Cesium and Strontium Uptake Utilizing a New Ternary Solid-State

Article pubs.acs.org/jced

Cesium and Strontium Uptake Utilizing a New Ternary Solid-State Supramolecular Adsorbent under a Framework of Group Partitioning Anyun Zhang,* Yining Wang, and Jinying Li College of Chemical and Biological Engineering, Zhejiang University, No.38 Zheda Road, Hangzhou 310027, P. R. China ABSTRACT: To simultaneously remove Sr and Cs from a simulated highly active liquid waste, a new ternary solid-state supramolecular adsorbent BnPC6DTXAD-7 (BnPDTX7), based on organic chelating agents 25,27-bis(1-n-propoxy)calix[4]26,28-crown-6 (BnPC6), 4,4′(5′)-di-tert-butyldicyclohexano-18-crown-6 (DT), and polymeric carrier XAD-7 was prepared for the first time. Scanning electronic microscopy and Brunauer−Emmett−Teller analysis demonstrated that BnPDTX7 is a macroporous adsorbent. The adsorption of 20 metals onto BnPDTX7 were studied. Under the optimal adsorption conditions of 3.0 M HNO3, 298 K, and contact time of 60 min, the adsorption capacity toward Cs and Sr were 0.1396 mM/g and 0.1289 mM/g, respectively. The experimental data fit well with the Langmuir model, indicative of the monolayer adsorption mechanism. The thermodynamic parameters, ΔH°, ΔG°, and ΔS° of Cs and Sr adsorption and other valuable chemical engineering data were obtained. BnPDTX7 proves to be a promising adsorbent for the simultaneous removal of Sr and Cs because of its high selectivity, fast kinetics, and high stability.

1. INTRODUCTION The management of highly active liquid waste (HLW) generated from the Purex process is very challenging, because HLW is highly radioactive with multicomponents that contain many harmful radionuclides such as Am, Cm, Sr, and Cs.1 Nowadays the disposal method of HLW, placed in a landfill after simple treatment, is defective.2 90Sr and 137Cs are the major high-heat-release nuclides, once the heat released from 137 Cs and 90Sr accumulate and destroy the vitrified waste form, some radionuclides will be released, which is harmful to the environment.3 At this point, removal of Cs and Sr from HLW is vital to reduce heat sources and enhance disposal safety. In addition, the recovered Sr and Cs can be utilized as a beta irradiation source for use in a 90Sr battery and other radiostrontium generators,4 in hospitals, and in material fields, etc. However, being radioactive alkali metals, Sr and Cs are hard to separate by forming complexes with traditional functional agents, and the relevant reports on chemical separation parameters are few. Thus, seeking a reliable removal method or technology has long been a significant work to promote efficient treatment and disposal of highly active liquid waste. Cs was strongly and selectively recognized by calix[4]arenecrown-6 families,5−10 while Sr was removed based on the supramolecular recognition agent with some derivatives of crown ethers such as dicyclohexyl-18-crown-6 (DCH18C6) and 4,4′(5′)-di-tert-butyldicyclohexano-18-crown-6 (DT).11−16 Some extraction processes such as cesium extraction (CSEX),17 caustic-side solvent extraction (CSSX),18 and strontium extraction (SREX)19 used to remove Sr or Cs were reported. However, research on the simultaneous separation of Cs and Sr which was of significant interest for its great advantages on © XXXX American Chemical Society

simple devices and elimination of regenerated liquid waste, was relatively few. The FPEX (fission product extraction) process was proposed in 2005; DT and calix[4]arene-bis(tertoctylbenzo-crown-6) were used as extractants for the simultaneous extraction of Sr and Cs from 1 M nitric acid media.20 Rais studied the extraction of Cs and Sr with 25,27bis(1-octyloxy)calix[4]arene-26,28-crown-6 and DCH18C6.21 Although solvent extraction is effective and promising, it has some obvious disadvantages: (1) modifiers including 1-(2,2,3,3tetrafluoropropoxy)-3-(4-sec-butylphenoxy)-2-propanol (Cs7SB), methyloctyl-2-dimethylbutaneamide (MODB), and others needed to introduce into organic phase to efficiently eliminate emulsification or the third-phase, and (2) regenerated liquid waste accumulated massively due to utilization of a large amounts of organic extractants, modifiers, and diluents. Compared to solvent extraction, extraction chromatography has attracted increasing attention recently, because it requires less instruments and equipment, and it can reduce greatly the amount of organic solvents used for washing and stripping.22,23 Strontium/cesium partitioning from HLW by extraction chromatography (SPEC) and group partitioning of strontium and cesium by extraction chromatography (GPSC) were developed on the basis of silica-based adsorbents containing 1,3-[(2,4-diethylheptylethoxy)oxy]-2,4-crown-6-calix[4] arene (Calix[4]arene-R14) such as (Calix[4]arene-R14+M)/SiO2− P, (DtBuCH18C6+M)/SiO2−P, and Calix[4]@DBC/SiO2− P.24,25 These studies demonstrated that the process is feasible and reliable. Received: December 3, 2016 Accepted: February 8, 2017

A

DOI: 10.1021/acs.jced.6b01007 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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dichloromethane were stirred for 90 min at 298 K, and then the mixture was evaporated at 318 K. It was not until all of the solvents distilled out of the flask that the compounds BnPC6 and DT dispersed homogeneously in the pores of XAD-7. At last it was dried in vacuum at 353 K for 4 h and the white powdered adsorbent BnPDTX7 was obtained. The mass ratio of BnPC6, DT, and XAD-7 is 1.4:1:9. BnPDTX7 and XAD-7 can be observed by scanning electron microscopy (SEM, SU-8010), and the total pore volume and specific surface area were measured by Brunauer−Emmett− Teller (BET, Beckman Coulter Co., USA). The results of Figure 2 and Table 1 indicated the material XAD-7 is porous,

The chelating agent, Calix[4]arene-R14, in Calix[4]@DBC/ SiO2−P provides the binding sites to Cs. However, the complicated synthetic route and hydrophobility resulting from the long carbon chain at the lower rim substituting group in Calix[4]arene-R14, brought along some drawbacks such as low yield, and poor dispersion.26,27 It makes the Calix[4]@DBC/ SiO2−P application in the GPSC process restricted; thus to build a novel adsorbent is valuable. For this purpose, a hydrophilic agent 25,27-bis(1-n-propoxy)calix[4]arene-26,28crown6 (BnPC6) was synthesized for the first time. A new ternary solid-state supramolecular adsorbent, BnPC6DTXAD-7 (BnPDTX7), by compositing BnPC6 to remove Cs and DT to remove Sr on carrier XAD-7, was prepared to be applied in the GPSC process. Compared with Calix[4]arene-R14, BnPC6 has some significant advantages: (1) lower cost and higher yield in organic synthesis; (2) excellent hydrophilicity so that a modifier is no longer needed to be introduced. The adsorption of 20 typical metals onto BnPDTX7 was evaluated by batch experiments. Some important adsorption data and thermodynamic parameters were obtained.

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. The simulated highly active liquid waste consisting of 20 typical metals (such as Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Fe, Co, Ni, Ru, Mo, Pd, Zr, La, Y, Yb, Nb) was prepared by dissolving the corresponding nitrates, and the initial concentration of the tested metals was about 0.5 mM. All of the nitrates used in this experiment are analytical reagent. DT shown in Figure 1a was obtained from Alfa Aesar with purity of more than 97% and used as received without further Figure 2. SEM of (a) XAD-7 and (b) BnPDTX7; nitrogen adsorption and desorption isotherms (c) and distribution of pore diameter (d).

Table 1. Physicochemical Data of XAD-7 and BnPDTX7 sample

specific surface area (m2/g)

most probable diameter (nm)

pore volume (cm3/g)

XAD-7 BnPDTX7

348.5 187.6

29.48 30.66

0.7035 0.6435

Figure 1. Molecular structure of BnPC6 (a) and DT (b).

and its pore volume decreased after impregnation with the recognition agents, demonstrating that BnPC6 and DT were successfully impregnated into the pores of XAD-7. The pore structures of BnPDTX7 afforded abundant space for permeation of metal ions, which makes it possible for Cs and Sr to bind with the supramolecular recognition agents BnPC6 and DT, respectively. 2.3. Adsorption Experiments. The batch of experiments with a ratio of 3.0 mL of aqueous phase to 0.15 g of solid phase were performed using a MM-10 model thermostatic bath (TAITEC, Japan) at 298 K except for the experiments of the temperature effect. After separation, metal content in the aqueous phase was analyzed by a AA240 model atomic absorption spectrometer (AAS, Varian, USA) for Na, K, Rb, and Cs and a 730-ES model inductively coupled palsma- optical emission spectrometer (ICP-OES, Varian, USA) for other metals. The bleeding of BnPDTX7 in the aqueous phase after adsorption was evaluated by the content of total organic carbon (TOC), which was measured using a 5000 model TOC-VCPN analyzer (Shimadzu, Japan). The distribution coefficients were calculated by the following equation:

purification. XAD-7 was provided by Acros Organics. The agent BnPC6 shown in Figure1b was synthesized10,28,29 with a purity of more than 99%. BnPC6. Element analysis calcd: C, 69.38%; H, 7.52%. Found: C, 69.12%; H, 7.23%. ESI-MS: [BnPC6]+ = 710.4, [BnPC6+NH4]+ = 728.1, [BnPC6+Na]+ = 733.0, [BnPC6+K]+ = 748.9. 1H NMR (400 MHz, CDCl3, TMS), ppm: δ = 0.717− 0.753 ppm, t, OCH2CH2CH3, 6H; δ = 1.291−1.345 ppm, OCH2CH2CH3, 4H; δ = 3.401−3.429 ppm, t, ArOCH2 CH 2 OCH 2 CH 2 OCH 2 , 8H; δ = 3.520−3.529 ppm, t, ArOCH2CH2OCH2CH2OCH2, 4H; δ = 3.592−3.620 ppm, t, OCH 2 CH 2 CH 3 , 4H; δ = 3.661−3.680 ppm, t, ArOCH2CH2OCH2CH2OCH2, 4H; δ = 3.714−3.716 ppm, t, ArO(CH2CH2O)2CH2, 4H; δ = 3.773 ppm, s, ArCH2Ar, 8H; δ = 6.763−6.848 ppm, t, ArH, 4H, δ = 7.010−7.100 ppm, t, ArH, 8H. The synthetic yield of BnPC6 was 37%. All other analytical grade reagents were used as received without further purification. 2.2. Preparation of Ternary Adsorbent. A novel ternary solid-state supramolecular adsorbent, BnPDTX7, was prepared based on a vacuum sucking technology which was proposed by our group.30 A mixture of BnPC6, DT, activated XAD-7, and B


Journal of Chemical & Engineering Data Kd =

C(b) − C(a) C(a)

×

V m

Article

(1)

where Kd shows the distribution coefficients of the tested metals (cm3/g). C(a) denotes the initial metal concentrations and C(b) represents the metal concentrations after adsorption (mg/L). V shows the volume of initial aqueous solution, and m is the weight of adsorbent.

3. RESULTS AND DISCUSSION 3.1. Effect of Contact Time. Effect of contact time on the adsorption of some typical metals onto BnPDTX7 was examined in 2.0 M HNO3 solution with an initial metal content of 0.5 mM. The results in Figure 3 showed that the Figure 4. Adsorption of metals by BnPDTX7 as a function of HNO3 concentration (phase ratio, 0.15 g/3.0 mL; contact time, 60 min; shaking speed, 150 rpm; temperature, 298 K; C0 = 0.5 mM).

all of other tested metals), which was consistent with the study of the effect of contact time. Generally, the physical and chemical properties of Rb are very close to those of Cs, so Rb was always separated along with Cs. Ba has the same behavior as Sr. To determine the separation behavior, the separation factors (SFs) of Cs+/Mn+, Sr2+/Mn+ (Mn+: all of other tested metals) at different HNO3 concentrations were calculated and listed in Table 2. The SFs of Cs/K, and Sr/K were adopted as an alternative measurement of the separation selectivity. The values of SFCs/K and SFSr/K exceeded 100 at 3.0 to 4.0 M HNO3, meaning Cs and Sr can be effectively separated simultaneously at 3.0 M−4.0 M HNO3. Besides, the solvent of actual HLW is HNO3 of about 3.0 M, and BnPDTX7 can be used without HNO3 concentration adjustment, which is beneficial to reducing regeneration waste excessively and eliminating secondary contamination. In summary, the appropriate HNO3 concentration is deemed to be 3.0 M; effective group separation of Cs and Sr can be achieved by BnPDTX7 at 3.0 M HNO3. To evaluate separation efficiency of BnPDTX7 in 3.0 M HNO3 solution, separation factors of Cs, Sr, Rb, and Ba with other metals are listed in Table 3. Separation factors of Cs+/ Mn+ and Sr2+/Mn+ (Mn+: all other tested metals) are larger than 100 in 3.0 M HNO3 with the exception of Rb and Ba. So it is feasible to remove Cs and Sr simultaneously from HLW by BnPDTX7 at 3.0 M HNO3. Although the separation factor of Cs+/Sr2+ is only 1.76 at 3.0 M HNO3, completely separating Cs and Sr from each other can still be achieved by multistep extraction chromatography. It is well-known that trivalent lanthanide metals have similar physical and chemical properties. Consequently adsorption behaviors of lanthanide metals onto BnPDTX7 can be deduced from those of La, Nd, and Yb. The experimental results indicated that La, Nd, and Yb exhibited no adsorption onto BnPDTX7. That is, lanthanide metals, from La to Lu almost have no effect on adsorption for Sr and Cs onto BnPDTX7. Furthermore, the similar chemical properties of rare earth elements and actinides resulting from lanthanide contraction and actinides contraction make it possible to infer the actinides adsorption behavior, so BnPDTX7 has almost no uptake onto Am and Cm. On the basis of the analysis above, the adsorption behavior of 32 typical metals is clear. The relationship between ionic radius and selectivity of BnPDTX7 in 3.0 M HNO3 is shown in Figure 5. As we all know, the adsorption mechanism of calix[4]arene-crowns is

Figure 3. Adsorption of metals by BnPDTX7 as a function of contact time (phase ratio, 0.15 g/3.0 mL; HNO3 concentration, 2.0 M; shaking speed, 150 rpm; temperature, 298 K; C0 = 0.5 mM).

coefficients of distribution of Sr and Cs close to the equilibrium value were achieved within 60 min of contact, and that the adsorption kinetics was relatively fast.31 The major uptake of Cs occurred in the first 60 min, and the equilibrium Kd value of Cs reached at 162.0 cm3/g. Moreover, the Kd value of Cs is over 100 after 10 min of contact, meaning that it can still separate Cs over others efficiently with short contact times if desired. The Kd value of Sr reached equilibrium at 65.97 cm3/g after 60 min of contact time. It is obvious that the Kd values vary with different metal ions under the same experimental conditions, the order of the distribution coefficients at 298 K is Cs+ > Sr2+ > Rb+ > Ba2+ > K+ > Mn+ ≈ 0 (Mn+: all of other tested metals). 3.2. Effect of HNO3 Concentration. Effect of HNO3 concentration on the adsorption of some typical metals onto BnPDTX7 is shown in Figure 4. Results indicated that BnPDTX7 had high selectivity and adsorption ability for Cs and Sr. Moreover, Rb and Ba showed weak uptake due to their resemblance of ionic radius with Cs and Sr. As can be seen, the Kd of Cs increased in the range of 0.4 to 2.0 M HNO3, and then decreased with a further increase of HNO3 concentration from 3.0 to 6.0 M. Therefore, the maximum Kd(Cs) is 170.2 cm3/g at 2.0 M HNO3, while the Kd value of Sr increased from 0.4 to 6.0 M HNO3 continuously and the Kd(Sr) value rose to 89.69 cm3/g at 6.0 M HNO3. The maximum Kd value of Rb and Ba are 24.77 cm3/g and 21.78 cm3/g, respectively, at 3.0 M HNO3, and yet Kd values of all other tested metals remained below 1.0 cm3/g in HNO3 from 0.4 to 6.0 M. Adsorption selectivity of BnPDTX7 obeys the following order: Cs+ > Sr2+ > Rb+ > Ba2+ > K+ > Mn+ ≈ 0 (Mn+: C



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Table 2. Separation Factors of Cs and Sr with Others in the Range of 0.4 to 6.0 M HNO3 Cs 0.4 M 1M 2M 3M 4M 5M 6M

Sr

SFCs/Sr

SFCs/Rb

SFCs/Ba

SFCs/K

SFCs/Pd

SFSr/Rb

SFSr/Ba

SFSr/K

SFSr/Pd

5.53 5.57 3.32 1.76 0.75 0.36 0.25

6.11 6.68 6.86 5.41 6.02 3.45 6.54

6.25 8.06 7.81 6.44 3.64 2.46 2.25

26.27 73.08 31.51 305.5 112.5 102.5 278.3

24.54 68.46 136.9 1618 1989 312.5 35.27

1.10 1.19 2.06 3.06 7.99 9.43 25.5

1.12 1.44 2.34 3.64 4.84 6.72 8.79

4.740 13.10 9.46 172.8 149.4 280.0 1085

4.431 12.27 41.11 914.8 2641 853.5 137.6

Table 3. Separation Factors of Cs, Sr, Rb, Ba with Others in 3.0 M HNO3 Cs SFCs/X SFSr/X SFRb/X SFBa/X SFCs/X SFSr/X SFRb/X SFBa/X

0.57 0.18 0.16 Ba 6.4 3.6 1.2

Sr

K

Co

Li

Mg

Ca

Ru

Pd

Mo

1.77

305.6 172.8 56.4 47.4 Ni

993.4 561.7 183.4 154.2 Nd

2129 1204 393.1 330.6 Y

552.4 312.3 102.0 85.77 La

975.7 551.7 180.1 151.5 Fe

3212 1816 593.0 498.7 Yb

1618 914.8 298.7 251.2 Na

1664 940.6 307.1 258.3 Zr

1377 778.4 254.2 213.8

1029 581.8 189.9 159.8

1865 1055 344.3 289.6

1179 666.7 217.7 183.1

1450 819.7 267.6 225.1

2129 1204 393.1 330.6

2113 1215 372.2 328.3

0.33 0.27 Rb 5.42 3.06 0.84

974.2 550.8 179.9 151.3

Figure 6. Adsorption of metals by BnPDTX7 as a function of temperature (phase ratio, 0.15 g/3.0 mL; HNO3 concentration, 3.0 M; shaking speed, 150 rpm; contact time, 60 min; C0 = 0.5 mM).

Figure 5. Influence of ionic radius on selectivity of BnPDTX7 in 3.0 M HNO3 (phase ratio, 0.15 g/3.0 mL; contact time, 60 min; shaking speed, 150 rpm; temperature, 298 K; C0 = 0.5 mM).

eters of Cs and Sr adsorption were calculated by eqs 2 and 3),35 and the results are listed in Table 4.

that the ionic radius should be catered to the sizes of crown ether cavity.32 Namely, the uptake of Rb and Ba by BnPDTX7 was due to their similar radius sizes with Cs and Sr. In conclusion, except for Rb and Ba, all of other 28 metals have no impact on the adsorption of Sr and Cs. That is, BnPDTX7 has high selectivity to remove Sr and Cs from 3.0 M HNO3 solution. 3.3. Effect of Temperature. Effect of temperature on the adsorption of some typical metals onto BnPDTX7 was investigated in the range of 298 to 318 K (Figure 6). Results followed the trend of decreasing Kd for Sr and Cs with increasing temperature from 298 to 318 K. The Kd of Cs decreased from 94.07 cm3/g at 298 K to 58.67 cm3/g at 318 K, while the Kd of the Sr counterpart decreased from 58.34 cm3/g to 33.10 cm3/g. Experimental data indicated that the adsorption of BnPDTX7 is exothermic.33,34 The thermodynamic param-

ln Kd = −ΔH °/(RT ) + ΔS°/R

(2) (3)

ΔG° = ΔH ° − T ΔS° 3

where Kd represents the distribution coefficient, cm /g; R denotes the universal gas constant; ΔH° is the standard enthalpy change, kJ/mol; ΔS° is the standard entropy change, J/(mol·K); ΔG° is the standard Gibbs free energy change, kJ/ mol. As can be seen from Table 4, the fact that ΔG° < 0 indicated the adsorption of Sr and Cs on BnPDTX7 is spontaneous. While ΔH° > 0 confirmed that the adsorption reaction is exothermic. In conclusion, adsorption of Sr and Cs ions onto BnPDTX7 was exothermic and spontaneous. 3.4. Competition Adsorption Mechanism. Both Calix[4]arene-crown and crown ether are oxygenated comD



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Table 4. Thermodynamic Parameters of Cs and Sr Adsorption onto BnPDTX7 ΔG° (kJ/mol) metal ions

298 K

303 K

308 K

313 K

318 K

ΔS° (J/(mol·K))

ΔH° (kJ/mol)

Cs Sr

−11.26 −10.07

−11.30 −9.960

−11.04 −9.606

−10.47 −9.347

−10.77 −9.252

−36.45 −45.22

−22.19 −23.58

pounds. These two agents not only selectively recognize some alkali metal and alkali earth metal ions, but also associate with the HNO3 molecule through hydrogen bonding.20,36,37 Moreover, HLW is a 3.0 M HNO3 solution which contains alkali metal and alkali earth metal ions. It is inevitable that there is a competition between adsorption and protonation. As shown in Figure 4, the Kd of Cs is being increased in the range of 0.4 to 2.0 M HNO3 concentration due to the association of Cs ion and BnPC6. That is, proton competition did not prevent the recognition, and adsorption of Cs played a main role by forming CsNO3·BnPC6,10 which is described in the following stoichiometric equation: Cs+ + NO3− + BnPC6 ⇌ CsNO3·BnPC6

Figure 7. Adsorption mechanism of BnPDTX7 in HNO3 solution.

(4)

Then Kd of Cs decreases with a further increase of HNO3 concentration from 3.0 to 6.0 M. The adsorption ability of Cs onto BnPDTX7 is weakened by the HNO3 molecule competition, in which the magnitude is related to the concentration of HNO3 in the solution.36 HNO3 + BnPC6 ⇌ HNO3·BnPC6

where Ce (mM) shows the equilibrium concentrations of Sr or Cs; and qe (mM/g) represents the equilibrium adsorption capacity; KL and KF demonstrate constants of Langmuir and Freundlich, respectively; qm (mM/g) denotes the monolayer adsorption capacity, and 1/n shows the heterogeneity factor. Parameters of Langmuir and Freundlich models for the adsorption of Cs and Sr onto BnPDTX7 are summarized in Table 5. Results indicated that the equilibrium data fit the Langmuir model better with higher correlation coefficients than the Freundlich model.39,40 The maximum capacity qmax of Cs and Sr are 0.1396 mM/g and 0.1289 mM/g, respectively, which are close to the theoretical values (Cs, 0.1480 mM/g; Sr, 0.1310 mM/g). Lower deviations between the experimental and calculated data indicated that the Langmuir model is well applicable. That is, the adsorption process of BnPDTX7 is monolayer adsorption, and intermolecular forces between the adsorbed ones is negligible. A Langmuir adsorption process can be classified by the separation factor RL, which is defined by eq 10.41 1 RL = 1 + KLC0 (10)

(5)

It dominated for the chemical adsorption of Cs with BnPC6 in the range of 0.4 to 2.0 M HNO3 and for the association of BnPC6 with HNO3 in excess of 3.0 M HNO3. The high hydrophilicity resulted from the six oxygen atoms of DT making it easy to associate with HNO3 through hydrogen bonding. Thus, the competitive reactions are formed between the molecular recognition with Sr 2+ and the intermolecular association with HNO3, and the results show that the competition leads to an increase in Kd values, meaning DT was not protonated extensively at high acidities. The DT and Sr2+ complex forms Sr(NO3)2·DT,38 which is described in following stoichiometric equation: Sr 2 + + 2NO3− + DT ⇌ Sr(NO3)2 ·DT

(6)

HNO3 + DT ⇌ HNO3·DT

(7)

where KL is the Langmuir constant related to the energy of adsorption (mg−1) and C0 is the initial concentration (mg/L). RL = 0 indicates an irreversible adsorption isotherms; RL < 1 and RL > 1 indicate a favorable and an unfavorable adsorption, respectively; RL = 1 represents a linear adsorption isotherm. As shown in Table 5, RL(Cs) and RL(Sr) range from 0 to 1, indicating a favorable adsorption of Cs and Sr on BnPDTX7. 3.6. Stability. The total organic carbon (TOC) content of BnPDTX7 leakage from HNO3 solution was studied, and the results are illustrated in Figure 9. The TOC content increased with increasing HNO3 concentration according to a linear equation: [TOC] = 10.42 [HNO3] + 115.7, which resulted from the intermolecular association of HNO3 with BnPC6, DT by hydrogen bonding through eqs 4 and 5). The TOC concentration of BnPDTX7 in 3.0 M HNO3 was 132.2 ppm, which verified the stability and acid resistance of BnPDTX7, and to some degree, testified to the strong combination between compounds (BnPC6, DT) and carrier (XAD-7). The bleeding percentage of BnPDTX7 in 3.0 M HNO3 was 0.13%.

It showed that in the tested HNO3 concentration range, the chemical adsorption of Sr with DT always dominated while the association of DT with HNO3 was insignificant. The adsorption mechanism of BnPDTX7 in HNO3 medium is shown in Figure 7. There is an adsorption competition between protonation and chemical complexation in the medium of HNO3 solution. CsNO3·BnPC6 and HNO3· BnPC6 is interconverted to a dynamic equilibrium, so as to form Sr(NO3)2·DT and HNO3·DT. 3.5. Adsorption Isotherms. The adsorption isotherms of Sr and Cs onto BnPDTX7 were described by the Langmuir model (eq 8) and Freundlich model (eq 9), and experimental results are presented in Figure 8. Ce C 1 = + e qe KL·qm qm ln qe = ln KF +

1 ln Ce n

(8) (9) E



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Figure 8. Adsorption isotherms of Cs (a) and Sr (b) onto BnPDTX7; Langmuir plots (c) and Freundlich plots (d) of Sr and Cs (phase ratio, 0.15 g/ 3.0 mL; contact time, 60 min; shaking speed, 150 rpm; temperature, 298 K; C0 = 0.5 mM; HNO3 concentration, 3.0 M).

Table 5. Adsorption Parameters of Cs and Sr onto BnPDTX7 Freundlich model

Langmuir model

metal ions

qm,exp (mM/g)

KF

1/n

R2

KL

qm,cal(mM/g)

R2

Sr Cs

0.1396 0.1289

0.6844 1.043

0.4859 0.4821

0.9659 0.7884

208.0 753.8

0.1480 0.1310

0.9918 0.9952

proved an effective method for achieving simultaneous removal of Cs/Sr. To replace Calix[4]arene-R14 containing adsorbent in the GPSC process, BnPC6, a more hydrophilic agent with excellent supramolecular ability for Cs, was synthesized, and a corresponding ternary solid-state supramolecular adsorbent BnPDTX7 was prepared. BnPDTX7 was used through impregnation and immobilization along with DT into the pores of the carrier XAD-7 without introducing any modifier. BnPDTX7 has a selectivity order in all systems as follows: Cs+ > Sr2+ > Rb+ > Ba2+ > K+ > Mn+ ≈ 0 (Mn+: all of other tested metals), which is attributed to a match between the sizes of the crown ether cavity and the ionic radius. It was also deduced that almost 28 metals have no impact on the adsorption of Sr and Cs, verifying a high selectivity of BnPDTX7. The short equilibrium time of Sr/Cs demonstrated relatively fast adsorption kinetics. The adsorption process was strongly HNO3 concentration-dependent, and 3.0 M HNO3 was selected as the optimum acidity concentration according to the separation factors analysis. The negative values of ΔH° and ΔG° revealed that the adsorption process of Sr and Cs ions onto BnPDTX7 was exothermic and spontaneous. The adsorption behavior fitted with the Langmuir model well, indicative of the monolayer adsorption mechanism. Low TOC values verified the stability and acid resistance of BnPDTX7 in solution. Besides, some valuable parameters in chemical separation engineering of Cs and Sr under GPSC process were obtained. The experimental results demonstrate that in HNO 3 medium, such as in HLW or other wastewater, BnPDTX7 is

Figure 9. TOC content of BnPDTX7 bleeding at different HNO3 concentration solutions (phase ratio, 0.15 g/3.0 mL; contact time, 60 min; shaking speed, 150 rpm; temperature, 298 K; C0 = 0.5 mM).

Compared to the experimental results obtained in the GPSC process, BnPDTX7 had the following advantages due to the use of hydrophilic BnPC6 with a shorter carbon chain than lipophilic Calix[4]arene-R14: (1) the adsorption efficiency of Cs(I) was higher, (2) the preparation of BnPDTX7 was easier, and (3) the economy is better in the adsorption process. It is a benefit to use BnPDTX7 in the removal of HLW from HNO3 medium.

4. CONCLUSIONS Removing Sr and Cs from HLW is one of the most challenging problems that has not been solved yet. The GPSC process F



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a promising adsorbent of highly selective removal of Sr and Cs in the GPSC process.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Anyun Zhang: 0000-0001-5816-3812 Funding

This work was financially supported by the National Natural Science Foundation of China (Nos. 91126021 and U1407115) and the Zhejiang Provincial Natural Science Foundation of China (No. LY17B060004). Notes

The authors declare no competing financial interest.

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REFERENCES

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