3.2.1. !. Solvent extraction method

The procedure was as follows: A series of 5 mL samples of seawater were doped with tracer quantities of 60Co labelled [Co(CN)6]3- and pH adjusted in the range pH1-7 by addition of HCl. Each sample was then contacted in a separator funnel with 5 mL of organic phase (kerosene) containing the extraction agent. The funnel was then vigorously shaken by a mechanical shaking machine for a predetermined time of 3 min at room temperature, which is sufficient to reach extraction equilibrium. After phase separation (by gravity), 1 mL samples were extracted from both phases and measured for radioactivity.

For the high gamma energies of 60Co (1173 keV and 1332 keV), correction for differences in counting efficiency due to sample density is not needed. In addition, when equal volumes of the two phases are used in the counting procedures, the distribution ratio (D value) can be determined from the raw counting rates by the formula:

R

D = -^ (21)

Raq

where Rorg is the net counting rate of the organic sample and Raq is the net counting rate of the aqueous sample.

The quantity extracted from one extraction operation of aqueous and organic phases is expressed as E(%) by the formula:

%E = —D—— 100% (22)

+ D V

org

where Vaq and Vorg are the volumes of the aqueous and organic solutions, respectively.

For high values of D, it is possible to obtain a substantial volume reduction and enrichment factor by using a volume ratio VqVog = 10 or higher. For equal volumes of the organic and aqueous phases, the formula simplifies to:

% E = -^~ -100% (23)

1 + D

For even further enrichment, it is necessary to strip the activity back to an aqueous phase with a high stripping efficiency and perform a new extraction sequence on this strip solution with a volume ratio Vaq/Vorg >> 1. As an example, for D = 10 and Vaq/Vorg = 10, it is possible to obtain a volume reduction of 10 and an enrichment factor of 5 after one extraction. After a stripping with 100% efficiency and a new extraction with the same volume ratio, a volume reduction of 100 and an enrichment factor of 25 will be obtained. For higher D values, the enrichment factor will be higher.

Therefore, the stripping efficiency was also screened with a few stripping agents.

The extractions performed here are based on so-called ion pair formation and the extraction agents are all amines of different kind, which, as a rule, have been conditioned with 0.1M H2SO4. Data for extraction and stripping systems are found in Table 7.