Sorption of radiocobalt on acid-activated sepiolite: effects of pH, ionic strength, foreign ions, humic acid and temperature

The acid-activated sepiolite (ASEP) was prepared by physical purification and acid activation of natural sepiolite, and was characterized by XRD, FT-IR, SEM and N₂ adsorption–desorption. The prepared ASEP was applied for the sorption of ⁶⁰Co(II) from aqueous solutions. The sorption of ⁶⁰Co(II) from aqueous solutions by ASEP was investigated as a function of contact time, solid content, pH, ionic strength, foreign ions, humic acid (HA) and temperature. The results indicated that the sorption of ⁶⁰Co(II) on ASEP was strongly dependent on pH values. At low pH, the sorption of ⁶⁰Co(II) was dominated by outer-sphere surface complexation or ion exchange, whereas inner-sphere surface complexation or surface precipitation was the main sorption mechanism at high pH. The presence of HA increased the sorption of ⁶⁰Co(II) on ASEP at low pH values, and reduced the sorption at high pH values. The Langmuir and Freundlich models were applied to simulate the sorption of ⁶⁰Co(II) at three temperatures of 298, 318 and 338 K. The thermodynamic parameters ( $ Updelta G^circ ,,;Updelta S^circ $ and $ Updelta H^circ $ ) calculated from the temperature dependent sorption isotherms indicated that the sorption of ⁶⁰Co(II) on ASEP was an endothermic and spontaneous process. ASEP has a great application potential for cost-effective disposal of ⁶⁰Co(II) from large volumes of aqueous solutions.