DFT and 2D-CA methods unravelling the mechanism of interfacial interaction between amino acids and Ca-montmorillonite

Applied Clay Science 183 (2019) 105356

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Research Paper

DFT and 2D-CA methods unravelling the mechanism of interfacial interaction between amino acids and Ca-montmorillonite

T

Hai-long Lia, Liang Biana,b, , Fa-qin Donga, , Wei-min Lia, Mian-xin Songa, Jia-nan Niea, Xiao-nan Liua, Ting-ting Huoa, Hong-ping Zhanga, Bing Xua, Frank S. Riehlea, Shu-hui Suna ⁎

⁎

a

Key Laboratory of Solid Waste Treatment and Resource Recycle, School of Environment and Resource, Fundamental Science on Nuclear Wastes and Environmental Safety Laboratory, Southwest University of Science and Technology, Mianyang 621010, Sichuan, China b Institute of Gem and Material Technology, Hebei GEO University, Shijiazhuang 050000, Hebei, China

ARTICLE INFO

ABSTRACT

Keywords: Adsorption 2D-CA methods Orbital coupling Interfacial interaction

We explored the effect of contact time on the interfacial interaction mechanism of amino acids (AAs) connected to the aluminol group (AlOH) and interlayer Ca ions of Ca-montmorillonite (CaeMt) in an aqueous solution using density functional theory (DFT) and two-dimensional correlation analysis (2D-CA) technology. The results showed that these interactions include electrostatic (or van-der-Waals) interaction, cation exchange and hydrophilic interaction. In particular, the electrostatic (or van-der-Waals) interaction between the –COO−(H) (and –NH3+) groups of the AAs and surface negative O atoms of CaeMt were found to be the main interaction leading to the adsorption behaviour of AAs onto CaeMt. With increasing contact time, the Ca-d0 orbital splitting (dx2+y2 → dx2+y2 + dZ2) not only changes the orbital coupling between the Ca-d0 and O-2p4 orbitals (Ca-dx2+y2-O2p4 → Ca-dx2+y2 + dZ2-O-2p4) but also enhances the formation of Ca+–COO−(H) p-p σ (neutral: glycine and serine) and Ca+-NH3+ p-p π (charged: glutamate and arginine) hybrid orbitals, as well as the cation exchange (AlOH-Ca + AAs) that mainly contributes to the short-range van-der-Waals interaction. Furthermore, the H-1 s (H2O) orbital is degenerate, which in turn enhances the orbital overlap of H-1 s (H2O) with O-2p4 (-HOCO) and N-2p3 (-NH3), leading to the formation of hydrated clusters: -NH3·(H2O)+ and –HOCO·(H2O)−. The hydrophilic interaction (AlOH-H2O + AAs) mainly contributes to the long-range electrostatic interaction. The results of the study provide a new perspective to understand the adsorption process of AAs onto clay mineral surfaces.

1. Introduction The interfacial interaction of amino acids (AAs) with clay minerals, as a potentially important process in the natural environment, is closely related to the adsorption kinetics of AAs onto mineral surfaces in soils and sediments (Zaia, 2004; Lambert, 2008; Yu et al., 2013; Bu et al., 2019; Bu et al., 2017). The contact time, which is the most important factor in adsorption kinetics of AAs onto clay minerals, not only reflects the changes in the ionic species (neutral→zwitterionic) of the AAs and charge characteristics (protonation/deprotonation) of the mineral surfaces, but also reflects the changes in the chemisorption process (edge/ surface adsorption→interlayer adsorption) (Swadling et al., 2013;

Pagel-Wieder, et al., 2007; Dong et al., 2018). Note that the contact time for minerals to adsorb different AAs (serine, glycine, arginine and glutamate, etc.) under equilibrium conditions can be different in a natural environment (Zaia et al., 2008; Friebele et al., 1980). For example, the contact time for adsorption equilibrium in the case of serine, glycine, arginine and glutamate adsorbed onto montmorillonite (Mt) was approximately 4 h, 2 h, 2 h and 4 h, respectively (Hedges and Hare, 1987). These results reflect the differences in the reaction mechanism and the nature of the interaction between AAs and minerals on the time scale of the process. The causes of these differences are related to the transition mechanism of the interactions, including electrostatic (or van-der-Waals) interaction, cation exchange, and hydrophilic

Abbreviations: Ca-Mt, Ca type montmorillonite; AA, Amino acid; DFT, density functional theory; GGA, generalized gradient approximation; PBE, Perdew Burke Ernzerhof; DNP, double numerical integration with polarization; PAW, potential projector augmented wave; PDOS, partial densities of states; ε(ω), dielectric functions; CB, conduction band; VB, valence band; dEad/dN, absolute average adsorption energy; CASTEP, Cambridge Sequential Total Energy Package; -COO–, carboxyl; -NH3+, amino; AlOH, aluminol group; e−, electron; h+, hole ⁎ Corresponding authors at: Key Laboratory of Solid Waste Treatment and Resource Recycle, School of Environment and Resource, Fundamental Science on Nuclear Wastes and Environmental Safety Laboratory, Southwest University of Science and Technology, Mianyang 621010, Sichuan, China. E-mail addresses: [email protected] (L. Bian), [email protected] (F.-q. Dong). https://doi.org/10.1016/j.clay.2019.105356 Received 1 January 2019; Received in revised form 24 October 2019; Accepted 28 October 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.

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interaction (Ramos and Javier Huertas, 2013; Arora et al., 2016). Thus, the study of the transition mechanism of interaction with contact time contributes to a better understanding of the kinetic process and the adsorption behaviour of AAs with clay mineral surfaces (Yu et al., 2013; Ramos and Javier Huertas, 2013). Naturally abundant clay minerals (montmorillonite, kaolinite, saponite, etc.) in soils can absorb various biomolecules (amino acids, proteins, purines, pyrimidines, etc.) in the natural environment (Yu et al., 2013; Lambert, 2008; Polubesova et al., 2010). Among these clay minerals, CaeMt has a great adsorption capacity for AAs due to its high cation exchange capacity (88–90 meq·100 g−1) and specific surface area (~800 m2·g−1) (Kalra et al., 2000; Newman et al., 2002; Gu et al., 2011; Laby, et al., 1962). Recently, the adsorption kinetic studies of AAs onto CaeMt allowed for interpreting the interaction mechanism involved in the adsorption reactions. For example, Ramos reported that glycine adsorption was dominated by complexation of the carboxylate group of zwitterionic glycine by edge and surface sites owing to electrostatic interaction at low glycine concentrations (1.0–30.0 mmol·L−1) (Ramos and Javier Huertas, 2013; Jaber et al., 2014). With increasing contact time, when the edge and surface sites were saturated, the adsorption of AAs through the cation exchange interaction in the interlayer space became the prevailing factor (Ramos and Javier Huertas, 2013). However, the maximum adsorption capacity (186 mmol·L−1) of adsorbed glycine exceeded the one resulting from cation exchange of Mt. (35–40 mmol·L−1) in the pH-neutral environment (Hedges and Hare, 1987; Farias et al., 2014). The reason for this is that the increasing of contact time led to change of the pH of the solution, which affected the surface charge of Mt. and the degree of ionization of AAs, leading to a change in the electrostatic (or van-der-Waals) interaction and hydrogen bond formation (Tran and James, 2012). These kinetic studies can be very useful to explain the possible interfacial interaction at different adsorption stages. However, it is difficult to explain the reason for the transition of the interfacial interaction with contact time, which becomes an obstacle to research the adsorption mechanism related to changes of the CaeMt surface structure and the molecular structure of the AAs (Zhao and Burns, 2012). Furthermore, adsorption of AAs onto clay minerals that occurs quickly (such as, the adsorption of lysine and glutamic acid by Mt. reached apparent equilibrium in < 60 min) (Wang and Lee, 1993; Ding and Henrichs, 2002; Shaker et al., 2012) and the transition of the interfacial interaction of AAs + Mt. are still unclear because of the limitations of experimental techniques. Recently, computational modelling studies have shown the potential to provide significant insight into this question, how the Mt. was adsorbed bio-molecules at the molecular and atomic level (Zhao and Burns, 2012; Berghout et al., 2008; Joshi and Aldersley, 2013; Mignon et al., 2009). For AAs incorporated into the Mt. surface, an interesting study by Newman provided new insight into the interaction of a model Mt. and AAs (tyrosine, phenylalanine) using a molecular dynamics (MD) method (Newman et al., 2002). The simulation provided detailed insight into the arrangement of the counterion and water in the interlayer of Mt. and suggested possible guest-layer interaction (cation exchange, electrostatic interaction and hydrogen bonds) (Yu et al., 2000; Katti et al., 2005). E. Escamilla-Roa used density functional theory (DFT) to qualitatively study the adsorption behaviour of glycine onto Mt (Khoury et al., 2010). The calculations showed that glycine was adsorbed as a zwitterionic form in the interlayer through cation exchange (K+-glycinium), electrostatic interaction and hydrogen bond formation (Roa-Escamilla et al., 2017; Ho et al., 2012). However, the interfacial interaction can be dynamic due to the chemical reactions in the adsorption process (Zhao and Burns, 2012). The transition mechanism of interaction occurring at the Ca-Mt + AAs interfaces is still far from a general understanding, especially at the electronic level. To solve this issue, Bian and co-workers used DFT and two-dimensional correlation analysis (2D-CA) methods to quantitatively study the orbital degenerate/split and electronic transition mechanism of minerals in the process of externally environmental (temperature, doping content, etc.)

accumulation (Bian et al., 2015a, b, c). These investigations provided a novel pathway for the adsorption process of AAs onto Mt. at the electronic scale with increasing contact time. Therefore, these computational modelling approaches enable us to use fully atomistic large-scale classical MD and DFT to explore the interaction mechanism of CaMt + AAs and to use 2D-CA methods to analyse the transition mechanism of interfacial interaction during the adsorption process. The present study is expected to have significant relevance in understanding the transition mechanism of cation exchange and hydrophilic interaction between amino acids and clay minerals. 2. Computational details 2.1. Structural models The Mt. structures include interlayer cations (e.g. Ca2+, K+ and Na+) and a 2:1 layer structure of phyllosilicates, which have one octahedral sheet sandwiched between two tetrahedral sheets (Parolo et al., 2012; Bu et al., 2019; Yuan et al., 2013). A unit cell model of CaeMt (Ca[Al4][Si8O20](OH)4) was used in this paper, with crystal lattice cell parameters (a = 4.80–5.0 Å, b = 8.30–8.70 Å, c = 13.90–14.50 Å and all angles equal to 90°) that were consistent with the experimental reported values, as shown in Table 2 (Jaber et al., 2014; Yu et al., 2013; Kitadai et al., 2009; Cuadros et al., 2009, Fonseca et al., 2018). In these models, only isomorphic substitutions of one Al3+-3s23p1 by Fe2+-3d64s2 (or Mg2+-2p63s2) in the O-sheet (octahedral sheet) were evaluated, namely, Ca-MtFe (or Ca-MtMg). In the interlayer of CaeMt, the free volumes were reduced (0.199 nm3 → 0.121–0.132 nm3), being chiefly occupied by Ca-3p64s2 and O-2p4 states, as illustrated in Table 1. The molecular diffusion paths of the AAs and water molecules increase approximately 1.6–1.8 times and for glycine molecules are 0.56–0.6 nm3. The AAs clusters, including glycine (C14N7P11H24, 4.98–5.03 nm3), serine (C18N7O15H31, 6.1–6.47 nm3), glutamate (C29N7O19H45, 5.4–6.1 nm3) and arginine (C27N19O11H51, 6.84–7.1 nm3), were established (see Fig. 1a). To study the adsorption of AAs on the surface of CaeMt, a 16 Å vacuum region in the z direction was employed in our calculation (Mignon et al., 2009). For both structures, a 2 × 1 × 1 super-cell was used, and the interlayer cations (Ca2+) were surrounded by four water molecules (each Ca2+ cation was surrounded by two water molecules) in the interlayer space in accordance with Escamilla's (Roa-Escamilla et al., 2017) and Fonseca's (Fonseca et al., 2018) works. The grand canonical Monte Carlo (GCMC) method via Adsorption Locator was used to adsorb different AAs (arginine, glutamate, glycine, serine) and four water molecules onto the surface of CaeMt. The optimized lattice constants (2 × 1 × 1 supercell) of Ca-Mt + AAs were a = 10.02–10.06 Å, b = 8.30–8.70 Å, c = 32.70–33.60 Å, α = β = γ = 89°~91°, as shown in Fig. 1b. 2.2. Methodology The Accelrys Material Studio software was used to perform all GCMC and MD simulations and quantum mechanical calculations. First, the GCMC simulations via the Adsorption Locator were used to calculate the interaction energy of Ca-Mt + AAs. Conjugate gradient methods were adopted to minimize the MD simulation initial configurations (GCMC simulation final configurations) (Zhang et al., 2018a, Table 1 Free volumes (F, nm3) of various AAs in CaeMt in an aqueous environment.

Pure Glycine Serine Glutamate Arginine

2

Ca-Mt

Ca-MtMg

Ca-MtFe

0.18 0.56 0.33 0.33 0.37

0.21 0.57 0.34 0.31 0.32

0.18 0.6 0.34 0.32 0.36

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Fig. 1. (a) Structural diagram, (b) structure model and (c) adsorption energy (eV) of various AAs and CaeMt in an aqueous environment, (d) total charge density (eV).

b; Zhao and Burns, 2012). Then, 1000 ps NVE (T = 298.0 K) and NPT (T = 298.0 K, pressure = 0.1 MPa) MD simulations via the Forcite were relaxed for the models of Ca-Mt + AAs and performed to reach the equilibrium state (final configurations) (Zhang et al., 2018a, b). The short-range van-der-Waals and long-range electrostatic interaction were simulated by the atom-based and Ewald+Group methods, respectively. Finally, the initial input in the DFT calculations was taken from the MD simulation final configurations. The structure optimization and properties calculation of the CaeMt and Ca-Mt + AAs were performed by means of quantum mechanical calculations based on DFT using the GGA (generalized gradient approximation) and PBE (PerdewBurke- Ernzerhof) exchange correlation functional (Bian et al., 2015a, b, c). The Dmol3 was used for structure optimization at a constant volume only in the preliminary calculations. The CASTEP was mainly used for property calculations (Roa-Escamilla et al., 2017). The highly accurate full potential projector augmented wave (PAW) method with ultrasoft pseudopotentials was used to describe the electron-ion interactions in the valence band region. Brillouin-zone integrations were calculated with a gamma-centered 3 × 3 × 3 Monkhorst-Pack k-point mesh. The convergence criteria for the energy, maximum force, maximum displacement and SCF tolerance was set as 1.0 × 10−5 eV/atom,

Table 3 Interaction energies for the different group pairs in the systems (kJ·mol−1).

Ca-Mt

Ca-MtMg

Ca-MtFe

Glycine Serine Glutamate Arginine Glycine Serine Glutamate Arginine Glycine Serine Glutamate Arginine

Adsorption energy

dEad/dN (AAs)

dEad/dN (H2O)

−3515.05 −3980.74 −4882.87 −5562.87 −3512.45 −3843.05 −9420.72 −4871.83 −3362.85 −3873.02 −8986.5 −4917.44

−3471.62 −3891.58 −4954.39 −5471.83 −3392.28 −3786.04 −9354.8 −4845.12 −3231.81 −3790.13 −8969.61 −4868.28

−48.03 −64.87 −14.13 −17.56 −55.72 −35.53 −13.13 −4.72 −52.25 −54.67 −9.2 −15.88

0.01 eV/Å, 1.0 × 10−4 Å and 1.0 × 10−4 eV/atom, respectively. The DNP numerical basis set with semi-core pseudopotentials was comparable to Gaussian 6–31 G(d, p), and its accuracy for describing hybridized bond strength has been tested (Bian et al., 2015a, 2015b, 2015c). The density matrix convergence threshold was set to 1 × 10−6. A Fermi smearing of 0.005 Hartree and a real-space cutoff of 0.45 nm

Table 2 Average lattice constants a, b, c (nm), bond lengths (nm) and bond angles (α, °) of different CaeMt obtained from experimental and theoretical methods. The lattice angles are 89–91° (α~β~γ), respectively.

a b c Si-O Al-O Ca-Ca αO-Al-O αO-Fe(Mg)-O αO-Si-O

Pure

Ca-Mt + AAs

Ca-MtFe + AAs

Ca-MtMg + AAs

Ref

0.48–0.5 0.83–0.87 1.39–1.45 0.16–0.17 0.17–0.18 1.39–1.45 90 – 100–101

0.51–0.53 0.88–0.91 3.27–3.36 0.16–0.17 0.17–0.18 0.27–0.46 59–62 – 110–112

0.51–0.52 0.88–0.9 3.27–3.3 0.15–0.16 0.18–0.19 0.28–0.52 72–75 72–78 102–116

0.51–0.52 0.89–0.91 3.3–3.36 0.16–0.17 0.18–0.19 0.45–0.75 53–72 62–66 109–118

0.51–0.52 0.88–0.9 1.2–1.6 0.16 0.17 1.2–1.6 89–91 – 100–101

(Ref: Jaber et al., 2014; Yu et al., 2013; Kitadai et al., 2009; Cuadros et al., 2009, Fonseca et al., 2018) 3

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Table 4 Mulliken charges (e) of various AAs in the interlayers of CaeMt.

Ca-Mt Ca-MtMg Ca-MtFe

-COO−(H) -NH3+ -COO−(H) -NH3+ -COO−(H) -NH3+

Pure

Glycine

−0.57~ − 0.29 −0.17~ − 0.16 −0.57~ − 0.29 −0.17~ − 0.16 −0.57~ − 0.29 −0.17~ − 0.16

< 10 ps −0.39 −0.33 −0.31 −0.23 −0.42 −0.16

Serine > 10 ps −0.43 −0.23 −0.42 −0.30 −0.48 −0.20

Glutamate

< 10 ps −0.41 −0.21 −0.35 −0.13 −0.51 −0.15

were also used to improve the computational performance. In general, the interaction process between AAs and CaeMt is not static in a certain external environment. However, in experiments, we could only measure the interaction at a certain time, and it was impossible to measure the transformation of the interaction in the adsorption process. For this purpose, according to the change in adsorption energy (the initial energy was transformed into long-range electrostatic interaction energy (~59.21 kJ·mol−1) and short-range van-der-Waals energy (~ − 98.29 kJ·mol−1), see Fig. 1c. The 2D-CA methods were used to describe the orbital fluctuation of Ca-Mt + AAs in 20 ps near the transition point (approximately 59.21 eV) of interaction energy. Thus, the contact time was distinguished by the changes in total-DOS (−1.0–2.0 eV) as follows: time = 0–10 ps, time = 10–20 ps, as shown in Fig. 2a. The transformation process of the interaction was explained by analysing the orbital fluctuation near the energy transition point at approximately 10 ps, and the corresponding time-areas of interaction were 0–10 ps (< 10 ps) and 10–20 ps (> 10 ps). Therein, the orbital fluctuation (synchronous (φ(e1,e2)) and asynchronous (ϕ(e1,e2))) intensity and range reflect the trend of orbital degeneracy/splitting of Ca-Mt + AAs. The PDOS was formally defined as the dynamic spectrum of a system associated with the application of an external perturbation (Bian et al., 2015a, 2015b, 2015c). If φ(e1,e2) × ϕ(e1,e2) > 0, the PDOS intensity variation observed for e1 predominantly occurred before that observed for e2, which implied that there was enhancement of the localized orbital coupling (Bian et al., 2015a, 2015b, 2015c). This enhancement of the localized orbital coupling could reflect the effect of Ca-Mt + AAs electron accumulation on the outer electron orbitals.

> 10 ps −0.26 −0.10 −0.33 −0.12 −0.48 −0.19

< 10 ps −0.31 −0.24 −0.39 −0.26 −0.39 −0.19

Arginine > 10 ps 0.20 −0.24 −0.37 −0.48 −0.01 −0.28

< 10 ps −0.41 −0.31 −0.37 −0.36 −0.52 −0.25

> 10 ps −0.33 −0.16 −0.32 −0.12 −0.53 −0.47

Javier Huertas, 2013; Dong et al., 2018; Zhao and Burns, 2012; RoaEscamilla et al., 2017). Before studying these interaction mechanism, we calculated the adsorption energy of the Ca-Mt + AAs systems, distinguishing the effect of surface charges of AAs (see Table 3). In general, it is a chemical adsorption process for highly negative charge surface area of CaeMt absorbing charged AAs (glutamate and arginine) with the electrostatic force. Meanwhile, the “double electronic layers” between charged AAs and Mt. produce some high energy density oxygen to absorb charged AAs with the short-range chemical adsorption (short-range van-der-Waals force), based on the smaller adsorption energy (Ca-Mt: −5471.83~ − 4882.87 kJ·mol−1, Ca-MtFe: −8986.50~ −4868.28 kJ·mol−1, Ca-MtMg: −9420.72~ − 4845.12 kJ·mol−1, see Table 3). However, the ionizable surface aluminol group (AlOH) has an amphoteric behaviour and can take up either a proton (H+) or an OH– depending on the nature of the AAs (Lambert, 2008). Compared to the charged AAs molecules, the active N+ ions of neutral AAs (glycine and serine) have tendencies to change from the surface hydrogen adsorption processes to the chemical adsorption processes owing to the long-range electrostatic interaction between –NH3+ (and –COO(H)−) group and AlOH group with relatively high adsorption energy (Ca-Mt: −3980.74~ − 3471.62 kJ·mol−1, CaMtFe: −3873.02~ − 3231.81 kJ·mol−1, Ca-MtMg: −3843.05~ − 3392.28 kJ·mol−1, see Table 3). The presence of single –COO−(H) and -NH3+ entities ensures charged binding sites for surface negative O atoms of CaeMt (Dong et al., 2018). The H-s orbital of AAs molecules enhances the surface potentials of C]O sp2 and NeH sp. hybrid orbital with the neighbouring C-p and N-p states. This will induce the surface O-2p states in CaeMt to form two new –C-O-O sp3 and –NH-O sp2 hybrid orbital with 0.04–0.45 e and 0.02–0.2 e lost; see Table 4. It promotes an active H atom to move away from the –OH (or –NH) bond. A new hydrogen bond forms, increasing the electron transfer path, and the broken bonds emerge in the charge transitions of the type C=O → C-O– and N3+ → N+. The –COO−(H) groups are seen to reside in the regions at approximately 0.12 nm (–C-OH) and 0.14 nm (–C=O) compared to the –NH3+ groups at 0.11 nm. In short, the N atom of the –NH3+ group is mainly interacting with surface O atoms by short-range van-der-Waals interaction, whereas the C]O bond of the –COOH group is interacting via long-range electrostatic interaction.

3. Results and discussion Since the binding interaction between the AAs and CaeMt plays a role in the solvation energy, it is expected to contribute to the final state of electronic relaxation of the system after electron ionization. Although the molecules were subjected to ionization lose one electron at a time on a timescale that is too short for the nuclear rearrangement, charge transfer from the solvent to the ionic state is expected to reduce the binding energy of the electron. Compared to the coordination number of AAs in bulk water, it is clear that the AAs molecules are not fully coordinated. The adsorption mechanism of AAs molecules and CaeMt are involved in a variety of electrostatic (or van-der-Waals) interaction, cation exchange and hydrophilic interaction (Ramos and

3.1. Electrostatic (or van-der-Waals) interaction on the surface of an AleO octahedron The Kohn-Sham electron band structure of CaeMt is evaluated

Table 5 Mulliken charges (e) of aluminium‑oxygen octahedrons.

Ca-Mt Ca-MtMg Ca-MtFe

O Al O Al Mg O Al Fe

Pure

Glycine

−1.12 2.01 −1.12 1.93 2.4 −1.16 2.01 1.03

< 10 ps −1.12 2.12 −1.13 2.04 2.23 −1.11 1.97 1.62

Serine > 10 ps −0.78 2.08 −0.81 2.02 2.20 −0.77 1.96 2.24

Glutamate

< 10 ps −1.16 2.08 −1.19 2.05 2.23 −1.04 1.97 1.15

4

> 10 ps −0.72 2.11 −0.71 2.00 2.23 −0.71 1.94 1.01

< 10 ps −1.09 2.06 −1.10 2.02 2.09 −1.05 1.94 0.79

Arginine > 10 ps −0.57 2.07 −0.63 2.00 2.14 −0.62 1.92 0.37

< 10 ps −1.05 2.07 −1.11 2.10 2.13 −1.05 1.95 1.32

> 10 ps −0.60 2.08 −0.59 2.13 2.20 −0.59 1.88 0.85

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Table 6 Elective electron/hole mass of CaeMt, Ca-Mt-Mg and Ca-Mt-Fe. Elective mass (×10−31 kg)

Ca-Mt

Glycine

< 10 ps > 10 ps < 10 ps > 10 ps < 10 ps > 10 ps

Ca-MtMg Ca-MtFe

Serine

Glutamate

Arginine

electron

hole

electron

hole

electron

hole

electron

hole

10.59 18.34 63.29 11.11 18.34 7.87

43.86 23.38 23.26 19.84 78.50 71.23

28.94 35.06 23.84 9.47 38.46 11.76

39.15 27.74 16.6 1.72 10.99 16.23

7.21 3.31 5.95 3.41 9.84 49.51

11.16 3.64 4.33 9.29 37.31 15.63

58.14 23.96 8.32 8.18 3.37 32.05

19.91 35.04 49.20 14.88 3.69 30.68

taking into account a specific path inside the Brillouin zone, which is included in the supporting information with high symmetry points depicted. The full band structure is shown at the top part of Fig. 2b-c together with the partial (per type of orbital) density of states (PDOS). Three deep bands (0.82 eV, 0.83 eV and 1.15 eV) can be seen in the band structure. Qualitatively, the direct gap is 0.82 eV, with a secondary indirect gap displaying a slightly larger energy of 1.15 eV and it is located between the G point at the valence bands (VB) and Q at the conduction band (CB). In this case, these peak structures reveal the electronic transitions between the O-2p uppermost VB and the lowest Ca-d CB just above the main band gap. Looking at the top of the VB that is derived from the O-2p4, Fe-3d5 and Mg-2p6 orbits, one can see that the electronic energy levels have a strong O-2p4 character (above 0.1 eV), with a much smaller contribution from the Fe-3d5, O-2p4 (–COO−(H)) and N-2p3 (–NH3+) levels. In previous reports (Lambert, 2008; Mignon et al., 2009; Mignon and Sodupe, 2012; Shi et al., 2013), the adsorption systems were found to be in agreement with various experimental observations pertaining to the relative adsorption of AAs in the presence of charge balancing. For example, the interaction between Mt. and lysine (our work, Dong et al., 2018) or glycine (Ramos and Javier Huertas, 2013) includes electrostatic interaction of the –NH3+/–COO– group and AlOH, vander-Waals force or hydrogen bond of the –NH3+/–COO– group with surface oxygen atoms. Because Mt. have permanent negative and variable surface charges, the –COO−(H) and –NH3+ groups affect the surface charge of CaeMt (Kitadai et al., 2009; Cuadros et al., 2009). The ionizable surface AlOH has an amphoteric behaviour and can take up the protonation/deprotonation processes, AlOH+H+ → AlOH2+ (–NH3+) and AlOH+OH– → AlO− + H2O (–COO−(H)). Regarding the AAs occupying outer-sphere free volumes and substituting the cationbridge of the CaeCa bonds, the –COO−(H) and –NH3+ combine with the surface O ions is explained by the PDOS curves for nearly degenerate conduction-band, see in Fig. 2. With the contact time increasing, the PDOS state shows the electron orbital variations arise from the fluctuating range shift to the Fermi point. One band (Ca-d0, O-2p4 orbital) shifts towards the low energy region in the conduction band (0.5–2.8 eV), while the other band (Ca-d0, Al-3p3, Ca-4 s2 orbital) shifts to the low energy region in the valence band (−0.5–2.5 eV), as indicated by the two arrows; see Fig. 3b. The band gap is reduced from 0.83 eV to 0.80–0.81 eV. Therein, the high energy density O-2pz state creates a part of the electron-hole defect pair (e−-h+) in which the orbital degeneracy decreases by approximately 11 electron·eV−1. The Al3+ states of AlOH can capture a part of the hole defect (h+) that is

consistent with the increase (0.06–0.11 e) of Mulliken charges (see Table 5). In short, the Al site charge balancing can be principally used to explain the change in the adsorption process. For example, with the contact time increasing, the Mg impurity can modify the partial excitation levels of the O states (VB region) at the octahedral Al-sites when the Mg2+ ion captures one electron (e−) from an O-2p4 state; as a result, the Mg charges will be reduced to 0.17–0.23 e as seen in Table 5. The Mg2+–O sp3 hybrid orbital rearranges to become a Mg+–O sp2 hybrid orbital. Whereas the O-2p4 orbital provides 0.01–0.02 e to ionize the –COO−(H) group (MgOH+e− → Mg (OH)−), the –NH3+ group captures 0.01–0.07 e from the O-p state of the negatively charged layer (MgOH2++2OH− + h+ → MgO− + 2H2O), which leads to a decrease of the effective hole mass (23.26 × 10−31 → 19.84 × 10−31 kg). Differently, Fe impurities in the surface Al-site modification reveal an AAs-driven adsorption based on CB holes increasing (effective hole mass: 7.87 × 10−31 → 71.23 × 10−31 kg), see Table 6. The p-p degenerate levels of the interlayer Fe orbitals suddenly increase from 1.00 electron·eV−1 to 2.00 electrons·eV−1. This originates from the FeeO hybrid orbital transition (sp2d → sp3d) and the corresponding increase of 0.29–1.5 e from changing the Fe charge state according to Fe2++h+ → Fe3+. Furthermore, the occupied Fe-d0 orbital affects the O-2p4 orbital energy by capturing 0.05–0.11 h+. Consequently, the different electrostatic (or van-der-Waals) interaction with octahedral (AleO) will be responsible for the different adsorption behaviours of the –COO−(H) and –NH3+ groups in the VB region. 3.2. Cation exchange at the interface In addition to the electrostatic (or van-der-Waals) interaction, the cation exchange between Ca and AAs is the second factor governing adsorption (Dong et al., 2018; Roa-Escamilla et al., 2017). Fig. 3a shows that the bottom of CB mainly consists of empty Ca-d0 states, which are partly filled by Ca-4 s2 electrons, but mostly benefit from electronic contributions from O-2p4 states. A close-up of the band structure is provided near the Kohn-Sham band gap, where we can see the VBmaximum and the VB-minimum occur at the G point. The intercalation levels of AAs at the bottom of the CB create a level vacancy, weakening the electron transfer rate of O-2p4 → Ca-4 s2. Such transition requires a charge compensation from the neighbouring active groups, which means that the –COO−(H) and –NH3+ groups of various AAs molecules transfer the O-2p4 and N-2p3 electrons to surface O-2p4 states according to the sudden enhancement in the H-1 s1 orbital. It should be noted that

Table 7 Mulliken charges (e) of Ca ions of CaeMt configurations. Pure

Ca-Mt Ca-MtMg Ca-MtFe

0.02–0.08 0.03–0.03 1.09–0.84

Glycine

Serine

Glutamate

Arginine

< 10 ps

> 10 ps

< 10 ps

> 10 ps

< 10 ps

> 10 ps

< 10 ps

> 10 ps

1.01 0.56 0.03

0.39 1.44 0.02

0.99 0.59 0.27

0.15 0.95 0.97

0.35 1.00 1.05

0.39 0.92 0.80

0.57 1.41 0.61

0.01 0.21 0.98

5

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Table 8 Mulliken charges (e) of water. Pure

Ca-Mt Ca-MtMg Ca-MtFe

−0.12 −0.12 −0.12

Glycine

Serine

Glutamate

Arginine

< 10 ps

> 10 ps

< 10 ps

> 10 ps

< 10 ps

> 10 ps

< 10 ps

> 10 ps

−0.45 −0.38 −0.10

−0.34 −0.39 −0.26

−0.32 −0.22 −0.31

−0.30 −0.27 −0.16

−0.27 −0.24 −0.30

−0.24 −0.23 −0.46

−0.35 −0.20 −0.34

−0.22 −0.27 −0.17

the sharp PDOS peaks at the top of the VB and the bottom of the CB are related to the localization of the wave functions corresponding to the highest occupied and unoccupied molecular orbitals between surface Ca (and O) atoms and active groups (–COO−(H) and –NH3+), as shown in Fig. 3b. The free energy barrier is highly cut off by the negatively charged layer hybrid orbital, therefore allowing free electron transfer in the local electron states (Roa-Escamilla et al., 2017). Consequently, the second adsorption factor is attributed to the Ca+–COO−(H) p-p σ (neutral: glycine and serine) and Ca+-NH3+ p-p π (charged: glutamate and arginine) hybrid orbitals at the CB region; see Fig. 3c. To fully and quantitatively understand the cation exchange process, we calculated the synchronous (φ(e1,e2)) and asynchronous (ϕ(e1,e2)) spectra using the DFT + 2D-CA technique, as shown in Fig. 4. It should be noted that the localized Ca-d0 state exhibits a band-position shift and the d0-orbital splitting at the bottom of CB is coupled with the contact time increasing. The Ca-d0 states show that the electron orbital variations are arising from the classical intensity changes of two highly overlapped bands with a fixed band position and a relatively linear shape. One band (1.8 eV) decreases in intensity quickly, while the other band (1.5 eV) increases in intensity gradually, as indicated by the two arrows. This splitting of the Ca-d0 orbital induces the O-2p4 orbital to move towards the high energy region of CB (1.0–3.0 eV), which in turn enhances the Ca-d0-O-2p4 d-p orbital hybridization. Due to the effect of orbital coupling, the suddenly enhanced H-1 s1 orbital gradually

induces the degenerate dx2+y2 orbital to split into dx2+y2 and dZ2 orbitals but with no net change of moment. This reflects that the integral intensity of the Ca-d0 energy level peak is kept at a constant value, so the peak height decreases gradually as the band width increases, as shown in Fig. 4a. It chiefly excites the O-2p4 orbital fluctuation to shift to the high energy region in VB, which in turn enhances the Ca-d0-O2p4 d-p orbital hybridization. The corresponding synchronous and asynchronous spectra show the fluctuation intensity (−2.0–3.0 eV) and fluctuation range (−1.0–3.0 eV) of the Ca-d0 orbital, as seen in Fig. 4b, c. According to the minimum energy principle, the dz2 and dx2+y2 orbitals will be more favourable than the dxy, dyz and dxz orbitals for containing the electron. With contact time < 10 ps, the Ca-dx2+y2 orbital preferentially hybridizes with the O-2pz orbital, producing π hybrid orbitals (Ca-dx2+y2-O-2pz sp3d bonding orbital). The d-p bonding orbitals contain weaker bound electrons (low-angular-momentum Cad0 and O-2pz orbitals), which forms for increasing the electronic transitions path, and the broken bonds emerge in the charge transitions of the type (e.g. C=O → C-O-). With contact time > 10 ps, the Ca-dx2+y2 orbital splits into dx2+y2 and dZ2, and the new Ca-dz2 orbital (approximately −0.5–0.5 eV) couples with O-2pz to form σ hybrid orbitals (Ca-dz2-O– 2pz sp3d2 bonding orbital), as shown in Fig. 4d. The O-2p electrons transfer from the single orientation (dx2+y2: 0.5–2.8 eV) to the two orientations of Ca-d0 (dz2: −0.5–0.5 eV; dx2+y2: 0.5–2.8 eV). Some portion of the Ca2+ electrons are annihilated in the O-2pz orbital, i.e.,

Fig. 2. (a) 2D-CA patterns of the total DOS, (b) and (c) band structure (and PDOS) at 0–10 ps and 10–20 ps. 6

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Fig. 3. (a) PDOS curves and 2D-CA patterns of the Ca-d0 orbital and (b) aluminium‑oxygen octahedron of CaeMt absorbing AAs, the corresponding intensities of the synchronous (Sy) and asynchronous (Asy) of 2D-CA patterns of the Ca-d0 orbital are shown in Supplementary material. (c) The illustration of orbital spitting behaviours and electron transfer characteristics between AAs and CaeMt.

Fig. 4. (a) The PDOS of the Ca-d0 orbital at 0–20 ps, (b) and (c) 2D-CA patterns of the Ca-d0 orbital at 0–10 ps and 10–20 ps, respectively. (d) Shows the illustration of hybridized orbitals between AAs and CaeMt. 7

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Fig. 5. Local density of states and dielectric function of Ca states in the interlayer of AA-Mt systems. (a) and (b) correspond to that of CaeMt, Ca-Mt-Mg and Ca-Mt-Fe at 0–10 ps and 10–20 ps, respectively.

Ca-charge changes from 0.21–0.92 e to 0.95–1.41 e. The Ca-dx2+y2-O2pz and Ca-dz2-O-2pz hybrid orbitals provide many electronic energy levels for the unpaired electronic transition from O-2p4 (–COO−(H)) and N-2p3 (–NH3+) to O-2p4 (AlOH group) orbitals in the CB (0–3 eV), releasing effective electrons to annihilate hole defects. The corresponding inter-atomic distance (–COO−(H) and –NH3+-Ca) was decreased (0.8 Å → 0.5 Å) and the surface potential (–COO−(H) and –NH3+-Ca) was increased (0.32 eV → 0.38 eV) (see Fig. 1d), which was consistent with our previous studies of the interaction of lysine adsorbed on the Mt. surfaces (Dong et al., 2018). To investigate the electronic transition in the interlayer of CaeMt, we calculated the dielectric constant via the Kohn-Sham inter-band energy gaps (Bian et al., 2015a, 2015b, 2015c). Therein, the effective interaction within the positively charged Mt. layers happens through divalent Ca2+ as charge-balancing ions. The Ca2+ ion (> 0.5 e) is offset by the mono-valent Ca+ (< 0.5 e) ion and the –COO−(H) and –NH3+ groups, and this charge disproportion of surface Ca is a key factor for degeneration of the charge density. The calculated ε in Fig. 5a shows that a partial Ca-s electron captures an outer sphere H-s (–COO−(H) and –NH3+) to jump into an empty d0 orbital with the contact time increasing. As the contact time increases, the Ca-d0 splitting into Cadx2+y2 and Ca-dz2 orbitals, some of the O-2p4 electrons jump into the empty Ca-3d0 orbitals. Such a split Ca-d0 orbital can easily hybridize with an O-2p4 orbitals, enhancing the d-p (Ca-3d0-O-2p4) orbital hybridization. The highly sensitive d-p hybrid orbital weakens the sp3 (CaeCa) hybrid orbital strength. Therefore, the empty d0 orbital is populated to be a 2d3/2 state of the Ca+ ion. The energy of the sp3 hybrid orbital (–COO−(H) and –NH3+) is lifted, and electrons associated with H+ prefer to fill the out-of-plane Ca-2p1/2 orbital. The Ca3p6 levels (CB) transfer to the Fermi point, correlating with the results of band gaps decreasing from 1.15 eV to 0.81–0.82 eV. Typically, Mg impurities create an electron-hole pair that can offset the positive-

negative ion recombination between the Ca and O atom. Indeed, the electronic transition can be affected by octahedral AleO due to the amphoteric behaviour of the AlOH. Fig. 5b, c shows that the Fe2+ (or Mg2+) impurity occupying the Al3+-site provides a hole defect to capture part of the active H+ (0.01–0.05 e or 0.03–0.1 e) of the –COO−(H) group, due to the appearance of a high excitation level (> Fermi point), whereas the –COO−(H) sp3 hybrid orbital changes to a sp2 hybrid orbital. Compared to the charge changes in –COO−(H), the active N+ ions of charged AAs tend to be N2+, resulting from the change of surface hydrogen adsorption (< −4180 kJ·mol−1) to double electric layer adsorption (> −4180 kJ·mol−1). This explains why the intercalation levels of AAs increase at 0–2 eV. Typically, we find high adsorption energy (−8986.5~ − 9420.72 kJ·mol−1) for the double electric layer (–NH4++AlOH−) of the glutamate-octahedral. An overview of static electron transfer processes shows that the Fe valence electrons occupying Al levels produce a high energy density oxygen state where the O2– loses approximately 0.02–0.07 e. The positive charges attract the electrons inside the outer-sphere Ca ions in a way such that the Ca+ ions are rapidly replaced by the mono-valent Ca2+ ions and a captured 0.06–0.93 h+, where the Ca-d0 → s charge transition appears, see Table 7. The 2d3/2 states of the Ca2+ ions are then populated as a consequence of the Fe-addition. The rate constant for reactions out of the excited 2p1/2 state of O-2p4 is found to be two or three orders of magnitude larger than the rate coefficients for reactions out of the two other states. Hence, the Ca+-NH3+ p-p π hybrid orbital has more effect on the van-der-Waals interaction than on the electrostatic interaction of the Ca+–COO−(H) p-p σ hybrid orbital in CaMtFe + AAs systems. 3.3. Hydrophilic interaction at the interface Additionally, the distribution of water throughout the models is 8

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another interesting aspect of this investigation. Previous reports have suggested that hydrogen bonding and hydrophobic interaction also play important roles during the adsorption of small bio-molecules onto the Mt. (Lambert, 2008; Ramos and Javier Huertas, 2013; Suter et al., 2012). With relatively low absolute average adsorption energy of H2O (dEad/dN(H2O): −64.87~ −4.72 kcal·mol−1), the water molecules dissociate from the surface outer-layers in order to provide an adsorption site for AAs molecules. The polar water molecules cut off the electron transfer paths of long-range CaeO bonds near 0.2 nm. Thus, two new –NH3+·(H2O) and –HOCO−·(H2O) hydrated clusters are formed, whose bond lengths are ~0.25 nm. In contrast, a high absolute average of dEad/dN(AAs) (−9354.8~ − 3471.62 kcal·mol−1, see in Table 3) was beneficial to partial ionization of the buried hydrophobic residues of AAs molecules, which leads to a greater unfolding of AAs due to the electrostatic repulsion. The enhancement of AAs-adsorption strength leads to more exposure of hydrophobic groups of AAs on the hydration surface of CaeMt, the water molecules and –NH3+ (and –HCOO−) groups are close to each other with the distance between H2O and –NH3+ (and –HCOO−) groups decreases (3.89–4.12 Å → 1.76–2.14 Å), and then hydrated –NH3+·(H2O) and –HOCO−·(H2O) clusters are formed. That promotes AAs close to the surface of CaeMt (distance of AAs~Ca-Mt: 7.72–7.95 Å → 6.94–7.16 Å). Therefore, the hydrophilic interaction decreases the adsorption energy (dEad/dN(H2O): −64.87~ − 4.72 kJ·mol−1) of the –NH3+ and –HCOO−(H) groups into the surface of the CaeMt. As shown in Fig. 6, water replacing cations become electron donors for providing valence electrons to the O-2p4 level. They can obviously cut off the hydrogen-bonding networks between the Ca (or O) atoms and active groups (–COO−(H) and –NH3+). As the contact time increases, the H-1 s state of H2O exhibits orbital degeneration at the bottom of the conduction (−1.2–2.3 eV), and the intensity of the corresponding peak was increased from 1.2 electrons·eV−1 to 1.5

Fig. 7. (a) the PDOS of the H2O-1 s orbital at 0–20 ps, (b) 2D-CA patterns of the H2O-1 s orbital at 0–10 ps and 10–20 ps, and the corresponding intensities of synchronous (Sy) and asynchronous (Asy) of 2D-CA patterns of the H2O-1 s orbital are shown in Supplementary material. (c) An illustration of the hybridized orbital between H2O-1 s and a –COO−(H) (and –NH3+)-p orbital.

electrons·eV−1, as shown in Fig. 7a. This enhanced H-1 s orbital of H2O at the Fermi point (−1.5–1.5 eV, see Fig. 6b) enhances the sp. orbital hybridization of H2O with –HOCO (H-1 s-O-2p4) and –NH3 (H-1 s-N2p3) group, respectively. The orbital hybridization form is then changed

Fig. 6. (a) PDOS curves and 2D-CA patterns of –COO−(H) and –NH3+ groups of various AAs and (b) H2O in the interlayer of CaeMt, the corresponding intensities of synchronous (Sy) and asynchronous (Asy) of 2D-CA patterns of –COO−(H) and –NH3+-p orbitals; see also Supplementary material. (c) Illustrates the hydrophilic interaction. 9

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from sp. (–NH3) and sp2 (–HOCO) to sp3 (–NH3·(H2O)+ and –HOCO·(H2O)−) orbital hybridization, as illustrated in Fig. 7b, c. The corresponding transfer of electrons increases from 0.12 to 0.47 e, and the surface potential increases from 0.28 to 0.32 eV, while the inter-atomic distance decreases from 5.3 Å to 1.8 Å. These active hydrogen states improve the ability of surface lone pair electrons of water molecules to adsorb an active electron from surface layer active groups of AAs molecules, being hydrated clusters: –NH3·(H2O)+ and –HOCO·(H2O)−. Therein, the sp2 and sp. states in the –C=O and -NH bonds hybridize to the H2O sp3 orbital, losing 0.11–0.47 e, see Table 8. These hydrated clusters can be seen as two adhesive layers, which are stabilized by electrostatic interaction with water and basal octahedral O atoms (RoaEscamilla et al., 2017). Furthermore, they can capture some surface layer free electrons even when the band gaps of the systems are 0 eV. Hence, the outer-sphere water molecules are also strongly adsorbed by AAs in the surface layers of CaeMt, and two new hydrated clusters are beneficial for the adsorption of various AAs.

Longshan Fund of Southwest University of Science and Technology (17QR004). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clay.2019.105356. References Arora, A.K., Jaswal, V.S., Singh, K., Singh, R., 2016. Chemical evolution and origin of life: a review. Chem. Biol. Lett. 3 (1), 9–17. Berghout, A., Tunega, D., Zaoui, A., 2008. Density Functional Theory (DFT) study of the Hydration steps of Na+/Mg2+/Ca2+/Sr2+/Ba2+-exchanged Montmorillonites. Clay Clay Miner. 58, 174–187. Bian, L., Dong, F.Q., Song, M.X., Dong, H.L., Li, W.M., Duan, T., Xu, J.B., Zhang, X.Y., 2015a. DFT and two-dimensional correlation analysis methods for evaluating the Pu3+-Pu4+ electronic transition of plutonium-doped zircon. J. Hazard. Mater. 294 (8), 47–56. Bian, L., Song, M.X., Dong, F.Q., Duan, T., Xu, J.B., Li, W.M., Zhang, X.Y., 2015b. DFT and two-dimensional correlation analysis for evaluating the oxygen defect mechanism of low-density 4f (or 5f) elements interacting with Ca-Mt. RSC Adv. 5 (36), 28601–28610. Bian, L., Xu, J.B., Song, M.X., Dong, F.Q., Dong, H.L., Shi, F.N., Zhang, X.Y., Duan, T., 2015c. First principles simulation of temperature dependent electronic transition of FM-AFM phase BFO. J. Mol. Model. 21 (4), 91. Bu, H.L., Yuan, P., Liu, H.M., Liu, D., Liu, J.Z., He, H.P., Zhou, J.M., Song, H.Z., Li, Z.H., 2017. Effects of complexation between organic matter (OM) and clay mineral on OM pyrolysis. Geochim. Cosmochim. Ac. 212, 1–15. Bu, H.L., Yuan, P., Liu, H.M., Liu, D., H, Z., Qin, Zhong, M, X., Song, H.Z., Li, Y., 2019. Formation of macromolecules with peptide bonds via the thermal evolution of amino acids in the presence of montmorillonite: insight into prebiotic geochemistry on the early Earth. Chem. Geol. 510, 72–83. Cuadros, J., Aldega, L., Vetterlein, J., Drickamer, K., Dubbin, W., 2009. Reactions of lysine with montmorillonite at 80 °c: implications for optical activity, h+ transfer and lysine- montmorillonite binding. J. Colloid Interface Sci. 333 (1), 78–84. Ding, X.L., Henrichs, S.M., 2002. Adsorption and desorption of proteins and polyamino acids by clay minerals and marine sediments. Mar. Chem. 77, 225–237. Dong, F.Q., Guo, Y.T., Liu, M.X., Zhou, L., Zhou, Q., Li, H.L., 2018. Spectroscopic evidence and molecular simulation investigation of the bonding interaction between lysine and montmorillonite: Implications for the distribution of soil organic nitrogen. Appl. Clay Sci. 159, 3–9. Farias, A.P.S.F., Tadayozzi, Y.S., Carneiro, Cristine E.A., Zaia, Dimas A.M., 2014. Salinity and pH affect Na+-montmorillonite dissolution and amino acid adsorption: a prebiotic chemistry study. Int. J. Astrobiol. 13, 259–270. Fonseca, C.G., Vaiss, V.S., Wypych, F., Diniz, R., Leitão, A.A., 2018. Investigation of the initial stages of the montmorillonite acid-activation process using DFT calculations. Appl. Clay Sci. 165, 170–178. Friebele, E., Shimoyama, A., Ponnamperuma, C., 1980. Adsorption of Protein and NonProtein Amino Acids on a Clay Mineral: a possible Role of selection in Chemical Evolution. J. Mol. Evol. 16, 269–278. Gu, C., Liu, C., Johnston, C.T., Teppen, B.J., Li, H., Boyd, S.A., 2011. Pentachlorophenol radical cations generated on Fe(III)-montmorillonite initiate octachlorodibenzo-pdioxin formation in clays: density functional theory and fourier transform infrared studies. Environ. Sci. Technol. 45 (4), 1399–1406. Hedges, J.I., Hare, P.E., 1987. Amino Acid Adsorption by clay minerals in distilled water. Geochim. Cosmochim. Ac. 51, 255–259. Ho, P.H., Mihaylov, T., Pierloot, K., Parac-Vogt, T.N., 2012. Hydrolytic activity of vanadate toward serine-containing peptides studied by kinetic experiments and DFT theory. Inorg. Chem. 51 (16), 8848–8859. Jaber, M., Georgelin, T., Bazzi, H., Costa-Torro, F., Lambert, J.F., Bolbach, G., Clodic, G., 2014. Selectivities in Adsorption and Peptidic Condensation in the (Arginine and Glutamic Acid)/ Montmorillonite clay system. J. Phys. Chem. C 118, 25447–25455. Joshi, P.C., Aldersley, M.F., 2013. Significance of mineral salts in Prebiotic RNA Synthesis Catalyzed by Montmorillonite. J. Mol. Evol. 76, 371–379. Kalra, S., Pant, C.K., Pathak, H.D., Mehta, M.S., 2000. Adsorption of glycine and alanine on montmorillonite with or without coordinated divalent cations. Indian J. Biochem. Biophys. 37, 341–346. Katti, D.R., Ghosh, P., Schmmidt, S., Katti, K.S., 2005. Mechanical properties of the sodium montmorillonite interlayer intercalated with amino acids. Biomacromolecules 6, 3276–3282. Khoury, G.A., Gehris, T.C., Tribe, L., Sánchez, R.M.T., Afonso, M.S., 2010. Glyphosate adsorption on montmorillonite: an experimental and theoretical study of surface complexes. Appl. Clay Sci. 50, 167–175. 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4. Conclusions In summary, we investigated the interfacial interaction mechanism of Ca-Mt + AAs using DFT and 2D-CA methods. The calculation results indicated that the significant adsorption factors were attributed to the electrostatic (or van-der-Waals) interaction (AlOH+AAs), cation exchange (AlOH-Ca + AAs) and hydrophilic interaction (AlOHH2O + AAs). The N atom of the –NH3+ group and the C]O bond of the –COOH– group were mainly interacting with surface O atoms by shortrange van-der-Waals force and long-range electrostatic interaction, respectively. With the contact time increasing, the Ca-d0 orbital splitting enhances the Ca-d0-O-2p4 p-d (Mt) orbital hybridization, as well as the Ca+–COO−(H) p-p σ (neutral glycine and serine) and Ca+-NH3+ p-p π (charged glutamate and arginine) orbital hybridization. This then enhanced the cation exchange between the interlayer Ca ions and the AAs, which mainly changed into short-range van-der-Waals interaction. In addition, the H-1 s state of H2O exhibited coupling of degenerated orbitals with increasing contact time, which leads to an enhancement of the s-p orbital hybridization of H2O with –HOCO and –NH3 to form hydrated clusters: –NH3·(H2O)+ and –HOCO·(H2O)−. The adhesive hydrated clusters as hydrophilic factors improve the adsorption of AAs on CaeMt and affects the long-range electrostatic interaction. Thus, the transformation of interaction occurring at the Ca-Mt + AAs interfaces depended on the changes of the short-range van-der-Waals force induced by the cation exchange (AlOH-Ca + AAs) and long-range electrostatic interaction affected by the hydrophilic interaction (AlOHH2O + AAs). To better understand the adsorption kinetics of AAs onto clay minerals in soils and sediments, further investigation will be focused on the effects of pH and temperature on the interlayer interaction of the Mt. + AAs system via Car-Parrinello Molecular Dynamics. This work, however, provides useful information on how to determine the quantitative orbital coupling of the Mt. + AAs. This is important, not only from a theoretical point of view since it can also advance the practical understanding of the dynamic evolution process of AAs with clay mineral surfaces. Declaration of Competing Interest The authors declare that they have no known competing financialinterestsor personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments National Natural Science Foundation of China (41872039 and 41831285), the One-Thousand-Talents Scheme in Sichuan Province, Sichuan Science and Technology Program (2018JY0462), and 10

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