Bioremediation of mercury (II) from aqueous solution by gum karaya (Sterculia urens): A natural hydrocolloid

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 (ΔGο) indicated that the thermodynamically feasible, spontaneous nature of Hg2+ ions biosorption onto gum karaya hydrocolloid. The negative value of enthalpy change (ΔHο) and entropy change (ΔSο) 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.

Research Highlights
► The bioremediation of Hg2+ ions by gum karaya, a natural hydrocolloid was assessed.
► The biosorption of Hg2+ ions onto gum karaya fitted well with Langmuir isotherm model.
► Bioremediation mechanism follows ion-exchange, spontaneous and exothermic in nature.
► SEM analysis suggests the formation of Hg2+ ions capped biopolymer structures.