How to apply graphene and its composite materials in water treatment

Graphene (GE) is a periodic honeycomb two-dimensional carbonaceous material formed by hexagonal arrangement of sp2 hybridized carbon atoms. It has large specific surface area, high electron mobility and strong chemical stability characteristic. This paper focuses on the research progress of graphene and its composite materials in water treatment adsorbents and photocatalysts in recent years. Graphene and its composites have good adsorption effect on heavy metals, organic pollutants and other pollutants, and high adsorption capacity. After combined with photocatalytic materials, graphene effectively enhances the photocatalytic properties of composites due to its unique physical and chemical properties. Finally, the application of various graphene and its composite materials in water treatment was evaluated, and their application prospects in water treatment were prospected.

Introduction

Graphene (GE) is a periodic honeycomb two-dimensional carbonaceous new material formed by hexagonal arrangement of sp2 hybridized carbon atoms. In 2004, Geim and Novoselov of the Department of Physics and Astronomy at the University of Manchester in the United Kingdom obtained graphene for the first time by stripping graphite crystals with tape, and thus won the 2010 Nobel Prize in Physics. Graphene has unique physical and chemical properties and is one of the most robust materials in the world. It has a theoretical specific surface area of up to 2,630 m²/g, good thermal conductivity and high-speed electron mobility (200,000 cm²/ (V·s)). It can be used as an electrode material, sensors and hydrogen storage materials. At the same time, the graphite used to prepare graphene and its composite materials is widely available. Graphene and its composite materials are cheaper than carbon nanotubes, and the preparation process is simple. Many scholars have begun to study graphene and its composite materials in application of water treatment.

Application of graphene and its composite materials in water treatment

Graphene (GE) is a periodic honeycomb two-dimensional carbonaceous material formed by hexagonal arrangement of sp2 hybridized carbon atoms. It has large specific surface area, high electron mobility and strong chemical stability characteristic. This paper focuses on the research progress of graphene and its composite materials in water treatment adsorbents and photocatalysts in recent years. Graphene and its composites have good adsorption effect on metals, organic pollutants and other pollutants, and high adsorption capacity. After combined with photocatalytic materials, graphene effectively enhances the photocatalytic properties of composites due to its unique physical and chemical properties. . Finally, the application of various graphene and its composite materials in water treatment was evaluated, and their application prospects in water treatment were prospected.

Preparation and properties of graphene

In recent years, positive progress has been made in the preparation of graphene. The preparation process of graphene is usually graphite-graphite oxide-graphene oxide-graphene. The preparation process is shown in Fig. 2. Common graphene-based carbon materials include graphene, graphene oxide, and reduced graphene oxide. The preparation methods of graphene mainly include micro mechanical cleavage, chemical vapor deposition, epitaxial growth, colloidal suspension, and the like. The graphene used in water treatment is mostly prepared by chemical methods in consideration of the preparation method, preparation cost and scale, and the redox method is most widely used. Graphene oxide (GO) is usually obtained by chemical oxidation and ultrasonic preparation of graphite. Due to the wide range of graphite sources and low prices, graphene oxide is easy to mass produce. At the same time, graphene oxide has a large number of oxygen-containing groups such as hydroxyl group, carboxyl group and epoxy group. It is a hydrophilic substance and has good compatibility with many solvents, and is very suitable for application in water treatment.

The commonly used graphite oxidation methods currently reported include the Brodie method, the Standenmaier method, and the Hummers method. The basic principle is to first treat the graphite with a strong acid, form a graphite intercalation compound, and then add a strong oxidant to oxidize it. The Hummers method using concentrated H2SO4, NaNO3 and KMnO4 as oxidants is most commonly used. This method shortens the preparation time and improves the safety factor. This method is often used in water treatment applications. The graphene oxide can be reduced to graphene by a chemical method (using a reducing agent such as hydrazine hydrate, dimethylhydrazine, sodium borohydride, etc.), a thermal stripping method, an ultraviolet radiation method, a microwave method, or the like. The reduced oxygenated group on the surface of the graphene oxide is reduced to obtain reduced graphene oxide (RGO), which increases the surface potential. Compared with graphene oxide, the adsorption capacity of anionic pollutants in water is enhanced.

Summary and outlook

The poor dispersibility of graphene is limited in the application in aqueous solution, and the surface of graphene oxide contains oxygen-containing groups such as hydroxyl group, carboxyl group and epoxy group, which has good hydrophilicity and high surface negative charge, and is suitable for treating metal ions and positively charged dye waste water has a good effect. The oxygen-containing group on the surface of the reduced graphene oxide is partially reduced, which increases the surface potential of the material, and can treat the negatively charged organic dye wastewater and some anionic pollutants on the surface. Graphene and its composite materials can play an effective role in photocatalytic performance, mainly due to the following three aspects. First, the specific surface area is large, and the adsorption capacity of the composite material is enhanced. Secondly, the higher electron transport ability can be used as an electron acceptor, delaying the recombination of electron-hole pairs and enhancing the photocatalytic activity of the composite. The most important thing is that graphene extends the spectral response of the composite to the visible region, reducing the band gap of the photocatalytic material.

From the current research, the main breakthrough in the application of graphene in water treatment is the cross-sectional study of materials, chemistry and environmental management. The research of novel graphene composite materials is mainly based on the characteristics of the materials themselves to remove pollutants, and by combining with graphene-based carbon materials to enhance the ability of materials in adsorption, electron transfer and reduction. The mechanism of the removal of contaminants by graphene and its composite materials is still unclear. From the mechanism, the graphene-based composite materials are explored for the removal of heavy metals, inorganic substances and organic pollutants in water, especially the removal of new refractory pollutants space. In addition, the stability of graphene and its composite materials needs to be improved. The preparation of stable graphene composite materials by volume is also a difficult problem that graphene is widely used in water treatment. The easy separation and environmentally friendly magnetic graphene composites have broad prospects for use in water treatment, but it takes time for graphene composites to be widely applied to water treatment projects.

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