1 Introduction
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]. At present, the treatment method is vitrification of HLLWs. Partition-transmutation [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. 137Cs, as a fission product that can strongly release the decay heat, needs to be separated from the HLLWs[6]. Also, 137Cs 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], solvent extraction[8-12], ion exchange [13,14] etc. While co-precipitation is of poor selectivity and solvent extraction is of poor radiation stability, inorganic ion exchange [15,16], being advantageous in selectivity, radiation stability and thermal stability, is a desirable way to separate cesium. Multivalent metal phosphate especially zirconyl phosphate [17,18] 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) [19-23] 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
2.1. Preparation of adsorbent[23]
Potassium pyrophosphate and ZrOCl2 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 HNO3 (~20 mL of 1 mol/L HNO3 for one gram adsorbent). It was washed with water again and dried for use.
2.2. Adsorption and elution experiment
2.2.1 Determination of distribution coefficient and ion exchange capacity
Certain amount of the absorbents was placed into a test tube, and 137Cs solution was added (In determining the ion exchange capacity, 133Cs solution using 137Cs as tracer was added. The 133Cs 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 (Kd, in mL/g) and ion exchange capacity (Q, in mmol/g) were calculated by Eqs.(1) and (2):
where, A0 and A are counting rates of Cs in the aqueous phase before and after adsorption equilibration (cps)， respectively; C0 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
A certain amount of absorbents was placed into a test tube, and 137Cs 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 ZrP2O7 was washed, and the cesium was eluted by the higher concentration of acid (5, 6, 8 and 10 mol/L HNO3). The activity after elution was analyzed with gamma spectrometry. The percentage elution was calculated by Eq.(3):
where, A0 and A1 are the counting rates of Cs in the aqueous phase before and after adsorption equilibration (cps). A2 is the counting rates of Cs after elution (cps). C0 and C1 are the cesium concentration in the aqueous phase before and after adsorption equilibrium, respectively (mol/L). C2 is the cesium concentration after elution (mol/L).
3 Results and Discussion
3.1. Cs adsorption on ZrP2O7
3.1.1 Static experiments
Fig.1 shows the Cs distribution coefficient as function of adsorption equilibrium time. The Cs adsorption on ZrP2O7 reaches equilibrium within 30 min. The adsorption rate is rapid.
The adsorption percentage and distribution coefficients at different HNO3 concentrations are listed in Table 1. Both the adsorption percentage and distribution coefficient decrease with increasing HNO3 concentrations.
| CHNO3 (mol/L) | 0.1 | 0.3 | 0.5 | 1.0 | 1.5 | 2.0 | 2.5 |
|---|---|---|---|---|---|---|---|
| Kd (×100 mL/g) | 28 | 10 | 6.7 | 4.7 | 3.4 | 2.8 | 2.2 |
| %A | 97 | 93 | 89 | 85 | 81 | 78 | 73 |
At 2mol/L HNO3, the adsorption percentage and distribution coefficient at 21°–80° are listed in Table 2. The adsorption distribution coefficients and adsorption rates decrease with increasing temperature.
| Temperature(°C) | 21.1 | 39.1 | 59.1 | 70.0 | 80.0 |
|---|---|---|---|---|---|
| Kd (mL/g) | 244 | 127 | 54.7 | 38.0 | 22.4 |
| %A | 75.3 | 61.3 | 40.6 | 32.2 | 21.9 |
For cesium adsorption at 2 mol/L HNO3, the cesium was eluted at 12°C, and the elution fractions were 46.7 % and 62.4% by 5 and 10 mol/L HNO3, respectively. And the elution fraction increased with elution temperature.
Portions of ZrP2O7 were weighed accurately and transferred into the test tubes, and then 5 and 10 mol/L HNO3, 0.5 and 1 mol/L NaOH, and 0.5 mol/L Na2CO3 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 ZrP2O7 before and after the treatment are listed in Table 3.
| ZrP2O7 | Before treated | 5 mol/L HNO3 | 10 mol/L HNO3 | 0.5 mol/L NaOH | 1 mol/L NaOH | 0.5 mol/L Na2CO3 |
|---|---|---|---|---|---|---|
| %A | 85 | 78 | 83 | 3 | 4 | Dissolved |
The results show that ZrP2O7 soaked with NaOH can hardly adsorb cesium, the percentage of Cs adsorption by ZrP2O7 soaked with HNO3 decreases slightly, and ZrP2O7 is dissolved in 0.5 mol/L Na2CO3. It can be seen that ZrP2O7 is stable at high temperature and in high HNO3 concentration.
It is preliminary speculated that −OH are presented on surface of ZrP2O7. Hydrogen of −OH may exchange with cesium ions so that ZrP2O7 can adsorb cesium. When ZrP2O7 is soaked with NaOH, the sodium ion exchanges with hydrogen of –OH, and cesium cannot be absorbed. This may be because zirconium of ZrP2O7 and carbonate can form a complex, so that it is dissolved in Na2CO3.
3.1.2 Dynamic experiments
In static experiments, ZrP2O7 shows good adsorption property, but 137Cs cannot be eluted well at room temperature. Furthermore, the metal ion is usually difficult to be eluted. Therefore, the Cs elution on ZrP2O7 is investigated in dynamic experiments.
3.1.2.1 Effect of nitric acid concentration on elution
ZrP2O7 was packed into a column of 0.35 cm ×14 cm in slurry state. Then 137Cs 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 HNO3. The results are shown in Fig 2. It can be seen that cesium is better eluted by 10 mol/L HNO3. This agrees with the static investigation. The Cs adsorption increases with decreasing HNO3 concentration, and the elution increases with HNO3 concentration.
3.1.2.2 Effect of temperature on elution
ZrP2O7 was packed into a column of Φ0.32cm×11.5 cm in slurry state, and the 137Cs solution in 2 mol/L HNO3 flew through the column at a flow rate of 0.12 mL/min. Finally, cesium was eluted by 8 mol/L HNO3 at 60°C and 12°C. The results are shown in 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 HNO3
Although cesium elution by 10 mol/L HNO3 is more efficient (Fig.2), the acid concentration is too high to protect equipment from corrosion. Therefore we tried to decrease the acid concentration. ZrP2O7 was packed into a column of Φ0.3 cm×11 cm in slurry state. The 137Cs solution in 2mol/L HNO3 flew through the column at a rate of 0.1 mL/min. Finally cesium was eluted by 8 mol/L HNO3 at 60°C. As shown in Fig 4, cesium can be eluted completely.
3.1.2.4 Extracting Cs from dissolved solution of the irradiated uranium
ZrP2O7 was packed into a column of Φ0.3 cm×11cm in slurry state. The solution dissolved with irradiated uranium in 2 mol/L HNO3 flew through the column at a rate of 0.4 mL/min. Then the column was washed by 2 mol/L HNO3. Finally cesium was eluted by 8 mol/L HNO3 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. Almost all the 137Cs in the U-dissolved solution, and most of the 95Zr and 95Nb, were adsorbed by the column, but 144Ce and 106Ru were not. After being eluted, almost all 137Cs was recovered in the solution, but 95Zr and 95Nb retained on the column. Cs can be separated from other fission products very well by ZrP2O7.
| Isotopes | Energy (keV) | Counts per second | Decontamination coefficient | ||
|---|---|---|---|---|---|
| Irradiated uranium | Washed solution | Eluted solution | |||
| 95Zr | 756.7 | 3.01 | 0.064 | 0.008 | 379 |
| 95Nb | 765.8 | 9.06 | 0.256 | 0.110 | 82.9 |
| 144Ce-144Pr | 133.5 | 995 | 1062 | 0.270 | 3.71×103 |
| 106Ru-106Rh | 511.82 | 55.8 | 57.0 | 0.038 | 1.48×103 |
| 137Cs | 661.7 | 321 | 0.120 | 323 | 1 |
3.2. Adsorption mechanism
3.2.1 Adsorption isotherm
This kind of adsorption isotherm is often described by Langmuir adsorption theory. The isothermal equation of Langmuir adsorption theory is expressed as follows [24]:
where, Q is the adsorption capacity (mmol/g), C is the measured metal concentration in aqueous phase (mmol/L), Qm 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:
If the Cs adsorption by ZrP2O7 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 ZrP2O7 at 30°C is shown in Fig.5. The data are in a good linear correlation. This shows that the Cs adsorption on ZrP2O7 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
The plot of lnKd as a function of 1/T is shown in 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 ZrP2O7 is chemical adsorption.
3.2.3 Experimental evidence of ion exchange reaction
The ion exchange of ZrP2O7 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). After adsorption, the cesium ion concentration decreased and the hydrogen ion concentration increased. Although the increase in hydrogen ion concentration (ΔCCs) is a little smaller than the decrease of the cesium ion concentration (ΔCH), due to probably measurement error, this suggests that hydrogen ions in the surface of ZrP2O7 are replaced by the cesium in the aqueous phase. So the adsorption is a chemical adsorption.
| CCs | CH | ΔCCs | ΔCH | ||
|---|---|---|---|---|---|
| B. | A. | B. | A. | ||
| 70.7 | 45.8 | 1.32 | 20.0 | 25.0 | 18.7 |
The reaction of −OH presented on the surface of ZrP2O7 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 HNO3 concentrations.
4 Conclusion
Zirconyl pyrophosphate (ZrP2O7) 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. The static experiments show that the Cs adsorption distribution coefficient is 2800 mL/g and the ion exchange capacity is 0.35 mmol/g. In dynamic tests, Cs can be well separated from other fission products. The Cs adsorption by ZrP2O7 is a monolayer and chemical adsorption. The adsorption mechanism is that the hydrogen in the surface of ZrP2O7 is exchanged by cesium. ZrP2O7 may likely be a selective ion exchanger for removal of 137Cs directly from strong acid HLLW.
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