Uranyl Arsenate Complexes in Aqueous Solution: Insights from First

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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Uranyl Arsenate Complexes in Aqueous Solution: Insights from FirstPrinciples Molecular Dynamics Simulations Mengjia He,† Xiandong Liu,*,† Jun Cheng,‡,§ Xiancai Lu,† Chi Zhang,† and Rucheng Wang† †

State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, P. R. China ‡ State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China § Department of Chemistry, University of Aberdeen, Aberdeen AB24 3UE, United Kingdom S Supporting Information *

ABSTRACT: In this study, the structures and acidity constants (pKa’s) of uranyl arsenate complexes in solutions have been revealed by using the first principle molecular dynamics technique. The results show that uranyl and arsenate form stable complexes with the U/As ratios of 1:1 and 1:2, and the bidentate complexation between U and As is highly favored. Speciation−pH distributions are derived based on free energy and pKa calculations, which indicate that for the 1:1 species, UO2(H2AsO4)(H2O)3+ is the major species at pH < 7, while UO2(HAsO4)(H2O)30 and UO2(AsO4)(H2O)3− dominate in acid-to-alkaline and extreme alkaline pH ranges. For the 1:2 species, UO2(H2AsO4)2(H2O)0 is dominant under acid-to-neutral pH conditions, while UO2(HAsO4)(H2AsO4)(H2O)−, UO2(HAsO4)(HAsO4)(H2O)2−, and UO2(AsO4)(HAsO4)(H2O)3− become the major forms in the pH range of 7.2−10.7, 10.7−12.1, and >12.1, respectively.

1. INTRODUCTION The uranyl ion (UO22+) widely exists in aqueous solutions due to its high solubility and mobility.1−6 Besides water,7−12 it can be coordinated with a wide range of ligands to form various uranyl complexes.13−28 Because arsenic concentration in uranium ores can be as high as 10 wt %,29−32 uranium and arsenic would be released together during mining, forming uranyl-arsenate complexes in solutions at first, then coadsorbed complexes on mineral surfaces, and eventually mineral phases.33−35 Therefore, exploring the thermodynamics properties of aqueous uranyl arsenate complexes is of great importance for understanding the mobility of uranyl in nature. Although numerous studies have investigated the cosorption of uranyl-arsenate onto various mineral surfaces,36−41 only a few studies have focused on their aqueous properties.42,43 Through the time-resolved laser-induced fluorescence spectroscopy (TRLFS) measurements, it has been suggested that UO2H2AsO4+ is the major uranyl arsenate species in the pH range of 1.5−3.5.42 Their microstructures were further investigated by Gezahegne et al. (2012),43 who proposed that the bidentate-coordinated 1:2 complex (i.e., UO2(H2AsO4)20) rather than UO2H2AsO4+ should be dominant at a pH of 2 by combining static density functional theory (DFT) calculation and extended X-ray absorption fine structure (EXAFS) fitting. However, the samples for EXAFS measurements were treated as shock-frozen, which may affect the sampling of the EXAFS signal. On the other hand, UO22+ and H3−xAsO4−x are highly susceptible to pH in solutions,3,44 resulting in a number of © XXXX American Chemical Society

uranyl-arsenate complexes. To identify the structure of these species at a certain pH needs the acidity constants (pKa). But these pKa values have not been determined as yet. An alternative to these issues can be given by the firstprinciples molecular dynamics (FPMD) technique, which has been widely used to study structures and atomistic thermodynamics of aqueous metal complexes in solution.45−57 The everincreasing computing power and development of efficient computing algorithm have made it possible to model many physical and chemical processes under complex chemical environments using the rather expensive FPMD. A number of sampling methods have been also developed to compute reaction free energies. For example, the constrained MD method has been validated in various systems to calculate dissociation free energy.7,28,49,58−62 The FPMD-based vertical energy gap method has been extensively applied to estimate the acidity constants of both organic and inorganic systems covering main group and transition metal elements.63−70 In this work, we undertake a systematic study of the structures, dissociation properties, and acidity constants of 1:1 and 1:2 uranyl arsenate complexes by using FPMD calculations. We employed the constrained molecular dynamics (MD) method and the FPMD-based vertical energy gap method to derive the thermodynamic properties of uranium-arsenate species. From our computational results, we have identified Received: January 15, 2018

A

DOI: 10.1021/acs.inorgchem.8b00136 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry the predominant U−As complexes over a wide pH range. The fundamental parameters and properties of uranyl arsenate species in this work provide new insights into the fate of uranium in As-bearing geo-fluids and the deposition of uranium arsenate minerals in nature.

ΔF(Q ) = −

2.1. Systems and Computational Details. In all initial configurations, each investigated complex was placed in the center of the box and surrounded by randomly inserted water molecules (listed in Table 1). The number of water molecules was derived from

ΔdpA° =

U/As = 1:2

arsenic acid

aqueous complex

number of water molecules 61 61 59 59 59 61

Vr =

+H2O

+H2O

H4B0_1 ⎯⎯⎯⎯⎯→ H4B0_2 ⎯⎯⎯⎯⎯→ H 2A+_1 + H 2AsO−4

∑ bonds

1

d η⟨ΔdpEAH⟩rη

(4)

(5)

1 kd(d − d0)2 + 2

∑ angles

1 kα(α − α0)2 2

(6)

where d0 and α0 are the equilibrium values of the bonding and angle bending terms of the proton in the protonated state, respectively. Details of these values for the simulated compounds are tabulated in Table S1. 2.4. Finite Size Effects. Finite size effects are an important issue in FPMD simulations. It was found that the correction to the free energy can be derived by generalizing the Born cavity model with periodic boundary condition (PBC):81−83

the equation of state of water at ambient T−P conditions.71 The simulation cells were three-dimensional (3D) periodically repeated cubic boxes with a side length of 12.43 Å. The uranyl arsenate complexes investigated are shown in Figure 1. H2−xA+1−x represents 1:1 species (i.e., UO2(H2AsO4)+ and its deprotonated products), while H4−xB−x means 1:2 species (i.e., UO2(H2AsO4)2 and its deprotonated products). The U−As coordination and proton positions were categorized into three configurations. The monodentate coordination represents only one bridging oxygen atom between U and As (designated as _2), and the bidentate coordination means two bridging oxygen atoms between U and As. The complexes containing bidentate coordination were further distinguished into two modes for the bridging O, the deprotonated mode (designated as _1) and the monoprotonated mode (designated as _3). To obtain the free energy changes of full reactions, we also calculated the pKa’s of arsenic acid. All FPMD simulations were performed using the CP2K/QUICKSTEP package.72 In this package, Goedecker−Teter−Hutter (GTH) pseudopotentials were used to calculate the interactions between core and valence states, and a hybrid Gaussian and plane waves (GPW) basis sets scheme was applied for the implementation of DFT.73,74 On the basis of our previous work, a Perdew−Burke−Ernzerhof (PBE) functional was employed for the exchange-correlation.75 DZVP basis sets were used for all elements. The plane wave cutoff was set to 400 Ry. Born−Oppenheimer molecular dynamics (BOMD) simulations were performed in NVT ensemble with the Nóse−Hoover thermostat76 at 330 K. This slightly elevated temperature was used to avoid the glassy behavior at the lower temperatures of liquid water.77 The time step was 0.5 fs. The production time for each system was 7.0−20 ps after a prior equilibration MD simulation for at least 3.0 ps. 2.2. Free Energy Calculation. In this work, a series of constrained FPMD simulations were carried out to enforce the breakages of the U−As bonds for H2A+_1 and H4B0_1. The free-energy changes of two processes (1) and (2) were obtained by integrating the average force (f) along the reaction coordinate (Q, or called collective variable (CV)) according to the thermodynamic integration relation (eq 3).78−80 For both H2A+_1 and H4B0_1, the distance of U and As was set as Q.

H 2A+_1 ⎯⎯⎯⎯⎯→ H 2A+_2 ⎯⎯⎯⎯⎯→ UO2 (H 2O)52 + + H 2AsO−4

∫0

where /R and /P represent the reactant state and the product state, respectively. The restrained harmonic potential Vr keeps the dummy hydrogen in a place resembling that in the reactant state:

The water numbers include the water molecules in the first shell of uranyl.

+H2O

(3)

0

/η = (1 − η)/R + η /P + Vr

a

+H2O

f (Q ′) dQ ′

where ΔdpE is the vertical energy gap, which is defined as the potential energy difference between the reactant state and the product state. η is a coupling parameter changing from 0 to 1 (i.e., from the protonated state to the deprotonated state). The subscript r means that the averages are evaluated over the restrained mapping potential:

Table 1. Aqueous Complexes and Corresponding Numbers of Water Molecules in Boxesa UO2(H2AsO4)+ UO2(HAsO4)0 UO2(H2AsO4)20 UO2(HAsO4)(H2AsO4)− UO2(HAsO4)22− H3AsO4/H2AsO4−

Q

2.3. Acidity Constants Calculation. The pKa’s of uranium arsenate complexes were calculated by using the FPMD-based vertical energy gap method,63,64,67 in which the proton will be transformed into a dummy atom gradually with a thermodynamic integration approach:

2. COMPUTATIONAL METHODS

U/As = 1:1

∫Q

ΔAL = q2ζEW /2εL − (1 − 1/ε)2πq2R2/3L3

(7)

in which L is the length of the cubic box, q means the charge of the ion, and ζEW represents the Madelung constant, ε is the water dielectric constant, and R is the ionic radius. The water dielectric constant (ε > 80) is so high that the first term can effectively vanish. Therefore, the second term is directly related to the finite size error; that is, the error is proportional to q2 − (q − 1)2 for the hydrolysis of Mq+. The error for the charged systems for M2+/M+ (q2 − (q − 1)2 = 3) is found to be within the statistical uncertainty of FPMD simulations with a box of 9.86 Å, for instance, the pKa of Cd(H2O)62+ (10.1 vs 9.92−10.4),84 Ni(H2O)62+ (10.4 vs 9.86−10.5).85 In this work, the maximal q change is from −2 to −3 (that is, q2 − (q − 1)2 = −5). With a length of 12.43 Å (L3 is twice larger than that of 9.86 Å), the finite size error is further reduced. Therefore, a box with a side length of 12.43 Å is large enough for the pKa calculations in this study. As for the free energy calculations for the dissociations of complexes, there is no electrostatics error during the dissociation process since the system charge does not change. The direct chemical bonding that we focused on means the structures ranging from contact ion pair (CIP) to the solvent-shared ion pair (SIP). The SIP form was taken as the end of the dissociation process, since the remaining interactions after SIP are electrostatics and hydrogen bonding which are much weaker than the U−As chemical bonds. In all simulations, the second hydration shells of the cations have been well included in the cell, which ensures that the error in the free energy calculations should be minimal. One can expect that a similar free energy curve will be obtained by using a larger system.

3. RESULTS AND DISCUSSION 3.1. Free Energies of Dissociation. The free-energy profiles for the dissociation processes of 1:1 and 1:2 species are shown in Figure 2, and their forces of constraint are plotted in Figure S1.

(1) (2) B

DOI: 10.1021/acs.inorgchem.8b00136 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Candidate structures of species for U/As = 1:1 and 1:2.

Figure 2. Free-energy profiles for the dissociation processes of H2A+_1 (a) and H4B0_1 (b).

The total free-energy change for H2A+_1 dissociation process is 16.1 kcal/mol (Figure 2a). The free energy goes up from 0 to 3.9 kcal/mol with the U−As distance increasing from 3.18 to

3.5 Å. After peaking at 3.5 Å, it falls to a local minimum (2.3 kcal/mol) at the U−As distance of 3.8 Å. At this point, one of the U−OAs bonds is broken, and the solvation shell of uranyl is C

DOI: 10.1021/acs.inorgchem.8b00136 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry filled with a solvent water molecule; i.e., the structure evolves to H2A+_2. From 3.8 and 4.8 Å, the free energy dramatically increases, reaching a peak of 17.3 kcal/mol. At the distance of 5.0 Å, a solvent water molecule enters the first shell of uranyl, forming one H-bond with one oxygen of the arsenate, which thus separates the uranyl and arsenate (the third structure in Figure 2a). The minimum at 5.2 Å is taken as the end of the dissociation process, where the constrained force reverses its sign (see its force curve in Supporting Information). These results show that uranyl ions have a high preference of forming uranyl arsenate complexes in As-bearing solutions. Since H2A+_1 has a lower free energy (2.3 kcal/mol) than H2A+_2 with a shallow energy barrier (1.6 kcal/mol), H2A+_1 should be the major UO2(H2AsO4)+ species. The overall free energy change for the dissociation of H4B0_1 is 21.8 kcal/mol (Figure 2b). At the U−As distance of 3.16 Å, uranium is coordinated with one water molecule. After an increase of 6.9 kcal/mol to 3.6 Å, the free energy change remains approximately constant at around 6.7 kcal/mol up to 3.9 Å. At this point, one water molecule enters the first shell of U, and the complex transforms into H4B0_2. The free energy barrier from H4B0_2 to H4B0_1 is relatively low (compared with the thermal energy of 0.6 kcal/mol). With the continuous uplift up to 22.3 kcal/mol, H4B0_2 transforms into arsenate and H2A+_1. This large free energy difference indicates that it is very difficult to break the U−As bond of H4B0_1. At 5.2 Å, the free energy curve shows a minimum with the free energy change of 21.8 kcal/mol. Therefore, both H2A+_1 and H4B0_2 are less favored than H4B0_1. As shown above, during the dissociation processes, solvent water molecules spontaneously enter the first shell of uranyl. In order to examine whether these events affect the calculated free energies (uranyl ion has a strong interaction with the first shell water7,20,22), we carried out additional simulations by taking a 1:1 U−As complex (i.e., Figure 2a) as an example. In those simulations, a second CV accounting for U−water correlation was employed to investigate the process of a solvent water molecule entering the first hydration shell of the uranyl ion. The calculations show that the free energies on the second CV are minimal (∼ −1 kcal/mol) and therefore demonstrate that the 1 CV approach used (i.e., U−As distance) is reliable for the free energy calculations (for the details see the Supporting Information). 3.2. Complexes Structures. For the 1:1 species (2 upper panels in Figure 3), the U−As distance of the monodentate complexes (around 3.8 Å) is obviously longer than those of the bidentate complexes (3.2−3.3 Å). Among these bidentate complexes, the U−As distances of H2A+_1 and HA0_1 (3.2 Å) are slightly shorter than those of H2A+_3 (3.3 Å) due to the presence of the proton on the bridging O. Similar to the 1:1 species, for the 1:2 species (3 lower panels in Figure 3), the average U−As distance (3.8 Å) in monodentate complexes is much longer than those in the bidentate ones, and the U−As distance in the deprotonated states (left three lower panels in Figure 3) is slightly shorter than that of H4B0_3 (3.3 Å). These observations show good consistency with previous static DFT calculations with continuum solvation models by Gezahegne et al. (2012).43 3.3. Acidity Constants. Table 2 and Table 3 list the pKa values of 1:1 and 1:2 aqua-ion complexes at the ambient conditions, respectively. Our previous pKa calculation of uranyl70 agrees with the experiment and previous calculations within 0.5 pKa units (5.8 vs 5.58−6.3 at 300 K and 4.0 vs 4.19−

Figure 3. Hydration structures of uranyl arsenate complexes in water. For clarity, some solvent waters have been removed. The dotted lines represent H-bonds. Color key: O = red, H = white, U = cyan, As = yellow.

4.24 at 373 K).62,86,87 The successive pKa’s of H3AsO4 (1.1 and 5.0, see details listed in Table S2) are consistent with the experimental data of 2.19 and 7.94.88 These comparisons validate our method for predicting the thermodynamics of U− As complexes. There are two kinds of proton-donating ligands for H2A+_2, that is, the OH of arsenate and the H2O of the first solvation shell of uranyl, with the pKa of 8.6 and 12.6, respectively. This indicates that the OH ligand of arsenate is easier to get deprotonated, and therefore it should be the first proton donor of H2A+_2. Note that the pKa’s of the H2O ligand for H2A+_2 (12.6) and HA0_2 (11.8) are relatively high, indicating that the proton from water ligand can hardly get deprotonated in the common pH range (4.0−9.0). Comparing the low pKa value for H2A+_3 (0.5) with that of H2A+_1 (5.0) and considering the same deprotonation products for H2A+_1 and H2A+_3, we rule out the structure of H2A+_3 and confirm that H2A+_1 is the stable one of the two complexes. Therefore, we only calculate the second pKa values for H2A+_1 and H2A+_2 (i.e., the pKa values of HA0_1 and HA0_2). The high pKa value of the H2O ligand of uranyl ion in H2B2−_1 (12.4) indicates that it is also D

DOI: 10.1021/acs.inorgchem.8b00136 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Calculated Energy Gap (eV), Integral (eV), and pKa Values of 1:1 Speciesa η=1

acid H2A+_1(HdAs) H2A+_2(HdAs) H2A+_2(HdW) H2A+_3(HdAs) HA0_1(HdAs) HA0_2(HdAs) HA0_2(HdW) a

17.87 18.11 18.39 17.53 18.21 18.13 18.36

η = 0.5

+ 0.01 ± 0.03 ± 0.03 ± 0.01 ± 0.01 ± 0.02 ± 0.01

16.03 16.28 16.82 15.80 16.47 16.26 16.65

η=0

+ 0.04 ± 0.05 ± 0.04 ± 0.01 ± 0.03 ± 0.03 ± 0.07

13.11 13.13 12.33 12.67 13.74 13.16 12.80

+ 0.02 ± 0.07 ± 0.3 ± 0.11 ± 0.21 ± 0.14 ± 0.09

integral 15.85 16.06 16.33 15.57 16.30 16.06 16.29

pKa value 5.0 ± 0.5 8.6 ± 0.8 12.6 ± 1.4 0.5 ± 0.3 12.9 ± 1.9 8.8 ± 0.8 11.8 ± 1.1

+ 0.03 ± 0.05 ± 0.08 ± 0.02 ± 0.11 ± 0.05 ± 0.06

The pKa values are given with the correction for the entropic contribution for equivalent protons.

Table 3. Calculated Energy Gap (eV), Integral (eV), and pKa Values of 1:2 Speciesa η=1

acid H4B0_1(HdAs) H4B0_2(HdAs) H4B0_3(HdAs) H3B−_1(HdAs) H3B−_2(HdAs) H2B2−_1(HdAs) H2B2−_1(HdW) a

18.19 18.16 17.59 18.13 18.21 18.05 18.39

± ± ± ± ± ± ±

η = 0.5

0.05 0.05 0.01 0.03 0.06 0.07 0.01

16.13 16.36 16.01 16.42 16.44 16.47 16.63

± ± ± ± ± ± ±

η=0

0.07 0.06 0.02 0.08 0.02 0.09 0.06

13.34 13.40 13.55 13.32 13.2 13.68 12.83

± ± ± ± ± ± ±

0.01 0.02 0.02 0.01 0.04 0.04 0.23

integral 16.01 16.16 15.86 16.19 16.2 16.27 16.29

± ± ± ± ± ± ±

pKa value

0.06 0.05 0.02 0.06 0.03 0.08 0.08

7.4 10.3 5.2 10.7 10.9 12.1 12.4

± ± ± ± ± ± ±

1.0 0.9 0.3 1.0 0.5 1.4 1.4

The pKa values are given with the correction for the entropic contribution for equivalent protons.

H3B−_1 + H+ → H4B0_1

hard to deprotonate the water ligand in the 1:2 species (Table 3). Therefore, similar to the 1:1 species, we only consider the deprotonation of the arsenate. On the basis of the higher pKa value of H4B0_1 (7.4) and the same deprotonation products for H4B0_3 and H4B0_1, one can deduce that H4B0_1 should be the dominant species. 3.4. Stability of U−As Complexes. To reveal the stability of uranium arsenate complexes, we calculated the free energies of full reactions (see below) according to the Hess law, which states that the free-energy change in a chemical reaction is independent of pathways. For example, combining reactions 8, 9, and 11 and canceling out the intermediate on both sides, we obtain the full reaction 11. Integrating the free energy changes in reactions 8, 9, and 11, we can obtain the change of free energy in reaction 11, i.e., ΔGd = ΔGa + ΔGb + ΔGc: +

H 2A _1 + 2H 2O →

UO2 (H 2O)52 +

+

H 2AsO−4 → HAsO24 − + H+

H3B−_1 + 2H 2O → HA0_1 + H 2AsO−4

(15)

H3B−_1 + 2H 2O → HA0_1 + H 2AsO−4

ΔGf = 7.5

ΔGi = −16.2

(17)

H 2AsO−4 → HAsO24 − + H+

ΔGc = 10.5

(18)

ΔGj = 12.4 (19)

(8)

H 2A+_1 + H 2O → H 2A+_2

(9)

HA0_1 + H+ → H 2A+_1

ΔG b = −7.5

(21)

(10)

H 2A+_2 → HA0_2 + H+

ΔGn = 13.0

(22)

HA0_1 + H 2O → HA0_2

ΔGo = 7.8

(23)

ΔG l = 2.3

(20)

ΔGr = ΔGp + ΔGg + ΔGq: H4B0_1 + H 2O → H4B0_2

in which ΔGa denotes the free energy change of H2A+_1 dissociation process, ΔGb can be derived from our pKa values of the corresponding complexes according to ΔG = pKakBT ln 10, and ΔGc can be calculated with the pKa’s of H3AsO4 (see details in Table S2). Similarly, we calculate the related free energies from the data of our known reactions, that is, ΔGh = ΔGe + ΔGf + ΔGg:

H 2A+_1 → HA0_1 + H+

H 2B2 −_1 + H+ → H3B−_1

H 2B2 −_1 + 2H 2O → HA0_1 + HAsO24 −

(11)

H4B0_1 + 2H 2O → H 2A+_1 + H 2AsO−4

ΔG h = 18.1 (16)

HA0_1 + 2H 2O → UO2 (H 2O)52 + + HAsO24 − ΔGd = 19.1

ΔG h = 18.1

ΔG0 = ΔGl + ΔGb + ΔGn:

ΔG b = −7.5 ΔGc = 10.5

(14)

ΔGj = ΔGh + ΔGi + ΔGc:

H 2AsO−4

ΔGa = 16.1

HA0_1 + H+ → H 2A+_1

ΔGg = −11.2

ΔGp = 6.6

(24)

H3B−_1 + H+ → H4B0_1

ΔGg = − 11.2

(25)

H4B0_2 → H3B−_2 + H+

ΔGq = 15.5

(26)

−

−

H3B _1 + H 2O → H3B _2

ΔGr = 10.9

(27)

ΔGu = ΔGr + ΔGs + ΔGt:

ΔGe = 21.8 (12)

H3B−_1 + H 2O → H3B−_2

ΔGr = 10.9

(28)

(13)

H 2B2 −_1 + H+ → H3B−_1

ΔGs = −16.2

(29)

E

DOI: 10.1021/acs.inorgchem.8b00136 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry H3B−_2 → H 2B2 −_2 + H+

ΔGt = 16.5

H 2B2 −_1 + H 2O → H 2B2 −_2

species in the fully protonated state is predominant at pH = 2, agreeing well with the results of Gezahegne et al. (2012). However, we find that UO2(H2AsO4)2(H2O)0 (the bidentatecoordinated complex in deprotonated mode of the bridging oxygen) is thermodynamically favorable at pH = 2 by using allatom MD, whereas Gezahegne et al. (2012) suggested the major component be the monoprotonated mode based on the EXAFS fitting and static DFT simulations. This discrepancy can be attributed to the shock-frozen liquid samples used for EXAFS in their study. Complexation at mineral−water interfaces can dramatically influence the transport and fixation of U and As on the Earth’s surface. The molecular-level speciation and structures derived here have formed a basis for understanding these processes. The pKa results show that arsenate moieties of the U−As complexes get deprotonated even in the acidic pH range, and therefore they can form inner-sphere complexes on Fe/Al (hydr)oxides. This explains why the adsorption of uranium on oxide minerals can be largely enhanced in the presence of arsenate.34,37,39,89 Moreover, the sorption model of arsenate on iron oxides is found to be a bidentate corner-sharing form.90 One can therefore deduce that U−As complexes can be adsorbed at these interfaces as this form. The H2O ligands of uranyl ions hardly deprotonate, and therefore, uranyl can hardly be adsorbed by forming direct U−O bonds with surface groups. With U−As accumulating on surfaces, uranyl-arsenate minerals would form, such as trögerite (UO2HAsO4·4H2O) and metazeunerite (Cu(UO2AsO4)2·8H2O). The underlying mechanisms for these processes are still poorly documented and thus deserve more experimental and simulation studies in the future.

(30)

ΔGu = 11.2

(31)

The free energy changes of the related reactions are listed in Table 4. The dissociation free energies (ΔGa, ΔGd, ΔGe, ΔGh, Table 4. Free Energy Changes for the Dissociation Reactions and the Reactions from the Bidentate to the Monodentate Style reactions

ΔG (kcal/mol)

H2A _1 + 2H2O → UO2(H2O)52+ + H2AsO4− HA0_1 + 2H2O → UO2(H2O)52+ + HAsO42− H4B0_1 + 2H2O → H2A+_1 + H2AsO4− H3B−_1 + 2H2O → HA0_1 + H2AsO4− H2B2−_1 + 2H2O → HA0_1 + HAsO42− H2A+_1 + H2O → H2A+_2 HA0_1 + H2O → HA0_2 H4B0_1 + H2O → H4B0_2 H3B−_1 + H2O → H3B−_2 H2B2−_1 + H2O → H2B2−_2

16.1 (ΔGa) 19.1 (ΔGd) 21.8 (ΔGe) 18.1 (ΔGh) 12.4 (ΔGj) 2.3 (ΔGl) 7.8 (ΔG0) 6.6 (ΔGp) 10.9 (ΔGr) 11.2 (ΔGu)

+

and ΔGj) are larger than 10 kcal/mol, indicating that the uranyl ion prefers forming complexes with arsenates regardless of the protonation states of arsenate. Also, the bidentate mode of U− As is thermodynamically favorable over the monodentate for both 1:1 and 1:2 species since ΔGl, ΔG0, ΔGp, ΔGr, and ΔGu are obviously higher than the thermal energy. Overall, H2A+_1 (i.e., UO2H2AsO4(H2O)3+), H4B0_1 (i.e., UO2(H2AsO4)2(H2O)), and their deprotonated products should be the major U−As complexes in aqueous solution. 3.5. Speciation Distributions of U−As Complexes. On the basis of the pKa’s for H2A+_1 and H4B0_1 (Table 2 and Table 3), the species distributions of H2A+ and H4B0 are derived (Figure 4). It is obvious that for the 1:1 species (in Figure 4a), H2A+ is predominant at pH < 5, while HA and A− become important in acid-to-alkaline (5−12.9) and extremely alkaline (>12.9) pH ranges, respectively. For the 1:2 species (in Figure 4b), the fully protonated complex (H4B0) is the overwhelming majority at pH < 6. With pH increasing, H3B−, H2B2−, and HB3− become predominant in the pH range 6− 10.7, 10.7−12.1, and pH > 12.1, respectively. Figure 4 indicates that the fully protonated species (i.e., UO2H2AsO4(H2O)3+ and UO2(H2AsO4)2(H2O)0) are predominant at pH = 2. Furthermore, the dissociation free energies suggest that the 1:2 species are more favorable than the 1:1 species. Our calculation thus indicates that the 1:2 bidentate

4. SUMMARY In summary, the structures and thermodynamic properties of the uranyl arsenate species have been systematically investigated by using FPMD simulations. The dissociation freeenergy changes have been calculated by using the constrained MD method, and the acidity constants have been derived with the FPMD-based vertical energy gap method. The free-energy results show that arsenate has a strong attraction for uranyl ion, and the complexation U−As is bidentate rather than monodentate. The pKa results show that the protons prefer binding on the distal oxygen rather than the bridging oxygen. The species distribution indicate that for the 1:1 system, UO 2 (H 2 AsO 4 )(H 2 O) 3 + exists at acidic pH, while UO2(HAsO4)(H2O)30 and UO2(AsO4)(H2O)3− are dominant in acid-to-alkaline and extreme alkaline pH ranges, respectively;

Figure 4. Aqueous species distributions of 1:1 species (a) and 1:2 species (b). Vertical lines represent the pH value of water. F

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Inorganic Chemistry for the 1:2 system, UO2(H2AsO4)2(H2O)0 is predominant at neutral pH, while UO 2 (HAsO 4 )(H 2 AsO 4 )(H 2 O) − , UO2(HAsO4)(HAsO4)(H2O)2−, and UO2(AsO4)(HAsO4)(H2O)3− are the dominating forms in the pH range of 7.2− 10.7, 10.7−12.1, and >12.1, respectively.

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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.8b00136. The parameters in the harmonic potential, the forces of constraint for U−As complexes, constrained MD simulations of a second CV accounting for U−water interaction, and the calculated vertical energy gap data and pKa’s of H3AsO4 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiandong Liu: 0000-0003-3966-3998 Jun Cheng: 0000-0001-6971-0797 Xiancai Lu: 0000-0001-8977-2661 Chi Zhang: 0000-0002-8225-3911 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the National Science Foundation of China (Nos. 41572027, 41425009), Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) under Grant No. U1501501, the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (No. 201228), Newton International Fellowship Program and the financial support from the State Key Laboratory at Nanjing University. We are grateful to the High Performance Computing Center of Nanjing University for allowing us to use the IBM Blade cluster system.

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