INTRODUCTION

Many toxic heavy metals are frequently discharged into the environment in the form of industrial wastewater, causing serious soil as well as water pollution. The wastewater discharged directly into natural water bodies is highly harmful to the aquatic ecosystems. Among these heavy metal species, Ni (II) has been frequently identified as a contaminant because of its high toxicity, mobility and enormous use in different industries, including electroplating and the manufacture of steel, pigments and storage batteries [1]. Although Ni (II) plays a pivotal role as a micronutrient in the synthesis of vitamin B12, its affluence over tolerance limits can cause cancer of the lungs, nose and bones as well as nausea, cyanosis, rapid respiration and etc. For these reasons, the necessity to removal of Ni (II) from industrial effluents has been emphasized before they are discharged [2-4]. The main source of nickel pollution in the water was derived from processes such as galvanization, smelting, mining, dyeing operations, batteries manufacturing and metal finishing. Trace amounts of nickel are beneficial to human organism as an activator of some enzyme systems, but if it is beyond the scope of normal levels, different types of diseases have been occurred. Evidently it is most important to remove these toxic metals from wastewater prior to their discharge into natural water bodies as they are non-biodegradable and persistent. Technologies, such as chemical reduction, ion-exchange, adsorption, reverse osmosis, electro dialysis and nanofiltration have been proposed for the removal of heavy metals [5-10]. However, most of these processes are relatively complicated or do not work efficiently to reduce metal concentrations below regulatory standards. Thus, the removal of heavy metal ions in an effective way is of great importance from the viewpoints of both scientific research and engineering application.

Recently, membrane techniques have been introduced to remove heavy metal ions from water. Nanosheet membranes have been also used for the removal of various heavy metals [11], because they can separate ions and molecules. Ion separation using nanosheet membranes is done via pores created in them [12]. Graphene nanosheet (GNS) and boron nitride nanosheet (BNNS) are examples of nanostructured membranes. GNS is a carbon based nanomaterial with layers of carbon atoms densely packed in a honeycomb crystal lattice composed of two equivalent carbon sub-lattices [13]. GNS have some significant advantages in the preparation of carrier matrix due to their high surface area and remarkable mechanical stiffness and it exhibits a number of intriguing unique properties, such as high surface area of over 2600 m²/g, large surface-to-volume ratio, high room temperature carrier mobility and conductance quantization. GNS has a wide range of possible membrane applications because of its ultimate thinness, flexibility, chemical stability and mechanical strength [14]. Recently, ion and gas separation through GNS by molecular dynamics (MD) simulation method have been studied [15-18]. The results of these works show that GNS with pores in its center can be efficiently utilized for separation technology.

For further investigation, BNNS was used also for the removal of nickel ions. A BNNS with a very high specific surface area exhibits excellent sorption performance for a wide range of oils, solvents, and dyes from water [19]. This membrane has unique properties, including a wide energy band gap, electrical insulation, ultraviolet photoluminescence, high thermal conductivity and stability, and high resistance to oxidation and chemical inertness [20-24]. This easy recyclability makes BNNS a good candidate for ion separation.

Perfect GNS and BNNS are impermeable to ions because there are no pores, and the electron density of their aromatic rings is enough to repel ions trying to pass through them. To pass ions through them, the drilling of pores is required. Functionalized pore in a nanosheet is obtained by passivating each atom at the pore edge using chemical functional groups. Herein, for nickel removal from an aqueous solution using a functionalized pore in center of GNS or BNNS, external electric field applied to the simulated system. We expect that our findings can be used to aid the design of nanostructured membranes for the removal of heavy metals for water treatment by nanostructure membranes such as GNS and BNNS.

SIMULATION METHOD

Fig. 1 displays an image of the simulation system. The size of the simulation box was 3×3×6 nm³. The box constituted of 1700 water molecules with 0.3 M NiCl₂ and a GNS or BNNS as a membrane was inside the box. 

The full geometric optimization of a functionalized GNS and BNNS was calculated by the density functional theory method to obtain atomic charges and their optimized structures. These calculations were done using the GAMESS [25] at the B3LYP level of theory with 6-311G basis sets. The obtained results from DFT calculations for the functionalized membranes are given in Table 1.

As shown in Fig. 2, there were 377 carbon atoms and 9 fluorine atoms in the GNS and 183 nitrogen, 183 boron and 12 fluorine atoms in the BNNS. During the simulations, these membranes were held fixed. The diameter of the pores in the GNS and BNNS were about 0.6 nm and 0.8 nm, respectively.

Periodic boundary conditions were applied in the three directions. Water was modelled by using the simple point charge model [26]. Intermolecular interactions were described by the 12-6 Lennard-Jones (LJ) potential together with a Coulomb potential. The system was initially equilibrated at 298 K with a coupling time of 0.1 ps^-1 for 1000 ps. In the modelling of sieving properties, our typical simulation runs were 5 ns long and obtained in the isobaric ensemble at the atmospheric pressure under applied electric field. The range of applied electric field was from 0 to 30 V, which is similar to other experimental studies in which the voltage is used to remove heavy metals [27]. The cutoff distance for nonbonding interactions was set up at 12 Å, and the particle mesh Ewald summations method was used to model the system’s electrostatics [28]. During simulations, all the GNS and BNNS atoms were held in fixed positions. A time step of 1 fs was employed. At the beginning of each simulation run, water molecules rapidly filled the pore of membranes. Then after a certain period of time, ions started enter the pore. All MD simulations were carried out at constant volume using the NAMD 2.10 [29] package with the CHARMM27 force field [30]. A Langevin thermostat and a hybrid Nose-Hoover Langevin piston were used to maintain the temperature and pressure of the system at 298 K and 1 bar, respectively. In this research, as in previous works [31-38], all analyses were performed using VMD 1.9.2 software [39]. The force field parameters for GNS and BNNS were obtained from [36,40] and nickel ions were modelled by using the parameters from reference [41].

The ion permeation from functionalized pores can be explained by calculating the potential of the mean force (PMF) [42]. The PMF was calculated by sampling the force experienced by ions that were placed at several positions along the z-axis of the box. In this work, each sampling window was run for 1 ns. The PMF of the ions was calculated using umbrella sampling [43] with either ion harmonically restrained in 0.1 Å steps in the axial z direction. Collective analysis was made using WHAM [44].

Table 1: Partial charges of GNS and BNNS atoms obtained from DFT calculations.

Fig. 1: A snapshot of the simulated system. The BNNS membrane with a functionalized pore in its center is located in the middle of simulation box (red: O, white: H, blue: Ni, and green: Cl).

RESULTS AND DISCUSSION

To removal of Ni²⁺, we used GNS and BNNS as a membrane with a functionalized pore in their centres under the influence of external voltage which was applied to the system, perpendicular to the pore of the membranes. With applying electric field, only nickel ions infuse through the functionalized pore of the membranes. Although GNS and BNNS have a large enough pore to accept nickel and chlorine ions, the results indicated that only Ni²⁺ permeates through these pores.

The MD results can be clarified by calculating the PMF of ions. We have done 50 ns MD simulation without applied electric field, to examine passage of ions in these conditions. It was observed that, any ions did not pass through the pores of GNS or BNNS. Therefore, we applied electric field for passage of ions from these pores. PMF can predict passing or rejecting of an ion across a path. Fig. 3 demonstrated the PMF for Ni²⁺ and Cl^- in the GNS and BNNS. As can be seen from Fig. 3, in both membranes, the energy barrier for chloride is higher than nickel. Thus, the chloride ion will not be able to cross the membrane. This is due to termination of functionalized pores of membranes by the negatively-charged atoms (fluoride atom), which favours the passage of cations.

Fig. 2: Functionalized pore of membranes. Left: BNNS and right is GNS (blue represents nitrogen, silver is boron, cyan is carbon and red is fluoride).

Fig. 3: The PMF for Ni2+ and Cl- ions in the GNS and BNNS systems.

 Also, the PMF for Ni²⁺ in the BNNS is lower than the GNS. This trend leads to more Ni²⁺ permeating from the pore of the BNNS, in effect of identical applied electric field to GNS and BNNS. The Ni²⁺ permeated from functionalized pores when 5 and 7.5 V electric fields were applied to the BNNS and GNS, respectively. Fig. 4 shows the number of Ni²⁺ passing through the functionalized pore of the GNS and BNNS. With the increment of the applied electric field, the number of Ni²⁺ passing was increased. As seen in Fig. 4, Ni²⁺ passes through the pore of the BNNS more than the GNS. This is due to the low energy barrier of Ni²⁺ in the BNNS compared to that in the GNS.

The time required for a nickel ion to pass through the pore (retention time) is an important parameter. Fig. 5 shows the retention time of a Ni²⁺ ion as a function of applied electric field. Increasing the electric field has decreased the retention time.

Figure 4. The number of Ni2+ passing through the pore of GNS and BNNS.

Fig. 5: Retention time for Ni2+ in various voltages in the GNS and BNNS.

Therefore, removal of heavy metal from the aqueous solution at high voltages happens quickly. The energy barrier in the PMF for Ni²⁺ in the pore of GNS is high compared to that of the BNNS pore. Thus, the retention time of the Ni²⁺ in the GNS system is greater than that of the BNNS. In the other words, nickel ions passes quickly through the BNNS pore.

To characterize the structure of water molecules around the ions, the radial distribution function (RDF) between the ion and water molecule was considered that it was obtained from the trajectory files of MD simulation. There is a clear maximum and minimum for each first RDF. These have been shown that a coordination shell of water exists around of ions. Fig. 6 shows the RDF Ni²⁺-water in the GNS and BNNS systems. The first peak corresponds to the distribution of the neighbour water molecules around the nickel ions. For the RDF peaks of Ni²⁺ in the GNS and BNNS systems, the position and magnitude of the first maximum and minimum are almost identical. This indicates that the hydration number of this ion in both systems is identical.

On the other hand, the structure of water molecules around the membranes in both systems is similar. The water molecules have been shown a tendency to accumulate in the region at a location about ±4.5 Å from the GNS and BNNS. This structure is shown by two visible peaks in the density profile on each side (see Fig. 7).

However, the number of Ni²⁺ passing through the GNS and BNNS is different. This indicates that structure of the water molecules were ineffective in the number of ions passing through the pores. Therefore, difference in the number of ions passing through the pores is related to the pore structure. This case is illustrated by the PMF, in which the barrier energy of Ni²⁺ ions in the BNNS is less than that of GNS. Thus, the retention time of the Ni²⁺ in the BNNS system is less than of the GNS system. Therefore, Ni²⁺ passes quickly through the pore of the BNNS.

Fig. 6: RDFs under various voltages: (a) RDF Ni2+- water in the GNS system, (b) RDF Ni2+- water in the BNNS system.

Fig. 7: Density profile of water molecules in the GNS and BNNS systems.

When water molecules pass across the functionalized pores of GNS and BNNS, they form a single file structure. As can be seen in Fig. 7, the structure of water molecules on both sides of the membranes was different from water molecules in the bulk region. The density profile was calculated from histograms of the number of water molecules. The water molecules displayed a tendency to accumulate in the region at a location about ±4.5 Å from the GNS and BNNS. In fact, the density of water on both sides of the membranes was higher than that in the bulk water. This phenomenon occurred for both types of membranes. In the region far away from the membranes, the density of water was about 1 g/cm³. The layered structure of water molecules in this region is due to the non-bonded interaction of membranes atoms, and water molecules.

CONCLUSION

MD simulations technique was used to investigate the removal of nickel ions as a heavy metal across functionalized pores of GNS and BNNS. Based on the results of PMF calculations, the GNS and BNNS were able to remove Ni²⁺ from aqueous solutions under applied electric field. Compared to the GNS, a BNNS can remove the Ni²⁺ from water efficiently. With increasing applied electric field to the considered systems, more ions could pass through the pore of membranes. In addition, with increasing applied voltage, the retention time of the ions decreased.

ACKNOWLEDGMENTS

Author thanks the Iranian Nanotechnology Initiative Council for the support provided.

CONFLICT OF INTEREST

The author declares that there are no conflicts of interest regarding the publication of this manuscript.