Biosorption by materials such as citrus peels could be a cost effective technique for removing toxic heavy metals from wastewater. Orange peels, lemon peels and lemon-based protonated pectin peels (PPP) had Langmuir sorption capacities of 0.7-1.2 mequiv./g (39-67 mg/g) of Cd per biosorbent dry weight. A potentiometric titration was interpreted using a continuous pK, spectrum approach. It revealed four acidic sites with pK, values of 3.8, 6.4, 8.4 and 10.7, and a total site quantity of 1. 14 mequiv./g. Sorption isotherms of untreated citrus peels showed an unusual shape with two plateau values. Protonated pectin peels on the other hand showed a typical Langmuir behavior with a higher sorption capacity than untreated peels. At lower pH, metal binding was reduced due to increased competition by protons. This was modeled using pl-l-sensitive isotherm equations. It was not necessary to assume four binding sites; using one site with pK, 3.8 and a quantity of 1. 14 mequiv./g was sufficient. It was possible to accurately predict metal uptake at one pH using the metal binding constant determined at a different pH. A 1: 1 stoichiometry model fit the sorption isotherms shape better than a 1:2 stoichiometry. For constant pH, the 1: 1 stoichiometry reduces to the Langmuir model. (c) 2007 Elsevier B.V. All rights reserved.