Adsorption of Nitrate and Bicarbonate on Fe-(Hydr)oxide - Inorganic

Article pubs.acs.org/IC

Adsorption of Nitrate and Bicarbonate on Fe-(Hydr)oxide Nancy Y. Acelas,*,† Cacier Hadad,‡ Albeiro Restrepo,‡ César Ibarguen,†,‡ and Elizabeth Flórez*,† †

Grupo de Materiales con Impacto, Mat&mpac. Facultad de Ciencias Básicas, Universidad de Medellín, Medellín, Colombia Instituto de Química, Universidad de Antioquia UdeA, Calle 70 No. 52-21, Medellín, Colombia

‡

S Supporting Information *

ABSTRACT: In this work, we used density functional theory calculations to study the resulting complexes of adsorption and of inner- and outer-sphere adsorption-like of bicarbonate and nitrate over Fe-(hydr)oxide surfaces using acidic, neutral, and basic simulated pH conditions. High-spin states that follow the 5N + 1 (N is the number of Fe atoms, each having five unpaired electrons) rule are preferred. Monodentate mononuclear (MM1) surface complexes are shown to lead to the most favorable thermodynamic adsorption for both bicarbonate and nitrate with −63.91 and −28.25 kJ/mol, respectively, under neutral conditions. Our results suggest that four types of regular and charged-assisted hydrogen bonds are involved in the adsorption process; all of them can be classified as closed-shell (long-range or ionic). The formal charges induce unusually short and strong hydrogen bonds. The ability of high multiplicity states of Fe clusters to adsorb oxyanions in solvated environments arises from orbital interactions: the 4s virtual orbitals in Fe have a large affinity for the 2p-type electron pairs of oxygens.

1. INTRODUCTION Aquatic environments and water sources are subject to evergrowing levels of stress, because heavy industrial and agricultural activities produce large amounts of pollutants, including inorganic anions, metallic ions, organic compounds, and organic material, among others. Because many of these inorganic anions (dihydrogen phosphate (H2PO4−), sulfate (SO42−), chloride (Cl−), bicarbonate (HCO3−), nitrate (NO3−), etc.) are harmful for plant, animal, and human life (specifically, low concentrations of NO3− cause two problems to human health: induction of blue baby syndrome (metahemoglobinemia) and the formation of cancer agents such as nitrosamines1) even at very low concentrations, their release into the environment is increasingly heavily regulated.2,3 As a matter of fact, the U.S. Environmental Protection Agency (U.S. EPA) has set strict limits to the concentrations of inorganic anions that can be tolerated in drinking and residual waters.4 Intensive use of fertilizers and pesticides in agriculture is the main source of anionic contamination in surface and underground water sources, leading to hypertrophication of rivers and lakes (mainly due to the high solubility of phosphate and nitrate).5 In addition to causing many environmental problems, high concentrations of inorganic anions lead, for example, to corrosion and operative problems in water treatment systems.6,7 To comply with regulations, several diverse technologies have been adopted by the residual water treatment industry: chemical precipitation, biological removal of pollutants, ionic exchange, solvent extraction, membrane separation, and electrochemical processes are among the most used.8 However, © 2017 American Chemical Society

these technologies are not without problems: low efficiency, high sensitivity to operating conditions, and high costs, among others, can be mentioned as their major drawbacks, which severely hamper meeting the standards.2 In recent years, because of their high selectivity, simplicity, and low operating costs, adsorption processes using a variety of materials have emerged as promising methods for the removal of inorganic anions from water samples.4 Hydrated iron oxide, Fe(hyd)oxide, for example, has been consistently studied as an adsorbent material, suitable for drinking and residual water purification. On the one hand, its high capacity to adsorb phosphate while being highly selective is well documented.4,9−14 On the other hand, not as much is known about the adsorption of nitrate and bicarbonate on Fe(hyd)oxide.13,14 Fe(hydr)oxide is a Lewis acid and is therefore prone to acid− base interactions and consequently to the adsorption of oxyanions in the form of Lewis bases via inner-sphere complexes.15−18 Complexation models are commonly invoked to describe the bonding between the anions and the surface of the metallic oxide.5 pH is the main regulating factor of the adsorption/desorption processes. Hydration conditions and charge in the surface of the oxide are also important.16 These effects have been studied using both experimental and theoretical methods.4,5,17−21 Monodentate, bidentate, and bridge modes were found for the adsorption of nitrate over aluminum oxide particles in water-free environments.21 When water is present, inner-sphere Received: March 1, 2017 Published: April 20, 2017 5455

DOI: 10.1021/acs.inorgchem.7b00513 Inorg. Chem. 2017, 56, 5455−5464

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Inorganic Chemistry

ASCEC algorithm, a stochastic-like search strategy. These candidate structures were subsequently optimized using traditional gradient-based minimization procedures. ASCEC (after its Spanish acronym Annealing Simulado Con Energı ́a Cuántica)24 is a simulated annealing optimization procedure, which uses a modification of the Metropolis acceptance test.25 Only the potential-energy surfaces for the solvated anions were stochastically searched. This sequence of procedures has been successfully applied to the study of wide variety of systems, including water clusters (H2 O) n , n = 4−7.26−29 The optimization of every complex, under different pH conditions, was performed at the DFT level of theory using the PBE0 functional and the DEF2TZVP basis set on O, H, N, and C atoms. The LANL2DZ relativistic electron core potential (RECP) was used for Fe. Harmonic vibrational frequencies for the optimized structures were calculated at the same level of theory to verify that they correspond to true minima in the respective potential energy surface (i.e., no imaginary frequencies). Frequencies were scaled by a factor of 0.961430 to correct for systematic errors. Gibbs free energies of anions adsorption were estimated for inner-sphere monodentate (MM) and bidentate (BB) bridging geometries and for outersphere H-bonded geometry. The Gibbs free energies were calculated as G = H − TS at 298.15 K and 1.0 atm. To account for the effect of hydrogen bonds in the explicit solvation of the various types of complexes, a total of six water molecules were added during the optimization process for each case. We have shown before18 that long−range solvent effects heavily influence adsorption free energies. To account for long-range implicit hydration, single-point energy calculations were performed on each optimized geometry in the integral equation formalism polarized continuum model (IEFPCM). IEFPCM calculations provided an estimate of the total free energy in solution (including non-electrostatic terms). The dielectric constant of bulk water (ε = 78.4) was used. Thus, both shortrange explicit hydration (six water molecules around each cluster) and long-range implicit hydration (IEFPCM calculation) were considered to account for solvation. All calculations were performed with the Gaussian 09 program.31 Topological properties of electron densities within the QTAIM framework were calculated using the AIM Studio package.32 It has been shown that, for clusters containing Fe3+ centers, stability is favored in high-spin states having five unpaired electrons per Fe ion; thus, 5N + 1 multiplicities are energetically preferred in systems containing N Fe atoms.33,34 To test this result, we calculated relative stabilities for all possible spin states up to 13 for the [Fe2(OH)4(H2O)6· (H2O)6]2+ cluster under acidic conditions. The results plotted in Figure 1 indeed show that the lowest-energy structure is the one corresponding to a total spin multiplicity of 11; in addition, negligible changes in geometries were observed. Thus, in what follows in this paper, we focus our discussion to this particular state.

(chemical bonding) and outer-sphere (electrostatic interactions) complexes are formed because of the displacement of nitrate ions by water molecules. Rahnemaie et al.5 concluded that adsorption of carbonate in goethite is dominated by bidentate inner-sphere complexes characterized via IR. Also, Beheshtian et al.7 found that adsorption of nitrate over singlewall nanotubes is thermodynamically favored by 1.30 eV in the gas phase and that it is endothermic when solvent is present. Experimental studies suggest that NO 3 − and HCO 3 − adsorption over Fe-(hydr)oxide happens via inner- and outersphere complexes.4,17,19,22 However, this topic is still debated, because the effect of pH over the thermodynamically favored adsorption mode is not sufficiently clear. IR and extended X-ray absorption fine structure (EXAFS) techniques do not provide enough information leading to thorough characterization of adsorption complexes; therefore, computational methods are a viable alternative to study adsorption problems.18,21,23 Nonetheless, the available literature of computational characterization of adsorption of anions over different surfaces is quite limited,7,16,18,20 and several fundamental aspects of the problem remain unexplored. The goals of this work are (1) to help in the understanding of the factors driving thermodynamic preferences for the adsorption of nitrate and bicarbonate over Fe-(hydr)oxide surfaces, (2) to understand the modes of adsorption in inner- and outer-sphere complexes as a function of pH, and (3) to characterize the nature of the intermolecular interactions involved in the adsorption complexes. To accomplish these goals, we use a variety of computational models such as density functional theory (DFT) with extended basis sets to calculate structures and energies, as well as the quantum theory of atoms in molecules (QTAIM) and natural bond orbitals (NBO) analysis to elucidate the nature of intermolecular interactions. We emphasize that this study is concerned only with the ability of Fe-(hydr)oxide surfaces to capture nitrate and bicarbonate when they are already present in water; many other relevant aspects of the problem of removing pollutants from water sources such as the origins of the contaminants and the dynamics of proton transfer are outside the scope of this work. According to established procedures (see below), we simulated pH conditions by changing the charge in the Fe-(hydr)oxide cluster.

2. COMPUTATIONAL DETAILS AND MODELS Following established models,16,18 iron oxide was simulated using a cluster comprised of two iron atoms in octahedral coordination with 10 oxygen atoms. Gibbs free energies for the adsorption of nitrate and bicarbonate over Fe-(hydr)oxide were estimated by using a series of stoichiometric relationships (see Supporting Information). To simulate various pH conditions we adjusted the OH−/H2O ratios in the molecular clusters as follows:16,18 clusters with 4/6 OH−-to-H2O ratios have a net +2 charge corresponding to acidic conditions, 1/1 ratios have net +1 charges corresponding to neutral conditions, and basic conditions correspond to 6/4 ratios and a net 0 charge. In this work, we focus on HCO3− and NO3−, because they are among the most abundant anions found at the pH conditions of residual waters. They were microsolvated with six water molecules to simulate their molecular properties in aqueous phase. To generate the most probable geometries of the relevant species involved in the adsorption processes, candidate structures for [NO 3 (H 2 O) 6 ] − , [HCO 3 (H 2 O) 6 ] − , [OH(H 2 O) 7 ] − , [(OH) 2 (H 2 O) 6 ] 2− , [OH(H 2 O) 6 ] − , (H 2 O) 8 , (H2O)7, and (H2O)6 were obtained by the use of the

3. RESULTS AND DISCUSSION 3.1. Structures and Vibrational Frequencies. All geometrical parameters in our calculations for the iron oxide cluster are in excellent agreement with the experimental values reported for goethite,35 including octahedral internal angles and Fe−O distances in the 1.932.0 Å range (Figure 2). On the one hand, the process of nitrate and bicarbonate adsorption over the Fe-(hydr)oxide cluster as inner-sphere complexes (BB and MM) is likely to occur via a ligand 5456


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conditions and to the coordination of the anions over the oxide. On the one hand, since analyzed samples usually contain mixtures of several adsorption modes, an IR spectrum rarely leads to unequivocal assignment of signals to individual structures.39 Computations, on the other hand, allow clear matching of specific bands to individual structures; thus, we calculated harmonic vibrational frequencies for all three modes of adsorption: mononuclear monodentate (MM1 and MM2), binuclear bidentate (BB), and H-bonded, depicted in Figure 3. Table 1 lists vibrational frequencies for free and adsorbed nitrate at different pH conditions. Free nitrate with D3h symmetry40 exhibits an asymmetric stretch band (ν3) at 1446.3 cm−1. Because of the loss of symmetry, this band splits into two (ν3,low and ν3,high) bands located at ∼1300 and 1500 cm−1 in the adsorbed MM or BB complexes of C2v symmetry. In addition, for the bare nitrate, ν1, the symmetric vibration is IR inactive, and the out-of-plane deformation band ν2 appears around 743.9 cm−1. The shift in the asymmetric stretch frequency (Δν3) is a useful tool to identify the coordination mode of nitrate when adsorbed over metallic oxides.21 Indeed, nitrate-adsorbed complexes exhibit IR bands in the 1475−1643, 1351−1427, and 1073−1153 cm−1 intervals, which are assigned to the ν3,high, ν3,low, and ν1, modes, respectively. A mapping of the calculated to experimental vibrational frequencies in Table 1 suggests that nitrate forms both inner- and outer-sphere complexes in the surface of the oxide in the presence of coadsorbed water; thus, both types of complexes may coexist under normal adsorption conditions. A similar conclusion was recently reported by Baltrusaitis et al.21 for the adsorption of nitrate over aluminum oxide surfaces. Table 2 lists calculated vibrational frequencies for free bicarbonate and for the three types of adsorption complexes under changing pH conditions. Strong bands for bare HCO3− appear in the region between 999.4 and 1509 cm−1 (C−O stretching). For BB, MM1, MM2, and H-bonded complexes, the bands show up at ∼1509/1049, ∼1469/1058, ∼1454/1043, and ∼1423/999 cm−1, respectively. The νC−O,asym/sym bands are of pivotal importance to understand the adsorption mode of bicarbonate over hydrated iron oxide. Bargar et al.39 compared experimental versus calculated frequencies for the adsorption of bicarbonate over goethite. They found that the experimental νC−O, asym/sym bands at ∼1530/∼1321 cm−1 match those of the inner-sphere complexes (MM and BB). For the complexes modeled here, νC−O, sym bands ∼1321 cm−1 are suggestive of the monodentate mode of adsorption (MM). On the one hand, for outer-sphere complexes, Bargar et al.39 reported experimental νC−O,asym/sym bands at ∼1469/∼1359 cm−1, which match our νC−O,sym bands

Figure 1. Relative stability of the [Fe2(OH)4(H2O)6·(H2O)6]2+ cluster under acidic conditions. All calculations using the PBE0 functional in conjunction with the DEF2TZVP basis set for O, H, N, and C atoms and the LANL2DZ relativistic electron core potential for Fe.

exchange mechanism in which an unprotonated oxygen atom belonging to either of the two anions is exchanged with an −H2O or with an −OH functional group in the surface of the hydrated iron oxide. On the other hand, adsorption of those anions as outer-sphere complexes (H-bonded) has been described as an electrostatic interaction between the oxygen atoms in the anions and the functional groups in the Fe(hydr)oxide surface, with no ligand exchange. The previous two mechanisms are very well-documented in the specialized literature.15−18 Tables 1 and 2 indicate that Fe−O(N) and Fe−O(C) distances for the inner complexes fall in the 1.95−2.29 Å range, while much larger separations (3.62−4.06 Å) are calculated for the outer-sphere complexes. These distances suggest that in innersphere complexes, Fe−O interactions should be considered as formal chemical bonds. Precise knowledge of the separation distances between adsorbed species and the Fe-(hydr)oxide cluster obtained from computational methods should be a very good way to test the accuracy of the calculations when comparing against experimental values; however, to the best of our knowledge, these experimental distances are not available for nitrate and for bicarbonate. Infrared (IR) spectroscopy is a widely used technique for structural characterization of anions adsorbed in water/oxide interfaces.38 It has the advantage of being sensitive to pH

Figure 2. DFT-calculated clusters of Fe-(hydr)oxide under acidic (charge +2), neutral (charge +1), and basic (charge 0) pH conditions. Red, yellow, and gray spheres denote O, Fe, and H atoms, respectively. The total spin multiplicity of 11 was considered for all cases (see text). 5457


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Table 1. Structural Parameters and Vibrational Frequencies (cm−1) for NO3− in the Complex Models Depicted in Figure 3 frequencies,a cm‑1

bond length (Å) complexes BB

MM1

pH conditions

O1−N

O2−N

O3−N

Fe−O1(O2)

ν3,high

ν3,low

ν1

acidic neutral basic acidic neutral

1.26 1.24 1.24 1.26 1.28 1.29 1.28 1.30 1.25 1.27 1.25

1.25 1.26 1.24 1.24 1.23 1.22 1.21 1.23 1.24 1.24 1.24

1.22 1.22 1.23 1.22 1.22 1.22 1.24 1.20 1.23 1.22 1.24

2.10(2.18) 2.29(2.17) 2.26(2.18) 2.11 2.16 2.24 2.07 3.62 3.84 4.04

1544.5 1532.0 1506.8 1514.3 1543.9 1544.8 1554.7 1642.7 1475.0 1528.8 1446.3ν3 1410 1546 1573 1594

1375.7 1389.2 1426.7 1387.1 1375.0 1368.4 1362.0 1350.8 1415.0 1363.7 743.9ν2 1246 1272 1319 1334

1137.1 1142.0 1153.3 1129.5 1110.8 1083.2 1110.4 1072.5 1133.8 1105.1 ia 1010 1020 1047 1039

MM2 H-bonded

basic acidic neutral basic

[NO3(H2O)6]− experimental

ν3,high, ν3,low, and ν1 correspond to calculated scaled frequencies (see text for further details). ν3 asymmetric stretch; ν2 out-of-plane deformation; ia: inactive; BB: bidentate binuclear; MM1: monodentate mononuclear complex bonded to one H2O surface functional group with one OH− group in the adjacent surface site; MM2: monodentate mononuclear complex bonded to one OH− surface functional group with one H2O group in the adjacent surface site. Experimental frequencies taken from the work of Baltrusaitis et al.21 Level of theory: PBE0/DEF2TZVP basis set on O, H, and N and LANL2DZ on Fe atoms. a

Table 2. Structural Parameters and Vibrational Mode Frequencies (cm−1) for HCO3− in the Complexes Models Depicted in Figure 3 frequencies,a cm‑1

bond length (Å) complexes BB

MM1

pH conditions

O1−C

O2−C

O3−C

Fe−O1(O2)

νC−O,asym

νC−O,sym

ν1

acidic neutral basic acidic neutral

1.26 1.24 1.24 1.25 1.30 1.29 1.28 1.24 1.25 1.24 1.25

1.27 1.25 1.26 1.25 1.22 1.22 1.22 1.27 1.24 1.25 1.25

1.29 1.36 1.34 1.34 1.33 1.34 1.36 1.33 1.36 1.38 1.37

1.96(1.95) 2.15(2.09) 2.08(2.04) 1.97 2.08 2.08 1.96 4.06 3.81 3.48

1509.1 1405.8 1438.2 1419.6 1468.5 1426.5 1454.3 1422.8 1412.2 1387.0 1404.7 1469 1530 1512

1433.6 1249.3 1376.4 1328.4 1308.9 1320.4 1317.7 1385.8 1384.8 1358.9 1241.1 1321 1319 1359 1392

1129.3 1049.2 1087.6 1065.3 1057.9 1056.2 1042.7 1072.0 1037.1 999.4 984.0 1062

MM2 H-bonded

[HCO3(H2O)6]− experimental

basic acidic neutral basic

a νC−O,sym, νC−O,asym, and ν1 correspond to calculated scaled frequencies (see text for further details). BB: bidentate binuclear; MM1: monodentate mononuclear complex bonded to one H2O surface functional group with one OH− group; MM2: monodentate mononuclear complex bonded to one OH− surface functional group with one H2O group in the adjacent surface site. Experimental frequencies taken from Bargar et al.36 and Chernyshova et al.37 Level of theory: PBE0/DEF2TZVP basis set on O, H, and C and LANL2DZ on Fe atoms.

for the H-bonded complexes located at ∼1422.8 and ∼1385 cm−1. On the other hand, Wijnja and Shulthess38 attributed νC−O,asym bands at 1462 and 1510 cm−1 to only mononuclear monodentate complexes when studying adsorption of bicarbonate on goethite. However, the presence of two νC−O,asym peaks implies at least two types of complexes. Our results strongly suggest that these two peaks correspond to BB and to MM1 complexes. Our calculated frequencies match the experimental observations in ranges within 20 to 50 cm−1, which is enough to tell apart all adsorption modes of bicarbonate over Fe(III) oxides. 3.2. Gibbs Free Energies of Adsorption. Accurate determination of Gibbs free energies of adsorption are

important to identify the thermodynamically favored adsorption modes under experimental conditions and to establish the affinities of the several species toward adsorption sites in the hydrated iron oxide. Gibbs free energies for each complex were calculated by subtracting the total energy of the reactant species from the total energy of the product species in aqueous phases. HCO3− and NO3− are species present in the experimental pH range from 5.0 to 7.0. Because of the computational cost of quantum mechanics calculations, currently, it is not possible to strictly simulate pH conditions by explicitly adding all molecular species to a macroscopic sample of the solution to be studied. Therefore, in this work, we accounted for the pHdependent surface charge by changing the number of H+ ions in 5458


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Figure 3. DFT calculated structures of inner-sphere and H-bond adsorption products of bicarbonate and nitrate on Fe-(hydr)oxide under neutral pH conditions. Red, yellow, blue, and gray spheres denote O, Fe, N, C, and H atoms, respectively. MM1: monodentate mononuclear complex bonded to one H2O surface functional group with one OH group in the adjacent surface site, and MM2: monodentate mononuclear complex bonded to one OH surface functional group with one H2O group in the adjacent surface site.

Figure 4. Gibbs free energies of bicarbonate (left) and nitrate (right) adsorption on various protonated Fe-(hydr)oxide surfaces at different pH conditions. MM1 and MM2 correspond to the MM complex, where the anions are bonded to a H2O or OH− surface functional group, respectively.

bicarbonate being the most thermodynamically favored of all processes (−63.91 kJ/mol). This result suggests an enhanced ability of water molecules to be replaced by HCO3− under neutral conditions when compared to −OH−. Under basic conditions, all absorptions are comparatively weaker. If the adsorption occurs in an MM fashion, only one active adsorption site is occupied by the incoming anion, whereas if adsorption happens in a BB mode, two active sites are occupied; thus, the ability of the Fe-(hydr)oxide surface to remove foreign anions is strongly favored by absorption in the MM mode. Consequently, neutral conditions should be ideal if large amounts of HCO3− or NO3− need to be removed from wastewaters under treatment. In this work, we argue that the low adsorption capability of Fe-(hydr)oxide toward bicarbonate or nitrate anions is related to the low thermodynamic favorability and not because these anions are not able to form inner-sphere complexes (MM and BB), as has been suggested in many reports.17 Speciation diagrams for these

the model cluster, as has been suggested in previous works.16,18,41 On the basis of the postulated reaction mechanisms (inner,and outer-spheres complexation), a series of stoichiometrically balanced equations (Table S1) were calculated to estimate the Gibbs free energies for adsorption of all anions on Fe(hydr)oxide clusters under different pH conditions. Three adsorption reactions were modeled for bidentate bridging (BB) anion, (bicarbonate and nitrate) on Fe-(hydr)oxide, considering acidic, neutral, and basic conditions, results are shown in Figure 4. Generally speaking, H-bonded complexes are either thermodynamically not favored or very weakly adsorbed regardless of the identity of the anion. Under acidic conditions adsorption of bicarbonate on the surface is strong on the BB mode (−60.89 kJ/mol), while the MM1 mode exhibits about half of the adsorption strength. For adsorption of nitrate, both modes are thermodynamically favored but with even smaller adsorption energies. Neutral conditions favor the MM1 mode for the adsorption of both anions, with adsorption of 5459


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Figure 5. Radial distributions (Å) for four types of hydrogen bonds. Bicarbonate (left) and nitrate (right) adsorptions are shown. Data taken from the PBE0 optimized geometries in states with spin multiplicities of 11. All pH conditions are included. HOHc refers to water molecules belonging to the molecular structure of the complex.

Figure 6. Topological analysis of electron densities. Logarithmic relationship for the [rO···H, ρ(rc)] pairs for all intermolecular interactions in the complexes. Data taken from the PBE0 optimized geometries in states with spin multiplicities of 11. All pH conditions are included. HOHc refers to water molecules belonging to the molecular structure of the complex. All R2 coefficients are larger than 0.99.

anions are very well-known;42 these clearly show that at neutral conditions the dominant species is HCO3−. 3.3. Analysis of Intermolecular Interactions. Hydrogen bonds (HBs) within complexes influence both structural and energetical preferences; thus, we analyze the nature of those interactions within the QTAIM43 and NBO44 frameworks. Four different types of HBs are detected after topological analysis of the electron distributions: (i) anion to solvation waters (HCO−3 , NO−3 )···HOH, (ii) anion to water molecules belonging to the structure of the complex (HCO−3 , NO−3 )··· HOHc, (iii) waters of solvation to waters of the complex, H2O···HOHc, (iv) water to water in different solvation shells, H2O···HOH. Figure 5 shows the distance distributions for

these types of HBs. Some important observations can be drawn: A variety of HB interactions are distributed in a range of ∼0.8 Å for both systems at different pH conditions. The 0.8 Å range of interactions for HBs leads to a wide structural variety concerning mainly the positions of solvation waters. In general terms, the radial distributions for both types of complexes exhibit similar shapes. Obviously, because there is only one anion molecule, and because the water molecules in the complex have little motion freedom, the least-frequent HBs are between ions and waters that belong to the complex (type ii). Interaction distances between ions and solvation water molecules (type i) for both anions have broad peaks in the 5460


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Figure 7. Energy quantities calculated at all BCPs for intermolecular interaction in the complexes. Data taken from the PBE0 optimized geometries in states with spin multiplicities of 11. All pH conditions are included. HOHc refers to water molecules belonging to the molecular structure of the complex.

Figure 8. Quantification of covalency of intermolecular interaction using Espinosa’s criterion.51

1.7−2.0 Å range; this result suggests that the effect of the formal charge is to strengthen the HBs leading to shorter distances of interaction. In addition, H2O··· HOHc distributions show that in the case of bicarbonate complex most contacts occur at ∼1.6−1.7 Å; meanwhile, in the case of nitrate complex, they are well-centered around 1.7 Å. Both observations are in marked contrast with the distributions for HBs in pure water clusters, where a well-defined peak centered around 1.9 Å is observed.26−28 One particularly sensitive and important issue in the study of molecular clusters is to shed light into the nature of the stabilizing intermolecular interactions. QTAIM is particularly well-suited to this end. Among the many QTAIM descriptors, we chose a few that have proven very useful to understand the intricacies of molecular interactions: (a) There are exponential relationships between the distances of intermolecular interactions and the electron densities at bond critical points (BCPs; Figure 6). These relationships hold regardless of the simulated pH and the identity of the ion; furthermore, all [rO···H, ρ(rc)] pairs obey similar logarithmic correlations as those obtained for pure water clusters and for other hydrogen bonded systems.26,45 Therefore, save the Fe···O contacts, all molecular interactions governing the microsolvation of nitrate and bicarbonate ions should be considered as hydrogen bonds. Figure 6 also clearly differentiates Fe··· O contacts from hydrogen bonds in that, as expected, they also follow exponential [rFe···O, ρ(rc)] relationships

with different trend lines. These results are in excellent agreement with earlier works that suggest that [rX···Y, ρ(rc)] correlations are expected for all types of binary interactions, regardless of the origin and magnitudes of individual contributions from molecular orbitals to the given contact.27,45,46 Also notice that these exponential relationships are useful in characterizing the relative strength of the interactions,47 as it is clearly seen that smaller concentrations of electron densities in the interaction regions lead to weaker hydrogen bonds. (b) The Laplacian of the electron density, ∇2ρ(rc), measures local concentration or depletion of charge.48 Thus, positive Laplacians at BCPs characterize local minima in the electron density, which leads to concentration of electron charge away from the critical point, toward the nuclei, indicating closed-shell (long-range, ionic, etc.) interactions, while negative Laplacians characterize local maxima in the electron density in the region intermediate between the interacting nuclei, indicating covalent interactions. It is known that in some cases, the signs of ∇ 2 ρ(r c ) and H(r c ) at BCPs give conflicting information; thus, alternative schemes combining the two criteria have been developed. In particular, Rozas and co-workers 49 suggested that weak-to-medium strength hydrogen bonds should be characterized by ∇2ρ(rc) > 0 and H(rc) > 0 simultaneously, while strong HBs should exhibit ∇2ρ(rc) > 0 and H(rc) < 0, and very strong HBs are those having ∇2ρ(rc) < 0 and H(rc) < 0. 5461


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2p-type electron pairs, while d orbitals experience negligible changes.

Accordingly, Rojas-Valencia and co-workers suggested to plot the two quantities against each other50 as shown in Figure 7; from there, it is seen that (i) all intermolecular interactions, including Fe−O bonds, have positive Laplacians. Thus, according to this criterion, they should be considered as being of closed-shell nature (long-range or ionic), (ii) there is a large number of strong, intermediate strength and weak HBs, which holds for all types of intermolecular contacts, and (iii) smaller Laplacians are associated with positive total energy densities. (c) The previous criteria are mostly qualitative. By local application of the virial theorem to BCPs, Espinosa and co-workers51 developed a model to quantify the degree of covalency of a chemical interaction that can be readily applied to hydrogen bonds. This criterion states that the |V(rc)|/G(rc) ratio defines closed (long-range, ionic, etc.) interactions in the [0,1] interval and intermediate (contributions from covalent and closed shell) interactions fall in the [1,2] interval, while covalent interactions are those for which |V(rc)|/G(rc) > 2. Figure 8 plots Espinosa’s ratio. These plots are consistent with the information drawn from Figure 7 in the sense that they show a number of closed-shell (long-range, ionic) interactions and a number of interactions in the intermediate range that are independent of the type of contact and of the identity of the ion; these plots also show unusually strong HBs, with |V(rc)|/G(rc) ratios in the proximity of 1.6, we assign these unusual strengths to the action of the formal charges, as has been thoroughly analyzed in the cases of water microsolvation of other charged species.52−55 NBO analysis of orbital interactions helps understanding the preference of high-spin states and provides a comprehensive picture of bonding. Our calculations show that in all cases (cluster, cluster + anion + solvent) d orbitals on Fe atoms are quasi-degenerate and are populated by ∼1 α electron each with some β contribution. This particular configuration leads to a situation where intermolecular bonding arises mainly from the virtual 4s orbitals in Fe interacting with 2p-type lone pairs in the oxygen atoms of the anions with overall little contribution from d orbitals in Fe. In this scenario, two types of Fe−O bonds are predicted by NBO; on one hand, α electrons in lone pairs lead to 2p → 4s charge transfer interaction (∼10 to 38 kcal/mol interaction energies). On the other hand, the remaining β electrons in lone pairs produce highly polarized Fe−O bonds by donating the 0.87β electronic charge to Fe. Notice that the donation of β electrons is the same for the complex and for the cluster, because the chemical environment of the Fe atom is the same in both cases. Indeed, the electron configuration for Fe atoms in the [Fe2(OH)4(H2O)6·(H2O)6]2+ cluster is 3d4.99α3d0.87β, which becomes 3d4.98α3d0.87β in the solvated complex. Remarkably, NBO shows that the 0.87β electronic charge is distributed among all 5 d orbitals, thus retaining the overall spin multiplicity for the entire cluster by contributions from other molecular orbitals. In this view, a more explicit description of the Fe center electron config3d1α0.21β 3d1α0.17β 3d1α0.08β 3d1α0.15β . In uration is 3d0.99α0.26β xy xz yz x2−y2 z2 summary, our NBO analysis beautifully explains why high multiplicity states of Fe clusters are prone to adsorb oxyanions in solvated environments: the 4s orbitals have a large affinity for

4. CONCLUSIONS We provide exhaustive characterizations of the potential-energy surfaces for the adsorption of nitrate and bicarbonate anions on model Fe-(hydr)oxide surfaces in aqueous environments. Our results allow the assignation of vibrational frequencies and spectral features to individual structures in various modes of adsorption; this information cannot be extracted from experimental spectra and is crucial to understand structural preferences of the adsorbed anions. High-spin states that follow the 5N + 1 (N is the number of Fe atoms, each having five unpaired electrons) rule are preferred. Our results show that monodentate complexes (MM1) are preferred for both anions under neutral pH conditions, and we support this claim by the calculated adsorption energies, which in addition lead to a marked preference for bicarbonate (−63.9 kJ/mol) over nitrate (−28.2 kJ/mol) adsorption in potential binary mixtures. Four types of intermolecular interactions are involved in the adsorption process; they are well-characterized as closed-shell (long-range and ionic) regular and charge-assisted hydrogen bonds, distributed in the [1.5, 2.3] Å interval. This 0.8 Å range allows a wide variety of structural possibilities. The formal charges induce unusually short and strong hydrogen bonds, and this is clearly evidenced in the |V(rc)|/G(rc) ratios close to 2 and in the negative values of the total energy density calculated for a sizable number of intermolecular interactions. NBO calculations nicely explain why high multiplicity states of Fe clusters are prone to adsorb oxyanions in solvated environments: the 4s virtual orbitals in Fe have a large affinity for 2ptype electron pairs, while d orbitals experience negligible changes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00513. List of Cartesian coordinates for all optimized geometries and calculated ΔGads (kJ/mol) of bicarbonate and nitrate on various protonated Fe-(hydr)oxide surfaces (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (N.Y.A.) *E-mail: elfl[email protected]. (E.F.) ORCID

Albeiro Restrepo: 0000-0002-7866-7791 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Univ. de Medellı ́n, for financing the project “720”, and to Univ. of Antioquia for financial support of the “Estrategia de Sostenibilidad”. N.Y.A. thanks “COLCIENCIAS” for the Ph.D. scholarship. We also like to thank an anonymous reviewer for providing very useful suggestions and for pointing out that high multiplicity sates are very relevant for the systems considered in this work. 5462


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Inorganic Chemistry

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Adsorption of Nitrate and Bicarbonate on Fe-(Hydr)oxide - Inorganic

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