Adsorption of fulvic-like organic ligands and their Cd and Pb complexes at a mercury electrode

Benzyliminodiacetic acid, pyridine-2-carboxylic acid and pyridine 2,6-dicarboxylic acid were used as model compounds of the components of natural pedogenic fulvic acids to study the most important factors related to their adsorbing and complexing properties which may affect the behaviour of their complexes at a mercury electrode. In the absence of metal ions, their adsorption properties were studied by alternating current polarography, at different pHs and ligand concentrations. The adsorption of their Pb and Cd complexes was studied by normal pulse polarography, in a low concentration range. Comparison of all the experimental data shows that adsorption of the free ligand and the complexes is always primarily due to the hydrophobic part of the ligand which tends to accumulate on the Hg surface. Any electrically charged group of the molecule (either positive or negative) increases its hydrophilicity and decreases its adsorbability. However, this effect plays a significant role only when the distance between the charged and hydrophobic groups is small. For instance, it was found to be much less important with the benzyliminodiacetic ligand than with the other two ligands. For all the ligands and complexes, 10−6M is the approximate limit below which the adsorption is small in the normal time-scale of a dropping mercury electrode, due to the small adsorption equilibrium constants of free or protonated ligands and to the slowness of the overall adsorption rate of the complexes. These results are compared to those obtained with pedogenic fulvic compounds.