1 Introduction
Chelating fibre sorbents have great advantages over liquid–liquid extraction for the recovery of elements in micro-amount (micro-elements) from natural and wastewater. They offer the possibility of selective sorption of microelement in the presence of elements in macro-amount, like alkali and alkaline-earth elements, with a high effective sorption [1,2]. Sorption capacity can be enhanced using ‘filled’ fibre chelating sorbents [3–5]. These materials are obtained by introducing the ‘filler’ (polymeric complexing sorbent) during the process of fibre formation. The content of the filler in the material amounts to 50% (mass) [5].
The basic aim of the present work is the preparation and the study of new nitrogen- and phosphorus-containing chelating fibre sorbents to be used for transuranium elements (TRU), lanthanides and technetium isolation from acidic or basic solutions. The goal of our researches was: (i) to synthesize the nitrogen-containing sorbents in the form of a fibrous ‘filled’ material for demonstration of their sorption and kinetic properties in the case of isolation of TRUs from complex acid systems; (ii) to select the one suitable for insertion into the polymeric matrix on the basis of liquid–liquid performance, and (iii) to prepare new sorbents.
2 Techniques
Composition, identification and purity monitoring of produced organic compounds were carried out using NMR spectrometry on the atoms of 31P (Bruker CXP-200) and on atoms of 1H in C6D6 (Bruker AMX-400). For the identification of the oxidation state of element, spectrophotometry (Shimadzu UV-3100 PC) was used. Measurement of pH was performed using a standard electrode from Hanna Instruments (Hanna-4400). A liquid-to-liquid ratio of 1, and a liquid-to-solid ratio (V:m) of 100 cm3 g–1 was used in liquid-liquid extraction and in batch experiment sorption. The contact time ranged from 5 min to 24 h at 20 °C. The distribution coefficients (DM, Kd) of the elements have been determined by measuring the activity of the solution containing appropriate radionuclides before and after extraction or sorption. Isotopes of 239Pu, 241Am, 233U, 152Eu, 137Cs, 231Pa, 237Np, and 99Tc were used. The range of the radionuclides concentrations was 10–6–10–5M. The fibrous ‘filled’ sorbents of POLIORGS – a family of chelating materials produced at the Vernadsky Institute RAS – were used.
3 Experimental results
3.1 Extraction studies
The derivatives of ethers of phosphoric acid, namely O-(2-alkyl) (diethylcarbamoylmethyl) phenylphosphinates (O2ADECMPP) were synthesized and examined for the liquid–liquid extraction of elements M (M = TRU, Ln and Tc) from nitric acid solutions. There is a non-linear increase of DM as a function of the carbon radical extension on these phosphoric-acid ethers when meta-nitrobenzotrifluoride (Ftoropol-723) or 1,2-dichlorethane was used as solvent. Maximums of DM are found at n = 9 (R = CnH2n+1) for all elements used. However, in all further experiments, the easily available and equally efficient O2EHDECMPP reagent was used (n = 8). The maximums of DM for Am(III) and Eu(III) extraction by O2EHDECMPP in 1,2-dichlorethane were observed at [HNO3] = 3 M for [E] = 0.1 M (with E = extractant), DAm = 1.45 and DEu = 1.02. For Pu(IV), and U(VI) extraction, the increase in the HNO3 concentration in the range 0.1–5 M leads to an increase in DM and, at [E] = 0.01 M, the values of DPu change from 0.48 to 35, and those of DU change from 0.057 to 4.8. On the contrary, for Tc(VII) extraction by O2EHDECMPP in 1,2-dichlorethane, the increase in the HNO3 concentration causes a noticeable decrease in DTc. In the [HNO3] range from 0.1M to 5 M, DTc decreases from 9.6 to 0.013 ([E] = 0.1 M).
Dependencies of the distribution coefficients for TRU`s, Ln and Tc(VII) on [E] were measured for 3 M HNO3. For Am(III), Eu(III), and Tc(VII), the increase in reagent concentration in the range 0.025–0.1 M leads to an increase in DAm from 0.057 to 1.45, in DEu from 0.055 to 1.02, and in DTc from 0.01 to 0.12. In logarithmic coordinates, the dependence slope of DAm and DEu versus [E] is close to 2, upon which we considered that the composition of the extracted complexes is: Am.2E and Eu.2E. For Tc(VII) extraction, the solvate number in similar conditions is about 1.5. Probably, in the extraction system, two different Tc complexes (mono- and bi-coordinated species) were formed. For Pu(IV) and U(VI), the increase in the reagent concentration in the [E] range from 10–4 to 0.01 M leads to an increase in the DM:DPu ratio from 0.08 to 11, and in DU from 0.02 to 3.2. Values of the solvate number calculated from the slopes of these bi-logarithmic plots for Pu(IV) and U(VI) are close to 1, and we considered that the composition of the extracted complex is: M·E.
3.2 Sorption studies
3.2.1 Granulated sorbents
On the basis of the O2ADECMPP compounds, new chelating granulated sorbents were synthesized, which were tested with respect to Pu(IV) isolation from nitric acid solutions. The immobilization of O2EHDECMPP was realized using two methods: impregnation and chemical fixation of the phosphinate-groups on the solid matrix. The impregnate was prepared using the so-called ‘dry method’. For this purpose, aliquots of O2EHDECMPP were brought into contact with the XAD-7™ pattern in chloroform solutions during 4 h under mixing. The mixture obtained was streamed and dried. The phosphorus content of the impregnate was 0.85% (mass) – (sorbent 1). The chemical fixation was provided using: (i) the Mekhaelis–Bekker reaction using the chloromethylated copolymer of styrene with divinylbenzene – the phosphorus content in the interaction reagent was 3.4% (in mass) (sorbent 2) –; (ii) the Arbouzov reaction – the content of phosphorus in the product was 7% (in mass) (sorbent 3). Pu(IV) sorption on sorbent 1 in nitric acid gives the larger values of Kd (Kdmax = 1760 cm3 g–1 in 5 M HNO3). The Kd values for Sorbents 2 and 3 changed from 1250 and 850 cm3 g–1 (in 0.03 M HNO3) to 850 and 750 cm3 g–1 (in 5 M HNO3), respectively. Complete isolation of Pu (IV) ions by phosphorus sorbents 1–3 is obtained after 2 h.
3.2.2 Filled fibrous sorbents
The ‘filled’ fibrous complexing sorbents POLYORGS 4-n with 3(5)-methylpyrazole groups and POLYORGS 17-n with 1,3(5)- dimethylpyrazole groups, which possess the anion exchanger properties, were tested for the Pu solution. We used also an anion exchanger with quaternary ammonium groups in the form of ‘filled’ fibrous material (AV-17-n). Only Pu among other actinide elements is completely sorbed by these ‘filled’ fibre sorbents from nitric acid solutions at 1–5 M within 10-min contact at 20 °C. The distribution coefficients of Pu (IV) (KdPu) were determined as 1.5 × 103, 103, and 9 × 102 cm3 g–1 by POLYORGS 17-n, 4-n and AV-17-n, respectively. The values of KdPu were not changed in the presence of NaNO3 concentration up to 1 M. For stripping of Pu(IV), we used 0.5–1.0M HNO3 solutions. Species of Am(III), Eu(III), U(VI), Pa(V), Np(V), Tc(VII), and Cs(I) were badly or not sorbed at all by these materials. The separation factors of Pu(IV) from other elements (f = KdPu/KdM) are listed in Table 1.
Separation factors of Pu(IV) from other elements (V:m = 100 cm3 g–1; τc = 2 h; 5 M HNO3)
| Radionuclides | Separation factors of Pu(IV) from other elements | |
| POLIORGS-4-n | POLIORGS-17-n | |
| Am(III) | 106 | 106 |
| Eu(III) | 106 | 102 |
| U(VI) | 106 | 0.75 × 102 |
| Pa(V) | 0.92 × 102 | 102 |
| Np(V) | 0.57 × 102 | 0.58 × 102 |
| Tc(VII) | 0.92 × 102 | 0.74 × 102 |
| Cs(I) | 106 | 3 × 102 |
The distribution coefficients concerning the pertechnetete ion Tc(VII) for sorption by fibrous ‘filled’ sorbents from acid, neutral and basic solution were measured. AV-17-n and POLYORGS-17-n quantitatively isolates Tc from low acidic as well as from neutral and alkaline media. Investigation of the sorption degree as a function of time showed that the sorption process is almost finished in 10–20 min at room temperature. Technetium (VII) desorbed with 3 M HNO3 solutions. The measured KdTc are listed in Table 2. The maximum of Tc sorption by fibrous sorbents AV-17-n and POLYORGS 17-n takes place for solutions with pH about 3–5 and in 0.1 M NaOH. Distribution coefficients are decreasing with increasing the acid concentration in the solution from 0.001 to 0.5 M HNO3 and from 0.1 to 1 M NaOH.
Distribution coefficients (Kd) for Tc(VII) sorption from solutions (V:m = 100 cm3 g–1; τc = 2 h)
| Solution | Sorbents | |
| AV-17-n | POLYORGS-17-n | |
| HNO3 (M) | Kd | |
| 10-3 | 3.7 × 10–4 | 1.0 × 10–3 |
| 10-2 | 1.3 × 10–4 | 2.5 × 10–3 |
| 0.1 | 1.2 × 10–3 | 2.2 × 10–3 |
| 0.2 | 670 | 300 |
| 0.5 | 380 | 200 |
| 3.0 | < 1 | < 1 |
| [NaOH] (M) | Kd | |
| 0.1 | 1.0 × 10-3 | 300 |
| 1.0 | 900 | – |
| [NaNO3] (M) | Kd | |
| 0.1 | 860 | 450 |
4 Conclusion
The results obtained show that ‘filled’ fibrous sorbents POLYORGS can be used for the isolation of plutonium from nitric acid solutions and for its separation from other elements. POLYORGS can be used also for concentration of technetium from low acidic and alkaline solutions. ‘Filled’ fibrous sorbents possess good filter capacity and fast kinetic properties provided by fibrous properties and by small-size filler particles.
For TRU elements, in comparison with the extraction capacity of O2ADECMPP and well-known phosphorous-containing compounds, the following ratio has been obtain: DAm(TBP):DAm(dialkylmethylphosphanates):DAm(O2ADECMPP):DAm(diphenyldibuthyl(carbamoylmethyl) phosphine oxide) = 1:5 × 101:2.5 × 104:5 × 104. For Tc(VII), the best extractant is O2ADECMPP.