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Adsorption of phenanthrene on Na-montmorillonite: A model study
Geoderma 169 (2011) 41–46
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Geoderma j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o d e r m a
Adsorption of phenanthrene on Na-montmorillonite: A model study A. Meleshyn a,⁎, D. Tunega b,c a b c
Institute for Radioecology and Radiation Protection, Leibniz Universität Hannover, Herrenhäuser Str. 2, D-30419 Hannover, Germany Institute of Soil Research, University of Natural Resources and Applied Life Sciences, Peter Jordan-Straße 82, A-1190 Vienna, Austria Institute for Theoretical Chemistry, University of Vienna, Währingerstrasse 17, A-1090 Vienna, Austria
a r t i c l e
i n f o
Article history: Received 31 May 2010 Received in revised form 6 September 2010 Accepted 19 September 2010 Available online 18 October 2010 Keywords: Polycyclic aromatic hydrocarbon Sorption Surface properties Free energy Sheet silicates
a b s t r a c t The role of clay minerals in the fate of polycyclic aromatic hydrocarbons (PAHs) in soil still remains an unresolved and controversial issue. Recent experimental studies report on PAH sorption capacity of clay minerals, which is either much smaller than or comparable to that of soil organic matter with water either inhibiting or not inhibiting PAH sorption. We studied adsorption of phenanthrene – a prominent PAH representative – on Namontmorillonite – a common member of the smectite group of clay minerals and often dominating component of soil inorganic matter – by means of molecular simulations incorporating free energy calculations. The study also included the effect of surface hydration on sorption. Phenanthrene adsorption site featured by Na+−π bonding was identified as the most favorable one on the dehydrated montmorillonite surface with other inner-sphere adsorption sites being less favorable by 23–32 kJ/mol and the desorption state being less favorable by 74 kJ/mol on the free energy scale. Upon montmorillonite hydration by a water film with a thickness of ~1 nm, however, phenanthrene desorption from the montmorillonite surface to the water–air interface becomes more favorable than phenanthrene direct adsorption. Our results support, therefore, experimental studies suggesting that PAHs adsorption on clays predominates in the dry soils but is negligible in the presence of water. Still, a consideration of higher phenanthrene surface coverages (to account for possible aggregation effect) or other exchangeable cations (to account for possible stronger cation − π bonding) in the future model studies may provide additional arguments pro or contra of this suggestion. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Polycyclic aromatic hydrocarbons represent an important environmental concern because of their high toxicity and potential carcinogenity. Their fate in the environment and their migration into groundwater strongly depends on their interactions with soil components. A direct contribution of clay minerals to the sorption of anthracene and phenanthrene, representative compounds of PAHs, by the soils is considered to be relatively small as compared to its partitioning into soil organic matter (Karimi-Lotfabad et al., 1996; Celis et al., 2006; and references therein). Indeed, it has been shown that an addition of water to soil is able to reduce the rate and extent of soil interaction with anthracene (Karimi-Lotfabad et al., 1996). This behavior indicated that water competed successfully with anthracene for active sites on mineral surfaces and a kinetic model that included inhibition of the active reaction sites by water gave good agreement with the data. Although clay minerals strongly bind hydrophilic organic contaminants from water, they also can effectively adsorb hydrophobic PAHs only after a modification by certain organic cations (El-Nahhal and Safi, 2004; Wiles et al., 2005; Changchaivong and Khaodhiar, 2009).
⁎ Corresponding author. Tel.: + 49 511 762 3069; fax: + 49 511 762 17917. E-mail address: [email protected] (A. Meleshyn). 0016-7061/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2010.09.018
It has been observed that PAHs sorption to hydrated mineral surfaces strongly depends on the type of exchangeable cations at mineral surfaces. For example, Ag+ modified montmorillonite adsorbed much more phenanthrene than alkali, alkaline earth, or even organically modified montmorillonites because of a specific bonding of a cation–π type (Zhu et al., 2004). Moreover, smectites have been found to retain large amounts of phenanthrene from water in yet another experimental study (Hundal et al., 2001). The latter study reported a larger phenanthrene affinity to Ca2+-montmorillonite as compared to K+- or Na+-montmorillonite. The mechanism of the retention of nonionic hydrophobic organic compounds in soil and in particular their partitioning to clay minerals remains still largely unclear. The present study aims at gaining detailed insight into this mechanism using obtained structural and energetic information at a molecular level by application of molecular modeling methods, which have been shown to successfully contribute to a progress in understanding of clay systems (Anderson et al., 2010). 2. Simulation Details The simulation cell with lateral dimensions of ~20.65 Å×~17.96 Å encloses eight unit cells of Na+-montmorillonite layer of Wyoming-type with the formula unit Na0.375 (Al1.75 Mg0.25) (Si3.875Al0.125) O10(OH)2 having unit cell area Auc =46.36 Å2. A simulation cell with such lateral
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dimensions has been shown to be representative of the macroscopic mineral system and not influenced by the artificial long-range symmetry of the imposed periodic lattice (Skipper et al., 1995). The saturation of montmorillonite's exchange sites with Na+ corresponds to the experimental procedure in the studies by Hundal et al., 2001, and Zhu et al., 2004. Starting from the dehydrated montmorillonite with the layer spacing of 9.6 Å, the thickness of its Na+–containing interlayer space was increased from 3 Å to 100 Å to prepare the simulation cell containing one montmorillonite layer of a thickness of 6.6 Å (Tsipursky and Drits, 1984). This ensures that as a result of the application of three-dimensional periodic boundary conditions to the simulation cell, two neighboring montmorillonite layers are separated enough by a vacuum slab with a thickness of 100 Å (see (Meleshyn, 2009) for the visualization of the very similar simulation cell for mica). Coordinates of layer atoms were calculated according to the method of Smoliar-Zviagina, 1993. Two tetrahedral Al/Si and four octahedral Mg/Al substitutions in the simulation box were uniformly distributes within the mineral layer with the symmetry C2 (Tsipursky and Drits, 1984). To study phenanthrene adsorption on the fully dehydrated montmorillonite surface, one phenanthrene molecule was positioned at an initial distance of 10 Å in a coplanar configuration with the surface (Fig. 1a, b). Na+ ions were initially positioned at a distance of 3.5 Å from the surface above tetrahedral and octahedral substitutions. To study the effect of microhydration of the montmorillonite surface on phenanthrene adsorption, 4, 8, and 16 water molecules per Auc (32, 64, and 128 molecules per complete simulation cell, respectively) were randomly distributed in a slab near the montmorillonite surface exposed to phenanthrene. Monte Carlo (MC) simulations according to Metropolis algorithm were carried out in a constant-NVT ensemble at 298 K using the TIP4P model for water (Jorgensen et al., 1983). OPLS-AA force field equations and potential parameters (Jorgensen et al., 1996) along with the procedure by Skipper et al., 1995, for rigid mineral layers were applied to calculate potential energies of interactions between montmorillonite, Na+ ions, TIP4P water, and phenanthrene. First,
phenanthrene structure was optimized with a quantum chemical method (hybrid B3LYP functional (Becke, 1993), an atomic basis set of triple zeta valence polarization (TZVP) quality (Schaefer et al., 1994)) using Turbomole program (Ahlrichs et al., 1989). The optimized geometry was kept rigid during the MC simulations, and partial atomic charges of phenanthrene atoms were assigned as shown in Scheme 1 and were obtained from the B3LYP/TZVP calculation using CHELPG method (atomic charges are fitted to reproduce molecular electrostatic potential) (Breneman and Wiberg, 1990). In the MC simulations, three-dimensional boundary conditions, a cut-off distance of 9 Å and the all-image convention were adopted for the short-range interactions, and the Ewald technique as modified for systems with a slab geometry (Yeh and Berkowitz, 1999) was applied to handle the long-range Coulomb interactions. A mean of ~1.4 × 106 MC cycles (a trial displacement of all Na+ cations as well as a trial displacement or rotation of all water molecules and the phenanthrene molecule were attempted during one MC cycle) were used for equilibration and 2 × 104 MC cycles were used for sampling structural properties, total potential energies, and potential energies of interaction between components of the simulated system. The latter energies could be readily calculated in the MC simulations due to a pairwise additive representation of intermolecular interactions in the OPLS-AA force field (an approximation was used additionally to separate contributions from different system components into the non-pairwise additive, reciprocal part of the Ewald sum). The equilibrium lateral positions of phenanthrene molecule obtained in the Metropolis runs for the considered water coverages were used as input for the potential of mean force (PMF) calculations according to the expanded ensemble density of states (EXEDOS) method (Kim et al., 2002) based on the Wang-Landau algorithm (Wang and Landau, 2001). For this purpose, the equilibrium conformation (tilt with respect to surface) and lateral position of phenanthrene molecule were kept fixed, whereas its height with respect to the montmorillonite surface was allowed to change. During such PMF calculation, the other species in the simulated system (Na+ ions and
Fig. 1. Snapshots of interfacial structures for phenanthrene on the fully dehydrated montmorillonite surface: side (a) and top (b) views of the initial configuration (with phenanthrene–montmorillonite and Na+–montmorillonite distances of 10 Å and 3 Å, respectively) as well as side (c) and top (d) views of the equilibrium configuration obtained with the Metropolis Monte Carlo method. Red, beige, and light blue spheres visualize O, Si, and Al atoms, respectively. Only silicon and basal oxygen atoms of the tetrahedral sheet exposed to phenanthrene are shown in (b) and (d). Na+ cations are displayed as violet spheres.
A. Meleshyn, D. Tunega / Geoderma 169 (2011) 41–46
Scheme 1. Partial atomic charges of phenanthrene atoms (White and green spheres visualize hydrogen and carbon atoms, respectively).
water molecules) were treated using Metropolis method as in the preceding Metropolis MC runs. Specifically, Na+ ions and water molecules were allowed to rearrange through arbitrary trial moves with the Metropolis acceptance ratio min{1,exp[–(Unew–Uold)/kT]}, where Uold and Unew are potential energies of the system before and after a trial move, k is the Boltzmann constant and T = 298 K. For trial moves of the phenanthrene molecule, the Wang-Landau acceptance ratio min{1,exp[–(Unew–Uold)/kT–(ln gnew(z)–ln gold(z))]} was applied, where gold(z) and gnew(z) are densities of the state z before and after a trial move. In the present study, z was defined as distance between center of mass of phenanthrene and the montmorillonite surface and was divided into bins with the width Δz = 0.01 Å. After each trial move of the phenanthrene molecule (one per MC cycle), the density g(z) of the visited state z was modified by a convergence factor f with initial value of e0.1 and the histogram of that state was increased by one. After each state was visited at least 100/(ln f)1/2 times (Zhou and Bhatt, 2005), ln f was halved, and a new EXEDOS cycle was started. A simulation was run for five EXEDOS cycles, sufficient for the calculation of free energy profiles for a single or several reaction coordinates at a constant temperature (Kim et al., 2002). The PMF expression –kT ln g(z) represents free energy profile in the direction perpendicular to the surface and this profile has been calculated at the end of the simulation (see (Meleshyn, 2010) for further details on PMF calculations).
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height of 9 Å equal –74 kJ/mol (poct) and –48 kJ/mol (ptet), respectively (black solid and dashed curves in Fig. 2). The higher affinity of phenanthrene to montmorillonite in the position poct than in the position ptet is related to the stronger binding of Na+ to the montmorillonite surface near a tetrahedral substitution than near an octahedral one. This stronger Na+ binding results in a smaller surface area sampled in the equilibrium state by Na+ (Fig. 2a) resulting in a weaker phenanthrene–Na+ interaction. It also results in a lower adsorption height of ~ 1.5 Å for Na+ adsorbed above a tetrahedral substitution as compared to that of ~ 1.85 Å for Na+ adsorbed above octahedral substitution as can be seen from one-dimensional atomic density profiles in Fig. 3a. Accordingly, Na+ ion in the latter position can be much more readily displaced by approaching phenanthrene molecule to favorably balance phenanthrene–Na+ and phenanthrene–montmorillonite interactions (regard that Na+ is strongly displaced from the position above octahedral Mg/Al substitution as a result of its coordination to phenanthrene as can be seen from a comparison of lateral Na+ distributions in the upper and left parts of Fig. 2a).
3. Results and Discussion 3.1. Phenanthrene interaction with the fully dehydrated montmorillonite surface Application of the Metropolis Monte Carlo method to the initial montmorillonite–phenanthrene configuration (Fig. 1a, b), which assumed a favorable coordination of planar phenanthrene with the planar basal surface of montmorillonite, revealed that phenanthrene actually prefers coordination with a Na+ ion adsorbed above a Mg2+/Al3+ substitution in the octahedral sheet of the montmorillonite layer (Fig. 1c, d). This site is labeled poct and corresponding equilibrium configuration of sorption is featured by Na+ ion fitting into the cavity of phenanthrene aromatic ring (with the most probable Na+–carbon distance of ~2.6 Å) and a tilt of ~20° between the normal vectors of phenanthrene and montmorillonite surface planes. The phenanthrene–Na+ interaction with a potential energy of –36 ±6 kJ/mol is dominated by electrostatic Coulomb attraction (-29 kJ/mol), whereas phenanthrene–montmorillonite interaction with a potential energy of –43±6 kJ/mol is dominated by van der Waals attraction (–41 kJ/mol). In PMF calculations, phenanthrene adsorption in its most stable equilibrium lateral position poct was compared to its adsorption at another possible site of Na+ sitting above a tetrahedral Al3+/Si4+ substitution. This lateral position is labeled ptet. Although adsorption minima of phenanthrene corresponding to these two lateral positions are characterized by the same height (distance between the center of mass of phenanthrene and the montmorillonite surface) of ~ 3.7 Å, the free energy differences between these states and a reference state at a
Fig. 2. Lateral density of Na+ (atoms/Å2, color box) sampled with the Metropolis algorithm in the state of thermodynamic equilibrium (a) and PMF profiles (kJ/mol) as functions of the distance between the center of mass of phenanthrene molecule and montmorillonite (b) for the dehydrated montmorillonite surface. For montmorillonite, only structural atoms (Si as circles and Al as a crossed circle in right bottom corner), basal oxygen atoms of the tetrahedral sheet (triangles) exposed to phenanthrene, and Mg2+ substitutions in the octahedral sheet (crossed squares) are shown in (a). PMF calculations were carried out for six different phenanthrene positions as denoted by a symbol “p” with a corresponding subscript index (see explanation in the related discussion). Phenanthrene structure is depicted by a chevron connecting the geometric centers of the three aromatic rings for each considered position. Full phenanthrene molecule is visualized only for the PMF path p3. The lateral Na+ distribution in (a) corresponds to the lowest free energy state in the position poct. In (b), the position of the basal plane of montmorillonite was set to z = 0 Å, PMF profiles were arbitrarily set to zero at z = 9 Å.
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0.03
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−3 Na , C densities (Å )
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c OH O density (Å−3) 2
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Na+, C densities (Å−3)
OH O density (Å−3) 2
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+
OH O density (Å−3) 2
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0 H2O/Auc 0.04
+ Na , C densities (Å )
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−3 Na , C densities (Å )
OH O density (Å−3) 2
a
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0
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0 18
z (Å) Fig. 3. Density profiles (atoms/Å3) of water oxygens, Na+ ions, and phenanthrene carbons at water coverages of 0 H2O/Auc (a), 4 H2O/Auc (b), 8 H2O/Auc (c), and 16 H2O/ Auc (d) as functions of the distance from the montmorillonite surface. These coverages correspond to 0, 32, 64, and 128 water molecules per phenenthrene molecule, respectively. The position of the basal plane of montmorillonite was set to z = 0 Å.
For better mapping of interactions of phenanthrene with the montmorillonite surface, four additional lateral positions (p1–p4 in Fig. 2a) were considered in further PMF calculations. The phenanthrene molecule was always kept in a coplanar conformation with the surface. In such conformation, Na+ cation does not mediate the interaction and phenanthrene can more closely approach the surface. The p1-p4 sites are characterized by the adsorption height of about 3.1–3.2 Å (Fig. 2b). However, the corresponding adsorption states are by 23–32 kJ/mol less favorable on the free energy scale than the most favorable state in the position poct. Notably, the position p3 featuring the best considered fit between the three hexagonal rings of phenanthrene and the three
underlying ditrigonal rings of the montmorillonite surface turned to be the second most favorable phenanthrene adsorption site, more favorable than ptet where Na+ is involved. This highlights the importance of a match between PAH adsorbate shape and the structure of adsorbent surface for favorable interaction.
3.2. Effect of surface microhydration on phenanthrene interaction with montmorillonite Upon water addition, the montmorillonite surface becomes increasingly covered with water molecules what leads to an increasing detachment of phenanthrene from the surface (Figs. 3 and 4). Whereas in case of the fully dehydrated montmorillonite surface phenanthrene molecule is more or less in a parallel configuration with the surface and an averaged distance of carbon atoms from the surface is about 4 Å (see one dimensional density profile of carbon atoms in Fig. 3), the situation starts to change with the increasing content of water molecules. The phenanthrene molecule turns to almost perpendicular configuration with respect to the montmorillonite surface at the water coverage of 4 and 8 H2O/Auc, respectively (Fig. 4a and b) and the number of carbon atoms below the distance of 4 Å to the surface decreases. This is also well demonstrated by one dimensional atomic density profiles of carbon atoms in Fig. 3b and c. Water molecules hydrate Na+ cations near the surface and a compact water layer is formed in this interfacial region (Fig. 4a and b). The increasing phenanthrene detachment from the surface is mirrored in the monotonically decreasing values of potential energy of phenanthrene–montmorillonite interaction (Table 1). Notably, differently from this rather gradual detachment, phenanthrene does not coordinate to Na+ any more already at the lowest considered water coverage (cf. corresponding values in Table 1) due to favorable hydration of Na+. The decrease of potential energy of phenanthrene interaction with the Na+-montmorillonite surface upon the surface hydration is partially compensated with an attractive interaction of phenanthrene with water molecules (Table 1). Albeit at a middle degree of water amount (8 H2O/Auc) phenanthrene is still attached to the montmorillonite surface, the formation of the second bound water layer advances (Figs. 3c and 4b) and a trend of complete detachment of phenanthrene with increasing amount of water is evident. It is shown in the model with the water coverage of 16 H2O/Auc, where a water slab bound to the surface is formed and phenanthrene becomes displaced to the water–air interface (Figs. 3d and 4c). Moreover, some of Na+ cations are fully hydrated and are released from the montmorillonite surface. This observation from the Metropolis MC simulations is confirmed by PMF calculations, which give additional insights into the changes of phenanthrene binding to montmorillonite and supply free energy values allowing a complete account of energetic and entropic effects of water addition to the montmorillonite–phenanthrene system. On a free energy scale, phenanthrene adsorption at a height of ~3.7 Å, ~4.7 Å, and ~5.7 Å is favored by 74, 43, and 14 kJ/mol (related to desorption state at a reference height of 9 Å, 11 Å, and 11 Å) for water coverages of 0, 4, and 8 H2O/Auc, respectively (Fig. 5, Table 1). However, at the latter coverage the phenanthrene inner-sphere adsorption (characterized by a direct contact between phenanthrene and montmorillonite atoms) becomes only 7 kJ/mol more favorable than phenanthrene outer-sphere adsorption (characterized by a single water layer separating phenanthrene and montmorillonite atoms) at a height of ~8.5 Å (second local minimum at the green curve in Fig. 5). This energy is comparable to a thermal energy of ~3.8 kJ/mol for gas phase molecules at ambient temperature and can be supplied through thermal motion of hydrating water molecules. At the highest water coverage (16 H2O/Auc), direct adsorption of phenanthrene on the montmorillonite surface becomes less favorable than phenanthrene desorption from montmorillonite. Indeed, a principally possible outer-sphere adsorption of phenanthrene at a height of ~6.3 Å becomes higher by 43 kJ/mol on the free energy scale
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Fig. 4. Side view snapshots of equilibrium interfacial structures for phenanthrene on the hydrated montmorillonite surface at water coverages of 4 H2O/Auc (a), 8 H2O/Auc (b), and 16 H2O/Auc (c) obtained with the Metropolis Monte Carlo method. Magenta spheres visualize Mg2+ ions (see caption to Fig. 1 for the other details).
Table 1 Potential energies (U) of phenanthrene interactions with Na+ ions, montmorillonite, and water as well as heights of phenanthrene inner-sphere adsorption states (h, measured as a distance between phenanthrene center of mass and the montmorillonite surface) and free energy difference (ΔF) between these states and the corresponding reference states (cf. Figs. 1b and 5). Water coverages are given as ratios of water-to-phenanthrene number densities. Nwater molecules/Nphenanthrene
Uphenanthrene-sodium, kJ/mol
Uphenanthrene-montmorillonite, kJ/mol
Uphenanthrene-water, kJ/mol
h, Å
ΔF, kJ/mol
0 32 64 128
–36 ± 6 –2 ± 6 –2 ± 3 0±2
–43 ± 6 –29 ± 6 –18 ± 6 –1 ± 1
– –24 ± 7 –38 ± 8 –34 ± 8
3.7 4.7 5.7 –
–74 –43 –14 –
than the phenanthrene desorption to a position at a height of ~12.6 Å at the water–air interface (Fig. 5). 4. Conclusions The surface of fully dehydrated (dry) montmorillonite is characterized by multiple adsorption sites for phenanthrene. In absence of
60
PMF (kJ/mol)
40 20 0 −20
0 H2O/Auc 4 H2O/Auc 8 H2O/Auc 16 H2O/Auc
−40 −60 −80
2
3
4
5
6
7
8
9
10
11
12
13
14
15
z (Å) Fig. 5. PMF profiles (kJ/mol) as functions of the distance between the center of mass of phenanthrene molecule and montmorillonite surface for water coverages of 0, 4, 8, and 16 H2O/Auc. Phenanthrene conformations and lateral positions, which were not allowed to change during the PMF calculations, were taken from the corresponding equilibrium structures obtained with the Metropolis Monte Carlo method and shown in Figs. 1c, 4a, b, and c, respectively. The position of the basal plane of montmorillonite was set to z = 0 Å, PMF profiles were arbitrarily set to zero at z = 11, 11, and 15 Å for coverages of 4, 8, and 16 H2O/Auc, respectively.
water, according the free energy scale the most favorable adsorption site is characterized by phenanthrene coordination to Na+ cation located above octahedral substitution in the montmorillonite structure. This supports the experimental suggestion of cation − π bonding facilitating phenanthrene adsorption on montmorillonite (Zhu et al., 2004). Consecutive microhydration of montmorillonite surface leads to an increasing detachment of phenanthrene from the montmorillonite surface. For the considered coverage of one phenanthrene molecule per 371 Å2 of the montmorillonite surface area, addition of 64 water molecules per phenanthrene molecule (corresponding to a phenanthrene concentration of 0.87 M) renders phenanthrene inner-sphere adsorption comparable with its outer-sphere adsorption. Doubling of this water content results in phenanthrene desorption from the montmorillonite surface into a state at the water–air interface (at a height of ~12.6 Å). These observations agree with the results of previous experimental studies (Karimi-Lotfabad et al., 1996; El-Nahhal and Safi, 2004; Celis et al., 2006; Changchaivong and Khaodhiar, 2009; and references therein) that PAHs adsorption on clays predominates in the dry soils but is negligible in the presence of high water content. The conclusion about a strong phenanthrene affinity to smectites by Hundal et al., 2001, is therefore not supported by the present results at least for phenanthrene surface coverages and phenanthrene aqueous concentrations below those considered here (1/371 Å-2 or 0.87 M, respectively) as well as for the considered exchangeable cation (Na+). Consideration of higher phenanthrene surface coverages and of other exchangeable cations to account for possible phenanthrene aggregation effects and to quantify the observed cation − π bonding is a task for subsequent model studies. Such studies can bring a contribution to solving a controversy in experimental observations of PAHs adsorption on hydrated clay minerals.
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References Ahlrichs, R., Bär, M., Häser, M., Horn, H., Kölmel, C., 1989. Electronic structure calculations on workstation computers: The program system Turbomole. Chem. Phys. Lett. 162, 165–169. Anderson, R.L., Ratcliffe, I., Greenwell, H.C., Williams, P.A., Cliffe, S., Coveney, P.V., 2010. Clay swelling – A challenge in the oilfield. Earth Sci. Rev. 98, 201–216. Becke, A.D., 1993. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 98, 5648–5652. Breneman, C.M., Wiberg, K.B., 1990. Determining atom-centered monopoles from molecular electrostatic potentials – the need for high sampling density in formamide conformational-analysis. J. Comp. Chem. 11, 361–373. Celis, R., De Jonge, H., De Jonge, L.W., Real, M., Hermosin, M.C., Cornejo, J., 2006. The role ofmineral and organic components in phenanthrene and dibenzofuran sorption by soil. Eur. J. Soil Sci. 57, 308–319. Changchaivong, S., Khaodhiar, S., 2009. Adsorption of naphthalene and phenanthrene on dodecylpyridinium-modified bentonite. Appl. Clay Sci. 43, 317–321. El-Nahhal, Y.Z., Safi, J.M., 2004. Adsorption of phenanthrene on organoclays from distilled and saline water. J. Colloid Interface Sci. 269, 265–273. Hundal, L.S., Thompson, M.L., Laird, D.A., Carmo, A.M., 2001. Sorption of phenanthrene by reference smectite. Environ. Sci. Technol. 35, 3456–3461. Jorgensen, W.L., Chandrasekhar, J., Madura, J.D., Impey, R.W., Klein, M.L., 1983. Comparison of simple potential functions for simulating liquid water. J. Chem. Phys. 79, 926–935. Jorgensen, W.L., Maxwell, D.S., Tirado-Rives, J., 1996. Development and testing of the OPLS All-Atom force field on conformational energetics and properties of organic liquids. J. Am. Chem. Soc. 118, 11225–11236. Karimi-Lotfabad, S., Pickard, M.A., Gray, M.R., 1996. Reactions of polynuclear aromatic hydrocarbons on soil. Environ. Sci. Technol. 30, 1145–1151.
Kim, E.B., Faller, R., Yan, Q., Abbott, N.L., de Pablo, J.J., 2002. Potential of mean force between a spherical particle suspended in a nematic liquid crystal and a substrate. J. Chem. Phys. 117, 7781–7787. Meleshyn, A., 2009. Cetylpyridinium chloride at the mica–water interface: Incomplete monolayer and bilayer structures. Langmuir 25, 881–890. Meleshyn, A., 2010. Adsorption of Sr2+ and Ba2+ at the cleaved mica–water interface: Free energy profiles and interfacial structure. Geochim. Cosmochim. Acta 74, 1485–1497. Schaefer, A., Huber, C., Ahlrichs, R., 1994. Fully optimized contracted Gaussian-basis sets of triple zeta valence quality for atoms Li to Kr. J. Chem. Phys. 100, 5829–5835. Skipper, N., Chang, F.-R., Sposito, G., 1995. Monte Carlo simulation of interlayer molecular structure in swelling clay minerals. 1. Methodology. Clays Clay Miner. 43, 285–293. Smoliar-Zviagina, B.B., 1993. Relationships between structural parameters and chemical composition of micas. Clay Miner. 28, 603–624. Tsipursky, S.I., Drits, V.A., 1984. The distribution of octahedral cations in the 2:1 layers of dioctahedral smectites studied by oblique-texture electron diffraction. Clay Miner. 19, 177–193. Wang, F., Landau, D.P., 2001. Efficient, multiple-range random walk algorithm to calculate the density of states. Phys. Rev. Lett. 86, 2050–2053. Wiles, M.C., Huebner, H.J., McDonald, T.J., Donnelly, K.C., Phillips, T.D., 2005. Matriximmobilized organoclay for the sorption of polycyclic aromatic hydrocarbons and pentachlorophenol from groundwater. Chemosphere 59, 1455–1464. Yeh, I.-C., Berkowitz, M.L., 1999. Ewald summation for systems with slab geometry. J. Chem. Phys. 111, 3155–3162. Zhou, C., Bhatt, R.N., 2005. Understanding and improving the Wang-Landau algorithm. Phys. Rev. E 72 025701(R). Zhu, D., Herbert, B.E., Schlautman, M.A., Carraway, E.R., Hur, J., 2004. Cation − π bonding: A new perspective on the sorption of polycyclic aromatic hydrocarbons to mineral surfaces. J. Environ. Qual. 33, 1322–1330.
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Geoderma j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o d e r m a
Adsorption of phenanthrene on Na-montmorillonite: A model study A. Meleshyn a,⁎, D. Tunega b,c a b c
Institute for Radioecology and Radiation Protection, Leibniz Universität Hannover, Herrenhäuser Str. 2, D-30419 Hannover, Germany Institute of Soil Research, University of Natural Resources and Applied Life Sciences, Peter Jordan-Straße 82, A-1190 Vienna, Austria Institute for Theoretical Chemistry, University of Vienna, Währingerstrasse 17, A-1090 Vienna, Austria
a r t i c l e
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Article history: Received 31 May 2010 Received in revised form 6 September 2010 Accepted 19 September 2010 Available online 18 October 2010 Keywords: Polycyclic aromatic hydrocarbon Sorption Surface properties Free energy Sheet silicates
a b s t r a c t The role of clay minerals in the fate of polycyclic aromatic hydrocarbons (PAHs) in soil still remains an unresolved and controversial issue. Recent experimental studies report on PAH sorption capacity of clay minerals, which is either much smaller than or comparable to that of soil organic matter with water either inhibiting or not inhibiting PAH sorption. We studied adsorption of phenanthrene – a prominent PAH representative – on Namontmorillonite – a common member of the smectite group of clay minerals and often dominating component of soil inorganic matter – by means of molecular simulations incorporating free energy calculations. The study also included the effect of surface hydration on sorption. Phenanthrene adsorption site featured by Na+−π bonding was identified as the most favorable one on the dehydrated montmorillonite surface with other inner-sphere adsorption sites being less favorable by 23–32 kJ/mol and the desorption state being less favorable by 74 kJ/mol on the free energy scale. Upon montmorillonite hydration by a water film with a thickness of ~1 nm, however, phenanthrene desorption from the montmorillonite surface to the water–air interface becomes more favorable than phenanthrene direct adsorption. Our results support, therefore, experimental studies suggesting that PAHs adsorption on clays predominates in the dry soils but is negligible in the presence of water. Still, a consideration of higher phenanthrene surface coverages (to account for possible aggregation effect) or other exchangeable cations (to account for possible stronger cation − π bonding) in the future model studies may provide additional arguments pro or contra of this suggestion. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Polycyclic aromatic hydrocarbons represent an important environmental concern because of their high toxicity and potential carcinogenity. Their fate in the environment and their migration into groundwater strongly depends on their interactions with soil components. A direct contribution of clay minerals to the sorption of anthracene and phenanthrene, representative compounds of PAHs, by the soils is considered to be relatively small as compared to its partitioning into soil organic matter (Karimi-Lotfabad et al., 1996; Celis et al., 2006; and references therein). Indeed, it has been shown that an addition of water to soil is able to reduce the rate and extent of soil interaction with anthracene (Karimi-Lotfabad et al., 1996). This behavior indicated that water competed successfully with anthracene for active sites on mineral surfaces and a kinetic model that included inhibition of the active reaction sites by water gave good agreement with the data. Although clay minerals strongly bind hydrophilic organic contaminants from water, they also can effectively adsorb hydrophobic PAHs only after a modification by certain organic cations (El-Nahhal and Safi, 2004; Wiles et al., 2005; Changchaivong and Khaodhiar, 2009).
⁎ Corresponding author. Tel.: + 49 511 762 3069; fax: + 49 511 762 17917. E-mail address: [email protected] (A. Meleshyn). 0016-7061/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2010.09.018
It has been observed that PAHs sorption to hydrated mineral surfaces strongly depends on the type of exchangeable cations at mineral surfaces. For example, Ag+ modified montmorillonite adsorbed much more phenanthrene than alkali, alkaline earth, or even organically modified montmorillonites because of a specific bonding of a cation–π type (Zhu et al., 2004). Moreover, smectites have been found to retain large amounts of phenanthrene from water in yet another experimental study (Hundal et al., 2001). The latter study reported a larger phenanthrene affinity to Ca2+-montmorillonite as compared to K+- or Na+-montmorillonite. The mechanism of the retention of nonionic hydrophobic organic compounds in soil and in particular their partitioning to clay minerals remains still largely unclear. The present study aims at gaining detailed insight into this mechanism using obtained structural and energetic information at a molecular level by application of molecular modeling methods, which have been shown to successfully contribute to a progress in understanding of clay systems (Anderson et al., 2010). 2. Simulation Details The simulation cell with lateral dimensions of ~20.65 Å×~17.96 Å encloses eight unit cells of Na+-montmorillonite layer of Wyoming-type with the formula unit Na0.375 (Al1.75 Mg0.25) (Si3.875Al0.125) O10(OH)2 having unit cell area Auc =46.36 Å2. A simulation cell with such lateral
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dimensions has been shown to be representative of the macroscopic mineral system and not influenced by the artificial long-range symmetry of the imposed periodic lattice (Skipper et al., 1995). The saturation of montmorillonite's exchange sites with Na+ corresponds to the experimental procedure in the studies by Hundal et al., 2001, and Zhu et al., 2004. Starting from the dehydrated montmorillonite with the layer spacing of 9.6 Å, the thickness of its Na+–containing interlayer space was increased from 3 Å to 100 Å to prepare the simulation cell containing one montmorillonite layer of a thickness of 6.6 Å (Tsipursky and Drits, 1984). This ensures that as a result of the application of three-dimensional periodic boundary conditions to the simulation cell, two neighboring montmorillonite layers are separated enough by a vacuum slab with a thickness of 100 Å (see (Meleshyn, 2009) for the visualization of the very similar simulation cell for mica). Coordinates of layer atoms were calculated according to the method of Smoliar-Zviagina, 1993. Two tetrahedral Al/Si and four octahedral Mg/Al substitutions in the simulation box were uniformly distributes within the mineral layer with the symmetry C2 (Tsipursky and Drits, 1984). To study phenanthrene adsorption on the fully dehydrated montmorillonite surface, one phenanthrene molecule was positioned at an initial distance of 10 Å in a coplanar configuration with the surface (Fig. 1a, b). Na+ ions were initially positioned at a distance of 3.5 Å from the surface above tetrahedral and octahedral substitutions. To study the effect of microhydration of the montmorillonite surface on phenanthrene adsorption, 4, 8, and 16 water molecules per Auc (32, 64, and 128 molecules per complete simulation cell, respectively) were randomly distributed in a slab near the montmorillonite surface exposed to phenanthrene. Monte Carlo (MC) simulations according to Metropolis algorithm were carried out in a constant-NVT ensemble at 298 K using the TIP4P model for water (Jorgensen et al., 1983). OPLS-AA force field equations and potential parameters (Jorgensen et al., 1996) along with the procedure by Skipper et al., 1995, for rigid mineral layers were applied to calculate potential energies of interactions between montmorillonite, Na+ ions, TIP4P water, and phenanthrene. First,
phenanthrene structure was optimized with a quantum chemical method (hybrid B3LYP functional (Becke, 1993), an atomic basis set of triple zeta valence polarization (TZVP) quality (Schaefer et al., 1994)) using Turbomole program (Ahlrichs et al., 1989). The optimized geometry was kept rigid during the MC simulations, and partial atomic charges of phenanthrene atoms were assigned as shown in Scheme 1 and were obtained from the B3LYP/TZVP calculation using CHELPG method (atomic charges are fitted to reproduce molecular electrostatic potential) (Breneman and Wiberg, 1990). In the MC simulations, three-dimensional boundary conditions, a cut-off distance of 9 Å and the all-image convention were adopted for the short-range interactions, and the Ewald technique as modified for systems with a slab geometry (Yeh and Berkowitz, 1999) was applied to handle the long-range Coulomb interactions. A mean of ~1.4 × 106 MC cycles (a trial displacement of all Na+ cations as well as a trial displacement or rotation of all water molecules and the phenanthrene molecule were attempted during one MC cycle) were used for equilibration and 2 × 104 MC cycles were used for sampling structural properties, total potential energies, and potential energies of interaction between components of the simulated system. The latter energies could be readily calculated in the MC simulations due to a pairwise additive representation of intermolecular interactions in the OPLS-AA force field (an approximation was used additionally to separate contributions from different system components into the non-pairwise additive, reciprocal part of the Ewald sum). The equilibrium lateral positions of phenanthrene molecule obtained in the Metropolis runs for the considered water coverages were used as input for the potential of mean force (PMF) calculations according to the expanded ensemble density of states (EXEDOS) method (Kim et al., 2002) based on the Wang-Landau algorithm (Wang and Landau, 2001). For this purpose, the equilibrium conformation (tilt with respect to surface) and lateral position of phenanthrene molecule were kept fixed, whereas its height with respect to the montmorillonite surface was allowed to change. During such PMF calculation, the other species in the simulated system (Na+ ions and
Fig. 1. Snapshots of interfacial structures for phenanthrene on the fully dehydrated montmorillonite surface: side (a) and top (b) views of the initial configuration (with phenanthrene–montmorillonite and Na+–montmorillonite distances of 10 Å and 3 Å, respectively) as well as side (c) and top (d) views of the equilibrium configuration obtained with the Metropolis Monte Carlo method. Red, beige, and light blue spheres visualize O, Si, and Al atoms, respectively. Only silicon and basal oxygen atoms of the tetrahedral sheet exposed to phenanthrene are shown in (b) and (d). Na+ cations are displayed as violet spheres.
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Scheme 1. Partial atomic charges of phenanthrene atoms (White and green spheres visualize hydrogen and carbon atoms, respectively).
water molecules) were treated using Metropolis method as in the preceding Metropolis MC runs. Specifically, Na+ ions and water molecules were allowed to rearrange through arbitrary trial moves with the Metropolis acceptance ratio min{1,exp[–(Unew–Uold)/kT]}, where Uold and Unew are potential energies of the system before and after a trial move, k is the Boltzmann constant and T = 298 K. For trial moves of the phenanthrene molecule, the Wang-Landau acceptance ratio min{1,exp[–(Unew–Uold)/kT–(ln gnew(z)–ln gold(z))]} was applied, where gold(z) and gnew(z) are densities of the state z before and after a trial move. In the present study, z was defined as distance between center of mass of phenanthrene and the montmorillonite surface and was divided into bins with the width Δz = 0.01 Å. After each trial move of the phenanthrene molecule (one per MC cycle), the density g(z) of the visited state z was modified by a convergence factor f with initial value of e0.1 and the histogram of that state was increased by one. After each state was visited at least 100/(ln f)1/2 times (Zhou and Bhatt, 2005), ln f was halved, and a new EXEDOS cycle was started. A simulation was run for five EXEDOS cycles, sufficient for the calculation of free energy profiles for a single or several reaction coordinates at a constant temperature (Kim et al., 2002). The PMF expression –kT ln g(z) represents free energy profile in the direction perpendicular to the surface and this profile has been calculated at the end of the simulation (see (Meleshyn, 2010) for further details on PMF calculations).
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height of 9 Å equal –74 kJ/mol (poct) and –48 kJ/mol (ptet), respectively (black solid and dashed curves in Fig. 2). The higher affinity of phenanthrene to montmorillonite in the position poct than in the position ptet is related to the stronger binding of Na+ to the montmorillonite surface near a tetrahedral substitution than near an octahedral one. This stronger Na+ binding results in a smaller surface area sampled in the equilibrium state by Na+ (Fig. 2a) resulting in a weaker phenanthrene–Na+ interaction. It also results in a lower adsorption height of ~ 1.5 Å for Na+ adsorbed above a tetrahedral substitution as compared to that of ~ 1.85 Å for Na+ adsorbed above octahedral substitution as can be seen from one-dimensional atomic density profiles in Fig. 3a. Accordingly, Na+ ion in the latter position can be much more readily displaced by approaching phenanthrene molecule to favorably balance phenanthrene–Na+ and phenanthrene–montmorillonite interactions (regard that Na+ is strongly displaced from the position above octahedral Mg/Al substitution as a result of its coordination to phenanthrene as can be seen from a comparison of lateral Na+ distributions in the upper and left parts of Fig. 2a).
3. Results and Discussion 3.1. Phenanthrene interaction with the fully dehydrated montmorillonite surface Application of the Metropolis Monte Carlo method to the initial montmorillonite–phenanthrene configuration (Fig. 1a, b), which assumed a favorable coordination of planar phenanthrene with the planar basal surface of montmorillonite, revealed that phenanthrene actually prefers coordination with a Na+ ion adsorbed above a Mg2+/Al3+ substitution in the octahedral sheet of the montmorillonite layer (Fig. 1c, d). This site is labeled poct and corresponding equilibrium configuration of sorption is featured by Na+ ion fitting into the cavity of phenanthrene aromatic ring (with the most probable Na+–carbon distance of ~2.6 Å) and a tilt of ~20° between the normal vectors of phenanthrene and montmorillonite surface planes. The phenanthrene–Na+ interaction with a potential energy of –36 ±6 kJ/mol is dominated by electrostatic Coulomb attraction (-29 kJ/mol), whereas phenanthrene–montmorillonite interaction with a potential energy of –43±6 kJ/mol is dominated by van der Waals attraction (–41 kJ/mol). In PMF calculations, phenanthrene adsorption in its most stable equilibrium lateral position poct was compared to its adsorption at another possible site of Na+ sitting above a tetrahedral Al3+/Si4+ substitution. This lateral position is labeled ptet. Although adsorption minima of phenanthrene corresponding to these two lateral positions are characterized by the same height (distance between the center of mass of phenanthrene and the montmorillonite surface) of ~ 3.7 Å, the free energy differences between these states and a reference state at a
Fig. 2. Lateral density of Na+ (atoms/Å2, color box) sampled with the Metropolis algorithm in the state of thermodynamic equilibrium (a) and PMF profiles (kJ/mol) as functions of the distance between the center of mass of phenanthrene molecule and montmorillonite (b) for the dehydrated montmorillonite surface. For montmorillonite, only structural atoms (Si as circles and Al as a crossed circle in right bottom corner), basal oxygen atoms of the tetrahedral sheet (triangles) exposed to phenanthrene, and Mg2+ substitutions in the octahedral sheet (crossed squares) are shown in (a). PMF calculations were carried out for six different phenanthrene positions as denoted by a symbol “p” with a corresponding subscript index (see explanation in the related discussion). Phenanthrene structure is depicted by a chevron connecting the geometric centers of the three aromatic rings for each considered position. Full phenanthrene molecule is visualized only for the PMF path p3. The lateral Na+ distribution in (a) corresponds to the lowest free energy state in the position poct. In (b), the position of the basal plane of montmorillonite was set to z = 0 Å, PMF profiles were arbitrarily set to zero at z = 9 Å.
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0.03
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+ Na , C densities (Å )
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OH O density (Å−3) 2
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0
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z (Å) Fig. 3. Density profiles (atoms/Å3) of water oxygens, Na+ ions, and phenanthrene carbons at water coverages of 0 H2O/Auc (a), 4 H2O/Auc (b), 8 H2O/Auc (c), and 16 H2O/ Auc (d) as functions of the distance from the montmorillonite surface. These coverages correspond to 0, 32, 64, and 128 water molecules per phenenthrene molecule, respectively. The position of the basal plane of montmorillonite was set to z = 0 Å.
For better mapping of interactions of phenanthrene with the montmorillonite surface, four additional lateral positions (p1–p4 in Fig. 2a) were considered in further PMF calculations. The phenanthrene molecule was always kept in a coplanar conformation with the surface. In such conformation, Na+ cation does not mediate the interaction and phenanthrene can more closely approach the surface. The p1-p4 sites are characterized by the adsorption height of about 3.1–3.2 Å (Fig. 2b). However, the corresponding adsorption states are by 23–32 kJ/mol less favorable on the free energy scale than the most favorable state in the position poct. Notably, the position p3 featuring the best considered fit between the three hexagonal rings of phenanthrene and the three
underlying ditrigonal rings of the montmorillonite surface turned to be the second most favorable phenanthrene adsorption site, more favorable than ptet where Na+ is involved. This highlights the importance of a match between PAH adsorbate shape and the structure of adsorbent surface for favorable interaction.
3.2. Effect of surface microhydration on phenanthrene interaction with montmorillonite Upon water addition, the montmorillonite surface becomes increasingly covered with water molecules what leads to an increasing detachment of phenanthrene from the surface (Figs. 3 and 4). Whereas in case of the fully dehydrated montmorillonite surface phenanthrene molecule is more or less in a parallel configuration with the surface and an averaged distance of carbon atoms from the surface is about 4 Å (see one dimensional density profile of carbon atoms in Fig. 3), the situation starts to change with the increasing content of water molecules. The phenanthrene molecule turns to almost perpendicular configuration with respect to the montmorillonite surface at the water coverage of 4 and 8 H2O/Auc, respectively (Fig. 4a and b) and the number of carbon atoms below the distance of 4 Å to the surface decreases. This is also well demonstrated by one dimensional atomic density profiles of carbon atoms in Fig. 3b and c. Water molecules hydrate Na+ cations near the surface and a compact water layer is formed in this interfacial region (Fig. 4a and b). The increasing phenanthrene detachment from the surface is mirrored in the monotonically decreasing values of potential energy of phenanthrene–montmorillonite interaction (Table 1). Notably, differently from this rather gradual detachment, phenanthrene does not coordinate to Na+ any more already at the lowest considered water coverage (cf. corresponding values in Table 1) due to favorable hydration of Na+. The decrease of potential energy of phenanthrene interaction with the Na+-montmorillonite surface upon the surface hydration is partially compensated with an attractive interaction of phenanthrene with water molecules (Table 1). Albeit at a middle degree of water amount (8 H2O/Auc) phenanthrene is still attached to the montmorillonite surface, the formation of the second bound water layer advances (Figs. 3c and 4b) and a trend of complete detachment of phenanthrene with increasing amount of water is evident. It is shown in the model with the water coverage of 16 H2O/Auc, where a water slab bound to the surface is formed and phenanthrene becomes displaced to the water–air interface (Figs. 3d and 4c). Moreover, some of Na+ cations are fully hydrated and are released from the montmorillonite surface. This observation from the Metropolis MC simulations is confirmed by PMF calculations, which give additional insights into the changes of phenanthrene binding to montmorillonite and supply free energy values allowing a complete account of energetic and entropic effects of water addition to the montmorillonite–phenanthrene system. On a free energy scale, phenanthrene adsorption at a height of ~3.7 Å, ~4.7 Å, and ~5.7 Å is favored by 74, 43, and 14 kJ/mol (related to desorption state at a reference height of 9 Å, 11 Å, and 11 Å) for water coverages of 0, 4, and 8 H2O/Auc, respectively (Fig. 5, Table 1). However, at the latter coverage the phenanthrene inner-sphere adsorption (characterized by a direct contact between phenanthrene and montmorillonite atoms) becomes only 7 kJ/mol more favorable than phenanthrene outer-sphere adsorption (characterized by a single water layer separating phenanthrene and montmorillonite atoms) at a height of ~8.5 Å (second local minimum at the green curve in Fig. 5). This energy is comparable to a thermal energy of ~3.8 kJ/mol for gas phase molecules at ambient temperature and can be supplied through thermal motion of hydrating water molecules. At the highest water coverage (16 H2O/Auc), direct adsorption of phenanthrene on the montmorillonite surface becomes less favorable than phenanthrene desorption from montmorillonite. Indeed, a principally possible outer-sphere adsorption of phenanthrene at a height of ~6.3 Å becomes higher by 43 kJ/mol on the free energy scale
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Fig. 4. Side view snapshots of equilibrium interfacial structures for phenanthrene on the hydrated montmorillonite surface at water coverages of 4 H2O/Auc (a), 8 H2O/Auc (b), and 16 H2O/Auc (c) obtained with the Metropolis Monte Carlo method. Magenta spheres visualize Mg2+ ions (see caption to Fig. 1 for the other details).
Table 1 Potential energies (U) of phenanthrene interactions with Na+ ions, montmorillonite, and water as well as heights of phenanthrene inner-sphere adsorption states (h, measured as a distance between phenanthrene center of mass and the montmorillonite surface) and free energy difference (ΔF) between these states and the corresponding reference states (cf. Figs. 1b and 5). Water coverages are given as ratios of water-to-phenanthrene number densities. Nwater molecules/Nphenanthrene
Uphenanthrene-sodium, kJ/mol
Uphenanthrene-montmorillonite, kJ/mol
Uphenanthrene-water, kJ/mol
h, Å
ΔF, kJ/mol
0 32 64 128
–36 ± 6 –2 ± 6 –2 ± 3 0±2
–43 ± 6 –29 ± 6 –18 ± 6 –1 ± 1
– –24 ± 7 –38 ± 8 –34 ± 8
3.7 4.7 5.7 –
–74 –43 –14 –
than the phenanthrene desorption to a position at a height of ~12.6 Å at the water–air interface (Fig. 5). 4. Conclusions The surface of fully dehydrated (dry) montmorillonite is characterized by multiple adsorption sites for phenanthrene. In absence of
60
PMF (kJ/mol)
40 20 0 −20
0 H2O/Auc 4 H2O/Auc 8 H2O/Auc 16 H2O/Auc
−40 −60 −80
2
3
4
5
6
7
8
9
10
11
12
13
14
15
z (Å) Fig. 5. PMF profiles (kJ/mol) as functions of the distance between the center of mass of phenanthrene molecule and montmorillonite surface for water coverages of 0, 4, 8, and 16 H2O/Auc. Phenanthrene conformations and lateral positions, which were not allowed to change during the PMF calculations, were taken from the corresponding equilibrium structures obtained with the Metropolis Monte Carlo method and shown in Figs. 1c, 4a, b, and c, respectively. The position of the basal plane of montmorillonite was set to z = 0 Å, PMF profiles were arbitrarily set to zero at z = 11, 11, and 15 Å for coverages of 4, 8, and 16 H2O/Auc, respectively.
water, according the free energy scale the most favorable adsorption site is characterized by phenanthrene coordination to Na+ cation located above octahedral substitution in the montmorillonite structure. This supports the experimental suggestion of cation − π bonding facilitating phenanthrene adsorption on montmorillonite (Zhu et al., 2004). Consecutive microhydration of montmorillonite surface leads to an increasing detachment of phenanthrene from the montmorillonite surface. For the considered coverage of one phenanthrene molecule per 371 Å2 of the montmorillonite surface area, addition of 64 water molecules per phenanthrene molecule (corresponding to a phenanthrene concentration of 0.87 M) renders phenanthrene inner-sphere adsorption comparable with its outer-sphere adsorption. Doubling of this water content results in phenanthrene desorption from the montmorillonite surface into a state at the water–air interface (at a height of ~12.6 Å). These observations agree with the results of previous experimental studies (Karimi-Lotfabad et al., 1996; El-Nahhal and Safi, 2004; Celis et al., 2006; Changchaivong and Khaodhiar, 2009; and references therein) that PAHs adsorption on clays predominates in the dry soils but is negligible in the presence of high water content. The conclusion about a strong phenanthrene affinity to smectites by Hundal et al., 2001, is therefore not supported by the present results at least for phenanthrene surface coverages and phenanthrene aqueous concentrations below those considered here (1/371 Å-2 or 0.87 M, respectively) as well as for the considered exchangeable cation (Na+). Consideration of higher phenanthrene surface coverages and of other exchangeable cations to account for possible phenanthrene aggregation effects and to quantify the observed cation − π bonding is a task for subsequent model studies. Such studies can bring a contribution to solving a controversy in experimental observations of PAHs adsorption on hydrated clay minerals.
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References Ahlrichs, R., Bär, M., Häser, M., Horn, H., Kölmel, C., 1989. Electronic structure calculations on workstation computers: The program system Turbomole. Chem. Phys. Lett. 162, 165–169. Anderson, R.L., Ratcliffe, I., Greenwell, H.C., Williams, P.A., Cliffe, S., Coveney, P.V., 2010. Clay swelling – A challenge in the oilfield. Earth Sci. Rev. 98, 201–216. Becke, A.D., 1993. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 98, 5648–5652. Breneman, C.M., Wiberg, K.B., 1990. Determining atom-centered monopoles from molecular electrostatic potentials – the need for high sampling density in formamide conformational-analysis. J. Comp. Chem. 11, 361–373. Celis, R., De Jonge, H., De Jonge, L.W., Real, M., Hermosin, M.C., Cornejo, J., 2006. The role ofmineral and organic components in phenanthrene and dibenzofuran sorption by soil. Eur. J. Soil Sci. 57, 308–319. Changchaivong, S., Khaodhiar, S., 2009. Adsorption of naphthalene and phenanthrene on dodecylpyridinium-modified bentonite. Appl. Clay Sci. 43, 317–321. El-Nahhal, Y.Z., Safi, J.M., 2004. Adsorption of phenanthrene on organoclays from distilled and saline water. J. Colloid Interface Sci. 269, 265–273. Hundal, L.S., Thompson, M.L., Laird, D.A., Carmo, A.M., 2001. Sorption of phenanthrene by reference smectite. Environ. Sci. Technol. 35, 3456–3461. Jorgensen, W.L., Chandrasekhar, J., Madura, J.D., Impey, R.W., Klein, M.L., 1983. Comparison of simple potential functions for simulating liquid water. J. Chem. Phys. 79, 926–935. Jorgensen, W.L., Maxwell, D.S., Tirado-Rives, J., 1996. Development and testing of the OPLS All-Atom force field on conformational energetics and properties of organic liquids. J. Am. Chem. Soc. 118, 11225–11236. Karimi-Lotfabad, S., Pickard, M.A., Gray, M.R., 1996. Reactions of polynuclear aromatic hydrocarbons on soil. Environ. Sci. Technol. 30, 1145–1151.
Kim, E.B., Faller, R., Yan, Q., Abbott, N.L., de Pablo, J.J., 2002. Potential of mean force between a spherical particle suspended in a nematic liquid crystal and a substrate. J. Chem. Phys. 117, 7781–7787. Meleshyn, A., 2009. Cetylpyridinium chloride at the mica–water interface: Incomplete monolayer and bilayer structures. Langmuir 25, 881–890. Meleshyn, A., 2010. Adsorption of Sr2+ and Ba2+ at the cleaved mica–water interface: Free energy profiles and interfacial structure. Geochim. Cosmochim. Acta 74, 1485–1497. Schaefer, A., Huber, C., Ahlrichs, R., 1994. Fully optimized contracted Gaussian-basis sets of triple zeta valence quality for atoms Li to Kr. J. Chem. Phys. 100, 5829–5835. Skipper, N., Chang, F.-R., Sposito, G., 1995. Monte Carlo simulation of interlayer molecular structure in swelling clay minerals. 1. Methodology. Clays Clay Miner. 43, 285–293. Smoliar-Zviagina, B.B., 1993. Relationships between structural parameters and chemical composition of micas. Clay Miner. 28, 603–624. Tsipursky, S.I., Drits, V.A., 1984. The distribution of octahedral cations in the 2:1 layers of dioctahedral smectites studied by oblique-texture electron diffraction. Clay Miner. 19, 177–193. Wang, F., Landau, D.P., 2001. Efficient, multiple-range random walk algorithm to calculate the density of states. Phys. Rev. Lett. 86, 2050–2053. Wiles, M.C., Huebner, H.J., McDonald, T.J., Donnelly, K.C., Phillips, T.D., 2005. Matriximmobilized organoclay for the sorption of polycyclic aromatic hydrocarbons and pentachlorophenol from groundwater. Chemosphere 59, 1455–1464. Yeh, I.-C., Berkowitz, M.L., 1999. Ewald summation for systems with slab geometry. J. Chem. Phys. 111, 3155–3162. Zhou, C., Bhatt, R.N., 2005. Understanding and improving the Wang-Landau algorithm. Phys. Rev. E 72 025701(R). Zhu, D., Herbert, B.E., Schlautman, M.A., Carraway, E.R., Hur, J., 2004. Cation − π bonding: A new perspective on the sorption of polycyclic aromatic hydrocarbons to mineral surfaces. J. Environ. Qual. 33, 1322–1330.