Understanding the performance of graphdiyne membrane for the separation of nitrate ions from aqueous solution at the atomistic scale.

A molecular dynamics simulation study is conducted to investigate the capability of the pristine graphdiyne nanosheet for nitrate ion separation from water. The removal of nitrate ion contaminants from water is of critical importance as it represents an environmental hazard. The graphdiyne is a carbon-based membrane with pore density of 2.4 × 1018 pores/m2 and incircle radius of 2.8 Å. We show that the efficient water flow is accurately controlled through fine regulation of the exerted hydrostatic pressure. The high water permeability of 6.19 L.Day-1cm-2MPa-1 with 100% nitrate ions rejection suggests that the graphdiyne can perform as a suitable membrane for nitrate separation. The potential of mean force analysis of the single water molecule and nitrate ion indicated the free energy barriers for nitrate of about 4 times higher than that of water molecules. The results reveal the weak interaction of the water molecules and the membrane which aid to high water flux.

Efficient Removal of Heavy Metals from Aqueous Solutions through Functionalized γ-Graphyne-1 Membranes under External Uniform Electric Fields: Insights from Molecular Dynamics Simulations.

Carbon-based nanosheet membranes with functionalized pores have great potential as water treatment membranes. In this study, the separation of Hg2+ and Cu2+ as heavy metal ions from aqueous solutions using a functionalized γ-graphyne-1 nanosheet membrane is investigated by molecular dynamics simulations. The simulation systems consist of a γ-graphyne-1 nanosheet with -COOH or -NH2 functional groups on the edge of pores placed in an aqueous solution containing CuCl2 and HgCl2. An external electric field is applied as a driving force across the membrane for the separation of heavy metal ions using these functionalized pores. The ion-membrane and water molecule-membrane interaction energies, the radial distribution function of cations, the retention time and permeation of ions through the membrane, the density profile of water and ions, and the hydrogen bond in the system are investigated, and these results reveal that the performance of -NH2-functionalized γ-graphyne-1 is better than that of -COOH-functionalized γ-graphyne-1 in the separation of Cu2+, while the Hg2+ cations encounter a high energy barrier as they pass through the membrane, especially in the -COOH-functionalized pore, due to their larger ionic radius and the smaller pore size of this membrane.

Carbon Nanotubes as Thermally Induced Water Pumps.

Thermal Brownian motors (TBMs) are nanoscale machines that exploit thermal fluctuations to provide useful work. We introduce a TBM-based nanopump which enables continuous water flow through a carbon nanotube (CNT) by imposing an axial thermal gradient along its surface. We impose spatial asymmetry along the CNT by immobilizing certain points on its surface. We study the performance of this molecular motor using molecular dynamics (MD) simulations. From the MD trajectories, we compute the net water flow and the induced velocity profiles for various imposed thermal gradients. We find that spatial asymmetry modifies the vibrational modes of the CNT induced by the thermal gradient, resulting in a net water flow against the thermal gradient. Moreover, the kinetic energy associated with the thermal oscillations rectifies the Brownian motion of the water molecules, driving the flow in a preferred direction. For imposed thermal gradients of 0.5-3.3 K/nm, we observe continuous net flow with average velocities up to 5 m/s inside CNTs with diameters of 0.94, 1.4, and 2.0 nm. The results indicate that the CNT-based asymmetric thermal motor can provide a controllable and robust system for delivery of continuous water flow with potential applications in integrated nanofluidic devices.