The behavior of cesium adsorption on zirconyl pyrophosphate

The behavior of cesium adsorption on zirconyl pyrophosphate 1 first-author 1 1 1 1 1 corresp Corresponding author. E-mail address:quejiqueji@163.com Corresponding author. E-mail address:quejiqueji@163.com quejiqueji@163.com 1 1 1 1 Nuclear and Radiation Safety Center, Ministry of Environmental Protection, Beijing 100082, China Nuclear and Radiation Safety Center, Ministry of Environmental Protection, Beijing 100082, China Zirconyl pyrophosphate (ZrP2O7) was prepared and Cs adsorption behavior was studied. Results show that the.distribution coefficient of Cs adsorption on ZrP2O7 was about 2800 mL/g Ion exchange capacity of ZrP2O7 was 0.35 mmol/g. In dynamic tests, Cs can be separated from other fission products very well by ZrP2O7. The ZrP2O7 was stable both at high temperatures and in high concentration of nitric acid. The Cs adsorption by ZrP2O7 is a monolayer and chemical adsorption. zirconyl pyrophosphate adsorbent cesium 1 Introduction 1 A large number of high level radioactive liquid wastes (HLLWs) are produced in spent nuclear fuel reprocessing. With complex composition and strong radioactivity, the final disposal of HLLWs has been of a great concern [ 1 1 ] . At present, the treatment method is vitrification of HLLWs. Partition-transmutation [ 2 2 - 5 5 2-5 ] can reduce the risk of long- term disposal of HLLWs. Chemical separation, a key technology of partition-transmutation, can not only combine with transmutation to reduce the harm of HLLWs, but also reduce the volume of waste to be disposed. 137 Cs, as a fission product that can strongly release the decay heat, needs to be separated from the HLLWs [ 6 6 ] . Also, 137 Cs can be used as a radiation source in γ-ray gamma ray well logging devices, flow meters, thickness gauges etc . The methods of cesium separation from HLLWs include co-precipitation [ 7 7 ] , solvent extraction [ 8 8 - 12 12 8-12 ] , ion exchange [ 13 13 , 14 14 ] etc . While co-precipitation is of poor selectivity and solvent extraction is of poor radiation stability, inorganic ion exchange [ 15 15 , 16 16 ] , being advantageous in selectivity, radiation stability and thermal stability, is a desirable way to separate cesium. Multivalent metal phosphate especially zirconyl phosphate [ 17 17 , 18 18 ] has received much attention for its good adsorption property to cesium. Zirconyl phosphate is of low adsorption capacity, but this can be increased by replacing phosphate with pyrophosphate. Many studies show that zirconyl molybopyrophosphate (ZMPP) [ 19 19 - 23 23 19-23 ] ionic sieve can separate cesium from HLLWs in good selectivity and large ion exchange capacity. However, zirconyl pyrophosphate is more simple, and more suitable for large-scale industrial production in the future. In this paper, we report the synthesis of zirconyl pyrophosphate and its adsorption property study. 2 Experimental 1 2.1. Preparation of adsorbent [ 23 23 ] 2 Potassium pyrophosphate and ZrOCl 2 were mixed in acid solutions. After complete precipitation, the solution acidity was adjusted. After 24 hours, the precipitate was filtrated. It was washed with water and dried at 40°C. The product was treated with 1 mol/L HNO 3 (~20 mL of 1 mol/L HNO 3 for one gram adsorbent). It was washed with water again and dried for use. 2.2. Adsorption and elution experiment 2 2.2.1 Determination of distribution coefficient and ion exchange capacity 3 Certain amount of the absorbents was placed into a test tube, and 137 Cs solution was added (In determining the ion exchange capacity, 133 Cs solution using 137 Cs as tracer was added. The 133 Cs concentration was measured by the atomic emission spectrometry before adsorption). After the adsorption reached equilibrium, the activity was analyzed with gamma spectrometry before and after adsorption. The distribution coefficient ( K d , in mL/g) and ion exchange capacity ( Q , in mmol/g) were calculated by Eqs.(1) and (2): <math display="block" id="M1"><mrow><msub><mi>K</mi><mi>d</mi></msub><mo>=</mo><mtext> </mtext><mrow><mo>(</mo><mrow><msub><mi>C</mi><mn>0</mn></msub><mo>−</mo><mi>C</mi></mrow><mo>)</mo></mrow><mi>V</mi><mo>/</mo><msub><mrow><mrow><mo>(</mo><mrow><mi>C</mi><mi>m</mi></mrow><mo>)</mo></mrow></mrow><mrow/></msub><mo>=</mo><mtext> </mtext><mrow><mo>(</mo><mrow><msub><mi>A</mi><mn>0</mn></msub><mo>−</mo><mi>A</mi></mrow><mo>)</mo></mrow><mi>V</mi><mo>/</mo><mrow><mo>(</mo><mrow><mi>A</mi><mi>m</mi></mrow><mo>)</mo></mrow><mo> </mo><mo>,</mo></mrow></math> <math display="block" id="M2"><mrow><mi>Q</mi><mo>=</mo><mtext> </mtext><mrow><mo>(</mo><mrow><msub><mi>C</mi><mn>0</mn></msub><mo>−</mo><mi>C</mi></mrow><mo>)</mo></mrow><mi>V</mi><mo>/</mo><mi>m</mi><mo>=</mo><mtext> </mtext><mrow><mo>(</mo><mrow><msub><mi>C</mi><mn>0</mn></msub><mtext> </mtext><mi>V</mi><mo>/</mo><mi>m</mi></mrow><mo>)</mo></mrow><mtext> </mtext><mrow><mo>(</mo><mrow><mn>1</mn><mo>−</mo><mi>A</mi><mo>/</mo><msub><mi>A</mi><mn>0</mn></msub></mrow><mo>)</mo></mrow><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mo>,</mo></mrow></math> where, A 0 and A are counting rates of Cs in the aqueous phase before and after adsorption equilibration (cps)， respectively; C 0 and C are Cs concentration in aqueous phase before and after adsorption equilibration, respectively (mol/L); V is volume of the aqueous phase (mL); and m is the weight of adsorbent (g). 2.2.2 Elution of cesium 3 A certain amount of absorbents was placed into a test tube, and 137 Cs solution was added. After equilibrium, the activity was analyzed with single channel gamma spectrometry before and after adsorption. The Cs concentration was represented as activity. The ZrP 2 O 7 was washed, and the cesium was eluted by the higher concentration of acid (5 , 6 , 8 and 10 mol/L HNO 3 ). The activity after elution was analyzed with gamma spectrometry. The percentage elution was calculated by Eq.(3): <math display="block" id="M3"><mrow><mtext>Percentage elution</mtext><mo>=</mo><msub><mi>C</mi><mn>2</mn></msub><mo>/</mo><mo stretchy="false">(</mo><msub><mi>C</mi><mn>0</mn></msub><mo>−</mo><msub><mi>C</mi><mn>1</mn></msub><mo stretchy="false">)</mo><mo>=</mo><msub><mi>A</mi><mn>2</mn></msub><mo>/</mo><mo stretchy="false">(</mo><msub><mi>A</mi><mn>0</mn></msub><mo>−</mo><msub><mi>A</mi><mn>1</mn></msub><mo stretchy="false">)</mo><mo>,</mo></mrow></math> where, A 0 and A 1 are the counting rates of Cs in the aqueous phase before and after adsorption equilibration (cps). A 2 is the counting rates of Cs after elution (cps). C 0 and C 1 are the cesium concentration in the aqueous phase before and after adsorption equilibrium, respectively (mol/L). C 2 is the cesium concentration after elution (mol/L). 3 Results and Discussion 1 3.1. Cs adsorption on ZrP 2 O 7 2 3.1.1 Static experiments 3 Fig.1 Fig.1 shows the Cs distribution coefficient as function of adsorption equilibrium time. The Cs adsorption on ZrP 2 O 7 reaches equilibrium within 30 min. The adsorption rate is rapid. The adsorption percentage and distribution coefficients at different HNO 3 concentrations are listed in Table 1 Table 1 . Both the adsorption percentage and distribution coefficient decrease with increasing HNO 3 concentrations. 1 1 C HNO3 (mol/L) 1 1 0.1 1 1 0.3 1 1 0.5 1 1 1.0 1 1 1.5 1 1 2.0 1 1 2.5 1 1 K d (×100 mL/g) 1 1 28 1 1 10 1 1 6.7 1 1 4.7 1 1 3.4 1 1 2.8 1 1 2.2 1 1 %A 1 1 97 1 1 93 1 1 89 1 1 85 1 1 81 1 1 78 1 1 73 At 2mol/L HNO 3 , the adsorption percentage and distribution coefficient at 21°–80° are listed in Table 2 Table 2 . The adsorption distribution coefficients and adsorption rates decrease with increasing temperature. 1 1 Temperature(°C) 1 1 21.1 1 1 39.1 1 1 59.1 1 1 70.0 1 1 80.0 1 1 K d (mL/g) 1 1 244 1 1 127 1 1 54.7 1 1 38.0 1 1 22.4 1 1 %A 1 1 75.3 1 1 61.3 1 1 40.6 1 1 32.2 1 1 21.9 For cesium adsorption at 2 mol/L HNO 3, the cesium was eluted at 12°C, and the elution fractions were 46.7 % and 62.4% by 5 and 10 mol/L HNO 3 , respectively. And the elution fraction increased with elution temperature. Portions of ZrP 2 O 7 were weighed accurately and transferred into the test tubes, and then 5 and 10 mol/L HNO 3 , 0.5 and 1 mol/L NaOH, and 0.5 mol/L Na 2 CO 3 solution were added, respectively. The test tubes were heated in a water-bath and kept at 90°C for 2 h. The percentages of Cs adsorption cesium by ZrP 2 O 7 before and after the treatment are listed in Table 3 Table 3 . 1 1 ZrP 2 O 7 1 1 Before treated 1 1 5 mol/L HNO 3 1 1 10 mol/L HNO 3 1 1 0.5 mol/L NaOH 1 1 1 mol/L NaOH 1 1 0.5 mol/L Na 2 CO 3 1 1 %A 1 1 85 1 1 78 1 1 83 1 1 3 1 1 4 1 1 Dissolved The results show that ZrP 2 O 7 soaked with NaOH can hardly adsorb cesium, the percentage of Cs adsorption by ZrP 2 O 7 soaked with HNO 3 decreases slightly, and ZrP 2 O 7 is dissolved in 0.5 mol/L Na 2 CO 3 . It can be seen that ZrP 2 O 7 is stable at high temperature and in high HNO 3 concentration. It is preliminary speculated that −OH are presented on surface of ZrP 2 O 7 . Hydrogen of −OH may exchange with cesium ions so that ZrP 2 O 7 can adsorb cesium. When ZrP 2 O 7 is soaked with NaOH, the sodium ion exchanges with hydrogen of –OH, and cesium cannot be absorbed. This may be because zirconium of ZrP 2 O 7 and carbonate can form a complex, so that it is dissolved in Na 2 CO 3 . 3.1.2 Dynamic experiments 3 In static experiments, ZrP 2 O 7 shows good adsorption property, but 137 Cs cannot be eluted well at room temperature. Furthermore, the metal ion is usually difficult to be eluted. Therefore, the Cs elution on ZrP 2 O 7 is investigated in dynamic experiments. 3.1.2.1 Effect of nitric acid concentration on elution 4 ZrP 2 O 7 was packed into a column of 0.35 cm ×14 cm in slurry state. Then 137 Cs solution flew through the column at a flow rate of 0.1 mL/min. Finally, cesium was eluted at 12°C by 10 and 8 mol/L HNO 3 . The results are shown in Fig 2 Fig 2 . It can be seen that cesium is better eluted by 10 mol/L HNO 3 . This agrees with the static investigation. The Cs adsorption increases with decreasing HNO 3 concentration, and the elution increases with HNO 3 concentration. 3.1.2.2 Effect of temperature on elution 4 ZrP 2 O 7 was packed into a column of Φ 0.32cm×11.5 cm in slurry state, and the 137 Cs solution in 2 mol/L HNO 3 flew through the column at a flow rate of 0.12 mL/min. Finally, cesium was eluted by 8 mol/L HNO 3 at 60°C and 12°C. The results are shown in Fig 3. Fig 3. Cesium is better eluted at 60°C. These agree with the static investigation. The Cs adsorption increases with decreasing temperature, and the elution increases with temperature. 3.1.2.3 Elution by 8 mol/L HNO 3 4 Although cesium elution by 10 mol/L HNO 3 is more efficient ( Fig.2 Fig.2 ), the acid concentration is too high to protect equipment from corrosion. Therefore we tried to decrease the acid concentration. ZrP 2 O 7 was packed into a column of Φ 0.3 cm×11 cm in slurry state. The 137 Cs solution in 2mol/L HNO 3 flew through the column at a rate of 0.1 mL/min. Finally cesium was eluted by 8 mol/L HNO 3 at 60°C. As shown in Fig 4 Fig 4 , cesium can be eluted completely. 3.1.2.4 Extracting Cs from dissolved solution of the irradiated uranium 4 ZrP 2 O 7 was packed into a column of Φ 0.3 cm×11cm in slurry state. The solution dissolved with irradiated uranium in 2 mol/L HNO 3 flew through the column at a rate of 0.4 mL/min. Then the column was washed by 2 mol/L HNO 3 . Finally cesium was eluted by 8 mol/L HNO 3 at 60°C. Activities of the U-dissolved solution, washed solution, and eluted solution were measured by HPGE gamma spectrometry. The results are given in Table 4 Table 4 . Almost all the 137 Cs in the U-dissolved solution, and most of the 95 Zr and 95 Nb, were adsorbed by the column, but 144 Ce and 106 Ru were not. After being eluted, almost all 137 Cs was recovered in the solution, but 95 Zr and 95 Nb retained on the column. Cs can be separated from other fission products very well by ZrP 2 O 7 . 1 1 Isotopes 1 1 Energy (keV) 1 1 Counts per second 1 1   1 1   1 1 Decontamination coefficient 1 1   1 1   1 1 Irradiated uranium 1 1 Washed solution 1 1 Eluted solution 1 1   1 1 95 Zr 1 1 756.7 1 1 3.01 1 1 0.064 1 1 0.008 1 1 379 1 1 95 Nb 1 1 765.8 1 1 9.06 1 1 0.256 1 1 0.110 1 1 82.9 1 1 144 Ce- 144 Pr 1 1 133.5 1 1 995 1 1 1062 1 1 0.270 1 1 3.71×10 3 1 1 106 Ru- 106 Rh 1 1 511.82 1 1 55.8 1 1 57.0 1 1 0.038 1 1 1.48×10 3 1 1 137 Cs 1 1 661.7 1 1 321 1 1 0.120 1 1 323 1 1 1 3.2. Adsorption mechanism 2 3.2.1 Adsorption isotherm 3 This kind of adsorption isotherm is often described by Langmuir adsorption theory. The isothermal equation of Langmuir adsorption theory is expressed as follows [ 24 24 ] : <math display="block" id="M4"><mrow><mi>Q</mi><mo>=</mo><msub><mi>Q</mi><mrow><mtext>m </mtext></mrow></msub><mi>b</mi><mi>C</mi><mo>/</mo><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><mtext> b</mtext><mi>C</mi></mrow><mo>)</mo></mrow></mrow></math> where, Q is the adsorption capacity (mmol/g), C is the measured metal concentration in aqueous phase (mmol/L), Q m is the saturated metal concentration in aqueous phase (mmol/L), and b is a constant. Eq.(4) can be transformed to a linear equation as: <math display="block" id="M5"><mrow><mn>1</mn><mo>/</mo><mi>Q</mi><mo>=</mo><mtext> </mtext><mn>1</mn><mo>/</mo><msub><mi>Q</mi><mtext>m</mtext></msub><mo>+</mo><mn>1</mn><mo>/</mo><mrow><mo>(</mo><mrow><mtext>b</mtext><msub><mi>Q</mi><mtext>m</mtext></msub><mi>C</mi></mrow><mo>)</mo></mrow></mrow></math> If the Cs adsorption by ZrP 2 O 7 can be described by Langmuir adsorption theory, the 1/ Q should be linear with 1/ C . The relationship between 1/ Q and 1/ C of Cs adsorption on ZrP 2 O 7 at 30°C is shown in Fig.5 Fig.5 . The data are in a good linear correlation. This shows that the Cs adsorption on ZrP 2 O 7 can be described by the Langmuir adsorption theory. Therefore, it is a monolayer adsorption. It may be chemical adsorption or physical adsorption. They should be identified by adsorption heat. 3.2.2 Adsorption heat 3 The plot of ln K d as a function of 1/ T is shown in Fig.6 Fig.6 , from which the adsorption heat can be estimated at −34.40 KJ/mol. The negative value shows the adsorption is an exothermic process, with which the adsorption distribution ratio decreases with increasing temperatures. And the value was in the range of chemical adsorption heat. So we concluded that the Cs adsorption on ZrP 2 O 7 is chemical adsorption. 3.2.3 Experimental evidence of ion exchange reaction 3 The ion exchange of ZrP 2 O 7 was investigated in low acid concentrations, so that the changes in pH in the aqueous phase before and after adsorption could be measured. The changes in hydrogen and cesium ion concentrations were compared before and after adsorption ( Table 5 Table 5 ). After adsorption, the cesium ion concentration decreased and the hydrogen ion concentration increased. Although the increase in hydrogen ion concentration (Δ C Cs ) is a little smaller than the decrease of the cesium ion concentration (Δ C H ), due to probably measurement error, this suggests that hydrogen ions in the surface of ZrP 2 O 7 are replaced by the cesium in the aqueous phase. So the adsorption is a chemical adsorption. 1 1 C Cs 1 1   1 1 C H 1 1   1 1 Δ C Cs 1 1 Δ C H 1 1 B. 1 1 A. 1 1 B. 1 1 A. 1 1   1 1   1 1 70.7 1 1 45.8 1 1 1.32 1 1 20.0 1 1 25.0 1 1 18.7 The reaction of −OH presented on the surface of ZrP 2 O 7 and Cs can produce hydrogen ions. So the concentration of nitric acid is high, reverse reaction will be carried out. That explain why distribution coefficients decrease with increasing HNO 3 concentrations. 4 Conclusion 1 Zirconyl pyrophosphate (ZrP 2 O 7 ) was synthesized by a simple co-precipitation method. It has a good chemical stability both at high temperature and in high concentration of nitric acid. 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New ecomaterial zireonyl molybopyrophosphate for cesium removal from HLLW . Acta Phys. Chim. Sin ., 16 , 952 - 955 ( 2000 ). DOI: 10.3866/PKU.WHXB20001016 http://doi.org/10.3866/PKU.WHXB20001016 New ecomaterial zireonyl molybopyrophosphate for cesium removal from HLLW X.C. Fu , W.X. Shen , etc . Physical Chemistry (Volume 2 ). ( Higher Education Press , Beijing , 2006 ), pp. 361 - 375 . Physical Chemistry The authors are thankful to colleagues in the project team of the Department of radiochemistry, China Institute of Atomic Energy. 10.1007/s41365-016-0054-1 Supported by the Defense Advanced Research Project. 2015-07-13 2015-09-09 2015-09-17 2016-05-10 2016-06-20 2021-04-11 © China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2016 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-016-0054-1 http://dx.doi.org/10.1007/s41365-016-0054-1 . 2016 Nuclear Science and Techniques 03 27