Comparative study of Zn(II) and Cd(II) ions adsorption on charged carbon nano tubes: molecular dynamics approach

A comparative study was performed on adsorption process of Zn(II) and Cd(II) ions using charged carbon nano tubes (CNTs). The results show that the adsorption behavior of Zn(II) and Cd(II) ions have different patterns. To understand the reason for this difference, the ion-water complex properties and the reduced density of water around CNT were studied. The results reveal that the water molecules make high-, low-, and bulk regions around CNTs, which can affect the ion movement toward CNT. The ion-water complex characteristics during the adsorption demonstrated that the Cd(II)-water complex has a large number of exchange events between 4 and 8 coordinate numbers where Zn(II) has only 6 coordinate numbers during the adsorption process. The Cd(II)-water complex was found less stable and therefore has faster translational dynamics. These properties help cadmium ion to be adsorbed faster into CNT despite its heavier weight.