Sodium-cesium extractions

Most of the studies carried out on cesium extraction conclude that the most efficient crown ethers for extraction of this cation are benzo-21-crown-7 derivatives. Like dicarbollides, these compounds need a synergistic agent or polar diluent modifier to allow cesium to be extracted from very acidic solutions. The resulting selectivity for cesium over sodium is low. Only dialkoxy-calix[4]arene-crown-6 and calix[4]arene bis(crown-6) compounds allow objectives to be fulfilled extraction of cesium at low-level concentration from acidic media.19... 

Sachleben et al. observed that for bis(alkoxy)calix[4]arene monocrown ethers, reducing the size of the alkoxy substituent from octyl to allyl increased the cesium extraction by 10%-30% and the cesium-to-potassium selectivity by 20%-40%, with little impact on cesium over sodium selectivity. A standard modeling approach was used to analyze the complementarity of the calix-crown cavity toward potassium and cesium. MM3 optimizations were performed by modifying the K+ and Cs+ complexes, replacing the 1,3-dioxybenzene-substituent with tert-butoxy, methoxy, or hydrogen groups. 

Luo et al.90 have described yet another approach to reducing the impact of ion exchange in metal ion extraction by neutral extractants in ILs, one which relies on modifying neither the structure of the IL nor the properties of the extractant. Instead, a sacrificial species that transfers in preference to the IL cation upon metal ion extraction (thereby reducing loss of the IL) is added to the IL phase. Ideally, the sacrificial species should exhibit no affinity for the extractant (in order not to interfere with extraction of the metal ion of interest) and be more hydrophilic than the IL cation (in order to favor its loss to the aqueous phase upon metal ion transfer). Tests with sodium tetraphenylborate indicate that its addition to a solution of a calix-crown ether in [C4mim+][Tf2N ] reduces the loss of the IL induced by cesium extraction by nearly one-quarter with no adverse effect on the efficiency of cesium extraction. 

CEA Cadarache and seven European universities were involved in a research programme to synthesize and test new macrocyclic extractants (crown-ethers and calixarenes). The main aim of this study was to selectively remove caesium, strontium and actinides from medium level liquid waste (MLLW) to decontaminate them to the extent that they can be disposed of in a near surface site. A new calixarene has been synthesized by the University of Parma, which has a cesium/sodium selectivity 100 times higher than that of the best current extractant for cesium. The large selectivity of this molecule has been explained theoretically. The results obtained for cesium extraction from simulated MLLW have been confirmed with real HLLW. Finally, new functionalized calixarenes have been also synthesized, which are more selective to actinides and lanthanides than the best extractant available on the market. 

Data on cesium extraction from cesium nitrate and nitric acid yielded the formation constants of CsN03Calix (o) and (CsN03)2Calix(o) the notation (o) refers to the organic phase. Addition of data on potassium extraction from potassium nitrate alone or mixed with cesium nitrate yielded the formation constant of KN03Calfac(o). Finally, addition of data on potassium and cesium extraction from sodium nitrate allowed us to calculate the formation constant of NaN03Calix(o). 

Direct Reduction with Metals. PoUucite can be directly reduced by heating the ore in the presence of calcium to 950°C in a vacuum (20), or in the presence of either sodium or potassium to 750°C in an inert atmosphere (21). Extraction is not complete. Excessive amounts of the reducing metal is required and the resultant cesium metal is impure except when extensive distiUation purification is carried out. Engineering difficulties in this process are significant, hence, this method is not commerciaUy used. 

Decomposition with Bases. Alkaline decomposition of poUucite can be carried out by roasting poUucite with either a calcium carbonate—calcium chloride mix at 800—900°C or a sodium carbonate—sodium chloride mix at 600—800°C foUowed by a water leach of the roasted mass, to give an impure cesium chloride solution that is separated from the gangue by filtration (22). The solution can then be converted to cesium alum [7784-17-OJ, CS2SO4 Al2(S0 2 24H20. Extraction of cesium from the poUucite is almost complete. Solvent extraction of cesium carbonate from the cesium chloride solution using a phenol in kerosene has also been developed (23). 

Cesium isotopes can be recovered from fission products by digestion in nitric acid, and after filtration of waste the radioactive cesium phosphotungstate is precipitated using phosphotungstic acid. This technique can be used to prepare radioactive cesium metal or compounds. Various processes for removal of Cs isotopes from radioactive waste have been developed including solvent extraction using macrocycHc polyethers (62) or crown ethers (63) and coprecipitation with sodium tetraphenylboron (64). 

In this procedure the soil sample (spiked with isotopic marker compounds) is processed in a two-part enrichment procedure (Fig. 5.3). In part I, a mixture of the sample and sodium sulphate is subject to solvent extraction, and the extract is, in the same process, passed through a series of silica-based adsorbents and then through the carbon/glass fibre adsorbent. The extract passes through the adsorbents in the following order potassium silicate, silica gel, cesium or potassium silicate, silica gel and finally an activated-carbon... 

One problem in refining cesium is that it is usually found along with rubidium therefore, the two elements must be separated after they are extracted from their sources. The main process to produce cesium is to finely grind its ores and then heat the mix to about 600°C along with liquid sodium, which produces an alloy of Na, Cs, and Ru, which are separated by fractional distillation. Cesium can also be produced by the thermochemical reduction of a mixture of cesium chloride (CsCl) and calcium (Ca). 

The debenzylated AT-boc-protected diamine (469 mg, 1 mmol), 3-bromopro-pionic acid (158 mg, 1 mmol), cesium carbonate (652mg, 2mmol) and lOmL of acetone were placed in a 50 mL round-bottomed flask equipped with a magnetic stir bar and a reflux condenser. The mixture was heated under reflux for 24 h. The volatiles were removed on a rotary evaporator and the residue was triturated with dichloromethane. The extracts were filtered and dried over anhydrous sodium sulfate after evaporation of the solvent on a rotary evaporator, a pale yellow foam was obtained which was purified by silica gel chromatography (eluent CH2Cl2/hexane, 1/2, v/v) to yield white crystals (362 mg, 67 %). m.p. 161-163 °C. 

Tetraphenylborate (TPB) was used at Savannah River to recover cesium from alkaline solutions, but attempts to treat HLW tanks with TPB resulted in the production of benzene (a TPB decomposition product) at levels that did not permit the safe operation of the process.8 Crown ethers and dicarbollides were proposed as extractants to remove cesium from acidic HAW, but these compounds are not selective enough to allow cesium to be removed from solutions containing large amounts of nitric acid or sodium nitrate.9 Dicarbollides were used in Russia at industrial scale to recover cesium from HAW, but the removal of cesium was only possible after partial denitration of the liquid waste.10... 

Extraction of Cesium and Sodium Cs/Na Selectivity and Competitive Extraction of Cesium in the Presence of an Excess of Sodium... 

Electrospray Ionization/Mass Spectrometry (ES/MS), a soft-ionization desorption technique using polar solvents such as water, methanol, or acetonitrile, was used for direct measurement of cations in solution. The first measurements carried out with mono or bis(crown-6) calix[4]arenes from an equimolar cation-extractant solution confirm that the calixarenes mono(crown-6) extract only one cesium cation. On the contrary, in the same conditions, bis(crown-6) calix[4]arenes can extract two cesium cations for a ratio Cs/BC6 equal to 2.5. The binuclear complex (composed of two cesium cations) is the major species. Cesium/sodium selectivity measurements implementing various mono or bis(crown-6) calix[4]arenes were in agreement with liquid/liquid results.42... 

Very high distribution ratios are obtained with ligands including one and two benzo units in the polyethylene chain, DCs = 45 and DCs = 56, respectively. This enhancement of distribution ratios is explained by a lesser extraction of sodium, linked to a better complementarity of these ligands with cesium (Table 4.6).46... 

For the calix[4]arene-bis(crown-6), the trends are the same on adding a benzo (BC5) or a naphtho group (BC10) in improving the extraction efficiency and the selectivity for cesium over sodium. However, the distribution ratios, although higher... 

In conclusion, calix[4]arene-bis(crown-6) and dialkoxy calix[4]arene crown-6, and especially their dibenzo derivatives, which display an important hydrophobic-ity, seem best suited for the extraction of cesium from very acidic media or media containing large amounts of sodium. In 2006, Mohapatra confirmed the high efficiency and selectivity of BC5 and BC10 for the extraction of cesium from simulated acidic high-activity level waste. The Cs distribution ratio follows the trend of diluent... 

Extraction of tetrahedral pertechnetate anion from aqueous solutions using several crown ethers is well known. The coextraction of cesium (or strontium) and technetium from nuclear waste by calix[4]arene-crown-6 has been reported from alkaline media. Although technetium in its common pertechnetate form does not complex directly with crown ethers, pertechnetate extraction may be facilitated by crown ethers as the coanion of sodium (for alkaline nitrate waste). Pertechnetate at trace levels in the waste may be more than a 1000-fold more extractable than the smaller nitrate anion in ion-pair extraction processes.87... 

This general phenomenon was observed when extracting cesium from neutral sodium nitrate as well as from alkaline solutions. In 1,2-DCE, however, the increase in DCs is less pronounced (possibly due to 1,2-DCE s higher polarity, which affords moderate DCs at low modifier concentrations). Nevertheless, both DCs and lij increase as a function of the concentration of the alcohol modifier 1, as shown in Table 4.18. 

Although solvent samples have been observed for approximately one year without any solids formation, work was completed to define a new solvent composition that was thermodynamically stable with respect to solids formation and to expand the operating temperature with respect to third-phase formation.109 Chemical and physical data as a function of solvent component concentrations were collected. The data included BC6 solubility cesium distribution ratio under extraction, scrub, and strip conditions flowsheet robustness temperature range of third-phase formation dispersion numbers for the solvent against waste simulant, scrub and strip acids, and sodium hydroxide wash solutions solvent density viscosity and surface and interfacial tension. These data were mapped against a set of predefined performance criteria. The composition of 0.007 M BC6, 0.75 M l-(2,2,3,3-tetrafluoropropoxy)-3-(4-.sw-butylphenoxy)-2-propanol, and 0.003 M TOA in the diluent Isopar L provided the best match between the measured properties and the performance criteria. 

Experiments suggested a potential cesium-stripping problem in the CSSX process due to the presence of nitrite in the waste stimulant. The true reason for the cesium-stripping problem was, in fact, the presence of surfactant (sodium mono- and dimethyl naphthalene sulfonate) that can partition into the organic phase on extraction and then retain cesium upon stripping. To overcome this drawback, the authors proposed a caustic wash with a moderate concentration of sodium hydroxide sufficient to remove enough of these lipophilic anions to rejuvenate the solvent, therefore,... 

A mixture of well-known extractants, di-(2-ethylhexyl)phosphoric acid (HDEHP) and CMPO, in n-paraffin was used for the study of combined extraction of different actinides (americium, plutonium, and uranium) and lanthanides (cerium and europium) and their separation from fission products (cesium, strontium, ruthenium, and zirconium).54 Combined extraction of MAs and lanthanides was studied together with group separation of MAs from lanthanides by selective stripping with a solution of diethylenetriaminepentaacetic acid (DTPA), formic acid, and hydrazine hydrate. This solution strips only MAs, leaving lanthanides in the organic phase. Subsequently, the lanthanides are stripped using a mixture of DTPA and sodium carbonate. 

Lamare, V., Bressot, C., Dozol, J. F., Vicens, J., Asfari, Z., Ungaro, R., and Casnati, A. Selective Extraction of Cesium at Tracer Level Concentration from a Sodium Nitrate Solution With Calixcrowns. Molecular Modeling Study of the Cs+/Na+ Selectivity, Sep. Sci. Technol. 32 (1997), 175-191. 

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