Experimental methods based on the radiotracer technique have been designed to study kinetic aspects of the interaction of humic substances (HS) with metal ions. The rate of dissociation of Eu-HS complexes was observed via the rate of the isotope exchange in the system 152 Eustable Eu -humic acid (HA) determined by the diaphragm method. The rate of exchange was found to be enhanced by decreasing pH value, increasing ionic strength and increasing degree of loading of HA with Eu, and was described with a simple 1-component kinetic model (mono-exponential decay function). Additional information on the kinetics has been derived from the electrophoretic mobilities of Eu depending on metal loading of HA and pH, determined by the free liquid/moving boundary electrophoresis with radiometric and spectrophotometric detection of the electromigration of the Eu/Eu-HA complex.

Diffusion is an important migration mechanism of trace elements and radionuclides in the biosphere, soils, clays, river sediments. Naturally, in the last few years there has increased also the relevance to diffusion of trace elements and radionuclides and methods of its determination in bentonite. Bentonite is proposed as sealing and backfill material in the near-field of deep repositories of spent nuclear fuel and of radioactive wastes. This paper presents capillary modification of measuring diffusivity, some of its results in porous natural materials and barrier materials of repositories for wastes.

Isotope exchange was employed to study dissociation of metal cations from their complexes with humic substances (HS). Dissociation of cation from HS controls the rate of isotope exchange between two identical metal-HS solutions (but for the presence of a radiotracer) divided by a dialysis membrane. The rate of isotope exchange of Eu/l~2Eu and Co/6~ in the systems with various HS was monitored as a function of pH, ionic strength, and the degree of HS loading with metal. The apparent rate of Eu-HS dissociation was found to be enhanced by decreasing pH, increasing ionic strength, and increasing metal loading. Co-HS dissociation was too fast to be followed by the method. For interpretation of the experimental kinetic data, the multiple first order law has been applied. Based on the results, a concept of HS as a mixture of two types of binding sites is discussed.