Trace metal–humate interactions. I. Experimental determination of conditional stability constants

The enhancement of mobility of radionuclides in the geosphere through complexation by humic substances is a source of uncertainty in performance assessment of radioactive waste repositories. Only very few data sets are available which are relevant for performance assessment of an underground repository for radioactive waste. Using the equilibrium dialysis-ligand exchange method developed at the Paul Scherrer Institut, conditional stability constants for the formation of complexes of Aldrich humic acid with Ca2+, NpO2+, Co2+, Ni2+, UO22+ and Eu3+ and complexes of Laurentian soil- and Suwannee River fulvic acid with Co2+, UO22+ and Eu3+ were measured. pH was varied between 5 and 10 and ionic strength between 0.02 and 0.2 M. The data are presented as equilibrium coefficients that are free from any model assumptions. The equilibrium coefficients increased in the order Ca2+congruent withNpO2+