The radiochemical separation methods

Radiochemists detect the presence of radionuclides in a sample either by their emitted radiation or by separating them according to their atomic mass.
It is important to understand which radionuclide is present, in order to assess how to manage the sample.
Many beta and alpha decaying radionuclides also emit gamma rays. If the energies of such gamma rays are high enough, they can be detected directly from the samples and therefore there is no need for radiochemical separations. Otherwise, a specific radiochemical separation procedure is necessary for isolating the alpha and beta radionuclides.
Radiochemical analyses allow both qualitative identifications and quantitative determinations. Radionuclides nearly always chemically behave identically to their corresponding stable isotopes. Radiochemical separation methods are mainly based on ordinary analytical procedures, thus employing chemical and physical principles that apply to their nonradioactive isotopes.
Some peculiar features of radioactive atoms

distinguish radioanalytical chemistry from ordinary analytical chemistry.

PRECIPITATION
The oldest radiochemical separation method is precipitation. Marie Curie used precipitation methods in the late 1890s to separate the radioactive elements, radium and polonium, from ore minerals. In the precipitation process, the sample is dissolved in a solution. The target radionuclide is separated from the sample solution, by making it precipitate as a solid compound and leaving the undesired and interfering elements in solution.
Being A the target radionuclide, for its precipitation as a compound A_nB_m, the solubility product (K_ps) must be exceeded:

$K_{ps}=[A]^n \cdot [B]^m$

where [A] and [B] are the concentrations of the cation and anion in the solution and n and m are the stoichiometric coefficients in the precipitation reaction:

$nA^{m+}+mB^{n-} \rightarrow A_n B_m \downarrow$

Because the concentrations of radionuclides in solution are generally very low by mass, their solubility products are seldom exceeded without the addition of a suitable carrier. The addition of a carrier, in the same chemical form as the target radionuclide is also necessary to prevent the radionuclide absorption on surfaces or other losses phenomena. A carrier is called an isotopic carrier when it is a compound containing a stable isotope of the target radionuclide, having the same chemical behavior. When a suitable isotopic carrier is not available, a nonisotopic carrier must be used, that is a stable compound with a chemical behavior analogous to that of the target radionuclide. When a nonisotopic carrier is used for precipitation, the process is called coprecipitation. For example, if the target radionuclide is ¹³¹I as iodide anion, an isotopic carrier is KI and a nonisotopic carrier is KB_r.
As an example, if we would like to determine the pure beta emitter C-14 content of a graphite sample, we could burn the sample generating ¹²CO₂ and ¹⁴CO₂, trap them in Ca(OH)₂ solution and then exploit the ¹⁴CO₂ precipitation as CaCO₃.

Ion exchange chromatography
A second radiochemical separation method is based on the ion exchange, by using organic cation and anion exchange resins (Figure 1). This method is called ion exchange chromatography.

Figure 1 - Ion exchange principle

A granular ion exchange is packed into a glass or plastic column. The solid sample is dissolved in a suitable solution. The sample solution is poured into the column, and the target radionuclide ions are adsorbed to the column by ion exchange. The column is then rinsed with a suitable eluent in order to remove nonsorbing radionuclides and any

other species from the column. The adsorbed radionuclide ions are then eluted out – usually with dilute acids or with complexing agents. The composition of the eluent can be adjusted so that radionuclides adsorbed with different strengths are separated from one another. Separation is based on the equilibrium of the ions between the two solid and liquid phases (ion exchanger and elution solvent, Figure 2).

Figure 2 - Ion exchange chromatography

Changing ionic strength, pH, composition of the eluent, also the separation of species with small differences in charge is achieved. Each column is

characterized by a loading capacity. A breakthrough curve reflects the concentration profile in the adsorber column and is therefore useful for a realistic estimation of the performance of an adsorbent. Important applications of ion exchange chromatography in radiochemistry are the lanthanides and the actinides separations. As an example, in a solution obtained by leaching a rock sample with acid, the separation of uranium from other natural alpha-emitting radionuclides (Th, Ra, Po) can be obtained by ion exchange in one step. By using a strongly basic anion exchange resin in concentrated hydrochloric acid, Uranium, along with Polonium, is retained by the column as metal chloride complexes, differently by Thorium and radium. Uranium is separated by elution with diluted HCl, while polonium still remains in the column. Since generally separation performance is typically better than in precipitation, no carrier is required in ion exchange chromatography. Moreover, it is not used because its presence could lead to column breakthrough faster.

SOLVENT EXTRACTION
A third method is Solvent extraction. Solvent extraction involves the transfer of the target radionuclide from one liquid phase to another one, immiscible with the first (Figure 3).

Figure 3 - Solvent extraction

A granular ion exchange is packed into a glass or plastic column. The solid sample is dissolved in a suitable solution. The sample solution is poured into the column, and the target radionuclide ions are adsorbed to the column by ion exchange. The column is then rinsed with a suitable eluent in order to remove nonsorbing radionuclides and any

The radioactive metal ion in the aqueous phase cannot be extracted to the organic phase as such, because its electroneutrality must be preserved in both phases. Furthermore, the metal ions form complexes containing water not usually soluble in organic solvents. Before the metal ion can be extracted from the aqueous phase to the organic solution, the metal ion must form a neutral molecule that is soluble in the organic phase. Such neutral molecules could be simple molecules or compounds, coordination complexes or chelates. Specific ligands can be used to form complexes or chelates. They have features that confer them a selectivity towards a specific metal family. In solvent extraction the extraction efficiency is described by the distribution ratio D, that is defined as the total concentration of the metal in the organic phase divided by its total concentration in the water phase:

$D=[A_{M}]_{org}/[A_{M}]_{acq}$

When the ligand is able to selectively extract the metal 1 in presence of a metal 2 (Figure 3), the selectivity is defined as the ratio between the distribution ratio of metal 1 over the distribution ratio of metal 2:

$SF_{M1/M2}=D_{M1}/D_{M2}$

Solvent extraction is the method used in the industrial reprocessing of spent nuclear fuel. U and Pu are separated from spent fuel by PUREX (Plutonium and Uranium Extraction) process. Solvent extraction is used also for the separation of ⁵⁵Fe from fallout samples collected within environmental monitoring of nuclear sites. The iron is first concentrated through precipitation as Fe(OH)₃. The precipitate is dissolved in 8M HCl, where the Fe(III) ion forms an [FeCl₄]- complex. This is then extracted fairly specifically into an organic phase containing di-isopropyl ether, being followed only by ¹²⁵Sb. Then, iron is retained by means of a suitable cation exchanger, not effective for Sb.

Extraction chromatography
A new method, accepted into wide use during the last decades of the last century, is extraction chromatography, which combines solvent extraction as the separation method with column chromatography technology used earlier in ion exchange.
The solvent extraction agents, functioning as the stationary phase, are impregnated into a porous inert support, either silica gel or an organic polymer.
The space between the beads provide passage for the mobile phase, normally nitric or hydrochloric acid, which contains the radionuclides to be separated. In extraction chromatography, as in ion

exchange chromatography, a small volume of the sample solution is poured into the column.
Those nuclides that do not transfer to the stationary phase are flushed out of the column. The elements that were retained in the stationary phase are then eluted by adjustment of the composition of the eluent.
One of the main application of extraction chromatography within radiochemistry regards the actinide separations.
Nowadays, thanks to the progresses achieved in this field, radiochemists have availability of several separation methods to efficiently determine the presence of radionuclides in different kinds of samples by routine analyses.

Figure 4 - Extraction chromatography