Kinetics-Controlled Separation Intensification for Cesium and

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Separations

Kinetics-controlled separation intensification for cesium and rubidium isolation from salt lake brine JIANFENG ZHANG, Liangrong Yang, Tingting Dong, Feng Pan, Huifang Xing, and Huizhou Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04820 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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Industrial & Engineering Chemistry Research

Kinetics-controlled separation intensification for cesium and rubidium isolation from salt lake brine Jianfeng Zhang a,b,‡, Liangrong Yang*,a, ‡, Tingting Dong a,b,Feng Pan a,

Huifang Xing a, Huizhou Liu*,a,c

a

Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P.R. China

b

c

University of Chinese Academy of Sciences, Beijing 100049, P.R. China

Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, P.R. China

Corresponding authors: Liangrong Yang, E-mail address: [email protected] Tel: +86 10 82544901; Fax: +86 10 62554264

Huizhou Liu, E-mail address: [email protected] Tel: +86 10 62554264; Fax: +86 10 62554264 ‡These authors contributed equally to this work.

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ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

The separation efficiency of cesium (Cs(I)) and rubidium (Rb(I)) from a synthetic brine solution containing potassium (K(I)) was improved via kinetics-controlled strategy. Specifically, 4-tert-butyl-2-(α-methylbenzyl) phenol (t-BAMBP) dodecane solution was partially saponified by NaOH solution firstly, and then was directly used in extraction without adjusting pH of feed solution to exceed 13. Compared with traditional process, much higher separation factors of Cs(I) and Rb(I) over K(I) (𝛽𝐶𝑠/𝐾 = 1550.7, 𝛽𝑅𝑏/𝐾 = 45.2) were gained due to the enhanced kinetics differences during ion exchanges. In addition, the consumption of NaOH was reduced by 70% ~ 97%, and a large amount of strong alkaline wastewater was avoided because of the recyclability of NaOH saponifying agent. For synthetic brine solution containing high concentration K(I), 99.1% Cs(I) and 86.5% Rb(I) were recovered after three-stage extraction. The novel process showed bright prospect on the extraction of Cs(I) and Rb(I).

Keywords: kinetics-controlled; separation; cesium; rubidium; salt lake brine; 4-tertbutyl-2-(α-methylbenzyl) phenol

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1. INTRODUCTION Cesium (Cs) and rubidium (Rb) are significant and expensive (the price of Cs and Rb is 15340 $/kg and 11180 $/kg, respectively) alkali metals which are widely used in aeronautics, television image devices, metal-ion catalysts, pharmacy, night-vision devices and fiber optic telecommunication systems 1-3. With the rapid development of perovskite solar cells, the demand for Cs and Rb has grown due to their necessity in stabilizing perovskite structures

4-6

. However, the output of Cs and Rb from ores can

not meet the increasing demand of the market. Furthermore, the price of Cs and Rb increased a lot due to the increasing cost of Cs and Rb ores 7-8. The salt lake brines are another important Cs and Rb sources with a high reserve in the world. Thus, rational exploration of Cs and Rb resources from salt lake brines has significant economic value and is also an research hot spot in the world. Compared with ores, extracting Cs(I) and Rb(I) from salt lake brines is a direct separation process from solution without leaching operation, so the consumption of energy and the contamination of environment can be reduced. However, in salt lake brines, the Cs(I) and Rb(I) concentrations are extremely low while the coexisting metal ions (K(I), Mg(II) etc.) concentrations are rather high 9. If K(I) is removed completely before the extraction of Cs(I) and Rb(I), a large proportion of Cs(I) and Rb(I) are taken away together with K(I) due to great similarity of their physical and chemical properties. These factors make it difficult to isolate and purify Cs(I) and Rb(I) from salt lake brines.

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Until now, various methods such as ion exchange and solvent (or solid phase) extraction have been investigated for the extraction of Cs(I) and Rb(I) in the last decades

10-23

. The solvent (or solid phase) extraction has become the most promising

method in the industrial field due to its advantages of simple operation, ease in continuous and large-scale separation et al. The substituted phenols and calixarene crown ethers are two kinds of widely-used extractant. However, calixarene crown ethers are usually used in the field of 137Cs(I) removal from strong acid nuclear waste. Moreover, extracting Cs(I) and Rb(I) through calixarene crown ethers still meets some problems, such as the moderate selectivity (𝛽𝐶𝑠/𝐾 ) and the difficulty in desorption. For example,

Luo

et

al.

dissolved

(BOBCalixC6) in ionic liquids to extract

calix[4]arene-bis(tert-octylbenzo-crown-6) 137

Cs(I) from aqueous solutions, quite high

distribution ratio of Cs(I) in organic and aqueous solution was obtained but the selectivity of Cs(I) over K(I) (𝛽𝐶𝑠/𝐾 = 28.2) was moderate

13

. Gujar et al. prepared

composite polymeric beads using bis(octyloxy)calix [4]arene−monocrown-6 to separate Cs(I) from aqueous feeds, the stability of composite beads was good but the absorbed Cs(I) needed to be washed three or four times with 6 mol/L HNO3 16. By comparison, substituted phenols are more favored in industrial field due to the relatively higher selectivity, lower cost and easier desorption property 14. 4-tert-butyl2-( α -methylbenzyl) phenol (t-BAMBP) is a representative substituted phenol (molecule structure is shown in Figure 1) widely used in Cs(I) and Rb(I) extraction 14, 24-26

. For example, Rais et al. extracted 137Cs(I) and 86Rb(I) from alkaline fission product 4


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by using t-BAMBP as extractant, and the results showed that the distribution ratio increased with the increase of aqueous solution pH. However, they didn’t investigate the effect of K(I) on the extraction of Cs(I) and Rb(I) 25. Liu et al. extracted Cs(I) and Rb(I) from neutral salt lake brines after the removal of K(I) via t-BAMBP. However, 1mol/L NaOH needed to be added into salt lake brines to adjust pH 26. Li et al. studied the effect of high concentration K(I) on the extraction of Cs(I) and Rb(I) from synthetic brine solution. The separation factors of Cs(I) and Rb(I) over K(I) were 139 and 11, respectively. Nevertheless, 0.1 mol/L NaOH still needed to be added into aqueous solution and the separation factors were moderate 14. The adding of NaOH into neutral salt lake brine to adjust pH will cause tremendous consumption of NaOH and serious environmental pollution due to the generation of plenty of strong alkaline wastewater. Moreover, small kinetics differences between Cs(I), Rb(I) and K(I) during extraction make the separation factors low. These drawbacks definitely restrict the application of t-BAMBP on the extraction of Cs(I) and Rb(I) from salt lake brines.

Figure 1. The molecular structure of t-BAMBP In this study, t-BAMBP organic phase was partially saponified by mixing with NaOH solution at first. Then the treated organic phase was used to extract Cs(I) and Rb(I) directly from neutral feed solutions without extra addition of NaOH. Much higher

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separation factors of Cs(I) and Rb(I) over K(I) were gained (𝛽𝐶𝑠/𝐾 =1550.7, 𝛽𝑅𝑏/𝐾 = 45.2) by controlling extraction time and the consumption of NaOH was reduced by 70% ~ 97%. In addition, extraction of Cs(I) and Rb(I) from synthetic brine solution containing high concentration K(I) was also studied.

2. EXPERIMENTAL 2.1. Reagents and solutions preparation Reagent t-BAMBP was purchased from Ruilekang Co. Ltd. China. Two types of feed solutions which contained Cs(I) and K(I) or Rb(I) and K(I) at their original concentrations of 5 mmol/L respectively were prepared by dissolving AR grade CsCl and KCl or RbCl and KCl in distilled water. The synthetic brine solution which contained 21.5 mg/L Cs(I), 206.7 mg/L Rb(I) and 4632 mg/L K(I) was prepared through the same way. Stripping solution was prepared by diluting 37% HCl in distilled water. The other chemicals were AR reagents and were used without further purification.

2.2.

Instruments and measurements

The concentrations of metal ions were determined by inductively coupled plasma optical emission spectrometer (ICP-OES, OPTIMA 7000DV, PekinElmer, USA).

2.3.

Experimental procedures

t-BAMBP was dissolved in diluent and mixed with NaOH saponifying agent in 50 mL centrifuge tube (the A/O ratio of 1:1). Intensive mixing was carried out in incubator shaker at the rate of 150 r/min. This treatment was indeed a process of partial t-BAMBP 6


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saponification by NaOH. The organic and aqueous mixture was centrifuged and the organic phase was separated to use and NaOH solution was saved for the next cycle. The treated organic phase and feed solutions which contained Cs(I) and K(I) or Rb(I) and K(I) at their initial concentrations of 5 mmol/L respectively or contained 21.5 mg/L Cs(I), 206.7 mg/L Rb(I) and 4632 mg/L K(I) were mixed in 10 ml centrifuge tube (the A/O ratio of 1:1) and shaken in incubator shaker at the rate of 200 r/min. After extraction, the aqueous phase was removed to determine the concentrations of ions and data was used to calculate the concentrations in organic phase. The distribution ratio (D), extraction percentage (E), separation factor ( β ) involved in this study were calculated according to the following equations: D=

𝐶0 𝑉0 − 𝐶1 𝑉1 𝐶1 𝑉1

E, % =

𝐶0 𝑉0 − 𝐶1 𝑉1 × 100% 𝐶0 𝑉0

β=

𝐷𝐶𝑠(𝑅𝑏) 𝐷𝐾

(1)

(2)

(3)

Where 𝐶0 , 𝐶1 represent the concentrations of metal ions in initial and raffinate solutions respectively. E represents the extraction percentage (%). 𝑉0 , 𝑉1 represent the volume of initial and raffinate solutions respectively. 𝐷𝐶𝑠(𝑅𝑏) is the distribution ratio of Cs(I) or Rb(I) between organic and aqueous phase, 𝐷𝐾 is the distribution ratio of K(I) between organic and aqueous phase. β is the separation factor of Cs(I) or Rb(I) over K(I). 7



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The treatment and extraction were both carried out at 298.15 K if there were no special instructions. The effect of diluent, saponifying agent and time on the extraction of Cs(I)/K(I) and Rb(I)/K(I) was given in supporting information.

3. RESULTS AND DISCUSSION 3.1 Effect of t-BAMBP concentration on the extraction of Cs(I)/K(I) and Rb(I)/K(I) The effect of t-BAMBP concentration on the separation efficiency was investigated. As shown in Figure 2 & 3, the separation factors (𝛽𝐶𝑠/𝐾 , 𝛽𝑅𝑏/𝐾 ) increased firstly and then decreased while the extraction percentages (ECs(I), ERb(I)) increased along with the increase of t-BAMBP concentration. The highest separation factors (𝛽𝐶𝑠/𝐾 = 721.3, 𝛽𝑅𝑏/𝐾 = 45.2) and relatively high extraction percentages (ECs(I) = 95.4%, ERb(I) = 52.0%) were obtained while the concentrations of t-BAMBP were 1.0 mol/L and 0.9 mol/L, respectively. The increase of extraction percentages (ECs(I), ERb(I)) is attributed to the extraction capacity enhancement of organic phase. However, while the concentration of t-BAMBP is high enough, the relative increment of EK(I) is higher than that of ECs(I) and ERb(I), resulting in a decrease of the separation factors ( 𝛽𝐶𝑠/𝐾 , 𝛽𝑅𝑏/𝐾 ). Considering the separation factors (𝛽𝐶𝑠/𝐾 , 𝛽𝑅𝑏/𝐾 ) and cost of t-BAMBP, 0.9 mol/L was selected as the optimal concentration for t-BAMBP in following study.

8


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Figure 2. (A) Separation factors of Cs(I) over K(I). (B) Extraction percentages of Cs(I) and K(I). Conditions: Ct-BAMBP = 0.2-1.2 mol/L, CNaOH = 0.9 mol/L, NaOH treatment time = 1 h, CCs(I) = C K(I) = 5 mmol/L, phase ratios of treatment and extraction (A/O) were both: 1:1, T = 298.15 K, extraction time = 3 min

Figure 3. (A) Separation factors of Rb(I) over K(I). (B) Extraction percentages of Rb(I) and K(I). Conditions: Ct-BAMBP = 0.2-1.2 mol/L, CNaOH = 0.9 mol/L, NaOH treatment time = 1 h, CRb(I) = C K(I) = 5 mmol/L, phase ratios of treatment and extraction (A/O) were both: 1:1, T = 298.15 K, extraction time = 3 min

3.2. Extraction isotherms In order to investigate the extraction capacity of t-BAMBP for different metal ions, different concentrations for Cs/K and Rb/K mixtures (5 - 95 mmol/L) were considered. 9



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In different metal ions concentrations, the extractable maximum amount of metal ions in organic extractant was shown in Figure 4. Ce (mmol/L) represents metal ions concentrations in aqueous phase at extraction equilibrium and qe (mg/g) represents the amount of metal ions which are extracted by per gram of extractant at equilibrium, respectively. As shown in Figure 4, the extraction amount of Cs(I) by t-BAMBP increased rapidly with the increase of equilibrium concentration of Cs(I) in an aqueous solution, which tended to a plateau when the equilibrium concentration of Cs(I) was higher than 27 mmol/L. The maximum extraction capacity of Cs(I) was about 15 mg/g at 298.15 K. The extraction amount of Rb(I) also increased with the increase of equilibrium concentration of Rb(I) in an aqueous solution. The maximum extraction capacity of Rb(I) was about 5.6 mg/g, which is reached when the equilibrium concentration of Rb(I) was higher than 60 mmol/L. The extraction capacity of K(I) was much lower than Cs(I) and Rb(I) in both of Cs/K and Rb/K mixtures. The effect of temperature (T) on the extraction was also studied and the results were listed in table 1. The initial concentrations of Cs(I), Rb(I) and K(I) are all 5 mmol/L. As shown in table 1, the extraction percentages of Cs(I), Rb(I) and K(I) decreased with the increase of temperature from 298.15 K to 318.15 K, which indicated that low temperature is favorable to extraction. Thus, 298.15 K (room temperature) was selected as the operation temperature.

10


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Figure 4. Extraction isotherms of metal ions. The initial concentrations of Cs(I), K(I) or Rb(I), K(I) in Cs(I)/K(I) or Rb(I)/K(I) mixtures were ranged from 5 to 95 mmol/L.

Table 1. Effect of temperature on the extraction percentages Cs(I)/K(I) [E(%)] Cs(I) K(I)

Rb(I)/K(I) [E(%)] Rb(I) K(I)

298.15

94.31

3.03

52.13

2.54

303.15

93.12

2.75

44.05

2.25

308.15

91.51

2.34

35.61

1.93

313.15

90.56

1.92

32.34

1.60

318.15

88.55

1.64

26.64

1.21

Temperature [K]

3.3. Extraction kinetics The extraction kinetics experiments were carried out to investigate the equilibrium time for the extraction of Cs(I), Rb(I) and K(I). As shown in Figure5A, initially, 𝛽𝐶𝑠/𝐾 decreased rapidly with the increase of extraction time and finally reached a plateau. From Figure 5C, it was also evident that the extraction of Cs(I) and K(I) reached

11



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equilibrium in 3 and 5 min, respectively. According to the equilibrium time, the extraction rate of Cs(I) was faster than that of K(I). As shown in Figure 5B, 𝛽𝑅𝑏/𝐾 decreased with the increase of extraction time and then reached a plateau. From Figure5C, it showed that the extraction of Rb(I) and K(I) reached equilibrium in 4 and 5 min, respectively. Here, qt represents the amount of metal ions which was extracted by per gram of extractant at different time t (min). Based on the equilibrium time, it was obvious that the extraction rate of Rb(I) was also faster than K(I). The extraction rate order was: Cs(I) > Rb(I) > K(I). In addition, the separation factors in nonequilibrium conditions are much higher than that in equilibrium state, which indicated that higher separation efficiency could be obtained by controlling extraction time.

12


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Figure 5. (A) Separation factors of Cs(I) over K(I). (B) Separation factors of Rb(I) and K(I). (C) Extraction kinetics of metal ions. Conditions: Ct-BAMBP = 0.9 mol/L, CNaOH = 0.9 mol/L, NaOH treatment time = 1 h, CRb(I) = C K(I) = 5 mmol/L, phase ratios of treatment and extraction (A/O) were both: 1:1, T = 298.15 K, extraction time = 1-6 min

Herein, the kinetics models which include pseudo-first order and pseudo-second order have been provided to analyze the kinetics data. The two models were expressed in the following equations, respectively: log(𝑞𝑒 − 𝑞𝑡 ) = log𝑞𝑒 −

𝑘1 t 2.303

𝑡 1 1 = + 𝑡 2 𝑞𝑡 𝑘2 𝑞𝑒 𝑞𝑒

(4)

(5)

Where qe (mg/g) and qt (mg/g) represent the amount of metal ions which were extracted by per gram of extractant at equilibrium and different time t (min), respectively. k1 (mg/g·min) and k2 (g/(mg·min)) are the rate constant of the pseudo-first-order and pseudo-second-order model, respectively. The kinetics data of Cs(I) were used in terms

13



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of two models and fitting plots were shown in Figure 6. The values of q e, k1, k2 were calculated based on the slope and intercept of fitting plots and listed in table 2.

Figure 6. The fitting plots of pseudo-first order rate model (A) and pseudo-second order rate model (B) for the extraction of Cs(I)

Table 2. Kinetic parameters of two rate models for the extraction of Cs(I) qe,exp

pseudo-first order model

pseudo-second order model

T(K) [mg/g] qe [mg/g] 298.15

3.13

0.451

k1 [mg/g·min]

R2

qe [mg/g]

k2 [g/(mg·min)]

R2

0.886

0.9229

3.15

4.831

0.9995

Based on the correlation coefficient (R2), experimental and calculated qe, the extraction process conforms to pseudo-second order kinetics model. The kinetics results were apparently different from that in the traditional process (the equilibrium time of all three ions was about 2 min according to Li et al.)

14

. The

comparison experiments were carried out. Two types of feed solutions which contained 14


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Cs(I) and K(I) or Rb(I) and K(I) with their original concentrations of 5 mmol/L respectively and 0.9 mol/L NaOH were prepared (traditional process). The extraction time was 1 min and 3 min for the Rb(I)/K(I) and Cs(I)/K(I) mixtures, respectively. Other experiments conditions were the same as mentioned above. The experimental results on the separation efficiency between traditional and novel processes were listed in table 3. Table 3. Differences on the extraction percentages and separation factors between traditional and novel processes Cs(I)/K(I) Process

E (%) Cs(I)

K(I)

Traditional

92.8

37.6

Novel

90.3

0.6

Rb(I)/K(I) 𝛽𝐶𝑠/𝐾

E (%)

𝛽𝑅𝑏/𝐾

Rb(I)

K(I)

21.4

77.2

33.9

6.6

1550.7

52.0

2.4

45.2

As shown in table 3, in Cs(I)/K(I) mixtures, the extraction percentages of Cs(I) in novel process were similar to traditional process while the extraction percentages of K(I) were much lower by comparison with traditional process. As a result, the separation factor 𝛽𝐶𝑠/𝐾 in novel process was about 72 times of that in traditional process. Likewise, the 𝛽𝑅𝑏/𝐾 in novel process was higher 6.8 times than that in traditional process. These results indicated the novel process enlarged the difference of the equilibrium time for the ion-exchange process compared to the traditional process. Herein, we can obtain the highest separation efficiency through the enhanced kinetic differences in novel process. 15



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3.5. Extraction of Cs(I) and Rb(I) from synthetic brine solution In the following sections, the extraction of Cs(I) and Rb(I) from synthetic brine solution was studied. The ions concentrations in residual liquid which was obtained from East Taijnar Plant were 20.0 mg/L Rb(I), 2.0 mg/L Cs(I), 19.39 g/L Na(I), 28.38 g/L Mg(II), 2.772 g/L K(I) and 0.685 g/L Li(I). 26 After solar evaporation, this solution composition could be concentrated 10 times. Mg(II) were removed by precipitation. Specifically, magnesium can be removed from brine solution through adding lime (CaO) to form magnesium and a calcium insoluble double salts. Calcium can be removed by adding oxalate. Then the magnesium and calcium precipitate can be separated by filtration.28 In the meanwhile, the concentration of K(I) could drop to about 5g/L due to the co-precipitation with Mg(II). 14 The concentration of Na(I) is also high, so the removal of majority of Na(I) firstly is a feasible way for the further treatment. Reducing the Na(I) content by participating NaCl was mentioned in a patent.28 In addition, the effect of Na(I) and Li(I) on the extraction of Cs(I) and Rb(I) is little based on the literature.

26, 27

Therefore, these metal ions were not included in the current synthetic

brine solution. At last, we prepared the synthetic brine solution with 21.5 mg/L Cs(I), 206.7 mg/L Rb(I) and 4632 mg/L K(I). As shown in Figure 7, in 1 min, 𝛽𝐶𝑠/𝐾 and 𝛽𝑅𝑏/𝐾 were 492.3 and 19.6 while ECs(I) and ERb(I) were 93.6% and 37.2%.

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Figure 7. (A) Separation factors of Cs(I) and Rb(I) over K(I) in synthetic brine solution. (B) Extraction percentages of Cs(I), Rb(I) and K(I) in synthetic brine solution. Conditions: Ct-BAMBP = 0.9 mol/L, CNaOH = 0.9 mol/L, NaOH treatment time = 1 h, and the concentrations of Cs(I), Rb(I) and K(I) were 21.5 mg/L, 206.7 mg/L and 4632 mg/L, respectively. Phase ratios of treatment and extraction (A/O) were both: 1:1, T = 298.15 K, extraction time = 1-4 min

3.6. Stripping of loaded organic extractant Acid liquors are generally used as strippant to unload the metal ions and regenerate the extractants

14, 16, 29

. A noteworthy issue that researchers focus on is that the

concentration of acid liquors which is needed in the stripping process. High concentration of acid liquors not only increase reagent cost but reduce the service life of equipment and cause environmental pollution due to highly corrosive behavior. HCl is the most conventional reagent that widely used in the stripping of metal ions. In this study, various concentrations of HCl in the range of 0.1 to 1 mol/L were used to strip Cs(I), Rb(I) and K(I) from loaded organic extractant. Stripping experiments were carried out at 313.15 K and the A/O ratio was 1:1. As shown in table 4, 0.5 mol/L HCl

17



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solution is effective for the stripping of all three ions. The acid liquor concentration required is much lower than that when calixarene crown ethers are used as extractant 16

. It shows that it is easy to strip the alkali metal ions from loaded t-BAMBP molecules.

Table 4. The stripping percentages of Cs(I), Rb(I) and K(I) by different concentrations of HCl

Cs(I)

Stripping percentage (%) Rb(I)

K(I)

0.1

98.5

98.1

94.7

0.5

99.3

98.9

95.5

1.0

99.5

99.2

96.4

CHCl (mol/L)

3.7. Multistage extraction and comparison between two processes After one stage extraction, 93.6% Cs(I) were extracted into organic phase but only 37.2% Rb(I) were recovered. To achieve higher recovery of Cs(I) and Rb(I) from high concentration K(I), multistage extraction was used. In the experiments, we carried out three-stage extraction. Fresh treated organic phase was used in each stage but raffinate at the former stage was used as feed solution for the next stage. The loaded organic phase was stripped by HCl solution at the ratio (A/O) of 1:3. As shown in table 5, 99.1% Cs(I) and 86.5% Rb(I) with only 12.9% K(I) were recovered through three-stage extraction. The concentration ratios of K(I) over Cs(I) and Rb(I) decreased from 215.4 and 22.4 in feed solution to 28.1 and 3.3 in final recovery solution, respectively, which resulting in a decrease of concentration ratios by 7.7 and 6.8 times, respectively.

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In order to reach the high recovery and high selectivity simultaneously, another two methods may be effective. The first one is to investigate synergistic extraction systems. Due to synergistic effect (solvation, coordination number of metal ions etc.), the recovery and selectivity may be improved; The second one is to explore novel extractant which can have a specific selectivity for Cs(I) or Rb(I) ions according to affinity interactions such as ion recognition interactions. The recovery and selectivity may be improved simultaneously by increasing the amount of extractant with the specific selectivity. Table 5. Concentrations and ratios of Cs(I), Rb(I) and K(I) in feed and final recovery solution

Solution

Concentration [mg/L] Cs(I) Rb(I) K(I)

Concentration ratio K(I)/Cs(I) K(I)/Rb(I)

Feed

21.5

206.7

4632

215.4

22.4

Final recovery

21.3

178.8

597.8

28.1

3.3

Recovery percentage (%) 99.1

86.5

12.9

Concentration times 7.7

6.8

In order to present the advantages of novel process more clearly, the comparison details between traditional and novel processes were listed in table 6. As shown in table 6, compared with one-off addition of NaOH in feed solution, lower concentrations of tBAMBP was needed but higher separation factors were obtained. Furthermore, the concentration ratios of K(I) over Cs(I) and Rb(I) decreased much more through multistage extraction. 19



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Table 6. Comparison between the traditional and novel processes

Process

Ct-BAMBP [mol/L]

Concentration times

Separation factors

K(I)/Cs(I)

K(I)/Rb(I)

𝛽𝐶𝑠/𝐾

𝛽𝑅𝑏/𝐾 11

Traditional14

1.07

5.14

5.12

139

Novel

0.9

7.7

6.8

492.3

19.6

3.8. Recyclability of organic extractant Recyclability of organic extractant is a significant consideration while solvent extraction is used. Therefore, recyclability experiments were carried out for five times. After stripping, the organic phase was removed to be treated by NaOH solution with the same conditions as mentioned above. Then, the treated organic phase was used to extract Cs(I) and Rb(I) from fresh synthetic brine solution. The results of extraction percentages were shown in Figure 8. As shown in Figure 8, no significant reduction for the extraction percentages of Cs(I) and only a slight decrease for the extraction of Rb(I) were observed while organic extractant recycled five times. This result shows that extractant can be reused in the extraction of Cs(I) and Rb(I) from synthetic brine solution. In alkaline aqueous solution, there was a slight loss of t-BAMMBP. However, according to the literature, the loss is low even at relatively high pH (less than 0.1 gram per liter of aqueous when pH is around 12.5).27 In addition, we could recover the slight loss of the extractant through some treatment process, such as the oil wash of raffinate and pH adjusting.

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Figure 8. Extraction percentages of Cs(I), Rb(I) and K(I) in recyclability of organic extractant. Conditions: Ct-BAMBP = 0.9 mol/L, NaOH treatment time = 1 h, and the concentrations of Cs(I), Rb(I) and K(I) were 21.5 mg/L, 206.7 mg/L and 4632 mg/L, respectively. Phase ratios of treatment and extraction (A/O) were both: 1:1, T = 298.15 K, extraction time = 1 min

4. CONCLUSIONS In this study, solvent extraction as the most effective method for the extraction of Cs(I) and Rb(I) from K(I) has been investigated. A novel process aiming at improving separation efficiency and reducing environmental contamination has been proposed. Specifically, 4-tert-butyl-2-(α-methylbenzyl) phenol (t-BAMBP) dodecane solution was partially saponified by NaOH solution firstly, and then was directly used in extraction without adjusting feed solution pH to exceed 13. Only 3.33% NaOH need to be added into the used saponifying agent to make it reused for the next cycle. The maximum extraction capacity of Cs(I) and Rb(I) was about 15 mg/g and 5.6 mg/g at 298.15 K while the extraction capacity of K(I) was much lower than Cs(I) and Rb(I). 21



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The extraction process conforms to pseudo-second order kinetics model. The kinetics differences were enhanced during ion exchanges, which indicated that we could get the highest separation efficiency by controlling extraction time. After three-stage extraction, 99.1% Cs(I) and 86.5% Rb(I) and only 12.9% K(I) were recovered from synthetic brine solution. The concentration ratios of K(I) over Cs(I) and Rb(I) decreased from 215.4 and 22.4 in feed solution to 28.1 and 3.3 in final recovery solution, respectively, which resulting in a decrease of concentration ratios by 7.7 and 6.8 times.

ACKNOWLEDGEMENTS We would like to thank the following financial support: National Key Natural Science Foundation of China (grant numbers U1507203), Chinese National Technology Research and Development Program (grant numbers 2015CB251402), Chinese National Natural Science Foundation (grant numbers 21676273), the Chinese High Technology Research and Development Program (grant numbers 2015CB251402, 17163-12-ZD-001-013-01) and the Youth Innovation Promotion Association, CAS (grant numbers 2016043).

SUPPORTING INFORMATION This information is available free of charge via the Internet at http://pubs.acs.org/.

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Separation factors and extraction percentages about effect of diluent, saponifying agent and time on the extraction of Cs(I)/K(I) and Rb(I)/K(I) were given in Figure S1, S2, S3, S4, S5, S6 respectively.

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Abstract graphic

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Kinetics-Controlled Separation Intensification for Cesium and

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