Study On Adsorption Of Heavy Metal Ions From Aqueous Solution With Modified Multi-walled Carbon Nanotubes

Carbon nanotubes, as a novel kind of carbon materials, have drawn widespread attentionsince their discovery because of their distinctive physical and chemical properties andpotential application in many fields. Now carbon nanotubes have been one of the hotspots inthe nanomaterial field. In this paper, multi-walled carbon nanotubes（MWCNT） were modifiedby oxidation with sodium hypochlorite（NaClO） solutions and were employed as adsorbents tostudy the adsorption characteristics of heavy metal ions from water.The optimum modification conditions of multi-walled carbon nanotubes determinedthrough single factor and orthogonal experiment are as follows: raw carbon nanotubes1.5g,100mL70%sodium hypochlorite（NaClO） solution, reaction temperature25℃, reaction time3hours. Adsorption experiment proved that the adsorption capacity of MWCNT for Cu²⁺andNi²⁺has boosted markedly after modification with sodium hypochlorite（NaClO） oxidation.Factors affecting adsorption performance of carbon nanotubes such as dosage ofadsorbents, pH value, temperture as well as initial concentration of heavy metal ions wereinvestigated. The results show that the adsorption effect will be improved with the increase ofthe pH value and dosage of adsorbents, especialy for modified carbon nanotubes; However,there would be a decline in the adsorption capacity of modified carbon nanotubes with theincrease of temperature; Modified carbon nanotubes have quite a high adsorption speed, itsadsorption for Ni²⁺and Cu²⁺could reach equilibrium in barely50and40minutes,respectively; Along with the initial concentration of Ni²⁺and Cu²⁺increase, their adsorptionamount on modified carbon nanotubes have increased and their adsorption percentage havedecreased.By analyzing the adsorption isotherms of the two metal ions, we found both Langmuirand Freundlich models could characterize the adsorption of the two metal ions by modifiedcarbon nanotubes well, thereinto Langmuir model would be better for Ni²⁺adsorption.Research on adsorption kinetics elucidate that both pseudo-first-order andpseudo-second-order kinetics models could match the adsorption of Ni²⁺and Cu²⁺onmodified carbon nanotubes favourably, and pseudo-first-order kinetics model fit better.Thermodynamics of Cu²⁺adsorption on modified carbon nanotubes were investigated.The Gibbs free energy change（ΔG⁰） of adsorption were negative at all temperatures, enthalpy change ΔH⁰=-5.396kJ/mol, entropy change ΔS⁰=67.099J/（mol·K）, indicating that thisadsorption process was spontaneous, exothermic and with an increase in entropy.Adsorption capacities of modified carbon nanotubes for Ni²⁺and Cu²⁺were compared. Apreliminary probe into competitive adsorption in Ni²⁺and Cu²⁺coexistence system was alsocarried out. Results indicated that Ni²⁺adsorption amount and adsorption percentage were allsmaller than Cu²⁺in single adsorption. In the dual competition system, Cu²⁺and Ni²⁺adsorption capacity had an obvious slump, and the Cu²⁺adsorption capacity is significantlybigger than Ni²⁺, which suggested that the adsorption stability of Cu²⁺is higher than Ni²⁺.Experiments showed that modified carbon nanotubes have a high adsorption capacity,and can be applied as a kind of hing-efficiency sewage treatment agent, so they have profoundprospect in enviroment protection.