Molecular Perspective of Radionuclides Separation by Nanoporous Graphene Oxide Membrane

1 Introduction

Isotopes of iodine play significant environmental roles including, a geochemical tracer (¹²⁹I), limiting micronutrient (¹²⁷I), and an acute radiotoxin (¹³¹I). ¹²⁹I is long-lived fission products that produced in significant amounts, this radioactive gas released into the environment by nuclear explosions, nuclear power plants, nuclear weapons and also naturally. Because I-129 is long-lived and relatively mobile in the environment, it is of particular importance in long-term management of spent nuclear fuel. The separation of such an important gas is an interesting subject of scientific investigations. Gas separation membranes allow one component in a gas stream to pass through faster than the others. There are many different types of gas separation membrane, including zeolites, silica and polymers [1,2]. As a novel one-atom-thick planar sheet of sp²-bonded carbon atoms, packed in a honeycomb lattice, graphene has received much attention in recent years in materials science. Significant progress has been made for the utilization of graphene nanopore in gas separation and purification [3,4]. Furthermore, graphene is easy to produce from natural graphite through chemical oxidation-dispersion-reduction procedure at a low cost. Through chemical oxidation, natural graphite can be effectively oxidized and exfoliated into GO with the introduction of oxygen-containing functional groups. The presence of oxygen-containing functional groups in GO and reduced graphene makes them as excellent supporters for enhancement of wide applications [5,6], to date, there are only a few reports in the separation of gas with GO nanopore

2 Body of article

The molecular permeation of three different diatomic gases, namely I₂, O₂ and N₂ through GO nanopore membranes which different pore sizes were investigated. We considered the Iodine-129 isotopes in the gas mixture system. For simplicity, throughout the text the I-129 is shown by the general I₂ symbol. All MD simulations were performed using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) package [7] and structures were visualized using the VMD package [8]. The all-atom optimized potentials for liquid simulations (OPLS-AA) was used for GO, which is able to capture essential many-body terms in the inter-atomic interactions, i.e. bond stretching, bond angle bending, van der Waals, electrostatic interactions and partial charges. The simulation was performed in an NVT ensemble for 5 ns. The Nosé-Hoover barostat and thermostat were applied to maintain the pressure and temperature at 1 atm and 300 K, with damping coefficients 1 ps^–1 and 0.1 ps^–1, respectively and time step of 0.2 fs was used during the simulation. Non-bonded van-der-Waals interactions were modeled in terms of 12−6 Lennard-Jones famous potentials with a cut-off distance of 1.2 nm. SHAKE algorithm was applied for the stretching terms between oxygen and hydrogen atoms of the GO to reduce high-frequency vibrations that require shorter time steps. The particle mesh Ewald (PME) method with a 10.0 Å real-space cutoff, 1.5 Å reciprocal space gridding, and the splines on the order of 4 with a 10^–5 tolerance was implemented to compute the electrostatic interactions. A simulation box with the dimensions of 30 Å × 30 Å × 80 Å was constructed and a GO nanopore membrane in the middle of the box in Cartesian coordinate origin (0, 0, 0) as the separator and dividing the simulation box with the height of 4 nm into two equal volume chambers, right and left chambers, see Figure 1.

Figure 1:

Initially MD snapshot of the gases loaded into the right chamber and the structures of the nanopores employed in the simulation, the colors assigned to each molecule: red, O₂; yellow, I₂ & blue, N₂, green, C; black, H).

Initially, an equal number of 15 molecules of I₂, O₂ and N₂ were loaded into the right chamber (see Figure 1), while the other side (left chamber) was a vacuum. Various sizes of pores were considered, which were called by the number of graphene ring units removed or partially opened, i.e., pore-7 (P-7), P-10, P-11, P-12, P-13, P-14.

3 Result and discussion

In this section, we analyze the plots of mean-squared displacement (MSD) for various pores, in the following diffusion coefficients were investigated.

3.1 MSD

Figure 2 shows MSDs versus time in the pore-12 of the GO nanopore membrane. Figure 2 demonstrates that, only O₂ cross through the membrane because the size of pore is bigger than kinetic diameter of O₂, and is smaller than N₂ and I₂ molecules.

Figure 2:

Calculated MSD vs. time for I₂, N₂ and O₂ in the pore-12 GO nanopore membrane.

3.3 The number of crossed gas molecules through graphene membrane

To find the effect of the pore size in passing gas molecules through the GO nanopore membrane, the numbers of gas molecules passed the membrane were calculated. The measure of number of passing gas to be a count of the number of molecules on one side to the other side of the Go nanopore membrane at the current time point. None of gas molecules could cross through p-7 GO nanopore membrane. When the pore size is increased to p-12, still I₂ could not pass the membrane. Figure 3 show total number of crossed gas molecules through GO nanopore membrane vs. time for I₂, N₂ and O₂ in the pore-12 of GO nanopore membrane.

Figure 3:

Total number of crossed gases through GO nanopore membrane vs. time for I₂, N₂ and O₂ in the pore-12.

3.4 Calculating flow of gases

The flow is used to characterize the membrane permeability quantitatively. The flow of I₂, O₂, and N₂ was calculated in various pore size models, as shown in Figure 4.

Figure 4:

Flow of gases passing through the GO nanopore membranes at different pore areas.

There was a prodigious increase in the flow of I₂ at P-12’s area, where the restriction of the size and shape to the N₂ permeation begins to vanish. After that, the flow of O₂ and N₂ molecules became reduced. In the case of the nitrogen and oxygenf molecules, the flow of nitrogen and oxygen molecules reduced at the pore-13, which is due to the different shape of the pore-12 and the pore-13. After that, the nitrogen and oxygen flow increases as the pore area increases.

4 Conclusion

We found that the graphene oxide nanopore membrane could be used to separate iodine-129 gas radionuclides from the air. Selectivity and permeability could be controlled by drilling different shapes and size of pores on a graphene membrane. The separation of I₂ from the air could be achieved by using pore-12 graphene oxide sheet. The restriction of the molecular orientation largely prohibits the permeation of I₂ molecules. When nitrogen and oxygen molecules are not blocked by the pores (e.g., pore-12), there are more oxygen molecules that permeate through the GO nanopore membrane than nitrogen molecules. However, in the pore-12, the best selectivity of I₂ from the air is achieved with the pore size barely bigger than the nitrogen and oxygen molecules and smaller than the I₂ molecules.