Surface of the Mineral Goethite - American Chemical Society

J. Phys. Chem. C 2007, 111, 877-885

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Quantum Chemical Adsorption Studies on the (110) Surface of the Mineral Goethite Adelia J. A. Aquino,*,†,‡ Daniel Tunega,†,§ Georg Haberhauer,§ Martin H. Gerzabek,‡ and Hans Lischka*,† Institute for Theoretical Chemistry, UniVersity of Vienna, Wa¨hringerstrasse 17, A-1090 Vienna, Austria, Institute of Soil Research, UniVersity of Natural Resources, and Applied Life Sciences, Vienna, Peter-Jordan-Strasse 82b, A-1190 Vienna, Austria, and Austrian Research Centers Seibersdorf, A-2444 Seibersdorf, Austria ReceiVed: August 1, 2006; In Final Form: October 23, 2006

The interaction between the (110) goethite surface and water, acetic acid, acetate, 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4-dichlorophenoxyacetate (2,4-D-), and benzene was studied by means of quantum mechanical calculations based on density functional theory (DFT) using the B3LYP approach. Furthermore, MøllerPlesset calculations to second order using the resolution-of-identity approach (RI-MP2) were also performed for the goethite/benzene interaction. Structural and energetic features of the surface complexes were evaluated using cluster models for the goethite surface derived from a periodic slab surface. Our investigations showed that the (110) goethite surface formed by three types of the hydroxyl groups offers a variety of possibilities for hydrogen bond formation with appropriate polar adsorbents. Calculated interaction energies for the water molecule on different sites are on the order of ca. -20 kcal/mol, a number that is in line with the number and type of hydrogen bonds formed. Comparison of the interaction energies for different interaction sites/clusters shows that these sites are energetically more or less equivalent. Slightly larger interaction energies were observed for acetic acid and 2,4-D in comparison with the goethite/water complexes. The deprotonated, anionic form of acetic acid and 2,4-D show even stronger interactions between -30 and -50 kcal/mol. The interaction with the nonpolar, aromatic benzene molecule is significantly weaker (estimated value is in the range of -5 to -9 kcal/mol) but still significant.

Introduction Goethite (R-FeOOH) is often formed as a secondary mineral through weathering processes by oxidation of other iron-rich minerals or as precipitate in swamps and bogs, and thus is a common component of soils. It is a widespread mineral, found in quantity all over the world. Although is it not as rich in iron as hematite, it is still an important iron ore. Goethite plays an important role for environmental processes as it possesses a strong affinity to a variety of contaminants and nutrients. It belongs to the group of ferric oxyhydroxides that are able to sorb large amounts of heavy metal cations, anions, and oxyanions1 and also organic pollutants (e.g., polycyclic aromatic hydrocarbons).2,3 The sorption capability of goethite is directly related to its surface structure and the composition of mineral particles. Goethite crystals vary in size from 10 to 1000 nm, having high specific surface areas (50-200 m2/g).4 In cases where goethite crystals are faceted, the dominant surfaces are (110), (021), and, to a lesser extent, (100), (010), and (001) as shown by high-resolution transmission electron microscopy and X -ray line broadening studies.5-10 Even though the bulk structure of goethite is relatively simple, the surface structure is complicated due to the existence of several types of adsorption surface sites. The specification of such surface sites has been the focus of numerous experimental and theoretical studies.11 * Corresponding authors. E-mail: [email protected] (A.J.A.A.); [email protected] (H.L.). † University of Vienna. ‡ University of Natural Resources and Applied Life Sciences, Vienna. § Austrian Research Centers Seibersdorf.

These studies showed that the surfaces of ferric oxyhydroxides are predominantly formed from hydroxyl groups. Several theoretical investigations have been performed on the goethite surface structure. A classical interatomic force field approach was used in calculations on proton binding to the goethite surface in the presence of aqueous solution.12,13 The same approach was used in the study of the goethite-water interface by Rustad et al.14 and, in combination with molecular dynamics, in the study of the surface charge on goethite nanoparticles by Rustad and Felmy.15 The goethite-water interface was also modeled by means of molecular dynamics using a different type of force field.16 Surface energies for different surface terminations perpendicular to the (010) plane have been calculated by means of ab initio periodic calculations.17 We did not find in the literature any ab initio calculations especially devoted to interactions of goethite surfaces, and therefore, there is still a strong need for reliable quantum chemical investigations on the structure and relative stability of different surface structures, in particular concerning the activity of surface adsorption sites. In this work we perform density functional theory (DFT) calculations to study the surface structure of goethite using a cluster approach with clusters containing four, six, and eight iron atoms. The structure of these clusters is derived from a periodic slab surface model that represents the most populated surface (110). It is well-known that molecular orbital (MO) calculations on systems containing 3d elements (in our case Fe) represent a serious problem.18 A large number of investigations have been devoted to this problem using ab initio molecular orbital calculations of iron-containing compounds. Practically

10.1021/jp0649192 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/13/2006

878 J. Phys. Chem. C, Vol. 111, No. 2, 2007

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Figure 1. The (110) slab surface model of goethite. S1, S2, and S3 are potential adsorption sites.

all of them focused on various iron complexes, predominantly mononuclear ones. Several papers have been published dealing with binuclear complexes in which oxo bridges were studied (see, e.g., refs 19-23), but no calculations on molecular complexes with more than two iron atoms could be found in the literature. The main aim of the present work is the study of adsorption complexes on goethite. We want to show the structural manifold of the hydroxyl groups of a goethite surface in their interaction with a set of adsorbents occurring in soil environments. For this purpose we have selected a series of molecular species containing small model molecules such as water, acetic acid, and acetate representing typical polar interactions in soils. Beyond that the interactions of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) and benzene with the goethite surface have been studied. The latter choice resulted from the aforementioned absorption capability of goethite concerning aromatic compounds. Structural and Computational Details The goethite structure consists of a network of distorted octahedra with Fe(III) cations in their centers that are connected via µ-oxo bridges. Half of these µ-oxo bridges are protonated, which results in the formation of one type of hydroxyl groups in the bulk structure. However, the surfaces of mineral particles contain more than one type of hydroxyl groups due to cutting through Fe-O bonds and their subsequent saturation with hydrogen atoms. As was already mentioned in the Introduction, the most frequently populated surface of goethite particles is the (110) surface. This surface contains three different OH sites at full bond saturation. Cluster models used in calculations were constructed from the (110) slab surface model presented in Figure 1. The surface of this model contains three different OH types. The standard IUPAC nomenclature is used to denominate these sites as follows: oxo (hydroxo) refers to an oxide (hydroxide) ion bound to one iron atom, µ-oxo (µ-hydroxo) refers to an oxide (hydroxide) ion bound to two iron atoms, and µ3-oxo (µ3-hydroxo) refers to an oxide (hydroxide) ion bound to three iron atoms. Three different types of hydroxyl groups are arranged in rows on the goethite surface, as can be seen in Figure 1. The vicinity of these rows forms three different interaction sites for hydrogen bond binding, as is schematically depicted in Figure 1. Site 1 (S1) is coordinated by hydroxo and µ-hydroxo, site 2 (S2) is coordinated by µ-hydroxo and µ3-

Figure 2. Structures of isolated clusters Fe4 (a), Fe6 (b), and Fe8 (c). Cartesian coordinates of the numbered oxygen atoms and of connected hydrogen atoms were optimized (see text for details).

hydroxo, and site 3 (S3) is coordinated by hydroxo and µ3hydroxo OH groups, respectively. Three types of clusters of increasing size were prepared containing four, six, and eight iron atoms (Figure 2) with the aim to model all three interaction sites. Further on, we will refer to them as clusters Fe4, Fe6, and Fe8, respectively. Cluster Fe4 was used as representative of sites S1 and S2, cluster Fe6 was used as representative of site S2, and cluster Fe8 was chosen to model site S3. The valences of broken bonds were completed with hydrogen atoms forming terminal hydroxyl groups in order to provide the overall neutral charge of the final cluster structures. To preserve the structure of the goethite surface, only the surface hydroxyl groups were optimized and the rest of the cluster geometry was kept fixed to conserve the octahedral coordination of the iron atoms as in the bulk (for details, see below). To evaluate the adsorption properties of these sites, the adsorption complexes with a water molecule, acetic acid, acetate, 2,4dichlorophenoxyacetic acid, and 2,4-dichlorophenoxyacetate were studied. For Fe6 the interaction with the benzene molecule was computed as well. In the structural configurations as they occur in goethite (the closest Fe-Fe distance is 3.02 Å), a spin-crossover coupling exists between iron centers. From experiments it is known that goethite is an antiferromagnetic material at room temperature. Owing to this fact, we should investigate our clusters as open shell systems in a low-spin-state configuration. However, since the stability of the antiferromagnetic phase is a property of the solid and not of the cluster, we decided to investigate also the high-spin state of all clusters. Since the clusters contain up to eight iron atoms, a large number of possible spin states exists. It is technically not possible to calculate all spin combinations. Thus we considered only the highest and lowest spin combinations of each cluster. These calculations were performed in a spin-unrestricted DFT formalism applying the B3LYP functional.24-26 The atomic basis set was of a split valence polarization quality (SVP).27 Additional calculations were performed with inclusion of diffuse functions on selected atoms

Chemical Adsorption on (110) Goethite Surface

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TABLE 1: Geometric Parameters (Å) of Isolated Iron Clusters at Low Spin (LSPIN), High Spin (HSPIN), and Closed Shell (CSHELL) Using B3LYP/SVP Approacha system method Fe4 Fe6 Fe8

a

LSPIN HSPIN CSHELL LSPIN HSPIN CSHELL LSPIN HSPIN CSHELL

R(O1-H)

R(O2-H)

R(O3-H)

R(O4-H)

R(O5-H)

0.969 0.969 0.973 (0.973) 0.984 0.988 1.001 (0.999) 0.969 0.967 0.981 (0.982)

0.971 0.971 0.980 (0.977) 0.971 0.972 0.972 (0.971) 0.969 0.967 0.977 (0.973)

0.968 0.970 0.969 (0.966) 0.968 0.966 0.969 (0.968) 0.970 0.968 0.969 (0.967)

0.970 0.965 0.966 (0.969) 0.967 0.966 0.977 (0.973) 0.987 0.987 0.969 (0.972)

0.969 0.967 0.969 (0.968) 0.977 0.977 0.977 (0.968)

R(O6-H)

R(O7-H)

R(O8-H)

R(O9-H)

0.968 0.969 0.979 (0.977) 0.972 0.971 0.969 (0.967) 0.988 (0.983) 0.975 (0.974) 1.003 (1.006)

Values in parentheses are results obtained with the SVP+sp basis set.

and using the TZVP basis set28 (see below). Møller-Plesset calculations to second order using the resolution-of-identity approach (RI-MP2)29,30 and the SVP basis set were performed for the interaction of benzene with the Fe6 cluster. The calculations were performed using the computer program TURBOMOLE.31 The main characteristic of exchange-coupled metal clusters is the weak interaction between two magnetic metal centers with unpaired electrons. In such systems, each individual spin of the metal site is coupled to a number of energetically closely lying electronic levels. From a theoretical view, multideterminantbased methods are required to describe such coupled spin systems. However, the use of these methods is restricted to small systems considering the high computational effort required. A way out of this impasse is the use of spin-unrestricted density functional method,32-34 which has been applied in both highand low-spin-state calculations. Since the clusters used in this work all contain an even number of iron atoms, the use of the spin-restricted closed shell formalism is also possible. The reasons for the choice of closed shell calculations are the following: (i) the closed shell approach for S ) 0 is standard, which guarantees the antiferromagnetic property as well as no spin contamination, and (ii) the calculations are faster than those for low spin and high spin. It is well documented that high-spin states are systematically favored in the B3LYP approach.35 These artifacts were not a direct concern in this work since we were primarily interested in interaction energies with adsorbents. The sensitivity of these interaction energies on different spin-coupling schemes was investigated systematically. A set of test calculations on lowand high-spin states and closed shell calculations were performed on all isolated goethite clusters and all interactions with the water molecule. In most cases only small differences in interaction energies to the low-spin calculations was observed. Based on the results obtained from these calculations, the remaining calculations were performed only in the closed shell formalism. To achieve self-consistent field (SCF) convergence turned out to be very difficult. We have optimized the three parameters available in the TURBOMOLE package controlling the convergence of the SCF procedure: a damping factor (determining the weight of the Fock matrix from the previous SCF step), orbital shift, and Fermi smearing (allowing fractional occupation near the HOMO-LUMO separation during the SCF procedure). The relatively weak intermolecular interactions calculated with the SVP basis set are accompanied by nonnegligible basis set superposition errors (BSSE). They often amount to more than 20% of the interaction energy. Unfortunately, BSSE counterpoise corrections in many cases led to unreasonable results. Therefore, only uncorrected interaction energies are given in this work. Another way to reduce the BSSE is to use bigger basis sets, which contain diffuse functions. In the effort

to reduce the BSSE, we used the same approach as in our previous investigations dealing with complexes of aluminum and specific organic acids36,37 or different hydrogen-bonded complexes.38 The SVP basis set was augmented with diffuse s and p functions on heavy atoms next to interaction sites. The exponents of these additional basis functions were obtained by dividing the smallest respective exponent of the SVP basis set by a factor of 3. This basis set is denominated SVP+sp, and its application was found to substantially reduce the BSSE.36-38 Additionally, we performed geometry optimizations for the cluster F4‚‚‚H2O and the two configurations of the cluster F6‚‚‚H2O using the TZVP basis set for atoms involved in the geometry optimization (see above). Results The presentation of results will be divided into four subsections: (i) the isolated iron clusters; (ii) interactions of clusters with water; (iii) interactions with acetic acid, acetate, 2,4dichlorophenoxyacetic acid, and 2,4-dichlorophenoxyacetate; and (iv) interactions with benzene. Isolated Clusters. The effect of using various spin states on optimized OH bond lengths is shown in Table 1. Corresponding optimized structures are displayed in Figure 2. Note that the following sites were optimized: one hydroxo, two µ-hydroxo, and one µ3-hydroxo groups for cluster Fe4 (Figure 2a); two hydroxo, two µ-hydroxo, and two µ3-hydroxo groups for cluster Fe6 (Figure 2b); and six hydroxo and three µ3-hydroxo groups for cluster Fe8 (Figure 2c). This was achieved by relaxation of the Cartesian coordinates of the numbered oxygen atoms (see Figure 2) and of connected hydrogen atoms as well as by fixing the coordinates of remaining atoms. Calculated OH bond lengths vary mostly around 0.97 Å. Only very small differences between the three types of OH groups are found. Slightly longer bond lengths were observed for the O1-H distance in the cluster Fe6 and for O9-H in the cluster Fe8, both in the closed shell calculations. This is due to the formation of internal hydrogen bonds within the clusters (e.g., between O9-H group and O5 in the cluster Fe8; see Figure 2c). Inclusion of diffuse functions to the atomic basis sets of hydrogen and oxygen atoms in the closed shell calculations had a minimal effect on the OH bond lengths. Differences in OH bond distances between low- and high-spin calculations are negligible; closed shell results are almost in all cases slightly longer than in the two other spin-coupling cases. Rustad et al.14 used the molecular mechanics method to calculate surface structures and proton binding energy and found bond lengths of 0.96, 1.00, and 1.04 Å for hydroxo, µ-hydroxo, and µ3-hydroxo groups, respectively, on a goethite surface. Our quantum chemical calculations do not confirm such significant differences for the three different types of OH groups. Interaction with Water. Taking into account that water can be considered a model molecule for specific solvation and


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Figure 3. Optimized structures of complexes Fe4‚‚‚H2O (a), Fe6‚‚‚H2O (b,c), and Fe8‚‚‚H2O (d). The stick model is used to characterize atoms and bonds of the goethite clusters involved in the interaction.

hydrogen bond formation, we analyzed interactions of goethite with one water molecule. Optimized closed shell structures are displayed in Figure 3. The proximity of the three different types of OH groups offers several combinations for hydrogen bonded interaction sites. In the construction of the surface complexes we tried to avoid interactions with the terminal OH groups (those which saturate dangling bonds). This goal could not be fully achieved for the smallest cluster Fe4. In the case of cluster Fe6 two different types of complexes were constructed (see Figure 3b,c; details are discussed later). The water molecule acts in both cases simultaneously as proton donor and proton acceptor. In our previous studies39,40 on the interactions of water molecules with the octahedral surface of the kaolinite layer (formed from surface hydroxyl groups), we have found that the dominant binding mechanism to the surface is hydrogen bond formation and that the surface OH groups are also able to act as proton donors and/or acceptors due to their flexibility. A similar flexibility can also be expected in the case of the goethite surface, which is also fully covered with the hydroxyl groups. In order to classify the nature of the formed hydrogen bonds, the label “HBD” is used if the water molecule behaves as a proton donor and the label “HBA” is used if the water molecule is a proton acceptor (see Figure 3). In all four cases investigated the formation of three hydrogen bonds between the water molecule and the surface hydroxyl groups is observed. Computed hydrogen bond distances, H‚‚‚O, are collected in Table 2 for all three spin states. In case of the complex Fe4 (Figure 3a) and the first complex of cluster Fe6 (Figure 3b), the water molecule forms two hydrogen bonds of HBD type and one hydrogen bond of the HBA type. Note that the water is proton donor to the hydroxo and µ-hydroxo OH groups and proton acceptor from the µ3hydroxo OH group in cluster Fe4. The HBA1 and the HBD1 hydrogen bonds (see Table 2) are of similar strength, comparing their lengths (about 1.7 Å). The third hydrogen bond, HBD2, is much weaker compared to the two other hydrogen bonds. It seems that if two hydrogen bonds are formed optimally the third

one is always limited due to a certain rigidity of the Fe-O bonded network on the surface. All calculations performed with different spin states give similar results with the hydrogen bond lengths varying within 0.1 Å. Adding diffuse functions to the atomic basis sets caused a small increase of the two stronger hydrogen bonds and a small shortening of the weak hydrogen bond HBD2. Closed shell calculations performed with the TZVP basis set for complexes Fe4‚‚‚H2O (Figure 3a) and Fe6‚‚‚H2O (Figure 3b) gave similar results for the hydrogen bond distances compared to the SVP+sp values (see Table 2). The biggest difference (0.3 Å) is observed for the length of the weakest hydrogen bond HBD2 of the Fe4‚‚‚H2O complex. The variation of the hydrogen bond lengths with respect to the various spin states is also reflected in the calculated interaction energies. In Table 3 results are presented for all three spin states using the B3LYP/SVP method. Additionally, B3LYP/SVP+sp results are given for the closed shell calculations as well. It can be seen that this variation is within 4.5 kcal/mol (difference between the low- and high-spin states) while the closed shell results are closer to the high-spin state. The interaction between the Fe4 cluster and the H2O molecule is rather strong, a fact that is reflected in the large interaction energy (in absolute value). If we estimate the basis set superposition error to be about 6-7 kcal/mol (as observed in our previous investigation of hydrogen bonds using the SVP basis set), 39 the interaction energy reduces to about -12 kcal/mol. This value is higher than the calculated interaction energy of -8 kcal/mol for an isolated water molecule adsorbed on the octahedral kaolinite surface using the cluster approach.40 This means that the goethite surface OH groups are more active in hydrogen bond forming than kaolinite OH groups. The interaction energies obtained with SVP+sp and TZVP basis sets (closed shell approach) are smaller than those ones obtained with the SVP basis set. This is an expected trend since the basis set superposition error is smaller for the bigger basis set as was already shown, for example, in our previous papers.36-38 Comparison of the cluster Fe6 (Figure 3b) with the Fe4 cluster shows that in the former case the water molecule forms the



TABLE 2: Hydrogen Bond Distances (Å) between Goethite Clusters and the Water Molecule Using the B3LYP/SVP Approach distance system Fe4‚‚‚H2O (Figure 3a) low spin high spin closed shella Fe6‚‚‚H2O (Figure 3b) low spin high spin closed shella

HBA1

HBD1

HBD2

1.70 1.70 1.63 (1.76/1.87)

1.78 1.68 1.79 (1.81/1.81)

2.44 2.53 2.43 (2.41/2.77)

1.76 1.73 1.85 (1.99/2.00)

1.84 1.84 1.91 (1.97/1.96)

2.39 2.19 2.29 (2.39/2.43)

distance system Fe6‚‚‚H2O (Figure 3c) low spin high spin closed shella Fe8‚‚‚H2O (Figure 3d) low spin high spin closed shella a

HBA1

HBA2

HBD1

1.60 1.81 1.84 (1.89/1.82)

1.68 2.05 2.19 (2.33/2.23)

2.07 1.76 1.76 (1.83/1.97)

1.92 1.95 1.96 (1.98)

2.39 2.08 2.26 (2.29)

1.71 1.71 1.70 (1.73)

Values in parentheses are results obtained with the SVP+sp/TZVP basis sets.

TABLE 3: Interaction Energies (∆E, kcal/mol) of the Water Molecule Adsorbed on Four Different Goethite Clusters Using the B3LYP/SVP Approach system Fe4‚‚‚H2O (Figure 3a) low spin high spin closed shella Fe6‚‚‚H2O (Figure 3b) low spin high spin closed shella a

∆E -16.4 -20.9 -19.2 (-16.5/-18.6) -18.3 -21.7 -17.5 (-13.2/-12.5)

∆E

system Fe6‚‚‚H2O (Figure 3c) low spin high spin closed shella Fe8‚‚‚H2O (Figure 3d) low spin high spin closed shella

-21.3 -24.6 -20.1 (-16.5/-15.6) -16.8 -17.8 -15.2 (-13.1)

Values in parentheses are results obtained with the SVP+sp/TZVP basis sets.

hydrogen bonds with two µ-hydroxo groups. This relatively small difference is an artifact of the smaller Fe4 cluster. The trends in the lengths of the hydrogen bonds for the complex in Figure 3b are similar to those in the case of the cluster Fe4 in Figure 3a (see Table 2). The two stronger hydrogen bonds, HBA1 and HBD1, are only slightly longer than corresponding hydrogen bonds in the complex of cluster Fe4, while the weak hydrogen bond, HBD2, is a little shorter than the corresponding hydrogen bond in the Fe4‚‚‚H2O complex. The effects of various spin states and of diffuse functions on the variation of the hydrogen bonds are similar to those in the case of the complex of the cluster Fe4. The structural similarity with cluster Fe4 results also in similar values of the interaction energies (see Table 3). The second interaction type for the water molecule with the Fe6 cluster is presented in Figure 3c. In this complex the H2O molecule has two hydrogen bonds of proton acceptor type with two µ3-hydroxo groups of the goethite surface, HDA1 and HBA2, and only one hydrogen bond of the proton donor type, HBD1, to the one µ-hydroxo group. Two of the three hydrogen bonds are shorter and one is longer (see Table 2). The closed shell and high-spin-state calculations give the HBA2 bond as the weakest one, while in the low-spin state the weakest hydrogen bond is HBD1. Also here we observe that two hydrogen bonds are formed optimally while the third one is restricted by the rigidity of the bonding network of the goethite surface. All three hydrogen bonds for this complex obtained with the SVP+sp basis set are slightly longer than those obtained with the SVP basis set. The hydrogen bond lengths calculated with the TZVP basis are similar to the SVP+sp values.

TABLE 4: Interaction Energies (∆E, kcal/mol) of Acetic Acid, Acetate, 2,4-D, 2,4-D-, and Benzene Adsorbed on Two Goethite Clusters Using the Closed Shell B3LYP Approach and Two Basis Sets ∆E system

figure

SVP basis

SVP+sp basis

Fe4‚‚‚HAc Fe6‚‚‚HAc Fe4‚‚‚Ac- a Fe6‚‚‚AcFe4‚‚‚2,4-D Fe6‚‚‚2,4-D Fe4‚‚‚2,4-DFe6‚‚‚2,4-DFe6‚‚‚C6H6

4a 4b 4c 4d 5a 5b 5c 5d 6

-22.7 -23.7 -55.4 -58.3 -20.9 -23.9 -38.2 -37.4 -2.6 (-13.1/-15.4)b

-25.3 -25.0 -43.4 -50.6 -21.1 -25.9 -32.1 -31.3 -4.4

a Proton transfer from the goethite surface to the Ac- anion. b In parentheses, single point/optimized MP2/SVP result.

Calculated interaction energies for the three spin states (Table 3) vary similarly as in the two previous cases. The results of Table 3 show that this second complex structure of Fe6‚‚‚H2O (Figure 3c) is more stable than the first one (Figure 3b). The same trend in the interaction energies as for the two previous complexes is observed (Table 3). The optimized structure of the complex of H2O with the biggest goethite cluster, Fe8, is shown in Figure 3d. The arrangement of hydrogen bonds is similar to that of the Fe6 cluster of Figure 3c. In this case the water molecule behaves as hydrogen acceptor in two of the hydrogen bonds formed between the water oxygen atom and the hydrogen atoms of the µ3-hydroxo groups of the cluster. The calculated bond lengths are collected in Table 2. As in the


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Figure 4. Optimized structures of different complexes with acetic acid and acetate: Fe4‚‚‚HAc (a), Fe6‚‚‚HAc (b), Fe4‚‚‚Ac- (proton transfer to the Ac- anion) (c), and Fe6‚‚‚Ac- (d) (distances in Å). The stick model is used to characterize atoms and bonds of the goethite clusters involved in the interaction.

previous cases, one hydrogen bond is significantly weaker than the remaining two, namely HBA2. Variation of the hydrogen bonds in the low- and high-spin states and interaction energies are similar and comparable to the results obtained for the smaller complexes. The discussion above shows that, in all cases except one, an agreement between the three approaches is good (within 0.1 Å for the bond lengths of the strong hydrogen bonds and less than 4.5 kcal/mol for the interaction energies). In the exceptional case of Fe6‚‚‚H2O (Figure 3c) the role of strong and weak hydrogen bonds was exchanged. However, the interaction energies were in the same range as specified above. Therefore, we decided to continue with the more time-efficient closed shell approach for the remaining interaction complexes. Interaction with Acetic Acid (HAc), Acetate (Ac-), 2,4Dichlorophenoxyacetic Acid (2,4-D), and 2,4-Dichlorophenoxyacetate (2,4-D-). Interaction energies for the complexes of the iron clusters with acetic acid, acetate, 2,4-D, and 2,4-Dcalculated as a closed shell system using the B3LYP/SVP and B3LYP/SVP+sp approaches are shown in Table 4. Corresponding structures obtained at the B3LYP/SVP+sp level of the theory are collected in Figure 4. Two hydrogen bonds are formed with acetic acid in the complexes in Figure 4a,b: (i) between the oxygen atom of the µ-hydroxo group and the hydrogen atom from the carboxyl group of the acetic acid and (ii) between the

hydrogen atom of the µ3-hydroxo group and the carbonyl oxygen atom of acetic acid. The hydrogen bonds involving the µ-hydroxo groups are stronger than those involving the µ3-hydroxo groups (1.60 and 1.57 Å vs 1.77 Å) in these two cases. In addition to the two relatively strong hydrogen bonds a weak one between the carbonyl group of the acetic acid and another surface hydroxyl group (µ-hydroxo and µ3-hydroxo, respectively) can be observed. Its length of about 2.50 Å is determined by the optimal configuration of the first two strong hydrogen bonds. In the complexes of the Fe4 and Fe6 clusters with the acetate anion, three hydrogen bonds are formed (Figure 4c,d). In the case of the Fe4 cluster these hydrogen bonds are formed between the hydrogen atoms from the hydroxo, µ-hydroxo, and µ3hydroxo groups and both oxygen atoms of the acetate anion. Note that in the optimized structure of this complex the µ3hydroxo hydrogen atom is transferred to acetate, creating the strongest hydrogen bond in this complex (1.51 Å). In the Fe6‚ ‚‚acetate complex (Figure 4d) the three hydrogen bonds are formed from the interaction between the hydrogen atoms of µ-hydroxo and µ3-hydroxo groups of goethite and the oxygen atoms of the acetate. In this case no proton transfer has been observed. The strongest hydrogen bond of this complex is the one formed from a µ3-hydroxo hydrogen and the oxygen atom from acetate (1.42 Å).



Figure 5. Optimized structures of different complexes with 2,4-D and 2,4-D-: Fe4‚‚‚2,4-D (a), Fe6‚‚‚2,4-D (b), Fe4‚‚‚2,4-D- (c), and Fe6‚‚‚2,4-D(d) (distances in Å). The stick model is used to characterize atoms and bonds of the goethite clusters involved in the interaction.

The surface complexes formed with the neutral 2,4-D molecule display behavior similar to that of the complexes of acetic acid, in terms of both geometric parameters (Figure 5a,b) and the energetics (Table 4). This similarity is expected since the dominant interacting group of the organic compounds (carboxyl) is the same in both cases. The strongest hydrogen bond is formed between the OH group of the carboxyl group and the µ-hydroxo group of the clusters Fe4 and Fe6, respectively. The lengths of these hydrogen bonds in the complex models (Figure 5a,b) are very similar. The second, weaker hydrogen bond in both complexes is also similar to the corresponding hydrogen bond in the complexes of acetic acid (Figure 4a,b). The third, very weak hydrogen bond of the carbonyl oxygen of the length ∼2.5 Å is formed, too, similarly to the acetic acid. Again, its bond length is determined by the two stronger hydrogen bonds. The interaction energies of the neutral molecules of acetic acid and 2,4-D are similar and are generally larger by about 3-5 kcal/mol than in the case of the water complexes. In contrast to the complexes of the neutral molecules, the complexes formed with the 2,4-D- anion differ more from the corresponding complexes of acetate (Figure 4c,d) than is

reflected in the structural parameters (Figure 5c,d) and calculated interaction energies (Table 4). In the case of the anionic form of 2,4-D, the chlorinated aromatic ring plays a more active role in the interactions with the goethite surface hydroxyl groups than in the case of the neutral 2,4-D form. Structures c and d of Figure 5 show the formation of two strong hydrogen bonds between the carboxylate group and two surface hydroxyl groups (hydroxo and µ-hydroxo in the cluster Fe4 and µ-hydroxo and µ3-hydroxo in the cluster Fe6). In the Fe4‚‚‚2,4-D- complex no proton transfer was observed during the optimization as was the case for acetate. In both complexes the third hydrogen bond is not formed as observed for the acetate complexes. Instead, the third hydroxyl group (µ-hydroxo in Fe4 and µ3-hydroxo in Fe6) is involved in a weak interaction with the Cl atom in position 2 in the aromatic ring (see dashed lines in Figure 5c,d). The Cl‚‚‚H distances are evidently shorter than in the case of neutral 2,4-D molecules (Figure 5a,b). Note that in all four cases (Figure 5a-d) the aromatic ring has a relatively stable position with respect to the carboxyl/carboxylate group, a fact that is displayed in Figure 5 by means of a second dashed line representing an intramolecular interaction of the chlorine atom and one hydrogen atom from the )CH2 group.


Aquino et al.

Figure 6. Optimized structure of the complex formed between Fe6 and benzene (two views). The dotted line represents approximate distance (in Å) between the benzene plane and the plane of the surface oxygen atoms.

In contrast to acetate, the formation of only two hydrogen bonds for the 2,4-D- anion has significant impact on the interaction energy. Comparing the absolute values of interaction energies, in both complexes of 2,4-D- they are reduced by about 15-20 kcal/mol in comparison to the acetate complexes (Table 4) but still are much larger than the interaction energies of neutral 2,4-D. It is worthwhile to point out that as in the case of hydrogenbonded complexes of water molecules, the µ3-hydroxo group is the exclusive hydrogen bond donor in all models of HAc/ Ac- and 2,4-D/2,4-D-, respectively. The change of the basis set from SVP to SVP+sp shows a relatively small increase in the interaction energy (up to 3 kcal/mol) for neutral species, whereas for both charged systems (Ac- and 2,4-D-) a strong decrease of up to 13 kcal/mol is observed. Generally, we consider the SVP+sp results to be our most reliable ones since they should exhibit a smaller BSSE and a better description of polarization effects. Interactions with Benzene. The geometry of the benzene complexes with the goethite surface, in our case represented by cluster Fe6 (see Figure 6), is completely different from that of the polar models. In this case the dominant component of the interaction energy will be of van der Waals type. Unfortunately, DFT-based methods usually do not describe this interaction well (see ref 41 and references therein). In order to obtain an assessment of the actual accuracy of our DFT/B3LYP calculations, we performed additional RI-MP2/SVP single point calculations using B3LYP/SVP geometries and RI-MP2/SVP geometry optimizations to compare with available literature results for the benzene dimer and the water/benzene complex.41,42 In the case of the benzene dimer, DFT/BLYP calculations show a purely repulsive interaction. CCSD(T) results increase the DFT interaction by 6.2 kcal/mol. For the water/ benzene complex the DFT interaction is attractive, but too weak. MP2 calculations lead to an increase by 3.2 kcal/mol. It has been shown by Hobza et al.43 that the MP2 method overshoots the CCSD(T) interaction energy by ∼2 kcal/mol. Since the application of the CCSD(T) method is quite prohibitive for our complexes, we stayed with the computationally more efficient MP2 method keeping in mind that reductions in the interaction energies have to be applied in this case. The optimized structure using the closed shell DFT/B3LYP approach is presented in Figure 6. The benzene ring is practically parallel to the goethite surface at a distance to the surface oxygen atoms of ∼3.4 Å. The position of the ring with respect to the goethite surface changed to the distance of ∼3.2 Å in the RIMP2/SVP geometry optimization calculation. This small short-

ening of the distance between the benzene molecule and the cluster was expected considering the fact that the B3LYP approach overestimates distances in weak interactions.42 Interaction energies are given in the last row of Table 4. B3LYP values are -2.6 and -4.4 kcal/mol for SVP and SVP+sp, respectively. RI-MP2/SVP gives -13.1 kcal/mol for the single point calculation on the B3LYP geometry and -15.4 kcal/mol for the RIMP2 optimized geometry. Considering BSSE effects and the above-mentioned overshooting of the MP2 method, we estimate the true interaction energy to lie in the range of 5-9 kcal/mol. Comparing the benzene interaction with those of the polar molecules given in Table 4, we find that the former one is significantly weaker due to the different character of the interaction (dispersion forces vs Coulomb interactions between polar systems). Nevertheless, as the MP2 results demonstrate, the interaction of goethite with benzene is still considerable, indicating that this mineral is an important soil component for the sorption of the polyaromatic hydrocarbons in soils.2,3 Conclusions Our investigations showed that the (110) goethite surface formed by three types of the hydroxyl groups offers a variety of possibilities for hydrogen bond formation with appropriate polar adsorbents. As model cases, the interaction of the goethite surface with a water molecule, acetic acid, and 2,4-D (neutral and deprotonated) were chosen. In all these cases two or three surface hydroxyl groups are actively involved in the interaction. Two OH types, hydroxo and µ-hydroxo, have sufficient flexibility for bending to allow them to act as proton acceptors, while the third type, µ3-hydroxo, acts only as proton donor due to its more pronounced rigidity. Calculated interaction energies on different sites are ca. -20 kcal/mol for the water molecule, a number that is in line with the number and type of hydrogen bonds formed. Slightly larger interaction energies were observed for neutral acetic acid and 2,4-D in comparison to the goethite/ water complexes. The deprotonated, anionic form of acetic acid and 2,4-D show even stronger interactions between -30 and -50 kcal/mol. The complexes of the 2,4-D- anion differ from those of the acetate anion. Comparison of the interaction energies of different interaction sites/clusters shows that sites S1-S3 are energetically more or less equivalent for extended goethite surface models. The aromatic ring actively participates in the interaction with the goethite surface groups that results in decreasing the interaction energy in comparison to acetate. Interactions with the nonpolar, aromatic benzene molecule are much weaker. However, the estimated interaction energy in the range of -5 to -9 kcal/mol is still significant. This result

Chemical Adsorption on (110) Goethite Surface rationalizes why goethite plays an important role in the retention of polyaromatic hydrocarbons in soils. Acknowledgment. This work was supported by a HerthaFirnberg fellowship for D.T. and by the Austrian Science Fund, Project No. P17967-N11. We are grateful for technical support and computer time at the Schro¨dinger III computer system of the computer center of the University of Vienna. References and Notes (1) Hayes, K. F.; Roe, A. L.; Brown, G. E. B., Jr.; Hodgson, K. O.; Leckie, J. O.; Parks, G. A. Science 1987, 238, 783. (2) Kanel, S. R.; Neppolian, B.; Jung, H. Y; Choi, H. EnViron. Eng. Sci. 2004, 21, 741. (3) Weigand, H.; Totsche, K. U. Soil Sci. Soc. Am. J. 1998, 62, 1268. (4) Schwertmann, U.; Cornell, R. M. Iron Oxides in the Laboratory; VCH: Weinheim, Germany, 1991. (5) Cornell, R. M.; Posner, A. M.; Quirk, J. P. J. Inorg. Nucl. Chem. 1974, 36, 1937. (6) Smith, K. L.; Eggleton, R. A. Clays Clay Miner. 1983, 31, 392. (7) Schwertmann, U. Clay Miner. 1984, 19, 9. (8) Amouric, M.; Baronnet, A.; Nahon, D.; Didier, P. Clays Clay Miner. 1986, 34, 45. (9) Smith, K. L.; Milnes, A. R.; Eggleton, R. A. Clays Clay Miner. 1987, 35, 418. (10) Torrent, J.; Barron, V.; Schwertmann, U. Soil Sci. Am. J. 1990, 54, 1007. (11) Sparks, D. L.; Grundl, T. J. ACS Symposium Series; American Chemical Society: Washington, DC, 1999; No. 715. (12) Felmy, A. R.; Rustad, J. R. Geochim. Cosmochim. Acta 1998, 62, 25. (13) Rustad, J. R.; Felmy, A. R.; Hay, B. P. Geochim. Cosmochim. Acta 1996, 60, 1553. (14) Rustad, J. R.; Felmy, A. R.; Hay, B. P. Geochim. Cosmochim. Acta 1996, 60, 1563. (15) Rustad, J. R.; Felmy, A. R. Geochim. Cosmochim. Acta 2005, 69, 1405. (16) Shroll, R. M.; Straatsma, T. P. Mol. Simul. 2003, 29, 1. (17) Rakovan, J.; Becker, U.; Hochella, M. F., Jr. Am. Mineral. 1999, 84, 884.

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Surface of the Mineral Goethite - American Chemical Society

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