An algal industrial waste from an agar extraction process was used as an inexpensive and efficient biosorbent for Pb2+ removal from aqueous solutions. This biosorbent was compared with the alga Gelidium, the raw material for agar extraction. The effects of various physicochemical parameters on Pb2+ biosorption were investigated in a batch contactor in order to explain the adsorption mechanism. Kinetic and equilibrium experiments were conducted at different values of pH (3.0, 4.0 and 5.3), ionic strength (0.001 and 0.1 M) and temperature (1045 degrees C). Uptake kinetics follows the pseudo-second-order Lagergren model and equilibrium is well described by Langmuir isotherm. Maximum biosorption capacities for Gelidium algae and algal waste were found to be 64 +/- 2 and 44 +/- 3 mg Pb2+/g biosorbent, respectively. Biosorption of Pb2+ was highly pH dependent, decreasing with pH. Good performances were obtained even under high ionic strength values. Changes in standard enthalpy and activation energy during biosorption were determined. (c) 2005 Elsevier Ltd. All rights reserved.