Magnesium Ion Gated Ion Rejection through Carboxylated Graphene Oxide Nanopore: A Theoretical Study

Abstract

While nanoporous graphene oxide (GO) is recognized as one of the most promising reverse osmosis desalination membranes, limited attention has been paid to controlling desalination performance through the large GO pores, primarily due to significant ion leakage resulting in the suboptimal performance of these pores. In this study, we employed a molecular dynamics simulation approach to demonstrate that Mg²⁺ ions, adhered to carboxylated GO nanopores, can function as gates, regulating the transport of ions (Na⁺ and Cl^-) through the porous GO membrane. Specifically, the presence of divalent cations near a nanopore reduces the concentration of salt ions in the vicinity of the pore and prolongs their permeation time across the pore. This subsequently leads to a notable enhancement in salt rejection rates. Additionally, the ion rejection rate increases with more adsorbed Mg²⁺ ions. However, the presence of the adsorbed Mg²⁺ ions compromises water transport. Here, we also elucidate the impact of graphene oxidation degree on desalination. Furthermore, we design an optimal combination of adsorbed Mg²⁺ ion quantity and oxidation degree to achieve high water flux and salt rejection rates. This work provides valuable insights for developing new nanoporous graphene oxide membranes for controlled water desalination.

Keywords: graphene oxide membrane; molecular dynamics simulation; water desalination.

Figure 1

The salt rejection rates of pores with different numbers of adsorbed Mg²⁺ ions. The solid lines are a guide to the eye.

Figure 2

The number of the Na⁺ ions (a), Cl⁻ ions (b), and salt ions (c) accumulating in a cylindrical region below and around the centers of the pores with different numbers of adsorbed Mg²⁺ ions as a function of the height of the cylinder h.

Figure 3

The PMF of Na⁺ ion (a) and Cl⁻ ion (b) along the z-axes of GO membranes with different numbers of adsorbed Mg²⁺ ions. The pore is located at z = 0.

Figure 4

The permeation time autocorrelation function of Na⁺ ion (C_Na(t)) (a), Cl⁻ ion (C_Cl(t)) (b), and salt ion (C_NaCl(t)) (c) in pores with different numbers of adsorbed Mg²⁺ ions as a function of time t.

Figure 5

The water flux of pores with different numbers of adsorbed Mg²⁺ ions. The solid line is a guide to the eye.

Figure 6

The salt rejection rate (a) and the water flux (b) of the nanoporous GO membranes with different numbers of adsorbed Mg²⁺ ions and oxidization degree R. The solid lines are a guide to the eye.

Figure 7

(a–f) show the number of the salt ions (water molecules) located inside the pore and the salt ion (water molecule) permeation time autocorrelation function at time t = 2 ps for each oxidization degree R. The figures along the parallel direction represent the pores with 0, 1, and 2 adsorbed Mg²⁺ ions. The solid lines are a guide to the eye.

Figure 8

(a) Side view of the gated system for investigating desalination through the GO membrane. The yellow and blue balls denote the sodium and chloride ions. (b) Atomistic structure of the nanoporous GO membrane with a variation in oxygen degree. One or two Mg²⁺ ions are placed near the carboxylic oxygen. The red, cyan, white, and pink spheres represent the oxygen, carbon, hydrogen, and magnesium atoms, respectively. (c) The black box in the figure denotes a cutout view of a cylindrical region with a diameter of 1.6 nm and a height of h below and around the center of the pore in the GO membrane.