Determination of conditional stability constants of cadmium-humic acid complexes in freshwater by use of a competitive ligand equilibration-solvent extraction technique

A technique for determining organic complexation of cadmium in freshwater u sing competitive ligand equilibration coupled with solvent extraction was e valuated. The method involves a competitive equilibration of the sample wit h potassium iodide and pyridine followed by extraction of the mixed cadmium -iodide-pyridine complex into a benzene phase. The final distribution of th e metal is measured by gamma ray spectrometry. The method's suitability was verified by performing extractions on the model ligand 8-hydroxyquinoline- 5-sulfonic acid (sulfoxine). which forms well-characterized complexes with cadmium. The speciation results so obtained were in excellent agreement wit h results calculated with a chemical speciation model. The method was appli ed to study complexation of cadmium with two commercially available humic a cids in reconstituted freshwater at relatively high cadmium concentrations to mimic polluted freshwaters. The obtained titration data were fitted to a one-site Langmuir adsorption model, yielding values of 10(6.59) and 10(6.5 2) for the respective conditional stability constants, and 0.57 and 0.90 mu M for the respective ligand concentrations.