I. INTRODUCTION
The selective partitioning of trivalent minor actinides (MA(III): Am(III) and Cm(III)) from the bulk of trivalent lanthanides (Ln(III)) present in high level liquid waste (HLLW) is important since some of the lanthanides exhibit large neutron-capture cross-sections, hence the decreased transmutation efficiency of MA in the partitioning and transmutation strategy [1]. Currently, several two-step partitioning processes based on liquid-liquid extraction were designed: Firstly, MA(III) and Ln(III) are extracted from HLLW using oxygen-donor ligands such as CMPO [2], DMDOHEMA [3], TRPO [4] and DIDPA [5]; Next, Am(III) and Cm(III) are selectively separated from Ln(III). However, the second step, MA(III)-Ln(III)-separation, is an exceptionally challenging task owing to their similar oxidation states, chemical properties, ionic radii, and their disadvantageous ratio of presence in HLLW [6]. This separation is not feasible using common oxygen-donor ligands, but soft nitrogen- or sulphur-donor ligands can achieve sufficient selectivity for MA(III) [7]. Among a variety of nitrogen and sulphur donor extracting agents developed, 2,6-bis(5,6-dialkyl-1,2,4-trizain-3-yl)pyridines (known as BTPs) turn out to be able to perform this difficult separation from highly acidic solutions (HNO3 > 1 mol/dm3) with good selectivity for MA(III) over Ln(III) [8]. Furthermore, BTPs follow the carbon-hydrogen-oxygen-nitrogen (CHON) principle, i.e., BTPs consist entirely of C, H, O, and N atoms, which makes them fully combustible to safe gaseous products, minimizing the generation of secondary waste [9].
A potential process application for actual MA(III) partitioning requires that extracting agent should be chemically stable, highly soluble in organic diluent, high selectivity towards MA(III), fast kinetics and high loading capacity [9]. However, so far the available BTPs suffer from one or more drawbacks in liquid-liquid extraction system [10]. This might result in forming a third phase, multi-stage extraction procedures and disposal of large volumes of organic wastes, which requires further optimization to comply with process constrains [10]. Therefore, a series of BTP derivatives where C-H bonds in the α-positions of the triazinyl rings are replaced by C-C bonds have been developed to improve the properties of extracting agents [7, 9, 10]. On the other hand, these problems can be alleviated by the use of alternative methods such as extraction chromatography [11-14]. Compared to liquid-liquid extraction method, the advantages of the MA(III) partitioning process based on extraction chromatography are the total insolubility of adsorbent in aqueous phase, China Academy of Engineering Physics, physical degradation, reduced radioactive waste inventory, compact equipment and no disposal of toxic organic solvents as waste [15]. Furthermore, compared to U and Pu, the minor actinides are significantly less abundant in the spent fuel, so the scale of the separation process for minor actinides from HLLW should be considerably smaller than that of the PUREX process [16].
To avoid the drawbacks of the liquid-liquid extraction process using BTPs as extracting agent and take full advantage of extraction chromatography method, we have reported a direct separation process of MA(III) based on extraction chromatography, which uses a single-column packed with silica/polymer-based isoHex-BTP adsorbent named as isoHex-BTP/SiO2-P (isoHex-BTP: 2,6-bis(5,6-diisohexyl)-1,2,4-triazin-3-yl)pyridine) (Fig. 1) [17, 18]. The isoHex-BTP/SiO2-P adsorbent, using silica/polymer composite support (SiO2-P) as the inert support and isoHex-BTP as the extractant, was prepared. SiO2-P support contains a macroreticular styrene-divinylbenzene copolymer which is immobilized in porous silica particles with pore size of 0.6 μm and mean diameter of 60 μm. The content of extracting agent isoHex-BTP reached up to 33.3% of the total mass of the adsorbent, so the adsorbent may overcome the limitation of the low solubility of BTPs in traditional aliphatic diluents. The other advantages of the partitioning process include its high adsorption selectivity and adsorption capacity, a minimal organic solvent, compact equipment, and the clean separation of MA(III).
In the present work, effects were made to study the initial nitric acid concentration and contact time on the adsorption of 241Am(III) and 152Eu(III) by isoHex-BTP/SiO2-P, with a batch adsorption method. The adsorption kinetics, isotherm and thermodynamics of stable nuclide Eu(III) (as a representative element of Ln(III) and a simulated element of MA(III)) by isoHex-BTP/SiO2-P were also determined.
II. EXPERIMENTAL
A. Materials
Trace amount of 241Am(III) and 152Eu(III) (1000 Bq/cm3 each) were prepared from their stocked solutions. Nitrate of Eu(III) (Eu(NO3)3⋅6H2O) reagent was of commercial reagent of analytical grade. The solutions of Eu(III) were prepared by dissolving Eu(NO3)3⋅6H2O into nitric acid solutions.
The synthesized isoHex-BTP reagent was of 99% purity. The isoHex-BTP/SiO2-P adsorbent was prepared as described in the previous studies of Wei et al. [19, 20] by impregnating isoHex-BTP molecules into the pores of the silica/polymer composite support (SiO2-P). isoHex-BTP/SiO2-P contained 0.5 g of isoHex-BTP in 1.0 g of SiO2-P, i.e. the content of extracting agent isoHex-BTP was as high as 33.3% of the total mass of the adsorbent.
B. Batch adsorption experiment
The adsorption of 241Am(III) and 152Eu(III) by the isoHex-BTP/SiO2-P adsorbent was studied in a batch adsorption mode. For each batch of experiment, 0.1 g isoHex-BTP/SiO2-P was taken in a glass vial with Teflon stopper. The 5-cm3 solution containing 241Am(III) and 152Eu(III) (1000 Bq/cm3 each) and nitric acid was added to the adsorbent. The mixture was shaken mechanically at 120 rpm for predetermined contact time at 298 K. The aqueous phase was filtrated through a membrane filter with 0.20 μm pore. The radioactivity of 241Am or 152Eu was measured by HPGe-γ-spectrometry (GEM70P-PLUS, Ortec). The distribution coefficient (Kd, cm3/g) was calculated by Eq. (1):
where A0 and Af (in Bq/cm3) are radioactivity of the radionuclides in initial and final aqueous phase, respectively; V is volume of the aqueous phase (cm3); and WR is weight of the isoHex-BTP/SiO2-P (g).
The effects of adsorption parameters including contact time (10 min–72 h), Eu(III) concentration and adsorption temperature (288±1, 298±1 and 308±1 K) on the adsorption of stable nuclide Eu(III) by isoHex-BTP/SiO2-P were studied in a batch adsorption mode as described above. The concentration of Eu(III) in aqueous phase before and after adsorption was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES: Shimadzu ICPS-7510). The equilibrium adsorption capacity, qe (mmol/g), and the adsorption capacity at time t, qt (mmol/g), were obtained using Eqs. (2) and (3).
where C0 (mmol/dm3) and Ce (mmol/dm3) are Eu(III) concentration in initial aqueous phase and at equilibrium, respectively; Ct (mmol/dm3) is Eu(III) concentration in aqueous phase at a given contact time t; V is volume of the aqueous phase (cm3); and WR is the weight of isoHex-BTP/SiO2-P (g).
Most of the experiments were repeated twice for better accuracy and blank experiments were performed. The experimental error was within ± 5%.
III. RESULTS AND DISCUSSION
A. Effect of initial nitric acid concentration on adsorption of 241Am(III) and 152Eu(III)
Figure 2 shows the distribution coefficients of 241Am(III) and 152Eu(III) towards isoHex-BTP/SiO2-P at nitric acid concentrations of 0.01–4 mol/dm3. From Fig. 2, the separation factors between 241Am(III) and 152Eu(III) were calculated as SFAm/Eu = 2;7;28;88;61;30 at [HNO3]initial = 0.01, 0.5, 1, 2, 3 and 4 mol/dm3, respectively.
It can be seen that the distribution coefficient values of 241Am(III) increased with initial nitric acid concentration due to the consumption of nitrate in complexation reaction between Am(III) and isoHex-BTP/SiO2-P, and reached the maximum value (Kd = 16246 cm3/g) at 3 mol/dm3 nitric acid, and then decreased slightly at > 3 mol/dm3 nitric acid due to the competition reaction between Am(III) and HNO3 with isoHex-BTP/SiO2-P [8]. High distribution coefficients (> 4869 cm3/g) of 241Am(III) indicated 241Am(III) quantitative adsorption from 2–4 mol/dm3 nitric acid medium. Meanwhile, 152Eu(III) showed weak adsorption within the experimental nitric acid concentration range of 0.01–4 mol/dm3. SFAm/Eu was over 60 in 2–3 mol/dm3 nitric acid solutions.
From Fig. 2, the adsorbed Am(III) can be stripped into dilute nitric acid (<0.1 mol/dm3) or distilled water because Am(III) showed almost no adsorption at ࣘ 1 mol/dm3 nitric acid and hence adsorption of Am(III) is readily reversible by changing the nitric acid concentration.
B. Effect of contac time on adsorption of 241Am(III) and 152Eu(III)
The distribution coefficient values of 241Am(III) and 152Eu(III) from 3 mol/dm3 nitric acid as a function of contact time are presented in Fig. 3. The separation factors between 241Am(III) and 152Eu(III) calculated form Fig. 3 are SFAm/Eu = 55;42;31;26;56;74 at 0.5, 1, 3, 6, 12 and 24 h, respectively. The distribution coefficients of 241Am(III) by isoHex-BTP/SiO2-P increased rapidly to 6535 cm3/g within the first 3 h of contact and the subsequently slow adsorption of 241Am(III) followed which continued for a relatively long period time until adsorption equilibrium was attained. The distribution coefficient of 241Am(III) reached up to 19413 cm3/g at 24 hours of contact, but the equilibrium state was not still attained. So the kinetics is considerably slow. Contrastively, Kd values of 152Eu(III) were always low and below 504 cm3/g within the experimental contact time range. SFAm/Eu was fairly high (>50) within 30 min and over 12 hours of contact.
From the results above, isoHex-BTP/SiO2-P exhibited a good selectivity and affinity for 241Am(III) over 152Eu(III) with relatively high separation factors at nitric acid concentration of 3 mol/dm3 within the experimental contact time of 0.5–24 h.
Prior to the column studies, it was required to study the adsorption kinetics, isotherm and thermodynamics to design MA(III) separation system by extraction chromatography method if suitable for application. Since the adsorption behavior of Eu(III) by BTP/SiO2-P adsorbent is similar to that of MA(III) owing to their similar coordination chemistry [21] and Eu(III) is a typical fission lanthanide present in HLLW, as a representative element of Ln(III) and a simulated element of MA(III), the adsorption of stable nuclide Eu(III) by isoHex-BTP/SiO2-P was studied to evaluate the adsorption kinetics, isotherm and thermodynamics in this work.
C. Adsorption kinetics of Eu(III)
To obtain the information on how the amount of adsorbed Eu(III) changes with contact time and about the process time required to achieve equilibrium between the aqueous and the adsorbent, the adsorption kinetics of Eu(III) by isoHex-BTP/SiO2-P was investigated. In this series of experiments, the initial concentration of Eu(III) was approximately 6 mmol/dm3 and the initial nitric acid was 3 mol/dm3. The adsorption capacity of Eu(III) by isoHex-BTP/SiO2-P against contact time at each temperature is given in Fig. 4. It was observed that the adsorption process occurred in two steps. The first step involved rapid Eu(III) uptake within the first 3 h of contact that was followed by a steady stage which finally reached an apparent plateau in 12 h at 298 K. Over 43%, 82% and 93% of the equilibrium adsorption capacity of Eu(III) occurred within the first 3 h at 288, 298 and 308 K, respectively. The sudden increase of qt at the very beginning of the process is attributed to an abundant availability of active sites on internal and external surface area of isoHex-BTP/SiO2-P. With the progressive occupancy of these sites by Eu(III), the process comes into a period of slower adsorption, during which the less accessible sites can be occupied by Eu(III).
The equilibrium adsorption capacity (qe,exp) of Eu(III) was 0.15 mmol/g at 288 K and 0.16 mmol/g at 308 K (Fig. 4 and Table 1).
| T(K) | Pseudo-first-order kinetic model | Pseudo-second-order kinetic model | Experimental qe,exp (mmol/g) | |||||
|---|---|---|---|---|---|---|---|---|
| qe,calc (mmol/g) | k1 (h-1) | R2 | qe,calc (mmol/g) | k2 (g/(mmol h)) | h (mmol/(g h)) | R2 | ||
| 288 | 0.11 | 0.08 | 0.936 | 0.14 | 3.56 | 0.07 | 0.989 | 0.15 |
| 298 | 0.06 | 0.13 | 0.574 | 0.16 | 13.16 | 0.33 | 1.000 | 0.16 |
| 308 | 0.04 | 0.19 | 0.915 | 0.16 | 25.57 | 0.70 | 1.000 | 0.16 |
To analyzed the adsorption kinetics of Eu(IIII) by isoHex-BTP/SiO2-P, the pseudo-first-order and the pseudo-second-order kinetic models were applied to the experimental data.
The linear form of the pseudo-first-order kinetic model can be written as Eq. (4) [22, 23]
where qe and qt (in mmol/g) are adsorption capacity of the metal ions at equilibrium and time (t), respectively; and k1 (h-1) is the adsorption rate constant.
The pseudo-first-order kinetic plots for Eq. (4) were made for Eu(III) adsorption by isoHex-BTP/SiO2-P at different temperatures (Fig. 5). The values of k1, the calculated equilibrium adsorption capacity (qe,calc) and the correlation coefficients (R2) at different temperatures were calculated (Table 1). R2 values for this model were fairly low (< 0.936), and qe,calc decreased with increasing temperature from 288 K to 308 K. The results conflicted with the experimental phenomena, so the experimental kinetic data could not fit to the pseudo-first-order kinetic model. The pseudo-first-order model fits experimental data well for an initial period of the first reaction step, but this model could not provide the best correlation for chemical adsorption process over long periods [23].
The experimental data were also applied to the pseudo-second-order kinetic model. The linear form of the model can be expressed as Eq. (5) [23]
where qt and qe (in mmol/g) are adsorption capacity of the mental ions at time t and equilibrium, respectively; k2 (g/(mmol h)) is the pseudo-second-order rate constant.
The kinetic data of t/qt versus t for Eu(III) adsorption at different temperatures are plotted in Fig. 6. The kinetic parameters including k2, the calculated equilibrium adsorption capacity (qe,calc), h (
From Table 1, the correlation coefficient of the pseudo-second-order kinetic model (> 0.989) was much higher than that of the pseudo-first-order kinetic model (< 0.936) at each temperature. Since chemisorption processes show a good compliance with the pseudo-second-order kinetic model and this model is more likely to predict the kinetic behavior of adsorption with chemical reaction being the rate-controlling step [23], it can be inferred that the adsorption of Eu(III) occurred by chemisorption and the chelation reaction between Eu(III) and isoHex-BTP /SiO2-P is the rate-controlling step of the adsorption process [8, 23]. From 288 K to 308 K, the calculated equilibrium adsorption capacities increased slightly (qe, calc = 0.14 and 0.16 mmol/g), whereas the rate constant k2 and initial adsorption rate h increased dramatically from 3.56 g/(mmol h) to 25.57 g/(mmol h) and from 0.07 mmol/(g h) to 0.70 mmol/(g h), respectively. The results were consistent with the experimental phenomena. This may be due to the increase in chemisorption rate as the rate-determining step and acceleration of diffusibility of Eu(III) in solution at higher temperatures.
D. Adsorption isotherms of Eu(III)
The adsorption capacity of Eu(III) is determined as a function of Eu(III) concentration at a constant temperature that could be explained in adsorption isotherms. The adsorption isotherm of Eu(III) by isoHex-BTP/SiO2-P was obtained at 3 mol/dm3 nitric acid, contact time of 72 h (to obtain the complete equilibrium state) by changing the initial concentration of Eu(III) within the range of 2–6 mmol/dm3, as shown in Fig. 7. The adsorption capacity of Eu(III) increased with the equilibrium concentration of Eu(III) in aqueous phase at different temperatures. Since the total existing adsorption sites of isoHex-BTP/SiO2-P were confined, adsorption finally reached an apparent plateau and saturation adsorption at higher concentration of Eu(III). The maximum adsorption capacity (qm,exp) of Eu(III) increased slightly by raising the temperature (Table 2).
| T(K) | Freundlich isotherm parameters | Langmiur isotherm parameters | Experimental qm,exp (mmol/g) | |||||
|---|---|---|---|---|---|---|---|---|
| nF | KF (mmol/g) | R2 | qm,calc (mmol/g) | KL (dm3/mmol) | RL | R2 | ||
| 288 | 4.14 | 0.13 | 0.850 | 0.16 | 4.50 | 0.10–0.04 | 0.998 | 0.15 |
| 298 | 4.22 | 0.14 | 0.638 | 0.17 | 5.89 | 0.08–0.03 | 0.992 | 0.16 |
| 308 | 7.28 | 0.15 | 0.566 | 0.17 | 7.73 | 0.07–0.02 | 0.996 | 0.16 |
The equilibrium relationship between Eu(III) concentration in the liquid phase and the concentration in isoHex-BTP/SiO2-P at individual temperature was analyzed by the Langmuir and Freundlich isotherm models [22].
The Freundlich equation is used to describe the adsorption of an adsorbate on a heterogeneous surface of an adsorbent. The logarithmic linear form of the model is given as Eq. (6) [23]
where KF and n are the Freundlich constants which relate to the adsorption capacity and the adsorption intensity, respectively. The plots of lg qe vs. lg Ce for Eu(III) adsorption by isoHex-BTP/SiO2-P are depicted in Fig. 8. The n and KF constants calculated from the slop and intercept of the lines at different temperatures are listed in Table 2.
The Langmuir model which is widely used for modeling adsorption data at equilibrium is valid for monolayer adsorption on a surface. Mathematically, the model may be represented in the linear form given in Eq. (7) [23]
where qe (mmol/g) is the equilibrium amount of an adsorbate adsorbed per specific amount of adsorbent, Ce (mmol/dm3) is the equilibrium concentration of an adsorbate in solution, qm (mmol/g) is the maximum adsorption capacity of metal ions required to form a monolayer onto an adsorbent surface, and KL (dm3/mmol) is Langmuir adsorption constant related to the adsorption affinity.
The fundamental characteristic of a Langmuir isotherm parameter (RL) can be expressed in terms of a dimensionless separation factor or an equilibrium parameter, which is defined by the following equation [22]:
where C0 (mmol/dm3) is the initial concentration of an adsorbate. According to the value of RL, the isotherm shape may be interpreted as follows:
• RL>1: unfavorable adsorption,
• RL=1: linear adsorption,
• 0<RL<1: favorable adsorption,
• RL=0: irreversible adsorption [22].
A plot of Ce/qe versus Ce would result in a straight line with a slope (1/qm) and interpret of (1/KLqm) as given in Fig. 9. Langmuir parameters for fitting adsorption data at various temperatures are listed in Table 2. The adsorption data was much better described with the Langmuir isotherm model (R2 ≥ 0.992) over the whole concentration range at various temperatures compared to the Freundlich isotherm model (R2 ࣘ 0.850) based on the correlation coefficient values. The better fit to the Langmuir model can be explained the monolayer adsorption of Eu(III) by isoHex-BTP/SiO2-P and each adsorptive site can be occupied only once in a one-on-one manner [22]. The maximum adsorption capacity (qm,calc and qm,exp) increase slightly with temperature. This may be attributed to the utilization of all available active sites for adsorption at higher temperatures. The adsorption constant KL increased by raising the temperature, indicating the adsorption affinity increasing. The value of RL (0 < RL < 1) reveals that the adsorption of Eu(III) by isoHex-BTP/SiO2-P is favorable.
E. Adsorption thermodynamics of Eu(III)
In order to explain the increase in adsorption with temperature (as described in the results of adsorption kinetics and isotherms), the thermodynamic parameters, i.e. change in enthalpy (ΔH), entropy (ΔS) and Gibbs free energy (ΔG) associated to the adsorption process were determined using the following equations [23, 24]:
where Kd is the distribution coefficient, T (K) is temperature and R (8.13 J/(mol K)) is the universal gas constant.
Figure 10 shows the relationship between 1/T and ln Kd. The calculated values are ΔH=5621 J/mol; ΔS = 54 J/(mol K); and ΔG = -9931, -10471 and -11011 J/mol at 288, 298 and 308 K, respectively. The negative ΔG value indicates that the process is spontaneous and feasible with high preference for Eu(III) by isoHex-BTP/SiO2-P. The increase in negative value of ΔG from -9931 J/mol at 288 K to -11011 J/mol at 308 K implies that adsorption tendency of Eu(III) by isoHex-BTP/SiO2-P increased as expected at higher temperatures. ΔH was positive, showing that the adsorption is endothermic, and heat was consumed to transfer Eu(III) from aqueous phase onto isoHex-BTP/SiO2-P and coordinate between Eu(III) and isoHex-BTP/SiO2-P. The positive value of ΔS suggests the increase in the degree of randomness at the solid-solution interface mostly encountered in Eu(III) binding due to the release of water molecules of the hydration sphere during the fixation of Eu(III) on isoHex-BTP/SiO2-P surface [22, 25]. Furthermore, positive ΔS value indicates that the adsorption process is probably irreversible and favored complexation adsorption. Although ΔH value was positive, ΔG value at each temperature was negative, indicating that the contribution of entropic term makes the free adsorption energy negative.
These results were in agreement with the results obtained from the pseudo-second-order kinetic model and Langmuir isotherm model.
IV. CONCLUSION
In this study, batch adsorption experiments showed that isoHex-BTP/SiO2-P exhibited remarkable selectivity towards 241Am(III) compared to 152Eu(III) at nitric acid concentration of 2–4 mol/dm3. Within the experimental contact time range of 0.5–24 h, isoHex-BTP/SiO2-P showed the relatively high selectivity for 241Am(III) over 152Eu(III) in 3 mol/dm3 nitric acid. Various factors such as contact time, Eu(III) concentration and temperature had impacts on adsorption of stable nuclide Eu(III) (as a representative element of Ln(III) and a simulated element of MA(III)) by isoHex-BTP/SiO2-P. The obtained adsorption data depending on contact time provided much better fitting for the pseudo-second-order kinetic equation with high correlation coefficient (R2 ≥ 0.989) than the pseudo-first-order kinetic model, indicating that Eu(III) adsorption by isoHex-BTP/SiO2-P occurred by chemisorption mechanism as a rate-controlling step in the adsorption process. However, the adsorption kinetics of Am(III) and Eu(III) by isoHex-BTP/SiO2-P is slow. Furthermore, the equilibrium adsorption capacity initial adsorption rate and adsorption rate constant increased with temperature from 288 K to 308 K. The experimental adsorption isotherm fitted better to the Langmuir isotherm model than the Freundlich isotherm model at various temperatures. The maximum adsorption capacity of Eu(III) and the Langmuir adsorption constant KL increased with temperature from 288 K to 308 K. The studies revealed that Eu(III) adsorption was spontaneous and endothermic process with an increase of entropy. It is expected that isoHex-BTP/SiO2-P has applicability in the separation of Am(III) from Eu(III) present in HLLW. Efforts are being made for more detailed property evaluation of isoHex-BTP/SiO2-P, and methods of improving the adsorption kinetics and the column separation experiments.
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