Improved cadmium sorption isotherms by the determination of initial contents using the radioisotope Cd-109

Cadmium (Cd) sorption isotherms were estimated by two different analytical approaches to assess the influence of initial Cd concentrations of soil mat rix on the sorption of added Cd. For the laboratory experiments a heterogen eous set of samples was collected to include a wide range of different init ial Cd concentrations. Comparison of both analytical methods (conventional analysis, radioanalysis) resulted in a strong conformity of Cd contents in solution at equilibrium. The calculated Cd concentrations in the soil solid phase differ according to the analytical approach for considering the init ial contents. The determination of the initial contents by the proposed rad ioanalytical method with Cd-109 resulted in long linear Freundlich-isotherm s, even in the low concentration range. Thus, radioanalysis seems to be me most suitable method to recognise the initial contents of Cd in soil. EDTA extractable Cd represent the initial concentrations, which are averaged ove r solid and liquid phase. However, depending on the sorption characteristic s of the soil these rates vary. In the investigated set of soil samples 52. 3 to 99.3% of Cd must be added to the solid phase.