Various approaches to the modeling of metal and radionuclide interactions with macromolecular ligands, proteins, polysaccharides
and humic substances in particular, their chemical and sorption equilibria, and the techniques used for their investigation
are concisely compared. To predict radionuclide mobility in the natural and semi-natural aqueous environment, the estimation
of the effective interaction constants, related to specific species of polyelectrolytes which are linked with the absorbancy
or absorbancy ratio in their electronic absorption spectra, should probably be preferred and developed as standard. For characterization
of the binding sites of specific molecular forms of polyelectrolyte ligands, the Scatchard plot analysis at macroconcentrations
of the metal and also, though not so effectively, the two-phase distribution at trace concentrations of radionuclide or metal
can be applied.
A method for determination of the composition of binary mixtures of a metal or radionuclide species by optimized repeated two-phase separations (SORTS) was proposed and theoretically substantiated. Its principle consists in repeated equilibration of two immiscible phases, one being the original liquid or solid matrix with minimal adjustment of its composition and varying the phase ratio (separation stage cut) as the optimized parameter. The batch separation technique may consist in the repeated solvent extraction or aqueous biphasic distribution, or in the replicate equilibration with solvent or leaching solution. Results of SORTS can be presented e.g. by Tukey box diagrams as the characteristic fingerprints of original species composition.
Membrane extraction, i.e. separation in double-emulsion systems, is analyzed theoretically as a three-phase distribution process. Its efficiency is evaluated from the point of view of chemical equilibria and diffusion transport kinetics. Optimization criteria of the process as a preconcentration technique are discussed. The main advantages of membrane extraction as compared with solvent extraction are in higher yields (for preconcentration) and higher capacity for recovery of solutes. No advantage is seen in applications for a mutual separation of solutes if the same, inexpensive reagent can be used in a conventional extraction process. All phenomena caused by thermodynamic instability of double emulsions and mutual contamination of the feed and receiving phases diminish its maximal efficiency, which should be characterized by the introduced parameters, pertraction factor (p) and multiplication factor (N). These factors are suitable for expression of efficient distribution, system capacity, process economics, and separation kinetics. The kinetics were approximated by means of a non-linear extraction isotherm.
The transport of radionuclides and metals in environment and living organisms is to a great extent controlled by multi-compartment, multi-stage liquid film diffusion and biological membrane transfer, often resulting in very selective and segregative process. The separative catalytic power of liquid membranes, and biomimetic membranes in particular is not always unambiguous at present state of art, but still promising. The scope of SIS '91 Conference was in the looking for interdisciplinary approach in that field of separation of ionic solutes.
Possibilities and problems of experimental differentiation of two chemical or particle forms of element M (Acentral atom or group, Bcomplex, metalorganic or bioinorganic species, colloid, etc.) between which mutual equilibria can exist at their partition in two-phase systems, when the gross (analytical) concentrations are determined in each phase (by AAS, NAA, or radiometrically) are outlined for: (1) static distribution (batch separation) between two immiscible phases, when both the partial concentrations of A and B and their distribution coefficients generally can be determined by a single or fractional separation, (2) dynamic distribution (chromatographic separation) between the peaks of two metal species, when the potentialities can be strongly affected by secondary equilibria or when the sample-eluent interactions can be assessed.
Radiolysis of nitrobenzene solution of cobalt(III) dicarbollide, which is used for solvent extraction of cesium from fission products results in enhanced extraction of niobium-95. The isomeric nitrophenols, 2,4-dinitrophenol, p-nitrosophenol and m-aminophenol exhibit antergism towards extraction of niobium cations. Synergistic effect is exhibited by 2,5-dinitrophenol, o- and p-aminophenol, o-nitroaniline and 2,4,6-trinitrophenol which are among the products of two-phase systems with nitrobenzene radiolysis. Two competing processes, complexation of niobium and protonation of ligand, both depending on the ligand benzene ring substituents, are discussed.
The effect of humic acid addition on the capacity of inorganic sorbents to sorb radioactive cesium and strontium was studied
on montmorillonites in a calcium and potassium form. The Sips isotherm for humic acid sorption and multisite distribution
model of ion sorption was found to suit well for the description of mobility of ions as a function of equilibrium humic acid
concentration as a single variable at given pH and type of silicate. Complexation of the ions was of minor importance at the
conditions investigated. Influence of humic acid on the specific radiocesium interception potential (SRIP) was also evaluated.
A standard analytical technique for determination of the partial ion exchange capacity of mineral or soil for selective uptake
of cesium, the specific radiocesium interception potential (SRIP) is formulated and theoretically discussed. The method is
based on the determination of a retained, leached or sorbed fraction of carrier-free cesium-137 in soil contacted with 0.01M
KNO3-0.01M Ag thiourea complex solution at the phase ratio solution:sorbent 10 ml:0.1 g. Reliability of the method is discussed
in connection of radiocesium carrier and humic substances presence.