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Graphene-Based Nanomembranes for Sustainable Water Purification Applications

Uluvangada T. Uthappa1, Dusan Losic2,3 and Mahaveer D. Kurkuri1

1JAIN University, Centre for Nano and Material Sciences, JAIN Global Campus, Bengaluru, Karnataka, 562112, India

2The University of Adelaide, School of Chemical Engineering, Adelaide, SA, 5005, Australia

3The University of Adelaide, ARC Research Hub for Graphene Enabled Industry Transformation, Adelaide, SA, 5005, Australia

1.1 Introduction

The global crisis on water scarcity ranks top among the world's major problems. Provided with recent statistics, two-thirds of the world population will face major difficulties by 2025 due to the lack of water facilities. Due to the rapid population expansion, industrial development, and water pollution, it incessantly depleted freshwater resources. The extreme consumption of freshwater resources, particularly for developing countries, due to the growth of agriculture and industrial activities completely dependent on water resources and continuous utilization leads to severe ecological disasters. Thus, the shortage of water resources is an alarming issue that needs urgent resolution. To address these problems, it is crucial to conserve water resources [1].

On the other hand, membrane technology is a separation process that permeates certain species to pass via membrane while controlling the others, which is completely dependent on the properties of the membrane material. Based on their properties, membranes can be categorized as dense and porous membranes according to the driving force and ionic and molecular movement transported through membranes. Usually, it is well recognized that diverse species permeate based on the specific diffusion rates via the membranes, which are generally influenced based on solubility, diffusivity, and the membranes' porosity, as represented in Figure 1.1.

It is believed that the smaller molecules might pass through the membrane and whereas others could be blocked based on the pore sizes of the membranes. The ceramic and polymers are the most widely used membranes for water purifications plants. Typically, ceramic membranes retain better thermal and mechanical properties. However, it is difficult to control the pore size accurately with low-cost approaches. In contrast, polymeric membranes are economically and extensively utilized in microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) process due to the rapid permeation and high selectivity. However, polymers are hydrophobic that can easily adsorb target pollutants that lead to membrane fouling. Compared to ceramic and polymeric membranes, the new astonishing classic example used in membrane technology is recently discovered graphene oxide (GO). Also, reduced graphene oxide (rGO) is an important alternative class that belongs to the graphene derivative that has been explored for the development of a new generation of membrane separation technology [2]. Since from past few years' research expansion in graphene known as two-dimensional (2D) carbon allotrope and their oxidized derivatives (GO) has rapidly developed by the drastic increase in publications and patents. It is important to mention that graphene and GO have distinctive favorable material advantages compared to carbon nanotubes (CNTs) concerning separation and purification technologies. GO can be easily fabricated and engineered with favorable size, surface chemistry/modification, and structural properties. Moreover, its industrial process consumes a lesser amount of energy, making their production cost economic (for example: 500-1000?MJ/kg by solvent exfoliation of graphite or chemical reduction of GO, when compared to CNTs 100?000?MJ/kg) [3].

Figure 1.1 Transport mechanism for dense (a) and porous membrane (b).

Source: Lyu et al. [2]. © 2018 Royal Society of Chemistry.

This chapter summarizes the very recent advances on graphene-based nanomembranes for water purifications. The properties of GO-based membranes and fabrication strategies of various graphene-based nanomembranes and their applications in desalination and dyes and some highlights on heavy metals treatment are elucidated and discussed. Finally, a conclusion and future perspectives have been provided. This current book chapter could help the researchers and experts in membrane technology working on GO-based nanomembranes for water treatment applications.

1.2 Graphene and GO-Based Membrane Characteristics and Properties

For the first time, graphene was witnessed in electron microscopes in 1962 supported on the metal surfaces. Further, it was again rediscovered in 2004 by Novoselov and Geim and awarded Nobel Prize in Physics 2010 due to their groundbreaking experiments on 2D material graphene. Since 2010, graphene has fascinated major attractions in several applications, including water purification. Graphene contains carbon atoms bonded to each other in hexagonal patterns. The monolayered and double-layered graphene is very thin, and so it can be regarded as 2D material. Graphene's flat based honeycomb arrangements are responsible for favorable characteristic features such as strongest, lightest, most conductive, transparent materials [4], high specific surface area (2600?m2/g), flexibility [5, 6], and large scale production from cheap raw material (graphite) [7]. GO is an oxidized form of graphene, made up of carbon atoms bonded in hexagonal honeycomb lattice structures. Based on strong oxidations surrounding environment and based on the synthetic protocol such as Hummers or Staudenmaier method, enormous groups of oxygen epoxide, hydroxyl, and carboxyl acid functional groups present in GO are represented in Figure 1.2.

Such diverse functional groups are responsible for better hydrophilicity and permit better dispersion of GO flakes in aqueous media. These substantial features significantly enable GO deposition from solution using water as an environmentally benign solvent. From the past few years, GO showed unlimited attention as a novel 2D membrane in water treatment applications due to its exceptional mechanical [8] and antifouling properties [9], atomically thin thickness, and outstanding dispersion in water [8]. Various composite membranes are obtained by mixing GO with different polymers and other surface enriched materials. Such studies have been carried to enhance surface properties such as conductivity, tensile strength, and or elasticity of materials []. These surface properties impart GO-based functionalized nanomembranes for advanced water treatment technologies. The selective permeation path of graphene-based membranes that allow separation via the nanopore structure originated within the basal plane of hexagonal crystalline structures.

Figure 1.2 Typical chemical structure of GO showing graphene sheet derivatized by phenyl epoxide, hydroxyl groups on the basal plane, and carboxylic group on the edge.

Figure 1.3 Selective permeation routes in nanoporous graphene and stacked sheets of GO. (a) Nanoporous graphene membrane and (b) stacked graphene oxide membrane.

Source: Dervin et al. [9]. © 2016 Royal Society of Chemistry.

On the other hand, ions or molecules are selectively transported or passed via interlayer spacing of 2D graphene-based materials. The stacked nanosheets of GO form a multilayer laminate that shows abundant mechanical strength for application in pressure-assisted water filtration due to strong hydrogen bonds among distinct sheets. The oxygen comprising functional groups deposited haphazardly beside the edges of GO sheets, which maintain both significant interlayer spacing between and empty spaces among non-oxidized areas, constituting a network of nanocapillaries within the film. These nanochannels allow penetration of water molecules and successive transport laterally on hydrophobic non-oxidized regions of membranes that could support the rapid flow of water molecules. Thus liquid, vapors, and gases are opposed, as shown in Figure 1.3. In addition, water-soluble oxygen functional groups present beside the GO sheets, which adsorb water molecules and further diffused between the non-polar hydrocarbon present on the backbone of GO. Such permeation of water escalates the interlayer spacing in the middle of stacked GO sheets and helps in water flux over nanochannels at a higher flow rate. It has also been recognized that smaller ions enter GO membranes considerably superior to simple diffusion. Conversely, enlargement of nanochannels during the hydrated state only accepts ions of analogous sizes [9].

1.3 Fabrication of Graphene-Based Nanomembranes for Water Treatment Applications

1.3.1 Desalination

The membrane-based desalination method is classified based on membrane pore size and rejection mechanisms [10]. Desalination is a process of removing salts from seawater or brackish water, and it is known to be the core technology for improving freshwater scarcity [11]. To improve the desalination process and enhance water treatments, innovative technological classes of membrane systems are advanced. However, to overcome the costs and energy demands accompanied by membrane treatment, advanced, innovative, and economical membranes are...