A computational study on the rejection of arsenic complexes by mean of carbon nano tubes

Molecular and ionic rejection is considered a fundamental aspect during the design of innovative membranes for waste water treatment. In this context, the choice of innovative materials for the membrane preparation with high performance is a crucial point. Nanostructured materials, such as carbon nanotubes (CNTs) which have shown amazing hydrodynamic properties [1,2,3], can represents a great opportunity. Thus, CNTs-composite membranes, can be considered promising solutions for the treatment of waste water. Although, the CNTs embedding in the membranes and parallel to the direction of water flow, constitutes the largest drawback to the development of CNTs-composite membranes, however the ability of CNTs to reject very low molecular weight solutes and ions represents a great chance to exploit. Thus, in this contribution, an computational analysis on the CNTs capability to reject arsenite and arsenate compounds is shown. The objective of this study is the diameters prediction of CNTs in order to obtain an efficient rejection both of arsenious and arsenic acids and related salts. In natural water the prevalent As(III) compounds are neutral while the As(V) compounds are negatively charged, therefore this computational study has been focus on these arsenic compounds. Armchair (n.n) type carbon nanotubes were used with internal diameter ranging between 1.6 nm and 0.4 nm. Neutral As(III) and charged As(V) structures were optimized using a quantum mechanics approach in the frame of Density Functional Theory. The calculated geometries were then used to evaluate the effective diameters by including the water molecules which coordinates each single arsenic compounds. The average molecular cross-sections were evaluated from the effective diameters, in addition to the molecular maximum and minimum sizes. Hence, these quantities were used to calculated the minimum cross-sections of the arsenic aqueous complexes. From previous studies on neutral and charged molecules, it is noted [4] that the ratio between the molecular minimum cross-section and the area of the CNT opening can be used to predict the diameter of CNTs in order to obtain an efficient rejection by size exclusion mechanisms of the considered arsenic complexes. Nerveless, aware that this geometrical criterion is reductive, here quantum mechanics trapping energies of the neutral As(III) and charged As(V) aqueous complexes were also calculated to complete the analysis of the rejection. The total trapping energy, associated to a specific packaging of hydrated complexes in the CNT, can be greater than energy of the arsenic aqueous complexes. These energy differences give information on the CNT capability to accept or reject the complexes. The results of the calculations show that though some CNTs can accommodate certain complexes of arsenic, however, the associated energy differences are remarkably positive. Hence, some As(III) and charged As(V) complexes can be encapsulated in the CNTs although their getting in required energy. This effect permits to use CNTs with larger diameters to reject these complexes with the consequences to increase the water flow through the CNTs-composite membranes.