Removal of divalent nickel cations from aqueous solution by multi-walled carbon nano tubes: equilibrium and kinetic processes

Release of heavy metals into water as a result of industrial activity may pose a serious threat to the environment. In this study, the potential of multi-walled carbon nano tubes (MWCNT) to remove Ni2+ cations from aqueous solutions was investigated in a batch reactor under different experimental conditions. The effects on the removal process of conditions such as initial concentration of Ni2+ ions, temperature, and adsorbent mass were investigated. Nickel uptake was quantitatively evaluated by use of the Langmuir, Freundlich, and Dubinin–Kaganer–Radushkevich isotherm models. For 20 mg/L initial Ni2+ cation concentration, adsorption capacity increased from 8.12 to 11.75 mg/g when the temperature was increased from 25 to 65 °C, an indication of the endothermic nature of adsorption process. In addition, the adsorption equilibrium was well described by the Langmuir isotherm model; maximum adsorption capacity was 17.86 mg/g Ni2+ cations on HNO3-treated MWCNT (t-MWCNT). The results obtained in this study show that adsorption of Ni2+ on t-MWCNT is a spontaneous and endothermic process. By use of second-order kinetic constants and the Arrhenius equation, the activation energy of adsorption (E a) was determined as 5.56 kJ mol−1.