Effect of pH, humus concentration and molecular weight on conditional stability constants of cadmium

The molecular weight distribution of DOC and cadmium in humic samples spiked with cadmium has been studied after changing the concentration and/or pH of the original humic sample. The ultrafiltration method, using standard Amicon ultrafiltration membranes with molecular weight cut-off values of 500, 10 3 and 10 4, was used for the separation of different molecular weight fractions. 109Cd was used for labelling the cadmium solutions. Humus concentration ranged from 2.7 to 13.6 mg DOC 1 −1, pH was varied from 5 to 7. Molecular weight distribution of DOC was found to be very dependent on experimental conditions. Some of the results are consistent with earlier proposed humus conformation model. Relative binding strength of single molecular weight fractions was expressed per mole and per weight basis, the fraction with nominal molecular weight in the range 10 3-10 4 being the strongest chelating agent. Conditional stability constants ( K) of cadmium humus complexes were calculated. An empirical equation for the calculation of K was derived.