Facile and Efficient Decontamination of Thorium from Rare Earths

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

pubs.acs.org/IC

Facile and Efficient Decontamination of Thorium from Rare Earths Based on Selective Selenite Crystallization Yaxing Wang,&,†,¶ Huangjie Lu,†,¶ Xing Dai,†,¶ Tao Duan,# Xiaojing Bai,§ Yawen Cai,† Xuemiao Yin,† Lanhua Chen,† Juan Diwu,† Shiyu Du,§ Ruhong Zhou,† Zhifang Chai,† Thomas E. Albrecht-Schmitt,∥ Ning Liu,*,& and Shuao Wang*,† &

Key Laboratory of Radiation Physics and Technology, Ministry of Education, Institute of Nuclear Science and Technology, Sichuan University, Chengdu 610064, P. R. China † State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, 215123 Suzhou, P. R. China # School of National Defence Science & Technology, Southwest University of Science and Technology, Mianyang 621010, China § Engineering Laboratory of Specialty Fibers and Nuclear Energy Materials, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, P. R. China ∥ Department of Chemistry and Biochemistry, Florida State University, 95 Chieftain Way, Tallahassee, Florida 32306, United States S Supporting Information *

ABSTRACT: The coexistence of radioactive contaminants (e.g., thorium, uranium, and their daughters) in rare earth minerals introduces significant environmental, economic, and technological hurdles in modern rare earth production. Efficient, low cost, and green decontamination strategies are therefore desired to ameliorate this problem. We report here a single-step and quantitative decontamination strategy of thorium from rare earths based on a unique periodic trend in the formation of crystalline selenite compounds across the lanthanide series, where Ce(III) is fully oxidized in situ to Ce(IV). This gives rise to a crystallization system that is highly selective to trap tetravalent f-blocks while all other trivalent lanthanides completely remain in solution when coexist. These results are bolstered by first-principles calculations of lattice energies and an examination of bonding in these compounds. This system is contrasted with typical natural and synthetic systems, where trivalent and tetravalent f-block elements often cocrystallize. The separation factors after one round of crystallization were determined from binary systems of Th(IV)/La(III), Th(IV)/Eu(III), and Th(IV)/Yb(III) to reach 2.1 × 105, 1.2 × 105, and 9 × 104, respectively. Selective crystallization of thorium from a simulated monazite composite yields a separation factor of 1.9 × 103 with nearly quantitative removal of thorium.

■

INTRODUCTION

Two types of energy- and chemical-intensive industrial processes are currently implemented to separate thorium from REEs. The first is biphasic liquid−liquid extraction-based purification. Here amino-bearing extractants that have preferential affinity for thorium over REEs are utilized.6,7 The largest concern associated with this strategy is the large amount of organic liquid waste generated that is also contaminated with uranium and thorium. The disposition of this waste remains an ongoing problem. The second method is ion-exchange chromatography that implements high separation factors, but holding times on the columns often prevent large-scale, continuous operation, and the resins are subject to regeneration and degradation. In light of these issues, many other approaches have been studied to optimize rare earth mining,8,9 separations,10−14 and cycle utilization,15−17 in order to mitigate the environmental impact.8,9,13 These methods include electro-

The demand for rare earth elements (REEs) is still rapidly increasing owing to their irreplacable roles in green and sustainable products in energy, military, and manufacturing. The supply chain has been a source of global controversy in recent years.1−3 One of the major issues in rare earth production is the coexistence of radioactive contaminants in REEs-bearing ores that creates significant difficulties in mining and waste handling. The principal deleterious contaminant is thorium, which imparts unwanted radioactivity to the ores ranging from 5.7 to 3224 pCi/g.4 China, with more than 90% of globle REEs supply, still has to mine radioactive REEs-bearing ores especially monazite, which typically contains considerable amounts of thorium at levels up to 20% by weight.5 This is particularly concerning because monazite has been a predominate source of REEs, and therefore, searching for efficient, low cost, and green chemical process for decontamination is highly desirable. © XXXX American Chemical Society

Received: November 2, 2017

A

DOI: 10.1021/acs.inorgchem.7b02681 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Result Summary of Th/Ln(La, Eu, Yb) Separation Experimenta SeO2

element

0.2 mmol

Th La Th La Th La Th La Th La

0.4 mmol 0.6 mmol 0.8 mmol 1.0 mmol

SeO2

element

0.2 mmol

Th Eu Th Eu Th Eu Th Eu Th Eu


SeO2

element

0.2 mmol

Th Yb Th Yb Th Yb Th Yb Th Yb


molar mass in reactants/ mmol

molar mass in products/mmol

0.1 ± 0.01 0.1 ± 0.01 0.1 ± 0.01 0.1 ± 0.01 0.1 ± 0.01 0.1 ± 0.01 0.1 ± 0.01 0.1 ± 0.01 0.1 ± 0.01 0.1 ± 0.01 molar mass in reactants/ mmol

0.0983 0.0012 0.0954 0.0102 0.0952 0.0172 0.0970 0.0182 0.0968 0.0166

0.1 ± 0.01 0.1 ± 0.01 0.1 ± 0.01 0.1 ± 0.01 0.1 ± 0.01 0.1 ± 0.01 0.1 ± 0.01 0.1 ± 0.01 0.1 ± 0.01 0.1 ± 0.01 molar mass in reactants/mmol

0.0931 ± 0.0082 0.0011 ± 0.0002 0.0946 ± 0.0035 0.0160 ± 0.0012 0.0959 ± 0.0002 0.0220 ± 0.0016 0.0947 ± 0.0030 0.0271 ± 0.0023 0.0954 ± 0.0015 0.0312 ± 0.0020 molar mass in products/mmol

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

± ± ± ± ± ± ± ± ± ±

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

± ± ± ± ± ± ± ± ± ±

0.0008 0.0002 0.0014 0.0011 0.0036 0.0002 0.0022 0.0010 0.0009 0.0005

molar mass in products/mmol

0.0984 0.0019 0.0976 0.0165 0.0985 0.0209 0.0943 0.0413 0.0958 0.0558

± ± ± ± ± ± ± ± ± ±

0.0030 0.0003 0.0028 0.0017 0.0018 0.0008 0.0030 0.0104 0.0029 0.0031

molar mass in wash solutions/mmol

4.0 3.9 4.0 4.6

crystallization purity

0.0010 ± 0.0002 0.1011 ± 0.0051 × 10−6 ± 4.4 × 10−7 0.0858 ± 0.0011 × 10−6 ± 5.5 × 10−8 0.0776 ± 0.0035 × 10−6 ± 1.5 × 10−7 0.0786 ± 0.0014 × 10−6 ± 2.7 × 10−7 0.0817 ± 0.0048

0.9928 ± 0.0011

molar mass in wash solutions/mmol

4.1 3.4 3.6 3.8

5.7 1.1 1.7 4.0

1 ± 4.4 × 10−6

208141.36 ± 3773.19

0.9020 ± 0.0035

1 ± 5.5 × 10−7

108807.56 ± 5984.65

0.8991 ± 0.0040

1 ± 1.5 × 10−6

104579.69 ± 3269.18

0.9071 ± 0.0029

1 ± 2.7 × 10−6

104194.10 ± 2703.20

0.9924 ± 0.0017

crystallization yield 0.9721 ± 0.0073

separation factor 3308.65 ± 183.08

0.9004 ± 0.0068

1 ± 7.9 × 10−6

119845.57 ± 6943.41

0.8693 ± 0.0085

1 ± 1.8 × 10−6

102207.75 ± 6560.19

0.8420 ± 0.0088

1 ± 2.2 × 10−6

71130.76 ± 4122.42

0.8237 ± 0.0069

1 ± 5.8 × 10−6

55053.29 ± 3724.76


0.0017 ± 0.0002 0.1009 ± 0.0025 × 10−6 ± 6.6 × 10−7 0.0853 ± 0.0038 × 10−5 ± 1.0 × 10−6 0.0776 ± 0.0070 × 10−5 ± 7.8 × 10−6 0.0596 ± 0.0096 × 10−5 ± 4.6 × 10−6 0.0437 ± 0.0038

separation factor 8226.87 ± 313.02

0.9401 ± 0.0053


0.0029 ± 0.0007 0.1025 ± 0.0026 × 10−6 ± 7.9 × 10−7 0.0817 ± 0.0079 × 10−6 ± 1.8 × 10−7 0.0787 ± 0.0039 × 10−6 ± 2.2 × 10−7 0.0726 ± 0.0055 × 10−6 ± 5.8 × 10−7 0.0677 ± 0.0065 molar mass in wash solutions/mmol

crystallization yield 0.9897 ± 0.0018

crystallization yield

separation factor

0.9857 ± 0.0021

0.9831 ± 0.0022

3094.36 ± 127.28

0.8882 ± 0.0110

0.9999 ± 6.6 × 10−6

89324.68 ± 4204.48

0.8634 ± 0.0060

0.9999 ± 1.0 × 10−5

33482.75 ± 1296.01

0.7564 ± 0.0438

0.9998 ± 7.8 × 10−5

8553.11 ± 182.86

0.6974 ± 0.0053

0.9996 ± 4.6 × 10−5

1872.07 ± 63.10

a The separation factors were calculated using the solid/aqueous model and the molar ratio of Th:Ln:Se in the original reactions is 1:1:n, n = 2, 4, 6, 8, 10. Solid samples were treated with dilute nitric acid.

minerals and synthetic phases that originates from similar hard−soft donor preferences and coordiantion chemistry.19,20 Herein, we report on a system that is highly selective for tetravalent f-block cations that does not carry lanthanide cations. This method provides a facile and environmental friendly thorium docomtamination strategy from rare earth elements.

chemical extraction, electrochemical metallurgy, and so on. However, these strategies still exhibit notable disadvantages of low-separation efficiencies, harsh operating conditions, and/or high costs. One of the oldest Ln(III)/Th(IV) (Ln = lanthanides) separation strategies is the selective precipitation using a variety of inorganic ligands, which possesses clear advantages of low energy input, low environmental impact, and facile operation. However, the separation efficiency is greatly limited by the coprecipitation and the separation factors are often low.7 We recently introduced a new lanthanide separation strategy based on selective borate crystallization that shows advantages over traditional fractional crystallization methods. This route capitalizes on an unusual periodic trend that amplifies differences between trivalent lanthanides in product formation.18 In principle, it is more feasible to separate tetravalent thorium from trivalent lanthanides according to their differences in charge densities. The challenge for this route, however, lies in the sacrifice of separation efficiency induced by cocrystallization or aliovalent substitution between Th(IV) and Ln(III) on the same lattice site as observed in many natural

■

MATERIALS AND METHODS

Note: Caution! Th-232 used in this study is an α emitter with the daughter of radioactive Ra-228. All thorium compounds used and investigated were operated in an authorized laboratory designed for actinide element studies. Standard protections for radioactive materials should be followed. Materials. All regents were purchased from chemical regent suppliers and used without further purification. Lanthanide nitrates including La(NO3)3·6H2O (99.99%), Ce(NO3)3·6H2O (99.99%), Pr(NO3)3·6H2O (99.99%), Nd(NO3)3·6H2O (99.95%), Sm(NO3)3· 6H2O (99.99%), Eu(NO3)3·6H2O (99.99%), Gd(NO3)3·6H2O (99.99%), Tb(NO3)3·6H2O (99.99%), Dy(NO3)3·6H2O (99.99%), Ho(NO3)3·6H2O (99.99%), Er(NO3)3·6H2O (99.99%), Tm(NO3)3· 6H2O (99.99%), Yb(NO3)3·6H2O (99.99%), and Lu(NO3)3·6H2O (99.99%) were provided from Energy Chemical Reagent Co., Ltd. B


Article

Inorganic Chemistry China. Th(NO3)4·6H2O (99.99%), and SeO2 (99.99%, Alfa Aesar Co.). Synthesis of Lanthanide Selenite/Selenate. Lanthanide nitrate Ln(NO3)3·6H2O (0.1 mmol) and SeO2 (0.1 mmol) were charged into a PTEF-lined Parr 4749 autoclave and then dissolved using 2 mL of deionized water. Subsequently, the samples were sealed and heated at 230 °C for 3 days followed by slow cooling to room temperature over a period of 1 day. The crystalline products were washed by deionized water and ethanol solution and then dried at room temperature. Five different phases of lanthanide selenite and selenate were obtained across the lanthanide series under identical reaction condition, as shown by powder X-ray diffraction patterns (PXRD) from Figures S1−S5. Crystallographic information is listed in Table S1. pH values were recorded for La, Ce, Pr, Eu, and Dy reactions before and after hydrothemal treatment. The pH values of starting solution are 1.66 (La), 1.58 (Ce), 1.62 (Pr), 1.54 (Eu), and 1.61 (Dy), repectively. After hydrothemal treatment, the pH of Ce solution is 1.00, significantly lower than others (1.40 for La, 1.34 for Pr, 1.22 for Eu, and 1.14 for Dy). Binary Lanthanide Separation and Lanthanide/Actinide Separation by Selenite Crystallization. La/Ce separation: La(NO3)3·6H2O (0.1 mmol), Ce(NO3)3·6H2O (0.1 mmol), and SeO2 (0.2, 0.4, 0.6, 0.8, or 1.0 mmol, respectively) were charged into a PTEF-lined Parr 4749 autoclave and then dissolved using 2 mL of deionized water. The samples were sealed and heated at 230 °C for 3 days followed by slow cooling to room temperature over 1 day. The resulting crystalline products were separated from the mother liquid solution in the autoclave, washed extensively with deionized water, and dried at room temperature. The mother liquid solution and the wash solutions were also collected and combined in 10 mL centrifuge tubes. Additional deionized water was added reaching a constant volume for further determination of the molar quantities of La and Ce. The crystallization products were characterized by the PXRD to identify the structure type (Figures S7). In order to precisely determine the molar quantities of La and Ce in solids, the crystalline solids were further dissolved in concentrated nitric acid by heated to 180 °C in a box furnace for 2 days and cooled to room temperature, and then diluted to 5% nitric acid solution before being quantitatively analyzed using ICP-MS or ICP-OES depending on the concentration. The solution samples were also analyzed in order to determine the separation factor. The other four binary separations of Ce/Pr, Th/La, Th/Eu, and Th/Yb were under the same procedure with La/Ce binary separation, and the structure type was determined by PXRD, as shown in Figures S8 and S9. The corresponding molar quantity of lanthanide and actinide elements is shown in Table S2 and Table 1. Separation factors were calculated using the following equation

are shown in Table S3. Separation factors were calculated using the following equation

S = [(nCe + n Th)/n Ln5]c *[nLn5 /(nCe + n Th)]m Crystallization yield are calculated as

Y = 1 − [(nCe + n Th)/n0]m

(4)

where nLn5 (nLn5 is the total molar mass of all lanthanides except Ce), nTh, nCe is the molar mass of Ce and Th in solution, n0 is the initial molar mass of Ce and Th charged into the autoclave. c represents the case in the solid products, and m represents the case in the mother liquid solutions. Crystallization percentage of thorium about all above separation tests are calculated using the following equation C = 1 − (n Th /n0)m

(5)

where nTh is the molar mass of thorium in solution, n0 is the initial molar mass of thorium charged into the autoclave. m represents the mother liquid solutions. Crystallization purity of solid about all above separation tests are calculated using the following equation

P = M Th /(M Th + MLn) or P = (MCe + M Th)/(MCe + M Th + MLn)

(6)

where MCe is the mass of Ce4+ in the solid, MTh is the mass of Th4+ in solid, and MLn is the total mass of all Ln3+ in the solid. Computational Models and Method. To illustrate the in situ oxidation process, two hypothetical oxidation processes are described as follow:

Ce3 +(H 2O)8 + 1/4O2 + H3O+ → Ce 4 +(H 2O)8 + 3/2H 2O (7) 3+

Ce (H 2O)8 + → Ce

2SeO32 −

+

+ H3O + 1/4O2

4+

(SeO32 −)2 (H 2O)4

+ 11/2H 2O

(8)

3+

For reaction (7), the Ce hydrate was assumed to be directly oxidized into Ce4+ by O2 oxidant in normal aqueous solution. For reaction (8), the Ce3+ was also assumed to be oxidized into Ce4+ by O2 oxidant, but with the assistance of SeO32− coordination. The final coordination environment of Ce4+ species in reaction (8) has been changed due to the introduction of SeO32−. This coordination environment is also in agreement with our experimental obtained crystal structure of LnSeO-2(Ce4+). The Gibbs free energy changes (ΔG) were analyzed by density functional calculation using Gaussian 09 program.21 Geometry structures of each reactant and product of reaction (7) and (8) have been fully optimized at B3LYP22,23/ SDD24,25 ∼ 6-31G*26 level in gas phase (the Stuttgart/Dresden relativistic pseusopotential with corresponding valence basis set (SDD) were used for Ce, the standard Gaussian-type 6-31G* basis set was used for O, Se, and H). Single-point energies were then performed based on each optimized reactant and product at different theory of level for calculating different terms in ΔG. The values of ΔG were calculated by

S = (nLn2 /nLn1)c *(n Ln1 /nLn2)m or S = (n Th /nLn3)c *(nLn3/n Th)m (1) Crystallization yield are calculated as

Y = 1 − (n Ln /n0)m or Y = 1 − (n Th /n0)m

(3)

(2)

where nLn1, nLn2, nLn3, nTh is the molar mass of different lanthanides and actinide, n0 is the initial molar mass of certain lanthanide or actinide charged into the autoclave. c represents the case in the solid products, and m represents the case in the mother liquid solutions. Separation of Thorium from Simulated Monazite System. Synthesis: The total molar quantity of Ln(NO3)3·6H2O and Th(NO3)4·6H2O is 0.2 mmol (∼20%La, ∼43%Ce, ∼4.5%Pr, ∼16% Nd, ∼3%Sm, ∼0.1%Eu, ∼1.5%Gd, ∼0.6%Dy, ∼0.2%Er, ∼0.1%Yb, ∼2.5%Y, ∼10%Th), and SeO2 (0.2, 0.4, 0.6, 0.8, or 1.0 mmol, respectively) were charged into a PTEF-lined Parr 4749 autoclave with 2 mL of deionized water. The sample were sealed and heated at 230 °C for 3 days followed by slow cooling to room temperature over 1 day. The next experimental procedures were the same as La/Ce binary separation. The structure type was determined by PXRD, as shown in Figures S10. The corresponding molar quantity of lanthanide elements

i

ΔG =

∑ product

i

G−

∑

G

reactant

G = εele + ΔGsov + E 0 + (H − TS) + 1.89kcal/mol (T = 298.15 K) ΔGsov = E sol − E gas where εele is the high-precision electronic energy calculated at B2PLYP27/SDD ∼ def2TZVP28,29 level; ΔGsov is the solvation energy calculated at M05-2X30/SDD ∼ 6-31G* level. Esol and Egas are singlepoint energies in liquid phase and gas phase, respectively. For calculating Esol, the implicit solvent model, SMD,31 was used with C


Article

Inorganic Chemistry water as the solvent. E0 is the zero-point energy calculated at B3LYP/ SDD ∼ 6-31G* level, which is consistent with the level of geometry optimizations. H − TS is a correction term to the Gibbs free energy (G) calculated at B3LYP/SDD ∼ 6-31G* level. H, T, and S indicate the enthalpy, temperature, and entropy. The 1.89 kcal/mol is a constant energy value, representing a 1 atm →1 mol free energy change from gas phase to liquid phase. Incorporation energy calculations. First-principles calculations are carried out by using the density functional theory (DFT) method32 implemented in CASTEP33,34 codes with the Perdew−Burke− Ernzerhof (PBE)35 functional under the spin-unrestricted generalized gradient approximation (GGA).35,36 The incorporation energies for the structure LnSeO-2 with thorium and lanthanides as coordination centers were calculated with the results shown in Figure S12 and Table S4. Crystal lattice energy [E(Lattice)] calculation. To evaluate the stability of different crystals, we calculated the crystal lattice energy E(Lattice) for the obtained crystal structure, including LnSeO-1(La3+), LnSeO-2(Ce4+), LnSeO-4(Eu3+), LnSeO-5(Yb3+) and ThSeO2(Th4+). In addition, we calculated the crystal lattice energies for three artificially constructed crystals, including Ce3+@LnSeO-1(La3+), Ce3+@LnSeO-4(Eu3+), and Ce3+@LnSeO-5(Yb3+). These three crystals were constructed by simply replacing the Ln3+ cations of LnSeO-1(La3+), LnSeO-4(Eu3+), and LnSeO-5(Yb3+) with Ce3+ cations. Although these three crystals cannot be experimentally obtained, they are more meaningful for theoretical comparisons on the stabilization of Ce3+ in different selenite matrixes. For each experimentally obtained and artificially constructed crystals, singlepoint energies were carried out for their periodic unit cells by DFT calculations using DMOL3 program.37,38 The PBE functional35 was employed in all calculations, and the DNP basis set37 (doublenumerical basis set plus polarization functions) was applied for all atoms. The DFT semicore Pseudopots 39 (DSPP, DFT-based pseudopotentials) core treatment was implemented for considering relativistic effects for heavy elements (Ln and Th), which replaced core electrons by a single effective potential. For LnSeO-1(La3+), LnSeO2(Ce4+), LnSeO-4(Eu3+), LnSeO-5(Yb3+), ThSeO-2(Th4+), Ce3+@ LnSeO-1(La3+), Ce3+@LnSeO-4(Eu3+) and Ce3+@LnSeO-5(Yb3+), the k-point was set to 12 × 7 × 14, 15 × 9 × 14, 11 × 11 × 14, 8 × 14 × 8, 15 × 9 × 14, 12 × 7 × 14, 11 × 11 × 14 and 8 × 14 × 8, respectively. These k-point settings can guarantee the specified k-point separation within 0.01 Å−1. The global cutoff was set to 6 Å, which is large enough for accurately calculating energies. Spin-restricted method was applied for LnSeO-1(La3+), LnSeO-2(Ce4+) and ThSeO-2(Th4+), because they have closed-shell electronic structures. Spin-unrestricted method was applied for other crystals. For spinunrestricted calculations, both spin ferromagnetic coupling and spin antiferromagnetic coupling conditions between metal cations were considered, and the lower-energy cases were used to calculate crystal lattice energies. The formula of crystal lattice energy is defined as

1 E(Lattice) = − ⎡⎣E(crystal) − n

Table 2. Calculated Crystal Lattice Energy crystals

E(Lattice) (eV)

LnSeO-1(La3+) LnSeO-2(Ce4+) LnSeO-4(Eu3+) LnSeO-5(Yb3+) ThSeO-2 (Th4+) Ce3+@ LnSeO-1(La3+) Ce3+@ LnSeO-4(Eu3+) Ce3+@ LnSeO-5(Yb3+)

46.48 84.59 54.89 57.79 77.60 48.01 47.88 47.65

ment. Inductively coupled plasma optical emission spectrometer (ICPOES) analysis of separation was conducted using a Thermo Scientific ICAP 7400 instrument. Single-crystal X-ray diffraction measurements were performed using a Bruker D8-Venture single crystal X-ray diffractometer equipped with a digital camera. The diffraction data were collected using a Turbo X-ray Source (Mo Kα radiation, λ = 0.71073 Å) adopting the direct-drive rotating anode technique and a CMOS detector under room temperature. The data frames were collected using the program APEX3 and processed using the program SAINT routine in APEX3. The structures were solved by the direct method and refined on F2 by full-matrix least-squares methods using SHELXTL-2014 program.40

■

RESULTS AND DISCUSSION Periodic Trend for Crystallization of Lanthanide Selenites/Selenates. The systematic investigation on hydrothermal reaction of Ln(NO3)3·xH2O (Ln = La−Lu except Pm) with SeO2 at 230 °C provides a unique periodic trend across the series on crystalline products as shown in Figure 1. Single

Figure 1. Depiction of the periodic trend for the formation of five different lanthanide or thorium selenites/selenates from hydrothermal reactions with SeO2. (a) Periodic trend of lanthanides and thorium for the crystallization products. (b) Depiction of the crystal structures. (c) Ln3+/Ce4+/Th4+ coordination geometries. The lanthanide/thorium centers are shown as cyan, orange, green, pink, or purple polyhedra/ spheres, oxygen as red spheres, SeO42−/SeO32− anions as yellow polyhedra.

∑ E(component)⎤⎦

(n = number of Ln) where E(crystal) is the total energy of each crystal, ΣE(component) is the energy sum of the total compositions in each crystal. The components of crystals include Ln3+/Ce4+/Th4+, SeO32−, SeO42−, OH−, and H2O. For calculating E(component), each component was placed into the center of a 50 Å × 50 Å × 50 Å periodic box and the geometry structures were optimized at gamma point. The calculated crystal lattice energies of each crystal are summarized in Table 2. Characterizations. Powder X-ray diffraction (PXRD) data were collected from 5 to 50° with a step of 0.02° and the time for data collection was 0.2−0.5 s on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.54056 Å) and a Lynxeye one-dimensional detector. Inductively coupled plasma mass spectrometry (ICP-MS) analysis of separation experiment was conducted using a Thermo Finnigan high-resolution magnetic sector Element 2 ICP-MS instru-

crystals suitable for X-ray analysis were collected and phases were identified by powder X-ray diffraction (PXRD) as shown in Table S1 and Figures S1−S5. The La reaction affords La2(SeO3)3 (LnSeO-1)41 as the pure product. In comparison, Pr, Nd, and Sm reactions result in the formation of two different products: selenite compounds Ln2(SeO3)3 (LnSeO-3) and mixed selenite and selenate compounds Ln2(SeO4)(SeO3)2(H2O)2 (LnSeO-5).42 Eu reaction yields mixed products of Eu3(SeO3)4(OH) (LnSeO-4) and LnSeO-542 while LnSeO-5 is isolated as the pure product from reactions starting from Gd. These crystallization products are highly D


Article

Inorganic Chemistry

Figure 2. Description of the process for the selective crystallization of tetravalent Th from REEs.

coordination, affording a crystallization system that is highly selective to trap tetravalent f-blocks. Selective Selenite Crystallization System for Tetravalent Lanthanide/Thorium Selenite. We noted that the crystal structure of Ce(SeO 3) 2 is isotypic to that of Th(SeO3)2,47 as shown in Figure 1b, further confirming the oxidation process. In addition, hydrothermal reaction of Th(NO3)4·6H2O and SeO2 under the same condition indeed results in the formation of pure Th(SeO3)2 product, initially suggesting this system may be applicable for the selective crystallization of tetravalent f-blocks over trivalent f-blocks and therefore provides a facile method for decontamination of thorium from REEs. In order to confirm such hypothesis, we proceeded on 11 binary crystallization reactions with combinations of two lanthanides in different valence states and thorium/lanthanides in different valence states as well as two lanthanides in the same +3 valence state but forming different products for comparison (La/Ce, Ce/Pr, La/Th, Th/Eu, Th/Yb, Nd/Dy, Eu/Dy, La/ Lu, La/Pr, La/Eu, and Nd/Eu in 1:1 molar ratio). The last six reaction combinations containing trivalent lanthanide products all resulted in the formation of pure LnSeO-5 phase (Figure S6). This is also true for the case of La/Lu, even though La does not yield this phase by itself. The ratio of Ln1:Ln2 in the solid products determined by inductively coupled plasma optical emission spectrometry (ICP-OES) is very close to 1, forming a uniform solid solution. However, the Ce/Ln and Th/ Ln reactions only yielded LnSeO-2 phases demonstrated by the PXRD analysis (Figure S7−S9), while the coexistence of Ce or Th significantly hinders the formation of their preferential phases for trivalent lanthanides. This confirms the selective crystallization of tetravalent Ce/Th phases, resulting in the concentration of Ln(III) in the solution portion, as shown in Figure 2. In order to precisely determine the separation factor, the crystalline solids were dissolved in nitric acid and the mother liquids were also collected. The atomic ratios of La/Ce and Ce/ Pr were quantitatively analyzed using inductively coupled plasma mass spectrometry (ICP-MS). The separation factors were calculated based on a biphasic solid−liquid model as illustrated in Supporting Information. The measured La/Ce and Ce/Pr separation factor are shown in Table S2 and Figure S11, reaching up to 5038.02 and 740.92, respectively. This leads to a one-step separation of tetravalent cerium from other trivalent lanthanides. Impressively for the cases of Th/Ln (Ln = La, Eu, Yb), as shown in Figure 3 and Table 1, the concentration of thorium in the mother liquid solution is remarkably low as result of the high crystallization yield, giving rise to the removal efficiency of thorium from solution of close to 100%. In addition, trivalent lanthanides almost completely remain in solution, further resulting in superior separation

reproducible from at least 10 parallel reactions. The crystal structures of all phases are illustrated in Figure 1b,c. The structure of LnSeO-1 can be best described as a porous 3D framework constructed by LnO10 polyhedra adopting the coordination geometry of capped triangular cupola.41 These polyhedra share edges forming 2D sheets, which are further bridged by selenite anions. The structure of LnSeO-3 is a relatively dense framework containing a combination of 10coordinate Ln(III) ions in distorted dodecahedron and 8coordinate distorted square antiprismatic polyhedra. The 3D framework of LnSeO-4 contains 9-coordinate Ln(III) ions adopting distorted tricapped trigonal prism geometry and a protonated μ3-hydroxo group. LnSeO-5 forms zigzagging chains that are constructed by 8-coordinate Ln(III) center and are further bridged by mixed selenite/selenate ligands. Tetravalent selenium is partially converted to hexavalent SeO42−, presumably by O2 in the air during the crystallization reaction. It is not surprising that all these four structures contain trivalent lanthanide metal centers given most Ln(III) ions are not redox-active under aerobic conditions. However, Ce is the only exception as all Ce(III) starting materials completely transform into Ce(SeO3)2 (LnSeO-2) containing Ce(IV) as the pure product demonstrated by PXRD analysis (Figure S2). This is quite unexpected since Ce(III) is generally more stable than Ce(IV) under normal condition (standard redox potential of Ce4+/Ce3+ couple is +1.70 V vs NHE). Full oxidation of Ce(III) to Ce(IV) often requires the presence of strong oxidants; despite a handful of examples were reported, where strong coordinating ligands showing strong preferences on coordinating to Ce(IV) over Ce(III) are able to significantly shift the reduction potential down for up to several hundred millivolts.43−45 Here, selenite is a highly polarized anion that exhibits strong coordination capability toward hard metal cations.46 In our case, no strong oxidant is present in the reaction and Ce(III) has to be oxidized by mild oxidant SeO2 and/or oxygen in the air; the later one is more possible because SeO2 is also partially oxidized during reaction forming LnSeO5. We therefore calculated the Gibbs free energy (ΔG) for the oxidation and crystallization reaction as well as the potential of Ce4+/Ce3+ couple as the result of selenite coordination. The calculated ΔG of reaction (7) is 62.75 kJ/mol. The quite positive ΔG demonstrates a clear thermodynamic barrier in this reaction. However, the calculated ΔG value of reaction (8) is −350.55 kJ/mol. The remarkable negative ΔG of reaction (8) implies that the thermodynamically favorable reaction is possible with the participation of SeO32−. Notably, O2 is still as the oxidant. Our calculation results clearly showed that the Ce3+ can be easily oxidized to Ce4+ by coordination of selenite. It was speculated that Ce(III) oxidation by O2 would become much more thermodynamically favorable after the selenite E


Article

Inorganic Chemistry

imental observation of the largest separation factor in binary Th/La compared to those of Th/Eu and Th/Yb. The results show that the crystal lattice energy of LnSeO-2 (Ce4+ and Th4+) is significantly higher than those of trivalent lanthanide compounds (LnSeO-1, LnSeO-4 and LnSeO-5), in agreement with the selective crystallization of tetravalent f-blocks. Decontamination of Thorium from Simulated Monazite System. In order to validate the practical application of this strategy, we tested the thorium decontamination efficiency using the simulated monazite composition with the presence of all 14 lanthanides.5,50 It should be noted that monazite is a phosphate salt and the predissolving of the sample in nitric acid or sulfuric acid is a required step for further processing. The PXRD results reveal that such a complicated reaction still affords pure crystalline products in the structure type of LnSeO-2 (Figure S10). Similar to the binary separation results, the ICP-analysis confirms that both thorium and cerium are enriched in the solid product while all other lanthanides almost entirely remain in solution. The separation results on the simulated experiments are summarized in Figure 4 and Table

Figure 3. Separation results of Th/La, Th/Eu, Th/Yb crystallization experiment. A, B, and C represent the molar distribution of Th/Ln in the starting material, solid product, and mother liquid solution, respectively. Olive: Thorium; Orange: Ln (La/Eu/Yb).

factors of 2.1 × 105, 1.2 × 105, and 9 × 104 respectively, after the optimization on the SeO2 content in the reaction (Table 1). These values are higher than the recently developed electrochemical metallurgy process (SFs up to 2 × 104).48,49 Mechanism Investigation. The selenite-coordinationinduced change in the thermodynamic barrier from Ce(III) to Ce(IV) promotes finding this selective crystallization system for tetravalent elements, as we discussed above. In addition, there are two key components that contribute to the extremely high separation factors observed between Th(IV) and Ln(III). First, Th(IV) and Ln(III) almost does not cocrystallize into the same lattice of LnSeO-2 phase, forming aliovalent substitution that is observed in many natural minerals and synthetic phases. In order to elucidate the origin of this unusual case, firstprinciples calculations were carried out to estimate the incorporation energies for the structure of LnSeO-2 with thorium and lanthanides (Figure S12). From the results, the incorporation energies of ThSeO-2 and CeSeO-2 are significantly higher than other lanthanides, supporting the observation that the lattice of LnSeO-2 is able to exclusively trap tetravalent f-blocks. Additionally, the predictions on the coordination bond strengths that can be reflected by the bond populations were also obtained in this work as listed in Table S4. In comparison with trivalent lanthanides, the coordination bonds for the tetravalent thorium and cerium atom with selenite ligands have a much higher bond order, which is also a consequence of their empty d−f valence shell configurations. On the other hand, the presence of tetravalent Ce/Th significantly depresses other trivalent lanthanides’ capabilities of the formation of their own preferential phases. We therefore calculated the lattice energy for all obtained crystal structures (Table 2). The definition of E(lattice) actually describes the process for the formation of crystals for different ions. Therefore, crystals with larger E(lattice) should have higher stability, and crystallization can be more accessible. The calculated values for experimentally obtained crystal structures unambiguously indicated the tetravalent f-element selenite compounds are more stable than those of trivalent lanthanide selenite, in agreement with the selective crystallization of tetravalent f-blocks. In addition, the energies of artificially constructed crystals are significantly lower than those of LnSeO-2(Ce4+) and ThSeO-2(Th4+), nearly equal to parent selenite matrixes of LnSeO-1, LnSeO-4, and LnSeO-5. It is a clear evidence that LnSeO-2(Ce4+) is a more stable phase, and also hints for why a trivalent cerium selenite compound does not form under the same reaction condition. Notably, the E(lattice) of LnSeO-1(La3+) is the smallest among the trivalent lanthanide selenite compounds, consistent with the exper-

Figure 4. Separation results of thorium from a simulated monazite composition. Open dots represent the crystallization yield of Th as a function of SeO2 amounts used in the reaction. The histogram shows the separation-factor values.

S3. The separation factors are dependent on the molar amounts of SeO2, leading to an optimal separation factor of 1.9 × 103 and the thorium decontamination percentage of close to 100%.

■

CONCLUSIONS Efficient and environmental friendly strategy to remove radioactive contaminants is a prerequisite for utilization of REEs. Although solvent extraction has been industrially implemented in the rare earth production, the secondary organic solvent pollution that is further cotaminated with uranium and thorium is currently one of the major envionmental concerns. The alternative ion-exchange chromatography method has many issuses to be fixed, including challenges in scaling up, continuous operation, resin regeneration, and cost. In general, the majority of investigations on thorium and lanthanides have shown paralleled coordination chemistry, leading to their cocrystallization in many inorganic solid compounds and natural minerals. What we show here demonstrates a unique crystallization system that is highly selective to trap tetravalent f-elements, whereas trivalent lanthanides almost entirely remain in solution when they coexist. This observation is based on the preferential coordination toward tetravalent f-cations over trivalent f-cations using highly polarized inorganic ligands (herein selenite), which F


Article

Inorganic Chemistry is further supported by the first-principle theoretic analysis on the lattice and bonding. We believe these observations may eventually lead to a highly efficient and green strategy for the decontamination of thorium from REE ores. A similar idea may be applied to the decontamination of uranium19,51−55 and daughter radioisotopes such as Radium-228 as well, which is currently under explored in our lab and would show some utility in the near future.

■

(2) Sholl, D. S.; Lively, R. P. Seven chemical separations to change the world. Nature 2016, 532, 435−437. (3) Humhpries, M. Rare Earth Elements: The Global Supply Chain; Congressional Research Service (CRS) Report Report No. R41347; CRS: Washington, DC, 2013; pp 1−27. (4) Environmental Protection Agency (EPA). Rare Earth Elements: A Review of Production, Processing, Recycling, and Associated Environmental Issues; EPA Report 600/R-12/572; EPA Office of Research and Development: Cincinnati, OH, 2012. (5) Long, K. R., Van Gosen, B. S., Foley, N. K., Cordier, D. The Principal Rare Earth Elements Deposits of the United StatesA Summary of Domestic Deposits and a Global Perspective; U.S. Geological Survey (USGS) Scientific Investigations Report 2010-5220; USGS: Reston, VA, 2010; pp 1−104. (6) Jordens, A.; Cheng, Y. P.; Waters, K. E. A review of the beneficiation of rare earth element bearing minerals. Miner. Eng. 2013, 41, 97−114. (7) Zhu, Z.; Pranolo, Y.; Cheng, C. Y. Separation of uranium and thorium from rare earths for rare earth production - A review. Miner. Eng. 2015, 77, 185−196. (8) Maes, S.; Zhuang, W.-Q.; Rabaey, K.; Alvarez-Cohen, L.; Hennebel, T. Concomitant Leaching and Electrochemical Extraction of Rare Earth Elements from Monazite. Environ. Sci. Technol. 2017, 51, 1654−1661. (9) Hatanaka, T.; Matsugami, A.; Nonaka, T.; Takagi, H.; Hayashi, F.; Tani, T.; Ishida, N. Rationally designed mineralization for selective recovery of the rare earth elements. Nat. Commun. 2017, 8, 15670. (10) Uda, T.; Jacob, K. T.; Hirasawa, M. Technique for enhanced rare earth separation. Science 2000, 289, 2326−2329. (11) Sun, X.; Luo, H.; Dai, S. Ionic liquids-based extraction: a promising strategy for the advanced nuclear fuel cycle. Chem. Rev. 2012, 112, 2100−2128. (12) Zhao, X.; Wong, M.; Mao, C.; Trieu, T. X.; Zhang, J.; Feng, P.; Bu, X. Size-selective crystallization of homochiral camphorate metal− organic frameworks for lanthanide separation. J. Am. Chem. Soc. 2014, 136, 12572−12575. (13) Bonificio, W. D.; Clarke, D. R. Environ. Sci. Technol. Lett. 2016, 3, 180−184. (14) Qi, X.-H.; Du, K.-Z.; Feng, M.-L.; Gao, Y.-J.; Huang, X.-Y.; Kanatzidis, M. G. J. Am. Chem. Soc. 2017, 139, 4314−4317. (15) Bogart, J. A.; Lippincott, C. A.; Carroll, P. J.; Schelter, E. J. An Operationally Simple Method for Separating the Rare-Earth Elements Neodymium and Dysprosium. Angew. Chem., Int. Ed. 2015, 54, 8222− 8225. (16) Bogart, J. A.; Cole, B. E.; Boreen, M. A.; Lippincott, C. A.; Manor, B. C.; Carroll, P. J.; Schelter, E. J. Accomplishing simple, solubility-based separations of rare earth elements with complexes bearing size-sensitive molecular apertures. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 14887−14892. (17) Fang, H. Y.; Cole, B. E.; Qiao, Y. S.; Bogart, J. A.; Cheisson, T.; Manor, B. C.; Carroll, P. J.; Schelter, E. J. Electro-kinetic separation of rare earth elements using a redox-active ligand. Angew. Chem., Int. Ed. 2017, 56, 13450−13454. (18) Yin, X.; Wang, Y.; Bai, X.; Wang, Y.; Chen, L.; Xiao, C.; Diwu, J.; Du, S.; Chai, Z.; Albrecht-Schmitt, T. E.; Wang, S. Rare earth separations by selective borate crystallization. Nat. Commun. 2017, 8, 14438. (19) Abraham, F.; Arab-Chapelet, B.; Rivenet, M.; Tamain, C.; Grandjean, S. Actinide oxalates, solid state structures and applications. Coord. Chem. Rev. 2014, 266, 28−68. (20) Eaton, T.; Lin, J.; Cross, J. N.; Stritzinger, J. T.; AlbrechtSchmitt, T. E. Th(VO3)2(SeO3) and Ln(VO3) 2(IO3) (Ln = Ce, Pr, Nd, Sm, and Eu): unusual cases of aliovalent substitution. Chem. Commun. 2014, 50, 3668−3670. (21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02681. Experimental and computational details, supplemental data (PDF) Accession Codes

CCDC 1568663−1568664 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail for N.L.: [email protected]. *E-mail for S.W.: [email protected]. ORCID

Yaxing Wang: 0000-0002-1842-339X Shiyu Du: 0000-0001-6707-3915 Ruhong Zhou: 0000-0001-8624-5591 Thomas E. Albrecht-Schmitt: 0000-0002-2989-3311 Shuao Wang: 0000-0002-1526-1102 Author Contributions ¶

(Y.W., H.L., X.D.) These authors contributed equally.

Notes

The authors declare the following competing financial interest(s): A Chinese patent on the presenting results has been filed by the authors and Soochow University.

■

ACKNOWLEDGMENTS We are grateful for funding supports from National Natural Science Foundation of China (21790370, 21790374, 21761132019, U1532259), the Science Challenge Project (JCKY2016212A504), China Postdoctoral Science Foundation (2016M591901), A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and “Young Thousand Talented Program” in China. Y.X.W. is supported by “The Fundamental Research Funds for the Central Universities” from Sichuan University. Support for T.E.A.-S. was provided as part of the Center for Actinide Science and Technology (CAST), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award Number DESC0016568.

■

REFERENCES

(1) Zaimes, G. G.; Hubler, B. J.; Wang, S.; Khanna, V. Environmental Life Cycle Perspective on Rare Earth Oxide Production. ACS Sustainable Chem. Eng. 2015, 3, 237−244. G


Article

Inorganic Chemistry

(40) Sheldrick, G.; SHELXTL, P, version 6.12, an integrated system for solving, refining, and displaying crystal structures from diffraction data; Siemens Analytical X-ray Instruments: Madison, WI, 2001. (41) Wickleder, M. S. Anhydrous selenites of lanthanum: Syntheses and crystal structures of La2(SeO3)3 and LaFSeO3. Z. Anorg. Allg. Chem. 2000, 626, 547−551. (42) Feng, M. L.; Mao, H. G. Synthesis and crystal structure of a new neodymium(III) selenate-selenite: Nd2(SeO4)(SeO3)2(H2O)2. J. Alloys Compd. 2005, 388, 23−27. (43) Wadsworth, E.; Duke, F. R.; Goetz, C. A. Present status of cerium(IV)-cerium(III) potentials. Anal. Chem. 1957, 29, 1824−1825. (44) Piro, N. A.; Robinson, J. R.; Walsh, P. J.; Schelter, E. J. The electrochemical behavior of cerium(III/IV) complexes: Thermodynamics, kinetics and applications in synthesis. Coord. Chem. Rev. 2014, 260, 21−36. (45) Bogart, J. A.; Lippincott, C. A.; Carroll, P. J.; Booth, C. H.; Schelter, E. J. Controlled redox chemistry at cerium within a tripodal nitroxide ligand framework. Chem. - Eur. J. 2015, 21, 17850−17859. (46) Zhu, L.; Zhang, L.; Li, J.; Zhang, D.; Chen, L.; Sheng, D.; Yang, S.; Xiao, C.; Wang, J.; Chai, Z.; Albrecht-Schmitt, T. E.; Wang, S. Selenium sequestration in a cationic layered rare earth hydroxide: a combined batch experiments and EXAFS investigation. Environ. Sci. Technol. 2017, 51, 8606−8615. (47) Xiao, B.; Langer, E.; Dellen, J.; Schlenz, H.; Bosbach, D.; Suleimanov, E. V.; Alekseev, E. V. Chemical and structural evolution in the Th-SeO32‑/SeO42‑ system: from simple selenites to cluster-based selenate compounds. Inorg. Chem. 2015, 54, 3022−3030. (48) Liu, Y.-L.; Ye, G.-A.; Yuan, L.-Y.; Liu, K.; Feng, Y.-X.; Li, Z.-J.; Chai, Z.-F.; Shi, W.-Q. Electroseparation of thorium from ThO2 and La2O3 by forming Th-Al alloys in LiCl-KCl eutectic. Electrochim. Acta 2015, 158, 277−286. (49) Liu, Y.-L.; Yan, Y.-D.; Han, W.; Zhang, M.-L.; Yuan, L.-Y.; Lin, R.-S.; Ye, G.-A.; He, H.; Chai, Z.-F.; Shi, W.-Q. Electrochemical separation of Th from ThO2 and Eu2O3 assisted by AlCl3 in molten LiCl-KCl. Electrochim. Acta 2013, 114, 180−188. (50) Chen, Z. Global rare earth resources and scenarios of future rare earth industry. J. Rare Earths 2011, 29, 1−6. (51) Lussier, A. J.; Lopez, R. A. K.; Burns, P. C. A revised and expanded structure hierarchy of natural and synthetic hexavalent uranium compounds. Can. Mineral. 2016, 54, 177−283. (52) Yang, W. T.; Parker, T. G.; Sun, Z. M. Structural chemistry of uranium phosphonates. Coord. Chem. Rev. 2015, 303, 86−109. (53) Andrews, M. B.; Cahill, C. L. Uranyl Bearing Hybrid Materials: Synthesis, Speciation, and Solid-State Structures. Chem. Rev. 2013, 113, 1121−1136. (54) Loiseau, T.; Mihalcea, I.; Henry, N.; Volkringer, C. The crystal chemistry of uranium carboxylates. Coord. Chem. Rev. 2014, 266−267, 69−109. (55) Liddle, S. T. The renaissance of non-aqueous uranium chemistry. Angew. Chem., Int. Ed. 2015, 54, 8604−8641.

T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision E.01; Gaussian, Inc.: Wallingford, CT, 2009. (22) Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652. (23) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (24) Dolg, M.; Stoll, H.; Preuss, H. Energy-adjusted ab initio pseudopotentials for the rare earth elements. J. Chem. Phys. 1989, 90, 1730−1734. (25) Cao, X.; Dolg, M. Segmented contraction scheme for small-core lanthanide pseudopotential basis sets. J. Mol. Struct.: THEOCHEM 2002, 581, 139−147. (26) Ditchfield, R.; Hehre, W. J.; Pople, J. A. Self-consistent molecular-orbital methods. IX. Extended Gaussian-type basis for molecular-orbital studies of organic molecules. J. Chem. Phys. 1971, 54, 724−728. (27) Grimme, S. Semiempirical hybrid density functional with perturbative second-order correlation. J. Chem. Phys. 2006, 124, 034108. (28) Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (29) Weigend, F. Accurate Coulomb-fitting basis sets for H to Rn. Phys. Chem. Chem. Phys. 2006, 8, 1057−1065. (30) Zhao, Y.; Schultz, N. E.; Truhlar, D. G. Design of Density Functionals by Combining the Method of Constraint Satisfaction with Parametrization for Thermochemistry, Thermochemical Kinetics, and Noncovalent Interactions. J. Chem. Theory Comput. 2006, 2, 364−382. (31) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. Universal Solvation Model Based on Solute Electron Density and on a Continuum Model of the Solvent Defined by the Bulk Dielectric Constant and Atomic Surface Tensions. J. Phys. Chem. B 2009, 113, 6378−6396. (32) Hohenberg, P.; Kohn, W. Inhomogeneous electron gas. Phys. Rev. 1964, 136, B864−B871. (33) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. J.; Refson, K.; Payne, M. C. First principles methods using CASTEP. Z. Kristallogr. - Cryst. Mater. 2005, 220, 567−570. (34) Segall, M. D.; Lindan, P. J. D.; Probert, M. J.; Pickard, C. J.; Hasnip, P. J.; Clark, S. J.; Payne, M. C. First-principles simulation: ideas, illustrations and the CASTEP code. J. Phys.: Condens. Matter 2002, 14, 2717−2744. (35) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (36) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, molecules, solids, and surfaces: applications of the generalized gradient approximation for exchange and correlation. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 46, 6671−6687. (37) Delley, B. An all-electron numerical method for solving the local density functional for polyatomic molecules. J. Chem. Phys. 1990, 92, 508−517. (38) Delley, B. From molecules to solids with the DMol3 approach. J. Chem. Phys. 2000, 113, 7756−7764. (39) Delley, B. Hardness conserving semilocal pseudopotentials. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 66, 155125. H


Facile and Efficient Decontamination of Thorium from Rare Earths

Recommend Documents