Atomic insight into water and ion transport in 2D interlayer nanochannels of graphene oxide membranes: Implication for desalination
Graphical abstract
Introduction
Nowadays, fresh water shortage has become a worldwide problem because of water pollution and waste [1,2]. Seawater, which covers 71% of the surface area of our earth, provides an adequate fresh water supply if advanced desalination method is developed. Therefore, in the past decades, many studies have been triggered to develop new methods for seawater desalination [3,4]. Among which, desalination through reverse osmosis (RO) membrane has become a widely applied technology due to its high-energy efficiency [[5], [6], [7]]. Currently, preparing advanced RO membranes with robust mechanical property, high separation performance (i.e. high water flux and high ion rejection rate), and low operation cost and energy consumption are the main developmental direction for seawater desalination technology [8,9].
Recently, nanofiltration RO membranes prepared from two-dimensional (2D) materials have presented great potential for desalination application [[10], [11], [12], [13]]. Especially for graphene oxide (GO), the interlayer gallery assembled from atomic-thin GO nanoflakes offers unique 2D nanochannels for fast water transport meanwhile rejecting the transport of other small molecules and ions [[14], [15], [16], [17], [18]]. Up to now, many theoretical [[19], [20], [21]] and experimental [[22], [23], [24]] studies have shown that GO membrane presents superior desalination performance. Overall, previous studies mainly focus on the regulation of interlayer spaces of GO membrane (e.g. chemically functional group modification and physically embedding nanoparticles) to pursue more efficient desalination. Although the increased interlayer space can enhance water permeability, the corresponding ion rejection rate will decrease. Typically, these regulating methods for interlayer space of GO membrane are not easy to realize, meanwhile the trade-off relationship between water permeability and ion rejection rate is another problem needing to be reconsidered.
GO is an oxidized form of graphene (GN) including epoxide, hydroxyl, and carboxylic acid groups on it [[25], [26], [27]]. Normally, the epoxide and hydroxyl groups mainly distribute on the base plane of GN, whereas carboxylic acid groups distribute on the edges. However, the distribution of epoxide and hydroxyl groups is not uniform on GN plane [25,28]. Yan et al. had proved that epoxide and hydroxyl groups tended to aggregate on GN plane [29]. In other word, GO nanosheet can be divided into oxidation and non-oxidation region, and therefore, the oxidation region presents island-like distribution. Considering the nonuniform distribution of oxygen-containing groups on GN sheet, the stacking ways of neighboring GO nanosheets (i.e. oxygen-containing groups facing each other, oxygen-containing groups with pristine GN patches facing each other, and pristine GN patches facing each other) during membrane preparation (e.g. vacuum filtration [30,31]) can influence the interlayer nanostructures. Meanwhile, the interlayer nanostructures govern the transport of water and ion in the 2D nanochannels between neighboring GO nanosheets. Therefore, the desalination performance of GO membrane can be improved through controlling the assembly structure of neighboring GO nanosheets. However, hardly any studies have been designed to investigate the influence of stacking ways of GO nanosheets on the corresponding desalination performance.
In this work, water and ion transport behavior in 2D nanochannels between neighboring GO nanosheets was studied by using atomic non-equilibrium molecular dynamics (MD) simulations. Specially, we considered the influences of the stacking ways of heterogeneously oxidized GO nanosheets on water and ion transport. The complicated cross-interactions between stacking ways and interlayer spaces of GO membrane were revealed. This work also provides a clear design principle to prepare GO membrane for high-efficient desalination by controlling the interlayer space and interlayer nanostructure of GO membrane.
Section snippets
Models
The studied model system is shown in Fig. 1A, where a 2D GO nanochannel is held between two fixed He slabs with cracks matching the interlayer space of the nanochannel. The He slab is selected because it has a weak interaction with water and ion, and therefore, the influence of slab adsorption effect on water and ion transport is ignorable. Actually, other fixed slabs can also be used like graphene slabs. However, there are strong nonbonding (vdW) interactions between water molecules and
Water flux
Fig. 2A–C shows the time evolution of the number of water molecules passed through the studied 2D GO nanochannels with different interlayer spaces, respectively. These curves present a nearly linear relationship between the passed water molecules and time. Therefore, water flux can be calculated through linear fitting of the curves in Fig. 2A–C, and the results are shown in Fig. 2D. It shows that the water flux increases with the increase of interlayer spaces. As the increase of interlayer
Conclusions
In this work, non-equilibrium MD simulations were preformed to investigate water and ion transport in 2D GO nanochannels with different interlayer space and nanostructure, intending to further improve the desalination performance of GO membrane by regulating the membrane structures. Our results reveal that the fast water and ion transport mainly occurs in the 2D GO nanochannel surrounded by non-oxidized region of GO nanosheets. At oxidized region, the strong electrostatic, vdW, H-bond, and
CRediT authorship contribution statement
Wen Li: Investigation, Conceptualization, Visualization, Writing - original draft. Lei Zhang: Formal analysis, Data curation. Xinyu Zhang: Conceptualization, Methodology. Mutian Zhang: Methodology, Conceptualization. Tengfei Liu: Conceptualization, Methodology. Shougang Chen: Supervision, Writing - review & editing, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is financially supported by the Fundamental Research Funds for the Central Universities (201813020, 201964009), China Postdoctoral Science Foundation (2018M640658, 2019T120611), Shandong Provincial Natural Science Foundation, China (ZR2019BB012), and National Natural Science Foundation of China (U1806223).
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