1 Introduction
Zirconium and hafnium, a pair of co-existing elements, are commonly used as engineering materials in nuclear industry for their specific nuclear properties. Zircaloy is generally used as structural components in reactor cores for its low thermal neutron capture cross-section, whereas hafnium alloy has been employed as controlling materials because of its 640 times higher neutron adsorption cross-section. The hafnium ratio in zirconium has a direct effect on the safety and efficiency of the reactor cores. Therefore, the nuclear grade zirconium sponge requires to contain hafnium of less than 100 ppm[1,2]. However, as brother symbiosis of the same subgroup, their chemical similarity is greater than that of any other two elements in the periodic table (atomic radius: Zr=0.145 nm, Hf=0.144 nm; ionic radius: Zr(IV)=0.074 nm, Hf(IV)=0.075 nm)[3]. The mutual separation of Zr and Hf is recently of intriguing interest in the area of nuclear industry.
Numerous processes for separating zirconium and hafnium have been conducted, and there are only three established technologies in industrial scale, i.e., the multiple crystallization of potassium zirconium fluoride, the solvent extraction using methyl isobutyl ketone (MIBK) or tributyl phosphate (TBP) or tri-n- octylamine (TOA) and the extractive distillation of the chlorides[4,5,6]. There are still many problems inherent inextricable in these processes, such as emulsification, flooding and loading limits, sophisticated technologies, phase disengagement, and large solvent consumption[7]. Recently, increasing environmental concern and energy consumption lead to the search for a new effective separation process.
One of the most efficient methods is the adsorption and separation based on solid phase, for example, the organic (synthetic/natural) or inorganic adsorbents can be treated as extraction chromato- graphy[8]. In extraction chromatography, an inert support either alone or in combination with a suitable diluent is impregnated with an organic extractant, producing a solid sorbent capable of selectively removing certain metal ions. In brief, the extraction chromatography combines the selectivity of the solvent extraction process with the simplicity and multistage character of the column chromatographic system, so it has some attractive advantages such as minimal organic solvent utilization, less waste accumulation, compact equipment, simple operation and simultaneous separation of multi-components. Hence, as an alternative separation method, the extraction chromatography used for separation of zirconium and hafnium is desirable[9,10].
An ideal extractant in separation process should consist only of C, H, O, and N atoms rather than S or P atoms to make them combustible to gaseous products after utilization. In recent years, a chelating agent, N, N, N′, N′-tetraoctyl-3-oxapentane-1, 5-diamide (TODGA) containing no S or P atoms is attracting extensive attentions as a promising extractant to separate high-valence metal ions. There have been many reports on its systematic adsorption ability to various elements[11,12,13,14,15,16]. A. Sh. Saleh et al. investigated the extraction behaviors of zirconium and hafnium by TODGA, indicating the effective extraction from HNO3 ≥ 3 mol/L[17]. Also, several other extractants abbreviated as BTP, Calix[4] arene-R14 and HDEHP were extensively used to separate Zr and Hf[18,19,20]. The present work mainly deals with preparing a silica-based macroporous TODGA adsorbent, exploring the optimum adsorption condition and mechanism of Zr(IV) and Hf(IV) from HCl solution.
2 Experimental
2.1 Materials
A neutral chelating agent, TODGA (more than 96.5 % purity), the BTP, Calix-arene-R14, and HDEHP (Kanto Chemical Co. Inc.) were used without further purification. The silica-based polymer support (SiO2-P) was synthesized as described previously[21]. The symbol (P) contained in the SiO2-P particles refers to the styrene–divinylbenzene (SDB) copolymer prepared by means of a polymerization reaction inside the macroporous SiO2 substrate. The Zr(IV) and Hf(IV) standard solutions (1000 mg·L–1) were from Alfa Aesar. Zirconium oxychloride (ZrOCl2∙H2O, 99.8%) and hafnium tetrachloride (HfCl4, 99.9%) were from J &K Scientific Ltd. Dichloromethane, dodecyl- benzenesulfonic acid, methanol, HCl, and NaOH standard solutions and other chemicals were all analytical reagent (Sinopharm Chemical Reagent Co. Ltd.). The distilled water was further purified using a Milli-Q (Millipore) system (18-ΜΩ resistance) and used throughout the experiments.
2.2 Preparation of adsorbents
In order to improve affinity between SiO2-P particles and extractant, the SiO2-P particles were activated by methanol as follows. The SiO2-P particles were mechanically mixed with the methanol in a conical flask for about 1 h at the 20 r/min, and separated using a sintering glass filter of 0.45 μm pore. The similar operation was repeated three times. The activated SiO2-P particles were dried in a vacuum oven at around 313 K for 12 h. The TODGA/SiO2-P was synthesized as follows.
The immobilization and impregnation of TODGA into the SiO2-P particles and the polymerization reactions inside SiO2-P particles were performed under the vacuum state. The weighted quantity of extractant was dissolved completely in dichloromethane (200 mL) and the double amount of the activated SiO2-P particles were weighed accurately, mixed into a conical flask (300 mL), and stirred mechanically for 120 min at room temperature. For Calix[4]arene-R14 adsorbent, it also needed to simultaneously add proportional dodecyl benzene sulphonic acid as a molecule modifier. Then the mixture was moved subsequently into a silicon-oil bath and further stirred at about 313 K for 120 min to impregnate and immobilize TODGA molecule into the pores of the SiO2-P particles. The operation temperature in synthesis process was kept using an EYELA OHB-2000 Model temperature controller (Tokyo Rikakikai Co. Ltd., Japan). The impregnation process was continued until almost all of organic solvent, i.e. dichloromethane, was evaporated. Following drying in a vacuum drying oven at around 313 K overnight, light yellow macroporous silica- based polymeric composite was obtained.
Thermal stability of the adsorbent was evaluated by a thermal gravimetry analyzer (Shimadzu T-60) at the operating temperature range from 30–600°C with a heating rate of 2ºC·min‒1 under oxygen atmosphere.
2.3 Batch adsorption experiments
The adsorption evaluation of the adsorbent was conducted using a thermostatic water bath shaker in a wide range of acidity , metallic ions concentration and temperature. All solutions used for adsorption were freshly prepared because both Zr(IV) and Hf(IV) metal ions are apt to hydrolyze and polymerize. The concentration of mineral acids in aqueous phase was investigated in the range of 0.1–8.0 mol/L and the operation temperature was controlled in the range of 298 K–323 K. The phase ratio was 0.05 g of adsorbent to 5 cm3 of solution. A weighed quantity of adsorbents was put into a 10 cm3 glass flask with 5 cm3 solution described above, then the mixture was shaken at 120 r/min for determined contact time under given temperature. After phase separation through a membrane filter with a mean pore of 0.45 µm, the concentrations of Zr(IV) and Hf(IV) metal ions before and after adsorption in aqueous phase were determined utilizing an ICP-AES instrument (Shimadzu ICP-75 10). The equilibrium adsorption capacity (Qe, mmol/g), distribution coefficient (Kd, cm3/g) and separation factor (SF) were calculated as follows.
where C0 and Ce denote the initial and equilibrium concentrations in aqueous phase (mg/L); M means the molar mass of Zr and Hf; m represents the mass of adsorbent, g; V(cm3) is the volume of the aqueous phase used in the experiment.
3 Results and discussion
3.1 Screening of adsorbents
In order to choose the optimum adsorbent to separate Zr(IV) and Hf(IV), adsorption selectivity with different adsorbents was firstly investigated in batch experiment under the same metal ions concentration in HCl solution. Fig.1 shows the variational distribution coefficient and seperation factor of Zr(IV) and Hf(IV) with different adsorbents. The results indicated that the TODGA adsorbent had the largest SF between Zr(IV) and Hf(IV), though the HDEHP adsorbent had the maximal adsorption capacity. As a result, the TODGA adsorbent was chosen as the candidate adsorbent in the next experiments for its excellent separation and friendly environment.
3.2 Characterization of adsorbents
The microscopic image were shown in Fig.(2).
The TG‒DTA curves for TODGA/SiO2-P indicated that there were obvious weight losses at about 300°C due to thermal decomposition of styrene- divinyl benzene (SDB) copolymer which could be abbreviated as the letter 'P’ in the SiO2-P. The overall weight loss of SiO2-P was estimated to be 16.9%, it meant that 16.9 wt% SDB had been polymerized inside the SiO2 substrate. As for TODGA/SiO2-P, the TG curve appeared two weight loss peaks at around 200°C and 300°C. This indicates that the TODGA mainly decomposed at about 200°C. The TG-DTA, composition of TODGA/SiO2-P adsorbent was determined as: TODGA, SDB and SiO2 share 33.1, 11.3 and 55.6 wt%, respectively. Generally, the adsorption capacity of adsorbents increases with the content of TODGA impregnated inside SiO2-P particles. However, it was observed that when the content of TODGA was higher than 33.3%, the redundant TODGA molecules could not enter the pores of the SiO2-P particles and only adhered on the particles surface[22], as shown in Fig.3. The physical and chemical parameters of the SiO2-P and TODGA/SiO2-P are listed in Table 1.
| Physical parameters of SiO2-P | Chemical component of TODGA/SiO2-P | ||||
| Mean pore size | 50 nm | Unit of analysis | SiO2 (wt%) | SDB(wt%) | Solvent (wt%) |
| Bead diameter | 40–60 µm | SiO2–P in synthesis | 81.6 | 18.4 | / |
| Spe Surface area | 80 m2/g | SiO2–P in TGA | 83.1 | 16.9 | / |
| Pore volume | 1.0 cm3/g | in synthesis | 54.4 | 12.3 | 33.3 |
| SDB inside | / | Adsorbents in TGA | 55.6 | 11.3 | 33.1 |
| SiO2-P support | / | / | / | / | / |
3.3 Effects of variety and concentration of acids on Kd
Generally, the extraction mechanism of TODGA as a neutral chelating agent to metallic ions is direct and uncomplicated. According to Hecke and Goethals’ report, diglycolamide (DGA) in the TODGA chelating agent contains three oxygen atoms that vigorously capture the metallic ions, so it acts as a tridentate ligand[23]. While various species such as M4+, MO2+, [M(OH)]3+, [M(OH)2]2+, [M(OH)3]+, [M(OH)4] and [M(OH)5]– may be yielded since Zr(IV) and Hf(IV) are both hydrolysable and polymeric in hydrochloric acid solutions (M represents Zr or Hf)[24,25]. The reactions of zirconium and hafnium in water were the subjects of much controversy. Aqueous solution chemistry and hydrolytic polymerization of their aqueous metallic ions have been reported in several papers[26,27,28,29]. So in order to estimate the optimum conditions for the separation between Zr(IV) and Hf(IV), the uptake experiment was carried out with three kinds of inorganic acids in the concentration range from 0.1 mol/L to 8 mol/L. A higher pH value was not examined because of the precipitation of Zr(IV) and Hf(IV) at pH >2[30].
As the results, the distribution coefficients of zirconium and hafnium as a function of acid concentration are shown in Fig.4.
It was obvious that the TODGA/SiO2-P adsorbent had a larger separation factor in the chloride system than those in the other two acid systems. The situation is complicated, one reason for this phenomenon may be explained as the stronger complexation of zirconium and hafnium metallic ions with the chloride ions at low acidities. So adsorption behaviors of the adsorbent in batch experiment were evaluated in hydrochloric acid solution.
Combining X-ray absorption spectros-copy (XAFS) and nano-electrospray mass spectrometry (ESI-MS), Walther et al. investigated polynuclear species of zirconium in acidic aqueous solution at 0 < pH < 3 for [Zr]=1.5–10 mmol/L. While the monomer remained a minor species, the degree of polymerization increased and the formations of tetramers, pentamers, octamers and larger polymers were observed with increasing pH[26]. Johnson et al. studied hydrolytic behaviors of ca. 0.05 mol/L Zr(IV) and Hf(IV) at the acidity of 0.5–5 mol/L HCl[27]. As a conclusion, Hf(IV) was probably either a trimer or tetramer and the degree of polymerization was relatively independent of Hf(IV) and HCl concentration in the range of 0.5–2 mol/L HCl. While at acidities of 0.2 mol/L or lower, Hf(IV) was more highly polymerized and polydisperse. In the range of 0–2 mol/L HCl, polymerization of Zr(IV) was similar. The principal difference is that Zr(IV) was more highly aggregated than Hf(IV) at the lower acidity.
As shown in Fig.5, the adsorption of Zr(IV) and Hf(IV) onto TODGA/SiO2-P adsorbent emerged a similarity and the Kd value tended to change greatly with a parabolic trend when increasing HCl concentration of 0.1‒6 mol/L. The maximum Kd value was obtained at about 0.8 mol/L and 0.5 mol/L HCl for Zr(IV) and Hf(IV), respectively. These results indicate that TODGA had a relatively greater extracting ability with the trimer or tetramer of Zr(IV) and Hf(IV) in HCl solution. At the same acidity, Zr(IV) had a higher Kd than that of Hf(IV) probably for the reason that Zr(IV) was more highly aggregated than Hf(IV) at the lower acidities mentioned above. Under the higher pH conditions, polymerization increased and the formation of tetramers, pentamers, octamers and larger polymers decreased the extraction of Zr(IV) and Hf(IV) hydrolyzed species[26,27]. The drop of Kd for both elements at higher HCl concentration may be due to a higher degree of protonation for the extractant. On the other hand, it was also possible that the high concentration HCl restricted hydrolyzing and polymerizing of Zr(IV) and Hf(IV) metallic ions to form trimer or tetramer. The findings indicate that the TODGA/SiO2-P adsorbent has stronger adsorption and higher selectivity for Zr(IV) over Hf(IV), and maximum separation acidity is 0.8–1.5 mol/L HCl.
3.4 Adsorption isotherms
Adsorption isotherms were obtained within a wide range of initial total concentrations at two different temperatures, as shown in Fig.6. From the results, we can derive the conclusion that the adsorbent had a larger adsorption capacity to Hf(IV) than Zr(IV) in a single component solution and adsorption capacity to both ions increased with temperature rising.
When adsorption achieved saturation, theoretical isotherm models of Freundlich equation and Langmuir equation were adopted to explore adsorption mechanism and calculate maximum capacity of the adsorbent. The Langmuir equation assumes that the adsorption process is a monolayer adsorption on a homogeneous surface. Meanwhile, the Freundlich equation is an empirical equation applied to monolayer adsorption on the heterogeneous surface [31,32]. They are expressed as below, respectively.
where Ce (mmol·dm–3) is the equilibrium concentrations of Zr and Hf in aqueous phase; Qe (mmol·g–1) and Qmax (mmol·g–1) denote the adsorption amounts at equilibrium state and the maximum adsorption state; KL (dm3·mmol–1) is the Langmuir constant which relates to the energy of adsorption; 1/n is the Freundlich isotherm exponent constant which relates to the adsorption intensity; Kf (mmol·g–1) is the Freundlich constant.
Figs.7A and 7B show the linearized Langmuir and Freundlich adsorption isotherms, respectively. The parameters of two isotherms obtained from experimental data and the related correlation coefficients (R2) are presented in Table 2. Better correlation coefficient (0.99) obtained from the Langmuir isotherm suggested that the adsorption of Zr and Hf were monolayer adsorption controlled by homogeneous active sites of the adsorbent. Since KL reflects the affinity between the active sites and metallic ion, the gradually increase of KL value in Langmuir isotherm with the temperature indicates a positive activation energy. Meanwhile, we can surely draw the conclusion that Zr(IV) had a more stronger affinity than Hf(IV) to the TODGA/SiO2-P adsorbent.
| Metal | T (K) | Langmuir isotherm | Freundlich isotherm | |||||
| Qe(mmol/g) | Qmax(mmol/g) | kL(L/mmol) | R2 | Kf(mmol/g) | n | R2 | ||
| Zr | 298 | 0.47 | 0.485 | 4.18 | 0.99 | 17 | 8.3 | 0.83 |
| 323 | 0.566 | 0.574 | 6.67 | 0.99 | 26 | 12.1 | 0.88 | |
| Hf | 298 | 0.414 | 0.425 | 2.73 | 0.99 | 26 | 6.2 | 0.72 |
| 323 | 0.485 | 0.49 | 6.05 | 0.99 | 43 | 8.1 | 0.84 | |
3.5 Adsorption thermodynamics
The values of standard ∆H° and ∆S° for Zr and Hf in adsorption reactions were calculated from the slopes and intercepts of the linear van’t Hoff equation as shown below (Fig.8).
The Gibbs free energy of the adsorption, ∆G° was calculated from
All the thermodynamic parameters are summarized in Table 3. As shown in the table, negative ΔG° values indicate that the adsorption processes took place spontaneously. The enthalpy change values for both metallic ions are positive, indicating that the adsorptions of zirconium and hafnium are endothermic processes. It is a notable phenomenon because adsorption processes for some trivalent rare earths using this adsorbent are exothermal[29]. The system is complex. One reasonable interpretation may be due to the facilitation of the metal ions hydrolysis when raising temperature. Also, the data |ΔH°|<|–TΔS°| in Table 3 showed that the adsorption processes were dominated by entropic changes rather than enthalpy changes. By comparing the enthalpy change values, it is obvious that temperature has a greater influence on the adsorption of Hf(IV) than that of Zr(IV), which could be controlled to increase the adsorption difference for better separation.
| T(K) | Zr | Hf | ||||||
| ΔH/kJ·mol‒1 | ΔS/J·mol‒1∙K‒1 | ΔG /kJ∙mol ‒1 | TΔS/kJ∙mol‒1 | ΔH/kJ∙mol‒1 | ΔS/J∙mol‒1∙K‒1 | ΔG/kJ∙mol‒1 | TΔS/kJ∙mol‒1 | |
| 298 | 18 | 109.5 | –14.6 | 32.6 | 49.8 | 202.9 | –10.5 | 60.3 |
| 308 | 18 | 109.5 | –15.7 | 33.7 | 49.8 | 202.9 | –12.5 | 62.3 |
| 318 | 18 | 109.5 | –16.8 | 34.8 | 49.8 | 202.9 | –14.6 | 64.4 |
4 Conclusion
Several kinds of macroporous silica-based adsorbents were synthesized and the TODGA/SiO2-P adsorbent was found to have a higher adsorption selectivity to Zr(IV) than that of Hf(IV). The adsorbent was characterized by means of SEM and TG-DTA. The uptake behaviors in three kinds of inorganic acid solutions were investigated and the adsorption mechanism was attempted to be explained. By fitting different isotherm models and calculating thermodynamic parameters, the adsorption procedures for both metals were found to be endothermic with a monolayer adsorption on a homogeneous surface. The TODGA adsorbent could be promising for the separation between Zr(IV) and Hf(IV) due to its good adsorptive selectivity in preliminary experiment.
