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Potentiometric mass titration (PMT) technique has been adapted to determine the pH pzc of four vegetable wastes: grape stalks, cork, yohimbe bark and olive stones wastes used for Cu(II) removal. The pH at the point zero charge (pH pzc ), determined by PMT, are compared with that obtained by two classical techniques: mass titration (MT) and immersion technique (IT). PMT has been found to be an easy and appropriate technique to determine pH pzc of the studied materials. From the results, the knowledge of sorbents pH pzc provides information about the possible attraction and repulsion between sorbent and sorbate but in any case enables to ensure that electrostatic force is one of the mechanisms that takes place in metal sorption.

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The sorption of lead and cadmium from aqueous solutions by grape stalk waste (a by-product of wine production) was investigated. The effects of the contact time, pH of the solution, ionic medium, initial metal concentration, other metal ions present and ligands were studied in batch experiments at 20 degrees C. Maximum sorption for both metals was found to occur at an initial pH of around 5.5. The equilibrium process was described well by the Langmuir isotherm model, with maximum grape stalk sorption capacities of 0.241 and 0.248 mmol g(-1) for Pb(II) and Cd(II), respectively, at pH around 5.5. Kinetic studies showed good correlation coefficients for a pseudo-second-order kinetic model. The presence of NaCl and NaClO(4) in the solution caused a reduction in Pb and Cd sorption, the latter being more strongly suppressed. The presence of other metals in the uptake process did not affect the removal of Pb, while the Cd uptake was much reduced. HCl or EDTA solutions were able to desorb lead from the grape stalks completely, while an approximately 65% desorption yield was obtained for cadmium. From the results obtained it seems that other mechanisms, such as surface complexation and electrostatic interactions, must be involved in the metal sorption in addition to ion exchange.