The mobility of toxic metal ions in nature is to a great extent controlled by complexation reactions at natural polyelectrolytes as humic acids, clay minerals or polycarboxylates. In all those systems the metal ions compete with the ubiquitous protons for the binding sites. Due to the very complex structure of the polyelectrolytes, the binding mechanism and the equilibria involved can be obtaines only by the evaluation of equilibrium combined with kinetic measurements. In this project we study the binding of nickel and cadmium ions to humic acids and clay minerals and discuss the results by comparing them with results obtained for model ligands as di- and tricarboxylates and salycilate.