Ultrasonic-assisted sodium hypochlorite oxidation of activated carbons for enhanced removal of Co(II) from aqueous solutions

Ultrasonic irradiation was employed to assist the modification of activated carbons (AC) by sodium hypochlorite, and tested for their ability to remove Co(II) from aqueous solutions. Modification of the activated carbon significantly enhanced their Co(II) adsorption capacity, which increased with the increasing NaOCl impregnating concentration. Ultrasonic-assisted impregnation can enhance the oxidation ability of the NaOCl solution, thus gave a higher adsorption capacity of Co(II) compared with the use of impregnation only. The Co(II) removal was a combination of adsorption and chemical precipitation. The optimal adsorption pH range for the native and modified AC was pH 7.0–8.5 and 4.5–8.5 respectively, above which precipitation of copper hydroxide became the main contributor to the cobalt removal. Cation exchange, electrostatic interaction, and surface complexation were demonstrated as the major mechanisms for the adsorption. Based on the XPS and FTIR analyses, Co(II) irons were considered to form monodentate charged complexes with the oxygen functionalities on the native AC, while they primarily formed multidentate coordination complexes within the modified carbon. The enhanced Co(II) removal by the ultrasonic-assisted NaOCl modified carbons was probably derived from their improved cation exchange capacity and higher content of adjacent aliphatic functional groups, which provided more complexation sites for cobalt.

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► NaOCl oxidation greatly improved the Co(II) adsorption on the activated carbons.
► Ultrasonic-assisted NaOCl oxidation further improved the Co(II) removal.
► Co(II) removal mechanisms on the native and modified carbons were clarified.
► Co(II) formed deferent type of complexes with the native and modified carbon.