Equilibrium, thermodynamic and kinetic studies were carried out for the biosorption of Pb(II) and Ni(II) ions from aqueous solution using the lichen (Cladonia furcata) biomass. Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm models were applied to describe the biosorption of the metal ions onto C. furcata biomass. The influences of pH, biomass dosage, contact time and temperature of solution on the biosorption were studied. Langmuir model fitted the equilibrium data better than the Freundlich isotherm. The monolayer biosorption capacity of the biomass was found to be 12.3 and 7.9 mg/g for Pb(II) and Ni(II) ions, respectively. From the D-R model, the mean free energy was calculated as 9.1 kJ/mol for Pb(II) biosorption and 9.8 kJ/mol for Ni(II) biosorption, indicating that the biosorption of both metal ions was taken place by chemical ion-exchange. Thermodynamic parameters, the change of free energy (Delta G degrees), enthalpy (Delta H degrees) and entropy (Delta S degrees) of the biosorption were also calculated. These parameters showed that the biosorption process of Pb(II) and Ni(II) ions onto C. furcata biomass was feasible, spontaneous and exothermic under studied conditions. Experimental data were also tested in terms of kinetic characteristics and it was found that biosorption processes of both metal ions followed well pseudo-second-order kinetics. (c) 2007 Elsevier B.V. All rights reserved.