A biomass for removing heavy metal ions from aqueous solutions has been investigated. The green alga Chaetophora elegans was characterized in terms of the number of binding sites and proton binding constant by potentiometric titration in different ionic media. The discrete site distribution model used, considering the algae cells suspensions as a mixture of monoprotic acids, allowed the characterisation of different classes of ionisable sites. Total concentrations of ionisable groups determined for intact (living) and homogenised (non-living) cells were significantly greater in the presence of chloride (32-80% for intact cells, 134-188% for homogenised cells). Only for homogenised cells no statistically significant differences (95% confidence level) were found for total group concentrations in nitrate medium with different ionic strength. The effect of the ionic strength was evident in the micrography of intact cells; the higher the ionic strength, the less preserved was the algae structure. pK, values were similar in nitrate medium with different ionic strength, indicating the presence of groups with the same nature. In chloride medium higher pKa values were found. Interactions of Cd(II), Pb(II), Ni(H) and Zn(II) were evaluated by voltammetric titration and the determined metal binding capacities increased in the order: Cd(II) < Pb(II) < Zn(II) < Ni(II). Binding electrostatic effects were more important for Ni(II), and covalent binding seemed to be more important for Zn(II). (c) 2004 Elsevier Ltd. All rights reserved.