (27) Zhu, R.; Molinari, M.; Shapley, T. V.; Parker, S. C. Modeling the Interaction of Nanoparticles with Mineral Surfaces: Adsorbed C60 on Pyrophyllite. J. Phys. Chem. A 2013, 117, 6602-6611. (28) Dequidt, A.; Devemy, J.; Malfreyt, P. Confined KCl Solution between Two Mica Surfaces: Equilibrium and Frictional Properties. J. Phys. Chem. C 2015, 119, 22080-22085. 13
(29) Hockney, R. W.; Goel, S. P.; Eastwood, J. W. Quiet High-Resolution Computer Models of a Plasma. J. Comput. Phys. 1974, 14, 148-158. (30) Bussi, G.; Donadio, D.; Parrinello, M. Canonical Sampling through Velocity Rescaling. J Chem. Phys. 2007, 126, 014101. (31) Nose, S.; Klein, M. L. Constant Pressure Molecular Dynamics for Molecular Systems. Mol. Phys. 1983, 50, 1055-1076. (32) Torrie, G. M.; Valleau, J. P. Non-Physical Sampling Distributions in Monte-Carlo Free-Energy Estimation-Umbrella Sampling. J. Comput. Phys. 1977, 23, 187-199. (33) Kumar, S.; Rosenberg, J. M.; Bouzida, D.; Swendsen, R. H.; Kollman, P. A. The Weighted Histogram Analysis Method for Free-Energy Calculations on Biomolecules. Ι. The Method. J. Comput. Chem. 1992, 13, 1011-1021. (34) Churakov, S. V. Mobility of Na and Cs on Montmorillonite Surface under Partially Saturated Conditions. Environ. Sci. Technol. 2013, 47, 9816-9823. (35) Rotenberg, B.; Marry, V.; Malikova, N.; Turq, P. Molecular Simulation of Aqueous Solutions at Clay Surfaces. J. Phys.: Condens. Matter 2010, 22, 284114. (36) Loganathan, N.; Yazaydin, A. O.; Bowers, G. M.; Kalinichev, A. G.; Kirkpatrick, R. J. Structure, Energetics, and Dynamics of Cs+ and H2O in Hectorite: Molecular Dynamics Simulations with an Unconstrained Substrate Surface. J. Phys. Chem. C 2016, 120, 10298-10310. (37) Ikeda, T. First-Principles-Based Simulation of Interlayer Water and Alkali Metal Ions in Weathered Biotite. J. Chem. Phys. 2016, 145, 124703. (38) Bowers, G. M.; Singer, J. W.; Bish, D. L.; Kirkpatrick, R. J. Alkali Metal and H 2O Dynamics at the Smectite/Water Interface. J. Phys. Chem. C 2011, 115, 23395-23407.
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Figure captions Figure 1. Initial and equilibrium configurations of 0.72 mol/L CsCl solutions in contact with the basal surfaces of kaolinite carrying different charge densities (indicated in the legends). With regard to the initial configuration (0 Cm-2), dashed red lines are used to schematically divide aqueous solutions into three regions. Cs+ and Cl- ions are presented as orange and green balls, respectively.
Figure 2. Numbers of adsorbed Cs+ and Pb2+ ions (Nad) for CsCl and PbCl2 solutions in equilibrium with the tetrahedral SiO4 surface of kaolinite with different charge densities. Figure 3. Potentials of mean force (PMF) for Cs+ and Pb2+ adsorption at the tetrahedral SiO4 surface of kaolinite with different charge densities. Figure 4. Numbers of adsorbed Na+ ions (Nad) and corresponding potentials of mean force (PMF) for the interaction of NaCl solutions and kaolinite with different charge densities. Figure 5. Schematic comparisons of Na+ vs. Cs+ adsorption driven by (a) point charge; (b) silanol model where the negative charge is produced by the deprotonation of surface silanol groups in clay minerals (Si-OH → Si-O-) and (c) electric field resulting from isomorphous substitutions within clay minerals, respectively.
15
Initial (0 Cm-2)
0 Cm-2
tetrahedral surface
bulk solution octahedral surface
0.01 Cm-2
0.04 Cm-2
0.07 Cm-2
0.14 Cm-2
Figure 1
16
0.18 mol/L
0.36 mol/L
0.72 mol/L
0.36 mol/L
0.72 mol/L
Nad (Cs+)
40
Inner Outer
30
20
10
0 20
0.18 mol/L
10
5
0. 01 0. 04 0. 07 0. 14 0. 00 0. 01 0. 04 0. 07 0. 14 0. 00 0. 01 0. 04 0. 07 0. 14
0 0. 00
Nad (Pb2+)
15
Inner Outer
Charge density (Cm-2) Figure 2
17
PMF (kJ·mol-1)
0 -20
Cs+
-40
0.00 Cm -2 0.01 Cm -2 0.04 Cm -2 0.07 Cm -2 0.14 Cm
-2
-60 -80
-100
PMF (kJ·mol-1)
0 -20
Pb2+
-40
-2
0.00 Cm -2 0.01 Cm -2 0.04 Cm -2 0.07 Cm -2 0.14 Cm
-60 -80
-100 -120 0
2
4
6
8
10
12
14
16
18
Distance to the surface (Å) Figure 3
18
15
0.36 mol/L
0.18 mol/L
0.72 mol/L
Inner Outer
Nad
10
5
0.
14
07 0.
04 0.
01 0.
14 0.
0.
07
04 0.
01 0.
0.
14
07 0.
04 0.
0.
01
0
Charge density (Cm-2)
-5
-1
PMF (kJ·mol )
0
-10
-2
0.01 Cm -2 0.04 Cm -2 0.07 Cm -2 0.14 Cm
-15 -20 -25 0
2
4
6
8
10
12
14
16
18
Distance to the surface (Å) Figure 4
19
Figure 5
20
Electric Fields within Clay Materials: How to Affect the Adsorption of Metal Ions Xiong Li, Hang Li, Gang Yang* College of Resources and Environment & Chongqing Key Laboratory of Soil Multi-scale Interfacial Process, Southwest University, Chongqing 400715, China
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