ABSTRACT The adsorption of nickel, cobalt and copper ions onto the surface of exopolysaccharides extracted from cyanobacterial mats has been studied to provide a sustainable and efficient approach for the remediation of heavy metals. The physical characterisation of the EPS surface, including SEM, EDX and FTIR, confirmed an irregular compact structure with multiple cracks, rough macropore structure and the existence of hydroxyl, amine, amide, polysulphide and carbonyl groups. The maximum adsorption capacity (q max 47.17 , 47.02 and 46.95 mg/g for Cu(II), Co(II) and Ni(II), respectively) was achieved at pH = 6, 0.3 g/L EPS dose, 120 min duration time and 35°C. The remediation affinity of EPS decreased in the order of Ni(II) > Co(II) > Cu(II). The Langmuir model (R 2 > 0.95) better fits the adsorption behaviour than the Freundlich model (R 2 < 0.90) and the Dubinin – Radushkevich isotherm model, indicating chemical adsorption between the biosorbent and metal ions. Kinetically, the pseudo-second-order model better describes adsorption than does the pseudo-first-order model. Finally, thermodynamic studies have demonstrated that the adsorption process is spontaneously endothermic, with positive enthalpy values (ΔH°) and negative free energy (ΔG°). The positive entropy values (ΔS°) indicate an increase in disorder at the solid-liquid interface during the adsorption process.
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