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Charge reversal and anion effects during adsorption of metal ions at clay surfaces: Mechanistic aspects and influence factors
Chemical Physics 529 (2020) 110575
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Charge reversal and anion effects during adsorption of metal ions at clay surfaces: Mechanistic aspects and influence factors Xiaoxiao Huang, Gang Yang
T
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College of Resources and Environment & Chongqing Key Laboratory of Soil Multi-scale Interfacial Process, Southwest University, Chongqing 400715, China
ARTICLE INFO
ABSTRACT
Keywords: Clay minerals Ion adsorption Charge reversal Electrostatic interaction Anion-specific effects
Charge reversal occurs frequently during adsorption of metal ions at clay surfaces, and recent experimental studies manifest the strong anion specificities during adsorption of metal ions at clay surfaces. Herein, mechanisms and influence factors of charge reversal and anion specificities have been addressed by DFT calculations. Charge reversal is attributed to the intrinsic electrostatic interactions from clay surfaces, which become more depleted at higher surface charges due to existence of more counterions. The tendency of charge reversal is negatively correlated with the amount of surface charges. Charge reversal can be caused by all metal ions while has a limited degree, even for high-valent metal ions. Anions take effects by forming ionic bonds with chargebalancing metal ions instead of forming direct interactions with clay surfaces, and the experimental observations are finely interpreted, such as anion specificities (Cl− < NO3− ≪ PO43−). Anion specificities be more pronounced at higher surface charges.
1. Introduction Clay minerals show the outstanding performances for adsorption of metal ions that is critical to the uptake of nutrients [1,2] and management of heavy and radioactive metal ions [3,4]; Meanwhile, the adsorptive metal ions modulate significantly the charge and chemical characteristics of clay minerals [5,6] and offer the sites for strong binding of water, organic matters, dyes and other substances [7,8]. Although significant progresses have been made for ion adsorption at clay surfaces, there are some critical issues remaining to be resolved. A large number of experimental studies have been conducted to understand the adsorption behaviors and mechanisms of metal ions at clay surfaces [9–13]. For adsorption of Mg2+, Ca2+, Cu2+, Zn2+ and Cd2+ at kaolinite, montmorillonite and illite surfaces, the effect of ionic concentrations can be described by the Langmuir isotherm, and the adsorption affinities of metal ions were found to rely strongly on the pH values [9,10]. The adsorption capacities of different divalent metal ions at kaolinite surface were determined that follow as Cu2+ < Zn2+ < Pb2+ < Cd2+ [13]. Kaolinite clays from Longyan (a city of China) remove Pb2+, Cd2+, Cu2+ and Ni2+ efficiently from wastewater, and pH increase favors the removal processes [11]. In addition to interpreting such experimental observations as indicated above, computer simulations provide valuable information about configuration, dynamics, adsorption affinity and reaction mechanism for
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clay systems that is otherwise almost inaccessible [14–18]. Adsorption of the uranyl(VI) ion at kaolinite surfaces was theoretically investigated, and the most favorable sites for adsorption were determined to be the partially deprotonated short-bridge AlOO(H) and long-bridge AlO-AlO(H) linkages [17]. Different factors may affect the adsorption of K+ at clay surfaces, and their respective contributions were evaluated by first-principles density functional theory (DFT) calculations, finding that surface charges play a critical role [18]. In the meanwhile, solvents exhibit a negligible effect for the adsorption configurations [19,20]. When the negative charges of clay surfaces have been compensated, adsorption of metal ions may continue to take place [21,22]. The state where more metal ions than needed to compensate the negative surface charges is referred to as charge reversal. Charge reversal is assumed to occur frequently for high-valent metal ions [23–25], while it was reported by Calero and Faraudo [26] that monovalent anions (SCN−, ClO4−) can also result in charge reversal at positively charged colloidal surfaces. Can monovalent metal ions result in charge reversal at negatively charged clay surfaces, what is the intrinsic mechanism to cause charge reversal, and which factors affect the results of charge reversal? To tackle these pending issues, first-principles DFT calculations were conducted for adsorption of low-valent K+ and high-valent Al3+, Ga3+ ions at charged-balanced clay surfaces. K+ and Al3+ are commonly found in clay systems, and a wide range of surface charges (σ = −0.07 ~ −0.42 C·m−2) were calculated, through which the
Corresponding author. E-mail address: [email protected] (G. Yang).
https://doi.org/10.1016/j.chemphys.2019.110575 Received 20 August 2019; Received in revised form 10 October 2019; Accepted 17 October 2019 Available online 19 October 2019 0301-0104/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics 529 (2020) 110575
X. Huang and G. Yang
relationship was established between the tendency of charge reversal and the amount of surface charges. Metal ions play a dominant role during the aggregation of negatively charged clay particles in electrolyte solutions, while anions also make a significant contribution and strong anion specificities (PO43− ≫ NO3− > Cl−) have been observed [27]. Argyris et al. [28] reported that Cl− can be adsorbed at the electroneutral silica surfaces, through the formation of H-bonds with surface silanol groups. A small amount of defects exist in clay surfaces [29–31], and defects were found to significantly promote the adsorption of metal ions [32]. In this work, defects of different sizes were constructed at negatively charged clay surfaces and as indicated by first-principles DFT calculations, clay surfaces even with these defects will not attach anions. Then a new mechanism for anion effects was proposed, where anions interact with the charge-balancing metal ions instead of being directly adsorbed at negatively charged clay surfaces. The proposed mechanism satisfactorily rationalizes the experimental observations, such as the difference between Cl− and NO3− [33] and the sequence of anion specificities [27]. The present results regarding charge reversal and anion-specific effects greatly promote the understanding of ion adsorption at clay surfaces.
siting of metal ions [18,32,38,39]. The adsorption energies of metal ions at clay surfaces are independent on the source of surface charges (isomorphically substituted and deprotonated) [18]. The surface areas of kaolinite models were estimated to be 227.16 Å2 [40], and a wide range of surface charges (σ = −0.07 ~ −0.42 C·m−2) were created in clay minerals by deprotonating the hydroxyl groups in the octahedral sheets, which were subsequently compensated by metal ions [18,36,39]. In order to study charge reversal, systems have to be overcharged, and cluster models that are more efficient than periodic boundary models [41,42] to treat charged systems were presently used (Fig. 1). A small amount of defects exist in clay surfaces [29–31] and defects of two different sizes were constructed for adsorption of anions. In line with previous works [18,41], the smaller defect was created by removal of one lattice Si atom (Fig. 1B) whereas for the larger defect (Fig. 1C), the lattice Si atoms neighboring the lattice Si atom that was removed during production of the smaller defect were leached as well. Then, the dangling O atoms in these defects were saturated by the H atoms forming the silanol groups (Si–OH).
2. Computational section
First-principles DFT calculations were performed using Gaussian09 software packages [43]. In accord to the previous works [27,37,40,44–47], clay models were divided into two regions that were simulated at different levels of theory. The hybrid B3LYP exchangecorrelation functional provides a good description of geometries, vibrational frequencies, and adsorption energies that show good agreement with higher-level calculated and experimental results [48–52]. The central hexagonal rings of the tetrahedral SiO4 surfaces of clay surfaces, along with adsorbents (metal ions and anions), became the choice for the high-level region and handled by the B3LYP/ 6–31 + G(d,p) method, whereas the rest were treated as the low-level region and described by the B3LYP/3-21G method. The 3-21G basis set is a good choice for structural optimizations of relatively large molecules [53–55] and more reliable than forcefield-based or semi-empirical methods [45] that were generally used for the low-level region. These two basis sets were combined in our previous works and have been verified to be efficient for different systems [27,37,40,45–47]. All
2.2. Methods
2.1. Models As aforementioned, kaolinite has been widely used for adsorption of metal ions, and pH increase (i.e., more deprotonation) favors the adsorption processes [11–13,17,19,34,35]. Kaolinite, with the chemical formula being Al2Si2O5(OH)4, is a type of layered clay minerals and comprised by alternating alumina octahedral and silicate tetrahedral sheets. Fig. 1 depicted the electroneutral clay model as used previously [32,36,37], consisting of 12 octahedral AlO6 and 12 tetrahedral SiO4 sites that mimics the 1:1-type clay minerals. The electroneutral clay model has a total of 156 atoms, including the H atoms to saturate the boundary O atoms. Metal ions are to be adsorbed at the tetrahedral surfaces of kaolinite. For all clay minerals including kaolinite and montmorillonite, the tetrahedral surfaces are always one of the most important arenas for
Fig. 1. A) Models for the electroneutral clay minerals: A) Regular surface, B) Small defect and C) Large defect. Color scheme: Si (blue), Al (pink), O (red) and H (white). 2
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adsorption processes in this study were associated with the formation of direct ionic bonds, and B3LYP functional obtains the comparable adsorption energies as CAM-B3LYP functional [56]. The hybrid quantities of B3LYP and long-range corrections proposed by Tawada and coworkers were integrated into CAM-B3LYP functional, which thus becomes accurate to describe non-covalent interactions. The adsorption energy of metal ions (M) at clay surfaces was calculated by [57],
Ead = Ead = (EClay
ESOL M
EClay
EM )
(EM
6H 2O
EM
(1)
6EH 2O )
where EClay, EM and EH2O stand for the electronic energies of the clay model, metal ion and water molecule, and EClay-M and EM-6H2O refer to the electronic energies for complexes of metal ions with clay models and water molecules, respectively. The coordination numbers of K+, Al3+ and Ga3+ with the first-shell water molecules were determined to be 6, and the subsequently added water molecules will enter into the second and higher coordination shells.
Fig. 3. Adsorption energies of one K+ ion (Ead) at clay surfaces that have already been charge-compensated by K+ ions. The adsorption energy of one K+ ion (Ead) on the electroneutral clay surface (σ = 0 C·m−2) is also shown.
3. Results 3.1. Charge reversal during K+ adsorption
surface charges are larger than −0.35 C·m−2, where the adsorption configurations should be especially unstable and the superfluous K+ ions more than to compensate the negative surface charges would not be adsorbed. Charge reversal is tentatively ascribed to the intrinsic electrostatic interactions from the electroneutral clay surfaces. The adsorption energy of one K+ ion onto the electroneutral clay surfaces (σ = 0 C·m−2) is calculated to be −0.68 eV, and the considerable Ead value corroborates the intrinsic electrostatic interactions from the electroneutral clay surfaces that clearly have no superfluous surface charges. Although the negative surface charges have been compensated, the intrinsic electrostatic interactions from the clay surfaces afford the additional capacity for adsorption of metal ions. For higher surface charges, more metal ions are required for charge compensation and meanwhile, the intrinsic electrostatic interactions from the clay surfaces may have already been depleted by these adsorptive metal ions, which may prevent the subsequent adsorption of metal ions as evidenced by positive Ead values. As indicated in Fig. 3, the adsorption energies of one K+ ion onto the charged-balanced clay surfaces (Ead) are getting more positive with increase of surface charges [58], and this trend remains tenable when including the electroneutral clay surfaces. Accordingly, the tendency of charge reversal has a negative correlation with the amount of surface charges.
+
Fig. 2 shows the adsorption configurations of one and two K ions at negatively charged clay surfaces (σ = −0.07 C·m−2). At −0.07 C·m−2, adsorption of two K+ ions changes the sign of surface charges from negative to positive, which is known to be charge reversal. Charge reversal is known to be pervasive for high-valent metal ions [23–25], while the adsorption energy of the second K+ ion at clay surfaces with σ = −0.07 C·m−2 (Ead) equals −0.63 eV and indicates that adsorption is energetically favorable. Accordingly, low-valent metal ions such as the presently investigated K+ can also result in charge reversal at negatively charged clay surfaces, which coincides with the adsorption of monovalent anions (SCN−, ClO4−) at positively charged colloidal surfaces [26]. To have a better understanding of charge reversal at negatively charged clay surfaces, a wide range of surface charges (σ = −0.07 ~ −0.42 C·m−2) have been subject to first-principles DFT calculations. The adsorption energies of one K+ ion at the chargecompensated clay surfaces (Ead) are calculated and depicted in Fig. 3. With increase of surface charges, the Ead values become more positive and it suggests the lesser likelihood of charge reversal for higher surface charges. The Ead values are −0.43, +0.02, +1.65 eV at σ = −0.14, −0.21 and −0.35 C·m−2, respectively. In consequence, charge reversal can take place when the negative surface charges are less than −0.14 C·m−2 while becomes almost impossible when the negative
Fig. 2. Adsorption structures for A) one and B) two K+ ions at negatively charged clay surfaces (σ = −0.07 C·m−2). Color scheme: Si (blue), Al (pink), O (red), K (purple) and H (white). 3
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Fig. 4. Adsorption structures for A) one Al3+ ion; B) one K+ ion, C) two K+ ions and D) three K+ ions at clay surfaces that have already been charge-compensated by Al3+ ions (σ = −0.42 C·m−2). Color scheme: Si (blue), Al (pink), O (red), K (purple) and H (white).
3.2. Charge reversal for high-valent metal ions
Because charge reversal has occurred, clay surfaces (σ = −0.42 C·m−2) with the adsorbed Al3+ ions now carry positive surface charges. The further adsorption of Al3+ ions will cause strong electrostatic repulsions and become especially difficult. As testified by first-principles DFT calculations, the subsequently adsorbed Al3+ ions will be pushed far away from the clay surfaces (σ = −0.42 C·m−2). In combination with the results of K+ adsorption on the K+-balanced clay surfaces (Fig. 3), it suggests that although ubiquitous, charge reversal has a rather limited degree, even for high-valent metal ions. The adsorption energies of the first, second and third K+ ions on the Al3+balanced clay surfaces are respectively +0.31, +2.16 and +4.17 eV, indicating that adsorption of the first K+ ion has already become unfavorable. It should be noted that the Al3+ ion carries three molar equivalent charges as the K+ ion. In consequence, higher-valent metal ions have an obviously larger tendency of charge reversal, with respect to both strength and quantity. The adsorption energy of one Ga3+ ion at the Ga3+-balanced clay surfaces (Ead) amounts to 0.05 eV and is slightly positive, totally different from the very favorable adsorption process for the Al3+ ion (Fig. 5). Despite that, the Ead value is less positive that that for adsorption of one K+ ion at the Ga3+-balanced clay surfaces (0.36 eV). The results further corroborate that higher-valent metal ions have an obviously larger tendency of charge reversal, since the Ga3+ ion also carries three molar equivalent charges as the K+ ion. In addition, although Al3+ and Ga3+ are both trivalent, their tendencies of charge reversal show divergences, and the Al3+ ion instead of the Ga3+ ion has a larger probability to result in charge reversal.
The adsorption structures of Al3+ and K+ ions at the Al3+-balanced clay surfaces (σ = −0.42 C·m−2) are depicted in Fig. 4. The adsorption energy of one Al3+ ion at the Al3+-balanced clay surfaces (Ead) is calculated to be −0.93 eV (Fig. 5) and is substantially negative, indicating the favorable adsorption process. Accordingly, the Al3+ ion is capable of resulting in charge reversal for clay surfaces with very high surface charges (σ = −0.42 C·m−2) while the K+ ion, as discussed above, has been prevented from further adsorption, in line with the previous observations that higher-valent metal ions correspond to a larger tendency for charge reversal [23–25].
3.3. Direct interaction of clay surfaces with anions?
Fig. 5. Adsorption energies of metal ions at clay surfaces that have already been charge-compensated by M3+ ions (σ = −0.42 C·m−2; M = Al, Ga as indicated in the legend).
As expected, the direct adsorption of anions at negatively charged clay surfaces seems impossible, and first-principles DFT calculations 4
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Fig. 6. Structures for adsorption of Cl− at defective clay surfaces (σ = −0.07 C·m−2): A) Smaller defect and B) Larger defect. Color scheme: Si (blue), Al (pink), O (red), Cl (green) and H (white).
The interaction energies of anions with K+ adsorbed at clay surfaces and in the corresponding potassium compounds are listed in Table 1. The Ein values of Cl−, NO3− and PO43− amount to −1.33, −1.40 and −8.14 eV for σ = −0.07 C·m−2 and −3.69, −3.96 and −15.25 eV for σ = −0.42 C·m−2, respectively. The Ein values are substantially negative, and it implies that interactions of anions with the K+-balanced clay surfaces occur very favorably and the proposed mechanism is reasonable. As compared to the Ein values in potassium compounds, the interaction strengths at negatively charged clay surfaces show a considerable reduction, due to the competing interactions with negatively charged clay surfaces that significantly decrease the affinities of K+ ions towards anions. At σ = −0.07 C·m−2, the Ein values may differ remarkably for the various anionic species and the interaction strengths ascend as Cl− < NO3− ≪ PO43−, exhibiting the exactly identical sequence as observed experimentally [27] and further evidencing the proposed mechanism. With increase of surface charges, the Ein differences among these anionic species are significantly magnified (Table 1), suggesting the stronger anion specificities at higher surface charges. At σ = −0.42 C·m−2, three K+ ions may be involved to form direct ionic bonds with anions and the profoundly enhanced interactions are produced than at σ = −0.07 C·m−2. As indicated in Table 1, Cl− and NO3− have the nearly identical interaction strengths with K+ in potassium compounds [60] while show the divergent effects at negatively charged clay surfaces. This theoretical prediction is again in excellent agreement with the recent experimental observations [27,33].
indicate that anions will be pushed far away from the clay surfaces, even with a small amount of surface charges (e.g., σ = −0.07 C·m−2). The results are consistent with the familiar EDL (electric double layer) theory stating that metal ions serve as counterions and are preferential to approach the negatively charged clay surfaces. It has been reported that a small amount of defects can be detected in the tetrahedral SiO2 surface of clay minerals [29–31]. Can the silanol groups of these defects accommodate the anionic species as those of the electroneutral silica surfaces [28]? To unravel the strong anion-specific effects observed during the aggregation of negatively charged clay particles, two defects of different sizes have been constructed at negatively charged clay surfaces (σ = −0.07, −0.42 C·m−2), see Fig. 1. Defects show the pronounced stabilization effects for adsorption of anions as for adsorption of metal ions [18], while the interaction mechanisms are different. For metal ions, defects have the similar interaction mode as regular surfaces and form direct ionic bonds with metal ions through the O atoms of the surface silanol groups, whereas anions are adsorbed through the formation of H-bonds with the surface silanol groups [28,59]. The adsorption structures of Cl− on two defective clay surfaces (σ = −0.07 C·m−2) are shown in Fig. 6. The adsorption energies (Ead) of Cl− are very positive and amount to 2.44 and 1.84 eV for the smaller and larger defects, respectively. Accordingly, increase of defect sizes is beneficial to the interaction with anions while the promoting effects are rather limited, and owing to strong electrostatic repulsions between anions and negatively charged clay surfaces, it seems impossible for the direct adsorption of anions, even if the negative surface charges are very small (σ = −0.07 C·m−2). When surface charges ascend to −0.42 C·m−2, the adsorption structures are not geometrically stable, and Cl− will be pushed far away from the clay surfaces. Accordingly, negatively charged clay surfaces behave distinctly from those of electroneutral counterparts [28,46], and the situation remains even with presence of defects.
4. Conclusions In the work, two key issues regarding ion adsorption at negatively charged clay surfaces (i.e., charge reversal and anion effects) have been addressed at a molecular level, and the main summaries are given as follows. Charge reversal is ascribed to the intrinsic electrostatic interactions from the electroneutral clay surfaces. In addition to negative surface charges, the intrinsic electrostatic interactions afford the additional adsorption capacity for metal ions. Low-valent metal ions (e.g., monovalent K+) can result in charge reversal at negatively charged clay surfaces. High-valent metal ions such as Al3+ and Ga3+ have a much larger tendency to result in charge reversal, with respect to both strength and quantity. For all metal ions, charge reversal has a very limited degree, and the degree may differ for homovalent metal ions. The tendency of charge reversal has a negative correlation with the amount of surface charges. Even with defects of different sizes, negatively charged clay surfaces will not attach anions directly. A new mechanism of anion effects has
3.4. Mechanism for anion effects Direct interaction of negatively charged clay surfaces (even with defects) seems difficult to rationalize the notable anion-specific effects observed experimentally [27,33]. To this end, a new mechanism is proposed and anions (Cl−, NO3− and PO43−) take effects by forming ionic bonds with the K+ ions that compensate the negative charges of clay surfaces, see Fig. 7. The interaction energies of anions with K+balanced clay surfaces (Ein) are calculated by,
Ein = EClay
M A
EClay
M
EA
(2)
where EA and EClay-M-A refer to the electronic energies for anions and their interacted structures with K+-balanced clay surfaces, respectively. 5
Chemical Physics 529 (2020) 110575
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Fig. 7. Structures for Cl−, NO3− and PO43− interactions with clay surfaces that have been charge-compensated by K+ ions: A, B, C) σ = −0.07 C·m−2 and D, E, F) σ = −0.42 C·m−2. Color scheme: Si (blue), Al (pink), O (red), K (purple), Cl (green), N (navy blue), P (orange) and H (white).
References
Table 1 Interaction energies (Ein, eV) of different anions with the K+-balanced clay surfaces (σ = −0.07, −0.42 C·m−2).a
σ = −0.07 σ = −0.42 Potassium compoundsb
Cl−
NO3−
PO43−
−1.33 (0.00) −3.69 (0.00) −4.85 (0.00)
−1.40 (−0.07) −3.96 (−0.27) −4.85 (0.00)
−8.14 (−6.81) −15.25 (−11.56) −29.86 (−25.01)
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a
The Ein differences using Cl− as benchmark are given in parentheses; Interaction energies for K+ with anions in the corresponding potassium compounds (KCl, KNO3 and K3PO4). b
been posed. Anions take effects by forming ionic bonds with the chargebalancing metal ions at clay surfaces and such interactions occur very favorably. In addition, the new mechanism satisfactorily interpret the experimental observations, such as the exactly identical anion specificity (Cl− < NO3− ≪ PO43−). Cl− and NO3− in potassium compounds have nearly the same interaction strengths with K+ while show clear differences at negatively charged clay surfaces. Anion specificities are predicted to be stronger at higher surface charges. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was sponsored by the Natural Science Foundation Project of CQ CSTC, China (cstc2017jcyjAX0145), Fundamental Research Funds for the Central Universities (XDJK2019B038), Innovative Research Groups of CQ, China (CXQT10006) and National Natural Science Foundation of China (21473137 and 41530855). 6
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Charge reversal and anion effects during adsorption of metal ions at clay surfaces: Mechanistic aspects and influence factors Xiaoxiao Huang, Gang Yang
T
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College of Resources and Environment & Chongqing Key Laboratory of Soil Multi-scale Interfacial Process, Southwest University, Chongqing 400715, China
ARTICLE INFO
ABSTRACT
Keywords: Clay minerals Ion adsorption Charge reversal Electrostatic interaction Anion-specific effects
Charge reversal occurs frequently during adsorption of metal ions at clay surfaces, and recent experimental studies manifest the strong anion specificities during adsorption of metal ions at clay surfaces. Herein, mechanisms and influence factors of charge reversal and anion specificities have been addressed by DFT calculations. Charge reversal is attributed to the intrinsic electrostatic interactions from clay surfaces, which become more depleted at higher surface charges due to existence of more counterions. The tendency of charge reversal is negatively correlated with the amount of surface charges. Charge reversal can be caused by all metal ions while has a limited degree, even for high-valent metal ions. Anions take effects by forming ionic bonds with chargebalancing metal ions instead of forming direct interactions with clay surfaces, and the experimental observations are finely interpreted, such as anion specificities (Cl− < NO3− ≪ PO43−). Anion specificities be more pronounced at higher surface charges.
1. Introduction Clay minerals show the outstanding performances for adsorption of metal ions that is critical to the uptake of nutrients [1,2] and management of heavy and radioactive metal ions [3,4]; Meanwhile, the adsorptive metal ions modulate significantly the charge and chemical characteristics of clay minerals [5,6] and offer the sites for strong binding of water, organic matters, dyes and other substances [7,8]. Although significant progresses have been made for ion adsorption at clay surfaces, there are some critical issues remaining to be resolved. A large number of experimental studies have been conducted to understand the adsorption behaviors and mechanisms of metal ions at clay surfaces [9–13]. For adsorption of Mg2+, Ca2+, Cu2+, Zn2+ and Cd2+ at kaolinite, montmorillonite and illite surfaces, the effect of ionic concentrations can be described by the Langmuir isotherm, and the adsorption affinities of metal ions were found to rely strongly on the pH values [9,10]. The adsorption capacities of different divalent metal ions at kaolinite surface were determined that follow as Cu2+ < Zn2+ < Pb2+ < Cd2+ [13]. Kaolinite clays from Longyan (a city of China) remove Pb2+, Cd2+, Cu2+ and Ni2+ efficiently from wastewater, and pH increase favors the removal processes [11]. In addition to interpreting such experimental observations as indicated above, computer simulations provide valuable information about configuration, dynamics, adsorption affinity and reaction mechanism for
⁎
clay systems that is otherwise almost inaccessible [14–18]. Adsorption of the uranyl(VI) ion at kaolinite surfaces was theoretically investigated, and the most favorable sites for adsorption were determined to be the partially deprotonated short-bridge AlOO(H) and long-bridge AlO-AlO(H) linkages [17]. Different factors may affect the adsorption of K+ at clay surfaces, and their respective contributions were evaluated by first-principles density functional theory (DFT) calculations, finding that surface charges play a critical role [18]. In the meanwhile, solvents exhibit a negligible effect for the adsorption configurations [19,20]. When the negative charges of clay surfaces have been compensated, adsorption of metal ions may continue to take place [21,22]. The state where more metal ions than needed to compensate the negative surface charges is referred to as charge reversal. Charge reversal is assumed to occur frequently for high-valent metal ions [23–25], while it was reported by Calero and Faraudo [26] that monovalent anions (SCN−, ClO4−) can also result in charge reversal at positively charged colloidal surfaces. Can monovalent metal ions result in charge reversal at negatively charged clay surfaces, what is the intrinsic mechanism to cause charge reversal, and which factors affect the results of charge reversal? To tackle these pending issues, first-principles DFT calculations were conducted for adsorption of low-valent K+ and high-valent Al3+, Ga3+ ions at charged-balanced clay surfaces. K+ and Al3+ are commonly found in clay systems, and a wide range of surface charges (σ = −0.07 ~ −0.42 C·m−2) were calculated, through which the
Corresponding author. E-mail address: [email protected] (G. Yang).
https://doi.org/10.1016/j.chemphys.2019.110575 Received 20 August 2019; Received in revised form 10 October 2019; Accepted 17 October 2019 Available online 19 October 2019 0301-0104/ © 2019 Elsevier B.V. All rights reserved.
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relationship was established between the tendency of charge reversal and the amount of surface charges. Metal ions play a dominant role during the aggregation of negatively charged clay particles in electrolyte solutions, while anions also make a significant contribution and strong anion specificities (PO43− ≫ NO3− > Cl−) have been observed [27]. Argyris et al. [28] reported that Cl− can be adsorbed at the electroneutral silica surfaces, through the formation of H-bonds with surface silanol groups. A small amount of defects exist in clay surfaces [29–31], and defects were found to significantly promote the adsorption of metal ions [32]. In this work, defects of different sizes were constructed at negatively charged clay surfaces and as indicated by first-principles DFT calculations, clay surfaces even with these defects will not attach anions. Then a new mechanism for anion effects was proposed, where anions interact with the charge-balancing metal ions instead of being directly adsorbed at negatively charged clay surfaces. The proposed mechanism satisfactorily rationalizes the experimental observations, such as the difference between Cl− and NO3− [33] and the sequence of anion specificities [27]. The present results regarding charge reversal and anion-specific effects greatly promote the understanding of ion adsorption at clay surfaces.
siting of metal ions [18,32,38,39]. The adsorption energies of metal ions at clay surfaces are independent on the source of surface charges (isomorphically substituted and deprotonated) [18]. The surface areas of kaolinite models were estimated to be 227.16 Å2 [40], and a wide range of surface charges (σ = −0.07 ~ −0.42 C·m−2) were created in clay minerals by deprotonating the hydroxyl groups in the octahedral sheets, which were subsequently compensated by metal ions [18,36,39]. In order to study charge reversal, systems have to be overcharged, and cluster models that are more efficient than periodic boundary models [41,42] to treat charged systems were presently used (Fig. 1). A small amount of defects exist in clay surfaces [29–31] and defects of two different sizes were constructed for adsorption of anions. In line with previous works [18,41], the smaller defect was created by removal of one lattice Si atom (Fig. 1B) whereas for the larger defect (Fig. 1C), the lattice Si atoms neighboring the lattice Si atom that was removed during production of the smaller defect were leached as well. Then, the dangling O atoms in these defects were saturated by the H atoms forming the silanol groups (Si–OH).
2. Computational section
First-principles DFT calculations were performed using Gaussian09 software packages [43]. In accord to the previous works [27,37,40,44–47], clay models were divided into two regions that were simulated at different levels of theory. The hybrid B3LYP exchangecorrelation functional provides a good description of geometries, vibrational frequencies, and adsorption energies that show good agreement with higher-level calculated and experimental results [48–52]. The central hexagonal rings of the tetrahedral SiO4 surfaces of clay surfaces, along with adsorbents (metal ions and anions), became the choice for the high-level region and handled by the B3LYP/ 6–31 + G(d,p) method, whereas the rest were treated as the low-level region and described by the B3LYP/3-21G method. The 3-21G basis set is a good choice for structural optimizations of relatively large molecules [53–55] and more reliable than forcefield-based or semi-empirical methods [45] that were generally used for the low-level region. These two basis sets were combined in our previous works and have been verified to be efficient for different systems [27,37,40,45–47]. All
2.2. Methods
2.1. Models As aforementioned, kaolinite has been widely used for adsorption of metal ions, and pH increase (i.e., more deprotonation) favors the adsorption processes [11–13,17,19,34,35]. Kaolinite, with the chemical formula being Al2Si2O5(OH)4, is a type of layered clay minerals and comprised by alternating alumina octahedral and silicate tetrahedral sheets. Fig. 1 depicted the electroneutral clay model as used previously [32,36,37], consisting of 12 octahedral AlO6 and 12 tetrahedral SiO4 sites that mimics the 1:1-type clay minerals. The electroneutral clay model has a total of 156 atoms, including the H atoms to saturate the boundary O atoms. Metal ions are to be adsorbed at the tetrahedral surfaces of kaolinite. For all clay minerals including kaolinite and montmorillonite, the tetrahedral surfaces are always one of the most important arenas for
Fig. 1. A) Models for the electroneutral clay minerals: A) Regular surface, B) Small defect and C) Large defect. Color scheme: Si (blue), Al (pink), O (red) and H (white). 2
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adsorption processes in this study were associated with the formation of direct ionic bonds, and B3LYP functional obtains the comparable adsorption energies as CAM-B3LYP functional [56]. The hybrid quantities of B3LYP and long-range corrections proposed by Tawada and coworkers were integrated into CAM-B3LYP functional, which thus becomes accurate to describe non-covalent interactions. The adsorption energy of metal ions (M) at clay surfaces was calculated by [57],
Ead = Ead = (EClay
ESOL M
EClay
EM )
(EM
6H 2O
EM
(1)
6EH 2O )
where EClay, EM and EH2O stand for the electronic energies of the clay model, metal ion and water molecule, and EClay-M and EM-6H2O refer to the electronic energies for complexes of metal ions with clay models and water molecules, respectively. The coordination numbers of K+, Al3+ and Ga3+ with the first-shell water molecules were determined to be 6, and the subsequently added water molecules will enter into the second and higher coordination shells.
Fig. 3. Adsorption energies of one K+ ion (Ead) at clay surfaces that have already been charge-compensated by K+ ions. The adsorption energy of one K+ ion (Ead) on the electroneutral clay surface (σ = 0 C·m−2) is also shown.
3. Results 3.1. Charge reversal during K+ adsorption
surface charges are larger than −0.35 C·m−2, where the adsorption configurations should be especially unstable and the superfluous K+ ions more than to compensate the negative surface charges would not be adsorbed. Charge reversal is tentatively ascribed to the intrinsic electrostatic interactions from the electroneutral clay surfaces. The adsorption energy of one K+ ion onto the electroneutral clay surfaces (σ = 0 C·m−2) is calculated to be −0.68 eV, and the considerable Ead value corroborates the intrinsic electrostatic interactions from the electroneutral clay surfaces that clearly have no superfluous surface charges. Although the negative surface charges have been compensated, the intrinsic electrostatic interactions from the clay surfaces afford the additional capacity for adsorption of metal ions. For higher surface charges, more metal ions are required for charge compensation and meanwhile, the intrinsic electrostatic interactions from the clay surfaces may have already been depleted by these adsorptive metal ions, which may prevent the subsequent adsorption of metal ions as evidenced by positive Ead values. As indicated in Fig. 3, the adsorption energies of one K+ ion onto the charged-balanced clay surfaces (Ead) are getting more positive with increase of surface charges [58], and this trend remains tenable when including the electroneutral clay surfaces. Accordingly, the tendency of charge reversal has a negative correlation with the amount of surface charges.
+
Fig. 2 shows the adsorption configurations of one and two K ions at negatively charged clay surfaces (σ = −0.07 C·m−2). At −0.07 C·m−2, adsorption of two K+ ions changes the sign of surface charges from negative to positive, which is known to be charge reversal. Charge reversal is known to be pervasive for high-valent metal ions [23–25], while the adsorption energy of the second K+ ion at clay surfaces with σ = −0.07 C·m−2 (Ead) equals −0.63 eV and indicates that adsorption is energetically favorable. Accordingly, low-valent metal ions such as the presently investigated K+ can also result in charge reversal at negatively charged clay surfaces, which coincides with the adsorption of monovalent anions (SCN−, ClO4−) at positively charged colloidal surfaces [26]. To have a better understanding of charge reversal at negatively charged clay surfaces, a wide range of surface charges (σ = −0.07 ~ −0.42 C·m−2) have been subject to first-principles DFT calculations. The adsorption energies of one K+ ion at the chargecompensated clay surfaces (Ead) are calculated and depicted in Fig. 3. With increase of surface charges, the Ead values become more positive and it suggests the lesser likelihood of charge reversal for higher surface charges. The Ead values are −0.43, +0.02, +1.65 eV at σ = −0.14, −0.21 and −0.35 C·m−2, respectively. In consequence, charge reversal can take place when the negative surface charges are less than −0.14 C·m−2 while becomes almost impossible when the negative
Fig. 2. Adsorption structures for A) one and B) two K+ ions at negatively charged clay surfaces (σ = −0.07 C·m−2). Color scheme: Si (blue), Al (pink), O (red), K (purple) and H (white). 3
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Fig. 4. Adsorption structures for A) one Al3+ ion; B) one K+ ion, C) two K+ ions and D) three K+ ions at clay surfaces that have already been charge-compensated by Al3+ ions (σ = −0.42 C·m−2). Color scheme: Si (blue), Al (pink), O (red), K (purple) and H (white).
3.2. Charge reversal for high-valent metal ions
Because charge reversal has occurred, clay surfaces (σ = −0.42 C·m−2) with the adsorbed Al3+ ions now carry positive surface charges. The further adsorption of Al3+ ions will cause strong electrostatic repulsions and become especially difficult. As testified by first-principles DFT calculations, the subsequently adsorbed Al3+ ions will be pushed far away from the clay surfaces (σ = −0.42 C·m−2). In combination with the results of K+ adsorption on the K+-balanced clay surfaces (Fig. 3), it suggests that although ubiquitous, charge reversal has a rather limited degree, even for high-valent metal ions. The adsorption energies of the first, second and third K+ ions on the Al3+balanced clay surfaces are respectively +0.31, +2.16 and +4.17 eV, indicating that adsorption of the first K+ ion has already become unfavorable. It should be noted that the Al3+ ion carries three molar equivalent charges as the K+ ion. In consequence, higher-valent metal ions have an obviously larger tendency of charge reversal, with respect to both strength and quantity. The adsorption energy of one Ga3+ ion at the Ga3+-balanced clay surfaces (Ead) amounts to 0.05 eV and is slightly positive, totally different from the very favorable adsorption process for the Al3+ ion (Fig. 5). Despite that, the Ead value is less positive that that for adsorption of one K+ ion at the Ga3+-balanced clay surfaces (0.36 eV). The results further corroborate that higher-valent metal ions have an obviously larger tendency of charge reversal, since the Ga3+ ion also carries three molar equivalent charges as the K+ ion. In addition, although Al3+ and Ga3+ are both trivalent, their tendencies of charge reversal show divergences, and the Al3+ ion instead of the Ga3+ ion has a larger probability to result in charge reversal.
The adsorption structures of Al3+ and K+ ions at the Al3+-balanced clay surfaces (σ = −0.42 C·m−2) are depicted in Fig. 4. The adsorption energy of one Al3+ ion at the Al3+-balanced clay surfaces (Ead) is calculated to be −0.93 eV (Fig. 5) and is substantially negative, indicating the favorable adsorption process. Accordingly, the Al3+ ion is capable of resulting in charge reversal for clay surfaces with very high surface charges (σ = −0.42 C·m−2) while the K+ ion, as discussed above, has been prevented from further adsorption, in line with the previous observations that higher-valent metal ions correspond to a larger tendency for charge reversal [23–25].
3.3. Direct interaction of clay surfaces with anions?
Fig. 5. Adsorption energies of metal ions at clay surfaces that have already been charge-compensated by M3+ ions (σ = −0.42 C·m−2; M = Al, Ga as indicated in the legend).
As expected, the direct adsorption of anions at negatively charged clay surfaces seems impossible, and first-principles DFT calculations 4
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Fig. 6. Structures for adsorption of Cl− at defective clay surfaces (σ = −0.07 C·m−2): A) Smaller defect and B) Larger defect. Color scheme: Si (blue), Al (pink), O (red), Cl (green) and H (white).
The interaction energies of anions with K+ adsorbed at clay surfaces and in the corresponding potassium compounds are listed in Table 1. The Ein values of Cl−, NO3− and PO43− amount to −1.33, −1.40 and −8.14 eV for σ = −0.07 C·m−2 and −3.69, −3.96 and −15.25 eV for σ = −0.42 C·m−2, respectively. The Ein values are substantially negative, and it implies that interactions of anions with the K+-balanced clay surfaces occur very favorably and the proposed mechanism is reasonable. As compared to the Ein values in potassium compounds, the interaction strengths at negatively charged clay surfaces show a considerable reduction, due to the competing interactions with negatively charged clay surfaces that significantly decrease the affinities of K+ ions towards anions. At σ = −0.07 C·m−2, the Ein values may differ remarkably for the various anionic species and the interaction strengths ascend as Cl− < NO3− ≪ PO43−, exhibiting the exactly identical sequence as observed experimentally [27] and further evidencing the proposed mechanism. With increase of surface charges, the Ein differences among these anionic species are significantly magnified (Table 1), suggesting the stronger anion specificities at higher surface charges. At σ = −0.42 C·m−2, three K+ ions may be involved to form direct ionic bonds with anions and the profoundly enhanced interactions are produced than at σ = −0.07 C·m−2. As indicated in Table 1, Cl− and NO3− have the nearly identical interaction strengths with K+ in potassium compounds [60] while show the divergent effects at negatively charged clay surfaces. This theoretical prediction is again in excellent agreement with the recent experimental observations [27,33].
indicate that anions will be pushed far away from the clay surfaces, even with a small amount of surface charges (e.g., σ = −0.07 C·m−2). The results are consistent with the familiar EDL (electric double layer) theory stating that metal ions serve as counterions and are preferential to approach the negatively charged clay surfaces. It has been reported that a small amount of defects can be detected in the tetrahedral SiO2 surface of clay minerals [29–31]. Can the silanol groups of these defects accommodate the anionic species as those of the electroneutral silica surfaces [28]? To unravel the strong anion-specific effects observed during the aggregation of negatively charged clay particles, two defects of different sizes have been constructed at negatively charged clay surfaces (σ = −0.07, −0.42 C·m−2), see Fig. 1. Defects show the pronounced stabilization effects for adsorption of anions as for adsorption of metal ions [18], while the interaction mechanisms are different. For metal ions, defects have the similar interaction mode as regular surfaces and form direct ionic bonds with metal ions through the O atoms of the surface silanol groups, whereas anions are adsorbed through the formation of H-bonds with the surface silanol groups [28,59]. The adsorption structures of Cl− on two defective clay surfaces (σ = −0.07 C·m−2) are shown in Fig. 6. The adsorption energies (Ead) of Cl− are very positive and amount to 2.44 and 1.84 eV for the smaller and larger defects, respectively. Accordingly, increase of defect sizes is beneficial to the interaction with anions while the promoting effects are rather limited, and owing to strong electrostatic repulsions between anions and negatively charged clay surfaces, it seems impossible for the direct adsorption of anions, even if the negative surface charges are very small (σ = −0.07 C·m−2). When surface charges ascend to −0.42 C·m−2, the adsorption structures are not geometrically stable, and Cl− will be pushed far away from the clay surfaces. Accordingly, negatively charged clay surfaces behave distinctly from those of electroneutral counterparts [28,46], and the situation remains even with presence of defects.
4. Conclusions In the work, two key issues regarding ion adsorption at negatively charged clay surfaces (i.e., charge reversal and anion effects) have been addressed at a molecular level, and the main summaries are given as follows. Charge reversal is ascribed to the intrinsic electrostatic interactions from the electroneutral clay surfaces. In addition to negative surface charges, the intrinsic electrostatic interactions afford the additional adsorption capacity for metal ions. Low-valent metal ions (e.g., monovalent K+) can result in charge reversal at negatively charged clay surfaces. High-valent metal ions such as Al3+ and Ga3+ have a much larger tendency to result in charge reversal, with respect to both strength and quantity. For all metal ions, charge reversal has a very limited degree, and the degree may differ for homovalent metal ions. The tendency of charge reversal has a negative correlation with the amount of surface charges. Even with defects of different sizes, negatively charged clay surfaces will not attach anions directly. A new mechanism of anion effects has
3.4. Mechanism for anion effects Direct interaction of negatively charged clay surfaces (even with defects) seems difficult to rationalize the notable anion-specific effects observed experimentally [27,33]. To this end, a new mechanism is proposed and anions (Cl−, NO3− and PO43−) take effects by forming ionic bonds with the K+ ions that compensate the negative charges of clay surfaces, see Fig. 7. The interaction energies of anions with K+balanced clay surfaces (Ein) are calculated by,
Ein = EClay
M A
EClay
M
EA
(2)
where EA and EClay-M-A refer to the electronic energies for anions and their interacted structures with K+-balanced clay surfaces, respectively. 5
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Fig. 7. Structures for Cl−, NO3− and PO43− interactions with clay surfaces that have been charge-compensated by K+ ions: A, B, C) σ = −0.07 C·m−2 and D, E, F) σ = −0.42 C·m−2. Color scheme: Si (blue), Al (pink), O (red), K (purple), Cl (green), N (navy blue), P (orange) and H (white).
References
Table 1 Interaction energies (Ein, eV) of different anions with the K+-balanced clay surfaces (σ = −0.07, −0.42 C·m−2).a
σ = −0.07 σ = −0.42 Potassium compoundsb
Cl−
NO3−
PO43−
−1.33 (0.00) −3.69 (0.00) −4.85 (0.00)
−1.40 (−0.07) −3.96 (−0.27) −4.85 (0.00)
−8.14 (−6.81) −15.25 (−11.56) −29.86 (−25.01)
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