The biosorption characteristic of mercury ions from aqueous solution using gum karaya (Sterculia urens), a natural hydrocolloid was explored as a function of pH, biopolymer concentration, contact time and temperature. Langmuir model fitted the equilibrium biosorption data better than Freundlich. Tempkin and Dubinin-Radushkevich isotherm models. The maximum biosorption capacity for gum karaya was observed to be 62.5 mg g(-1) as calculated by Langmuir model. Adsorption kinetic data were best fitted by pseudo-second-order kinetic model. The negative value of the Gibbs free energy change (Delta G degrees) indicated that the thermodynamically feasible, spontaneous nature of Hg2+ ions biosorption onto gum karaya hydrocolloid. The negative value of enthalpy change (Delta H degrees) and entropy change (Delta S degrees) revealed that biosorption of Hg2+ ions on gum karaya hydrocolloid to be an exothermic reaction and shows a decrease in the randomness at the solid/solution interface during the biosorption process of Hg2+ ions by the biopolymer. The biosorbed mercury ions on gum karaya hydrocolloid were desorbed (95%), by 0.1 M HCl. The metal interaction with biopolymer was assessed by FT-IR and SEM analysis. An irregular and porous structure was observed by SEM analysis before Hg2+ biosorption and upon Hg2+ biosorption showed metal capped structures. (C) 2011 Elsevier B.V. All rights reserved.