The influence of pH on the adsorption of heavy metals cadmium, manganese and cobalt was studied on a chemically modified clay. The isotherms were realized at pH 2; 3; 4 and 6.5. The heavy metals were applied as nitrates with initial concentrations ranging from 10 to 50 ppm. The adsorption was quantified according to the Freundlich model and the results show that there is a linear relationship between ln Kf and the pH whose slopes are respectively 0.55; 0.33 and 0.10 for cobalt, manganese and cadmium. The retention power decreases in the following direction: Cd > Co >Mn