Separation of Palladium along with Minor Actinides by isoBu-BTP

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Separation of Palladium along with Minor Actinides by isoBu-BTP/ SiO2‑P Adsorbent from High-Level Liquid Waste Qing Zou,† Yan Wu,† Qiding Shu,† Shunyan Ning,‡,§ Xinpeng Wang,‡,§ Yuezhou Wei,*,†,‡,§ and Fangdong Tang∥ †

School of Nuclear Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China Guangxi Key Laboratory of Processing for Non-ferrous Metallic and Featured Materials, Guangxi University, Nanning 530004, P. R. China § College of Resources, Environment and Materials, Guangxi University, Nanning 530004, P. R. China ∥ Shanghai Institute of Measurement and Testing Technology, Shanghai 201203, P. R. China ‡

S Supporting Information *

ABSTRACT: Aiming at the separation of Pd(II) along with MA(III) from high-level liquid waste, a porous silica based adsorbent isoBu-BTP/SiO2-P was prepared. The adsorption properties of isoBu-BTP/SiO2-P toward Pd(II) were investigated. It was found that with the increase of HNO 3 concentration, Pd(II) adsorption ability increased initially as HNO3 concentration increased until 1 M, then remained unchanged as HNO3 concentration further increased. The adsorption kinetics and isotherm of Pd(II) onto isoBu-BTP/ SiO2-P fit well with the pseudo-second-order rate law and Langmuir adsorption model, respectively. The adsorption thermodynamic parameters revealed that the adsorption process was spontaneous and endothermic. A hot test utilizing isoBuBTP/SiO2-P packed column was conducted and the simultaneous separation of Pd(II) and MA(III) was achieved from solution by using 0.01 M HNO3-0.1 M TU and 0.01 M HNO3-0.01 M DTPA as eluents, respectively. The Pd(II) with isoBu-BTP/SiO2-P complex was illustrated to be 1:1 type by HRMS, while 1:3 Eu(III) to isoBu-BTP complex was found by single-crystal X-ray diffraction.

1. INTRODUCTION As a member of platinum group metals (PGM), palladium is considered as one of the most important metals in many advanced technological applications, especially in fuel cells, autocatalysis, and oil processing.1,2 There is an increasing demand for palladium, while its abundance in the earth crust is very low (∼10−6%).3 Hence, the contradiction between demand and potential natural sources becomes increasingly sharp, and must be resolved by seeking alternate resources. On the other hand, the nuclear power plant (NPP), producing a kind of clean energy, has seen tremendous growth in recent years. Whatever development occurs for the NPPs in the future, the treatment of spent nuclear fuel becomes one of the most important and urgent issues nowadays in the nuclear fuel cycle. High level liquid waste (HLLW), generated from the PUREX process developed for separating uranium and plutonium from nuclear spent fuel, still contains significant amounts of heavy metals, such as minor actinides MAs (Np, Am(III), Cm(III), etc.), PGMs (Ru, Rh, Pd) and Ln(III) (mainly Ln lighter than Dy).4−6 MAs have long-term radiotoxicity, and thereby have attracted much attention for its separation to reduce the storage time.7 It should also be noted that the amount of fission© XXXX American Chemical Society

generated palladium is abundant. The content is estimated to be around 1.3 kg per ton of Light Water Reactor (LWR) spent fuel and 11.1 kg per ton of Fast Breeder Reactor(FBR) spent fuel, which could significantly cover the amount obtained naturally.4,5 Furthermore, the fission-generated palladium contains only one radioactive isotope (107Pd, 17 wt %, βemitter ∼35 keV, t1/2 ∼ 106 years) and could be tolerated in various fields.8,9 In addition, palladium would form separate phases, which would lead to destabilizing the vitrified products during the vitrification process of HLLW.9,10 In brief, efficient separation and recovery of palladium from HLLW is highly desirable. During the past three decades, the separation and recovery of palladium from HLLW has been extensively investigated. The conventional separation techniques including solvent extraction, electrodeposition, and precipitation, have been reviewed in great detail.3,6,11,12 Unfortunately, the industrialization of recovering palladium from HLLW has not yet been realized due Received: March 24, 2018 Accepted: May 28, 2018

A

DOI: 10.1021/acs.jced.8b00238 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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to the complexity of HLLW. In recent years, there has been growing interest in extraction chromatography, in which a solid adsorbent is used which is prepared by impregnating the organic extractant or functional group into or onto an inert support and is capable of selectively removing certain metal ions from liquid solution.13 Extraction chromatography not only makes it possible to achieve the highly efficient separation with no or very limited utilization of organic diluents, but also combines the high selectivity of solvent extraction with the simplicity and multistage character of a column operation system.14 Especially in HLLW partitioning, the radioactive waste would be concentrated to a minimal volume thereby facilitating its storage or disposal by using extraction chromatography.15,16 According to the recent literature, Ruhela et al. studied the recovery of palladium from HLLW by Amberlite XAD-16 resin functionalized with 2-acetyl amide and 2-acetylpyridine;17,18 Iyer et al. studied the separation of palladium from simulated HLLW using imino diacetamidegrafted styrene divinylbenzene resin;9 Bai et al. employed a macrocyclic ligand functionalized silica for the recovery of palladium from simulated HLLW;19 Zhang et al. investigated the adsorption behavior of BDIHTP/SiO2-P, BDIBTP/SiO2-P, and BisDiOTP/SiO2-P toward palladium;15,20,21 Liu et al. achieved a good recovery of palladium using isohexyl-BTP/ SiO2-P.22 Among them, R-BTP as shown in Figure 1 is a

Figure 2. Concept separation process of Pd(II) and MA(III) together from HLLW.

P adsorbent was conducted. The adsorption mechanisms of the adsorbent toward Pd(II) and Ln(III) were also investigated.

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. All the reagents used in this work such as nitric acid, thiourea (TU), and nitrates of Pd(II), Eu(III), etc. were of analytical grade. The purity of extracting agent, isoBu-BTP was 94%. Five grams of isoBu-BTP was impregnated into 10 g of silica/polymer composite support (SiO2-P) with pore size of 0.6 μm, pore fraction of 0.69, and mean diameter of 50 μm. In other words, isoBu-BTP/adsorbent mass ratio was 33.3%. P refers to the macroreticular styrene− divinylbenzene copolymer (SDB) and accounts for 17−18% of the total mass of SiO2-P. The detailed synthesis procedure has been described in previous studies.31 Five grams of isoBu-BTP was dissolved in a flask using dichloromethane as a diluent. Subsequently, 10 g of the dried SiO2-P particles was added to the solution and rotated mechanically for 2 h at 298 K. Afterward, the diluent was removed under reduced pressure by rotary evaporator and finally dried in vacuo. 2.2. Batch Adsorption Experiments. The adsorption behaviors of Pd(II) onto isoBu-BTP/SiO2-P were investigated by batch experiments. The solutions were prepared by dissolving metal nitrates into the required concentration of nitric acid, while isoBu-BTP/SiO2-P acted as the solid phase. The ratio of aqueous phase to solid phase was 5 cm3 to 0.1 g. The mixture was put into a glass vial, and then was shaken mechanically at 120 rpm at the required temperature for a designed contact time in a water bath. After filtration through a membrane filter with 0.45 μm pore, the concentrations of Pd(II) in aqueous solution were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES) instrument (ICP-7510, Shimadzu, Japan). The adsorption amount per unit adsorbent Q (mmol/g) was calculated as follows:

Figure 1. Chemical structure of R-BTP.

neutral multidentate chelating ligand containing soft-atom nitrogen, which is always likely to react with Pd(II) and form stable complexes. Meanwhile, R-BTP has been well studied for the separation of MA(III) from HLLW due to its excellent extraction selectivity toward MA(III) over Ln(III).23−26 The unique character of R-BTP/SiO2-P makes it possible to separate MA(III) and Pd(II) simultaneously in a single extraction chromatography system, which would greatly reduce the wastes produced in the partitioning process. Moreover, these silica-based adsorbents have exhibited a number of advantages such as strong acid and radiation resistance.14,27 Currently, several conceptual processes based on R-BTP/SiO2P such as modified MAREC process,22,28 MPS process,20,29 and SPMP process21 were proposed to separate MA(III) and Pd(II) together. The concept of those partitioning processes could be described as shown in Figure 2. Although preliminary investigations have been conducted, the simultaneous partitioning of MA(III) and Pd(II) has not been realized yet. In the present work, isoBu-BTP/SiO2-P was prepared for simultaneous partitioning of MA(III) and Pd(II). According to our previous work, the adsorbent exhibited good adsorption behaviors toward MA(III), good chemical stability in 3 M HNO3, and high adsorption selectivity toward Pd(II).25,30 Herein, this study mainly focused on the adsorption behaviors toward Pd(II) and the chromatographic separation of MA(III) and Pd(II). The effects of various parameters including initial concentration of HNO3, contact time, and adsorption temperature on the adsorption toward Pd(II) were examined. The simultaneous partition of Pd(II) and MA(III) by extraction chromatography using a column packed with isoBu-BTP/SiO2B


Journal of Chemical & Engineering Data Q = (Co − Ce) ×

V W

Article

(1)

where C0 and Ce (mM) are the concentrations of metal ions in the aqueous phase before and after adsorption, respectively. V (dm3) and W (g) represent the volume of the aqueous phase and the weight of the dry adsorbent, respectively. 2.3. Column Experiment. The simultaneous separation of Pd(II) and MA(III) from simulated HLLW was examined by a hot test using isoBu-BTP/SiO2-P packed column. The glass column with a dimension of 5 mm inner diameter and 500 mm effective length was used. The experiment was conducted at 298 K by circulating the thermostated water through the water jacket of the column. The flow rate in the operation was controlled to 0.1 cm3/min utilizing a NPG-50UL model pressure gage (Nihon Seimitsu Kagaku Co., Ltd., Japan) and a 2GN15K Model pressure limiter (Oriental Motor Co., Ltd., Japan). Before experiment, the column was equilibrated using 15 cm3 3 M HNO3 solution. Then the feed solution and different kinds of eluents subsequently were let to flow through the column. Effluents were collected every 15 min by an EYELA DC-1500 model autofractional collector (Tokyo Rikakikai Co., Ltd., Japan). The concentrations of metal ions in initial solution (C0) and effluents (C) were determined by ICP-AES mentioned above. The radioactivity of 241Am was measured by a NaI (Tl) γ-ray spectrometry (Seiko EG&G). 2.4. Adsorption Mechanism Studies. High resolution mass spectrometer (HRMS) analysis was used to investigate the coordination environment of the Pd-isoBu-BTP complex. Before the test, the isoBu-BTP/SiO2-P loaded with palladium was dried in vacuum at 323 K and then be washed by acetonitrile. Afterward, the acetonitrile solution containing the Pd-isoBu-BTP complex was used for the HRMS test. HRMS measurements of the complex and adsorbent were recorded on a Waters micromass ultraperformance liquid chromatography (UPLC) and quadrupole-time-of-flight (Q-TOF) premier mass spectrometer (MS) in the positive mode (capillary voltage, 3.0 kV; source temperature, 623 K; desolvation gas flow, 600.0 dm3/h; scan time, 1 s; interscan time, 0.02 s). The adsorption mechanism toward Ln(III) was investigated by single-crystal X-ray diffraction analysis. A 0.1 g sample of isoBuBTP was dissolved in acetonitrile (10 cm3), to which a solution of Eu(NO3)3·6H2O (0.3 g) in 1 M HNO3 (10 cm3) was added. After a stir under 298 K for 2 h, the mixture was kept at room temperature for a week until single crystals suitable for X-ray structure analysis occurred. Single-crystal Xray diffraction data of the Eu-isoBu-BTP complex was collected on a Bruker D8 Venture diffractometer equipped with a MicroSource X-ray generator using the Cu Kα (1.54178 Å) radiation at 173 K. Data reduction was performed within Bruker SAINT. The structure was solved using SHELXS 9732 while refinement was carried out on F2 by full matrix leastsquares techniques using SHELXL-2014.33 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were introduced at calculated positions as riding atoms. CCDC 1815956 contains the crystallographic data for EuisoBu-BTP complex. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http:// www.ccdc.cam.ac.uk/getstructures.

Figure 3. Effect of initial nitric acid concentration on isoBu-BTP/ SiO2-P adsorption toward Pd(II) ([Pd], 15 mM; temperature, 298 K; contact time, 24 h; shaking speed, 120 rpm).

adsorption of Pd(II) onto isoBu-BTP/SiO2-P. It was observed that the adsorption ability of isoBu-BTP/SiO2-P toward Pd(II) increased as acidity increased initially. After the HNO3 concentration reached 1 M, the adsorbent showed a saturated uptake of Pd(II). It is known that Pd(II) is a kind of soft metal ion which has strong ability of electron acceptance and thus forms stable complexes with BTPs, which contain nitrogen atoms bearing lone-pair electrons.23 At lower acidities, the increase of the adsorption ability may be due to the facilitation of nitrate ion in the adsorption reaction. On the other hand, isoBu-BTP as a base, will also be protonated in HNO3 solution and hence form competition.34 However, in this study, no marked weakening effect was performed even at the condition that the concentration of HNO3 was much larger than Pd(II). Thus, the results in Figure 3 revealed that the chemical complexation of Pd(II) with isoBu-BTP/SiO2-P is more competitive than the protonation of isoBu-BTP/SiO2-P at the examined HNO3 concentration range. Considering that the HLLW generated in the PUREX process is a HNO3 medium usually with a concentration around 3−5 M, isoBu-BTP/SiO2-P would be suitable for potential application. 3.2. Adsorption Kinetics. To evaluate the influence of contact time on Pd(II) adsorption in 3 M HNO3, the relationship between adsorption amount and contact time was investigated. Results were shown in Figure 4. As can be seen, the adsorption amount (Qt) increased quickly with increasing contact time in the first few hours, and then reached equilibrium within 12 h. According to previous studies, a reduction in particle size of the resin would contribute to a rapid diffusion, thereby the silica-based adsorbent such as CMPO/SiO2-P exhibited quite fast adsorption kinetics.20 In this case, the adsorption spent a long time to reach equilibrium, which could result mainly from the slow complexation of isoBuBTP/SiO2-P with Pd(II) in consideration of the small particle size of SiO2-P. The slow adsorption kinetics was also reported such as BDIBTP/SiO2-P,20 isoHex-BTP/SiO2-P,22 and Me2CA-BTP/SiO2-P.28 In addition, to understand the adsorption kinetics mechanism, the pseudo-first-order model and pseudo-second-order model were applied to analyze the experimental data.35 The pseudo-first-order model can be written as eq 2:

3. RESULTS AND DISCUSSION 3.1. Effect of HNO3 Concentration on Adsorption. Figure 3 shows the effect of initial HNO3 concentration on the

Q t = Q e(1 − exp(−k1t )) C

(2) DOI: 10.1021/acs.jced.8b00238 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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Figure 4. Adsorption kinetics, pseudo-first-order, and pseudo-secondorder rate fitting of Pd(II) adsorption onto isoBu-BTP/SiO2-P in 3 M HNO3 solution ([Pd], 15 mM; temperature, 298 K; shaking speed, 120 rpm).

Figure 5. Adsorption isotherms of Pd(II) onto isoBu-BTP/SiO2-P at 288, 298, 308, and 313 K in 3 M HNO3 solution (contact time, 24 h; shaking speed, 120 rpm).

change of adsorption amount upon increasing the temperature, the actual application of isoBu-BTP/SiO2-P was preferred at room temperature. To understand the adsorption characteristics clearly, the adsorption data were fitted with two basically theoretical isotherm models, that is, the Langmuir and the Freundlich models. The Langmuir model, as shown in eq 4,36 assumes that the adsorption reaction of the adsorbate is on a homogeneous, flat surface of an adsorbent with constant adsorption energy, and each adsorptive site can be occupied only once in a one-onone monolayer adsorption manner, while the Freundlich model describes adsorption on a heterogeneous surface with uniform energy as shown in eq 5.37

where Qt (mmol/g) is the adsorption amount at time t (h), Qe (mmol/g) is the equilibrium adsorption amount and k1 (h−1) is the adsorption rate constant of the pseudo-first-order model. The pseudo-second-order model can be written as eq 3: Qt =

k 2Q e2t 1 + k 2Q et

(3)

where k2 (g/(mmol·h)) is the adsorption rate constant of the pseudo-second-order model. The parameters were calculated by nonlinear fitting as shown in Figure 4 with the corresponding values listed in Table 1. As R2 values represent the correlation coefficient between fitting models and experimental data, the fitting results indicated that experimental data agreed well with the pseudo-second-order model. Furthermore, the calculated equilibrium adsorption amount (Qe) obtained from the pseudo-second-order model were well consistent with the experimental results. These findings suggested that Pd(II) adsorption on isoBu-BTP/SiO2P was reasonably fitted by the pseudo-second-order mode, which meant that the adsorption process was controlled by the chemisorption process. 3.3. Effect of Temperature on Adsorption Isotherms. Adsorption isotherms could reflect the relationship between the equilibrium concentration (Ce) of metal ions in solution and the equilibrium adsorption amounts (Qe) at a constant temperature. Herein, the adsorption isotherms of Pd(II) by isoBu-BTP/SiO2-P were obtained at 288, 298, 308, and 313 K, respectively. As presented in Figure 5, the adsorption amount increased as the equilibrium concentration of Pd(II) increased, and then reached an equilibrium state in each temperature. The Ce decreased while the adsorption amount increased with increasing temperature, indicating that higher temperature was more favorable for the adsorption. Considering the small

Qe =

KLQ maxCe 1 + KLCe

(4)

Q e = KFCe(1/ n)

(5)

where Ce (mM) is the equilibrium concentration, Qe (mmol/g) is the equilibrium absorption amount, Qmax (mmol/g) is the saturated adsorption amount, KL (mM−1) is the Langmuir constant, KF (mmol/g) and n are the Freundlich constant. The parameters calculated from Langmuir and Freundlich isotherms fitting were listed in Table 2. It was found that the correlation coefficient (R2) of the Langmuir model matches was higher than that of the Freundlich model at three different temperatures, meaning that the adsorption behavior of Pd(II) was better fitted by the Langmuir model. Moreover, the Qmax values calculated by the Langmuir model were very close to the experimental data. The results above indicated the adsorption of Pd(II) onto isoBu-BTP/SiO2-P could be more reasonably regarded as monolayer adsorption. Compared to other adsorbents, isoBu-BTP/SiO2-P has a reasonably high adsorption capacity for Pd(II) as shown in Table 3.

Table 1. Fitted Parameters of Pseudo-first-order Model and Pseudo-second-order Models for Pd(II) Adsorption pseudo-first-order model

pseudo-second-order

k1 (h−1)

Qe (mmol/g)

R2

k2 (g/(mmol·h))

Qe (mmol/g)

R2

expt Qe(mmol/g)

0.86

0.59

0.83

2.62

0.63

0.94

0.62

D



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Table 2. Fitted Parameters of Langmuir and Freundlich Models for Pd (II) Adsorption Langmuir model

Table 4. Calculated Thermodynamic Parameters for Adsorption of Pd(II) on isoBu-BTP/SiO2−P ΔG (kJ/mol)

Freundlich model

T (K)

Qmax (mmol/g)

KL (mM−1)

R2

n

KF (mmol/g)

R2

288 298 308 313

0.63 0.64 0.65 0.65

25.70 34.61 46.65 62.15

0.99 0.99 0.99 0.99

7.88 8.50 8.75 8.92

0.57 0.58 0.60 0.61

0.80 0.72 0.74 0.78

Qmax (mmol/g)

ref

adsorbents

IDAA SDVB

0.51

9

ACAM-XAD16

0.08

17

(AP-XAD 16)

0.08

18

functionalized silica adsorbent

0.78

19

isoHex-BTP/ SiO2-P Me2-CABTP/SiO2-P Tulsion CH95 isoBu-BTP/ SiO2-P

Qmax (mmol/g)

ref

0.89

22

0.80

28

0.19

38

0.64

this work

ΔS (kJ/(mol·K))

288 K

298 K

308 K

318 K

R2

25.41

0.17

−24.15

−25.87

−27.59

−28.45

0.99

spontaneous. The ΔG became more negative with the increase of the temperature, which meant a greater driving force at higher temperature. Furthermore, the positive ΔS reflected that the adsorption was probably irreversible and indicated the Pd(II) adsorption onto isoBu-BTP/SiO2-P was an entropydriving process. 3.5. Column Experiments. The chromatographic separation of Pd(II) and MA(III) was operated using simulated 3 M HNO3 HLLW solution containing typical FPs (Sr, Y, Ru, Rh, Pd, La, Ce, Pr, Nd, Sm, Eu, Gd, 1 mM, respectively) and 241 Am(III) (1000 Bq/cm3) as the feed solution. To achieve highly effective separation results, four different kinds of eluents, 3 M HNO3, 0.01 M HNO3, 0.01 M HNO3-0.1 M TU, and 0.01 M HNO3-0.01 M DTPA (diethylene triamine pentacetate acid) were utilized in the hot test as shown in Figure 7 and Table 5.

Table 3. Comparison of Qmax of isoBu-BTP/SiO2-P with the Qmax of some Reported Adsorbents adsorbents

ΔH (kJ/mol)

3.4. Adsorption Thermodynamics. As presented by the adsorption isotherms, it can be found that the adsorption properties of Pd(II) by isoBu-BTP/SiO2-P generally increased with the increase in temperature. In addition, to understand well the adsorption thermodynamics, the thermodynamics parameters for the adsorption process were calculated using the following thermodynamic equations:39 ln KL =

ΔS ΔH − R RT

(6) (7)

ΔG = ΔH − T ΔS −1

where KL is Langmuir constant (M ), R is the gas constant (kJ/(mol·K),) T is the absolute temperature (K), ΔS, ΔH, and ΔG are entropy change (kJ/(mol·K)), enthalpy change (kJ/ mol), and Gibbs free energy change (kJ/mol), respectively. The values of the thermodynamics parameters were obtained from the fitting curve of ln KL versus 1/T as shown in Figure 6 and listed in Table 4. As can been seen, ΔH was positive, suggesting that the adsorption process was endothermic in nature. The negative ΔG indicated that the adsorption was

Figure 7. Chromatographic separation of Pd(II) and MA(III) from simulated HLLW by isoBu-BTP/SiO2-P packed column at 298 K: (A) 3 M HNO3, (B) feed solution, (C) 3 M HNO3, (D) 0.01 M HNO3, (E) 0.01 M HNO3−0.1 M TU, (F) 0.01 M HNO3−0.01 M DTPA).

Table 5. Recovery Yields of Tested Elements

Figure 6. Fitting curve of ln KL versus 1/T. E

elements

B (%)

C (%)

D (%)

E (%)

F (%)

total (%)

Ce La Rh Ru Sr Y Pr Nd Sm Eu Gd Pd Am

25.5 48.0 57.5 53.3 0.7 0.8 0.3 0.5 0.3 0.0 1.5 0.1 0.0

70.6 46.6 42.6 48.0 1.8 2.1 2.5 3.4 2.2 1.6 1.7 0.7 0.0

1.2 0.8 0.5 1.3 92.6 89.7 88.5 87.3 87.4 83.5 84.1 2.1 0.0

1.7 0.3 0.7 0.3 0.8 3.2 2.0 1.6 0.3 0.5 1.1 84.7 0.0

0.9 0.5 0.3 0.1 0.7 0.1 0.8 1.9 1.6 2.2 2.7 2.6 94.6

99.9 96.2 101.6 103 96.6 95.9 94.1 94.7 91.8 87.8 91.1 90.2 94.7



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It was clear that La(III), Rh(III), Ru(III), and Ce(III) showed no or very weak adsorption and flowed through the column with the feed solution and the following 3 M HNO3 elution. On the other hand, Sr(II), Y(III), Pr(III), and the other Ln(III) heavier than Pr(III) performed stronger adsorption and need to be eluted by 0.01 mM HNO3. As illustrated in Figure 7, Pd(II) and MA(III) were strongly adsorbed by isoBu-BTP/ SiO2-P, with the result that they could not be effectively eluted by reducing the HNO3 concentration. With the supplement of 0.01 M HNO3-0.1 M TU, Pd(II) was eluted with a recovery yield of 84.7% due to the effective complexation of Pd(II) with the CS functional group in thiourea molecule.15 The sharp elution band of Pd(II) reflected a fast elution dynamic. It is known that S in the CS group is a soft Lewis base, which is softer than N in the isoBu-BTP. Therefore, TU showed stronger complexation with Pd(II) and the relevant elution mechanism was probably described as eq 8.40 As to 241Am, 0.01 M HNO3-0.01 M DTPA was used as the eluent. Similarly, DTPA is a multidenate acidic chelating agent and leads to the effective substitution of the complex of 241Am(III) with DTPA for the complex of 241Am(III) with isoBu-BTP as described in eq 9.24,41 Accounting for the behavior of the hot test, it revealed that effective separation of MA(III) and Pd(II) together from HLLW by the use of the BTP/SiO2-P materials have been achieved successfully.

Figure 9. HRMS spectrum of (a) neat isoBu-BTP and (b) the isoBuBTP loaded with Pd(II) (adsorption condition: ([Pd], 15 mM; 3 M HNO3; temperature, 298 K; contact time, 24 h; shaking speed, 120 rpm).

Q-TOF-MS spectra of the isoBu-BTP before and after adsorption. As shown in Figure 9a, the peak at m/z = 462.33 and 489.29 were assigned to [L+H] + and [L+CN+H]+, respectively, where L means isoBu-BTP. The MS spectrum of the Pd-isoBu-BTP complex was presented in Figure 9b. The peak at m/z = 612.23 was attributed to [Pd+L+COOH]+ and peak at m/z = 320.11 could be ascribed to [Pd+L+CH3CN +CH3OH]2+. Obviously, the MS results illustrated that the complex constitution was 1:1. Furthermore, considering the electricity neutralization, the reaction equation of the adsorption of Pd(II) with isoBu-BTP/SiO2-P needs the participation of NO−3 which was supposed as eq 10:

[Pd‐isoBu‐BTP/SiO2 ‐P](NO3)2 + 4TU = [Pd(TU)4 ](NO3)2 + isoBu‐BTP/SiO2 ‐P

(8)

[Am‐(isoBu‐BTP/SiO2 ‐P)3 ](NO3)3 + H5DTPA = Am‐H 2DTPA + 3(HNO3‐isoBu‐BTP/SiO2 ‐P)

(9)

3.6. Adsorption Mechanism. The Adsorption Mechanism of the Adsorbent Toward Pd(II). To obtain the ratio of palladium to isoBu-BTP in the complex, the influence of the added isoBu-BTP/SiO2-P amount on the Pd(II) adsorption was studied at constant Pd(II) concentration. As shown in Figure 8, the adsorbed amount of Pd(II) increased linearly with the added isoBu-BTP/SiO2-P with a linear slope close to 1, indicating 1 molecule of isoBu-BTP/SiO2-P uptake about 1 molecule of Pd(II), and the ratio does not change with the adsorbent increase. For a better understanding, the complex was further studied by MS analysis. Figure 9 shows the UPLC-

Pd2 + + 2NO−3 + isoBu‐BTP/SiO2 ‐Padsorbent → Pd(NO3)2 ·isoBu‐BTP/SiO2 ‐Padsorbent

(10)

The Adsorption Mechanism of the Adsorbent toward Ln(III). As Ln(III) plays very important role in HLLW partitioning, attempts were also made to investigate the adsorption mechanism of the adsorbent toward Ln(III) by single-crystal X-ray diffraction analysis. In this study, Eu(III) was selected as the representative element of Ln(III). Figure 10 shows the structure of [Eu(isoBu-BTP)3]3+ cation; the details for crystallographic data and structure was shown in Table 6. According to the results, a typical 1:3 Eu-BTP complex was presented, which was similar to other kinds of R-BTP.42−44 The central Eu(III) was coordinated by nine nitrogen atoms provided by three isoBu-BTP. Each BTP ligand coordinated with one nitrogen atom of the pyridine fragment (N4 in Figure 11) and one nitrogen atom of each triazine moiety (N3 and N6 in Figure 11). The three nitrogen donor atoms of the pyridyl moieties formed tricapped trigonal prismatic coordination geometry around the central metal atom. The Eu−N4 distances in the complex were in the range of 2.522−2.541 Å, which was a little shorter than the Eu−N3 or Eu−N6 distances with the range of 2.543−2.554 Å. The reason could be that the charge on the central nitrogen is more negative than the charges on the donor nitrogen atoms in the triazine fragment, with the result that Eu−N4 bond strength or covalence is stronger.45

Figure 8. Relationship between adsorbed Pd(II) and added isoBuBTP/SiO2-P amount ([Pd], 15 mM; 3 M HNO3; temperature, 298 K; contact time, 24 h; shaking speed, 120 rpm). F



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Figure 11. Structure of isoBu-BTP molecules and the position of nitrogen atoms.

respectively. The Pd(II) with isoBu-BTP/SiO2-P was illustrated to be the 1:1 type, while 1:3 Eu(III) to isoBu-BTP complex was found. The results obtained in this work are valuable for the recovery of Pd(II) along with the separation of MA(III) from HLLW. In short, isoBu-BTP/SiO2-P is a promising adsorbent for the separation process.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00238. CIF files for Eu-isoBu-BTP complex (CIF)

Figure 10. Cationic structure of Eu-isoBu-BTP complex with thermal ellipsoids drawn at the 50% level.

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4. CONCLUSIONS In this study, a macroporous silica based adsorbent, isoBu-BTP/ SiO2-P, was applied for the separation of palladium from HLLW along with MA. The adsorption properties of the adsorbent toward Pd(II) were investigated. In all the examined HNO3 concentration, isoBu-BTP/SiO2-P showed strong adsorption ability toward Pd(II) and the complexation between the adsorbent and Pd(II) was found to be more competitive than protonation of the adsorbent. The adsorption behaviors matched well with pseudo-second-order rate law and Langmuir isotherm adsorption model, indicating the adsorption process was controlled by chemisorption process in a monolayer way. A higher temperature would facilitate the adsorption process. The negative values of Gibbs free energy and positive enthalpy and entropy indicate that the adsorption process is spontaneous and endothermic in nature. In the column experiment, the simultaneous partitioning of Pd(II) and MA(III) from 3 M HNO3 simulated HLLW was achieved by using 0.01 M HNO30.1 M TU and 0.01 M HNO3-0.01 M DTPA as eluents,

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Qing Zou: 0000-0002-4792-3692 Funding

This work was supported by the National Natural Science Foundation of China (No. 11675102, 11705032) and Major Science and Technology Program for Water Pollution Control and Treatment (No. 2015ZX07406006-06) and Guangxi Natural Science Foundation (NO. 2017 GXNSFBA198175). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr Lingling Li (Instrumental Analysis Center of SJTU) for assistance with the single-crystal X-ray diffraction measurement.

Table 6. Bond Lengths (Å) and Angles (deg) for the Eu-isoBu-BTP Complex Eu−N14 Eu−N9 Eu−N6A N4A−Eu−N14 N9A−Eu−N14 N9−Eu−N14 N9−Eu−N9A N6A−Eu−N4 N6−Eu−N14 N6−Eu−N9A N3A−Eu−N14 N3A−Eu−N9A N3A−Eu−N6 N3−Eu−N4 N3−Eu−N6A

2.522(5) 2.543(4) 2.547(3) 121.03 (8) 64.19 (9) 64.19 (9) 128.38 (17) 137.29 (11) 73.67 (8) 78.95 (12) 135.35 (8) 77.11 (12) 78.00 (12) 63.92 (12) 78.00 (12)

Eu−N9 Eu1−N9A Eu−N3 N4−Eu−N14 N9A−Eu−N4A N9−Eu−N4A N6A−Eu−N14 N6A−Eu−N9A N6−Eu−N4A N6−Eu−N9 N3A−Eu−N4A N3A−Eu−N9 N3−Eu−N14 N3−Eu−N9A N3−Eu−N6

2.541(4) 2.543(4) 2.554(4) 121.02 (8) 74.50 (12) 135.73 (12) 73.66 (8) 86.94 (12) 137.28 (11) 86.94 (12) 63.92 (12) 147.41 (13) 135.35 (8) 147.41 (12) 127.45 (12) G

Eu−N9A Eu−N6 Eu−N3A N4−Eu−N4A N9A−Eu−N4 N9−Eu−N4 N6A−Eu−N4A N6A−Eu−N9 N6−Eu−N4 N6−Eu−N6A N3A−Eu−N4 N3A−Eu−N6A N3−Eu−N4A N3−Eu−N9 N3−Eu−N3A

2.541(4) 2.547(3) 2.554(4) 117.95 (16) 135.73 (12) 74.50 (12) 63.59 (12) 78.96 (12) 63.59 (12) 147.33(17) 72.92 (12) 127.45(12) 72.92 (12) 77.11 (12) 89.31(17)



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Separation of Palladium along with Minor Actinides by isoBu-BTP

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