Bio sorption of lead (Pb) from aqueous solution using citrus peels can provide an efficient and cost-effective solution for lead removal from industrial wastewaters. These peels contain the biopolymer pectin that has a strong affinity for metal ions. A better understanding of the chemistry behind these interactions can help in the preparation of commercial biosorbents using waste citrus peels. This study was conducted in order to obtain mechanistic information about Pb binding by low and highly methoxylated citrus pectins. Potentiometric titrations revealed that carboxyl groups are important contributors to negative charge of these biopolymers, which was further confirmed by a pH variability study. Pb binding by both low- and high-methoxylated pectins follows second-order kinetics, suggesting that each divalent metal ion binds to two monoprotic functional groups. Fourier transform infrared (FTIR) spectrometric results confirm that carboxylic acid groups are active participants in Pb binding by citrus pectin. (c) 2008 Elsevier B.V. All rights reserved.