Synthesis of multi-walled carbon nanotube–hydroxyapatite composites and its application in the sorption of Co(II) from aqueous solutions

Abstract In this paper, multi-walled carbon nanotube–hydroxyapatite (MWCNT–HAP) composites were synthesized and characterized by scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy and X-ray diffraction (XRD) and used as an original adsorbent for Co(II) sorption from aqueous solutions. The sorption of Co(II) on MWCNT–HAP composites was investigated as a function of contact time, pH, foreign ions, fulvic acid (FA), humic acid (HA) and temperature. The results indicated that K + , Mg 2 + and Ca 2 + ions restrained Co(II) sorption on MWCNT–HAP composites at low pH. In the whole pH ranges, anions (i.e., ClO 4 − , NO 3 − and Br − herein) made no obvious effect on Co(II) sorption, while F − ions dramatically enhanced Co(II) sorption. The presence of FA and HA enhanced Co(II) sorption on MWCNT–HAP composites at low pH values, but suppressed Co(II) sorption at high pH values. The Freundlich and Langmuir models were used to imitate the Co(II) sorption isotherms at three different temperatures. The thermodynamic data (∆ G 0 , ∆ S 0 , and ∆ H 0 ) counted from the temperature dependent sorption isotherms suggested that the sorption of Co(II) on MWCNT–HAP composites was a spontaneous and endothermic process. The high sorption capacity of Co(II) on MWCNT–HAP composites suggested that the MWCNT–HAP composites were suitable materials in heavy metal pollution cleanup.