Scavenger precipitation

The Scavengerfällung [ skævɪndʒə (r) - ] (also called "co-precipitation by adsorption ") is an in the radiochemistry used process for separating carrier-free (or arms) radionuclides from solutions by adsorption on the surface of a precipitate.

Direct precipitation of radionuclides

A precipitation reaction only occurs when the solubility product of the substance to be precipitated is exceeded. However, radionuclides are often present in such low molar concentrations that the solubility product cannot be exceeded even for poorly soluble compounds, which is why direct separation by precipitation is not possible.

For example, the solubility product of strontium sulfate (SrSO ₄ ) at a temperature of 25  ° C has a value of K _sp  = 3.44 · 10 ⁻⁷ mol ² / l ² :

${\displaystyle K_{\mathrm {sp} }=c(\mathrm {Sr^{2+}} )\cdot c(\mathrm {SO_{4}^{2-}} )=3{,}44\cdot 10^{-7}\;\mathrm {mol^{2}} \cdot \mathrm {l^{-2}} }$

This means that if the strontium sulfate  is to be precipitated with sulfuric acid (H ₂ SO ₄ ) with a concentration c = 1 mol / l, the strontium concentration must have a value of c (Sr ²⁺ ) = 3.44 · 10 ⁻⁷  mol / l significantly exceed so that precipitation can occur.

For a carrier-free radionuclide, the activity A corresponding to an amount of substance n results from the law of decay

${\displaystyle A=-{\frac {\mathrm {d} N}{\mathrm {d} t}}=\lambda \cdot N={\frac {\ln 2}{T_{\text{1/2}}}}\cdot N}$

and using Avogadro's constant N _A

${\displaystyle N_{\mathrm {A} }={\frac {N}{n}}=6{,}022\;140\;76\cdot 10^{23}\;\mathrm {mol} ^{-1}}$

to

${\displaystyle A=n\cdot {\frac {N_{\mathrm {A} }\cdot \ln 2}{T_{\text{1/2}}}}}$.

Therefore, a molar concentration of c  = 3.44 · 10 ⁻⁷  mol / l for a carrier-free ⁹⁰ Sr solution ( half-life : T _1/2  = 29.12  a  ≈ 9.2 · 10 ⁸  s ) corresponds to an activity concentration c _A of 1.6 · 10 ⁸  Bq / l:

${\displaystyle {\begin{aligned}c_{A}={\frac {A}{V}}&=c\cdot {\frac {N_{\mathrm {A} }\cdot \ln 2}{T_{\text{1/2}}}}\\&=3{,}44\cdot 10^{-7}\;\mathrm {mol} \cdot \mathrm {l^{-1}} \cdot {\frac {6{,}022\;140\;76\cdot 10^{23}\;\mathrm {mol} ^{-1}\cdot \ln 2}{9{,}2\cdot 10^{8}\;\mathrm {s} }}=1{,}6\cdot 10^{8}\;\mathrm {Bq} \cdot \mathrm {l^{-1}} \end{aligned}}}$

For comparison: The maximum permissible activity concentration of ⁹⁰ Sr in the water that is discharged from radiation protection areas into sewers is only c _A  = 4 · 10 ³  Bq / m ³  = 4 Bq / l according to Section 29 StrlSchV . The concentrations of carrier-free radionuclides that typically occur in wastewater or environmental samples are therefore much too low to be recorded by direct precipitation.

Co-precipitation of radionuclides

Coprecipitation through isomorphic replacement

One possibility for the precipitation of radionuclides is the addition of a stable (i.e. non-radioactive) carrier that behaves chemically like the radionuclide under consideration, so that the total solubility product can be exceeded. The radionuclide can then be precipitated together with the carrier.

Co-precipitation through adsorption (scavenger precipitation)

Bulky precipitation of iron (III) hydroxide

In contrast to coprecipitation by isomorphic replacement, scavenger precipitation offers the possibility of separating carrier-free radionuclides - especially if the use of a carrier is undesirable or not possible. The radionuclides are adsorbed on the surface of a freshly precipitated precipitate. The effectiveness of this co-precipitation by adsorption depends heavily on the size of the adsorbing surface of the precipitate. Therefore, voluminous precipitates (e.g. of hydroxides ) are particularly suitable for the adsorption of radionuclides.

The co-precipitation by adsorption is also influenced by the charge on the surface of the precipitate and by the charge of the radionuclide. For example, cationic radionuclides are more strongly adsorbed on a precipitate if it contains an excess of anions on its surface.

Otto Hahn summarizes the conditions of co-precipitation by adsorption in Hahn's rule of adsorption :

"An ion present in any dilution is strongly adsorbed on a precipitate if the precipitate has a charge opposite to the ion and the compound formed after adsorption is sparingly soluble or poorly dissociated in the given solvent."

For a scavenger precipitation, those precipitates are particularly suitable that form large surfaces, such as. B. the hydroxides of polyvalent metal ions (e.g. Fe ^III , Mn ^IV , Sb ^V or Sn ^IV ), salts of polyvalent metal ions with polybasic acids (e.g. phosphates , tungstates , arsenates of Zr, Ti, Sn ^IV , Cr ^III ) as well as salts of heteropoly acids , hexacyanoferrates, etc.

Individual evidence

1. ^ Karl Heinrich Lieser: Introduction to nuclear chemistry . 3. Edition. VCH, Weinheim 1991, ISBN 3-527-28329-3 , pp. 493-494 .
2. ↑ Lieselott Herforth , Hartwig Koch: practical course in radioactivity and radiochemistry . 3. Edition. Johann Ambrosius Barth, 1992, ISBN 3-335-00347-0 , p. 315-316 .
3. ↑ David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Solubility Product Constants, pp. 8-119.
4. ↑ CODATA Recommended Values. National Institute of Standards and Technology, accessed July 30, 2019 .  Value for the Avogadro constant. The value is exact.
5. ↑ Lieselott Herforth , Hartwig Koch: practical course in radioactivity and radiochemistry . 3. Edition. Johann Ambrosius Barth, 1992, ISBN 3-335-00347-0 , p. 316-318 .
6. ^ Karl Heinrich Lieser: Introduction to nuclear chemistry . 3. Edition. VCH, Weinheim 1991, ISBN 3-527-28329-3 , pp. 496 .
7. ↑ ^{a b} Lieselott Herforth , Hartwig Koch: Practical course in radioactivity and radiochemistry . 3. Edition. Johann Ambrosius Barth, 1992, ISBN 3-335-00347-0 , p. 318 .
8. ^ Karl Heinrich Lieser: Introduction to nuclear chemistry . 3. Edition. VCH, Weinheim 1991, ISBN 3-527-28329-3 , pp. 496-497 .
9. ^ Gregory R. Choppin, Jan-Olov Liljenzin, Jan Rydberg: Radiochemistry and Nuclear Chemistry . 3. Edition. Butterworth-Heinemann, 2001, ISBN 978-0-7506-7463-8 , pp. 243 .
10. ↑ Lieselott Herforth , Hartwig Koch: practical course in radioactivity and radiochemistry . 3. Edition. Johann Ambrosius Barth, 1992, ISBN 3-335-00347-0 , p. 318-319 .
11. ↑ Lieselott Herforth , Hartwig Koch: practical course in radioactivity and radiochemistry . 3. Edition. Johann Ambrosius Barth, 1992, ISBN 3-335-00347-0 , p. 319 .