How does graphene 'hack' water treatment work?

With the continuous growth of the world population and the aggravation of water pollution, sewage treatment has attracted unprecedented attention. People are actively looking for ways to treat and utilize sewage. Based on the advantages of large specific surface area, high strength, good chemical stability, strong modifiable property and good electrical conductivity, graphene can not only adsorb organic solvents, heavy metals and other pollutants in water, but also act as a catalyst carrier to catalyze the degradation of pollutants in water. Therefore, graphene has been widely studied as a sewage treatment material.

The indiscriminate discharge and discharge of industrial wastewater, urban household garbage, and pesticide spraying have aggravated the shortage of fresh water resources, which are already few and cannot be used by people. Every year a large number of waste water to China's environmental protection industry brings great pressure. Water pollution is becoming more and more serious, which makes sewage treatment receive unprecedented attention.

People are actively looking for ways to treat and utilize sewage. Based on the advantages of large specific surface area, high strength, good chemical stability, strong modifiable property and good electrical conductivity, graphene can not only adsorb organic solvents, heavy metals and other pollutants in water, but also act as a catalyst carrier to catalyze the degradation of pollutants in water. Therefore, graphene has been widely studied as a sewage treatment material.

Graphene adsorption of organic matter in water

Graphene is the thinnest and hardest material in the world. It was discovered in 2004.

Graphene materials can be used to adsorb organic pollutants in water, such as organic dyes, hydrocarbons, crude oil, pesticides and some natural organic substances.

Professor wang xiangke et al. conducted a large number of experiments to functionalize the surface of graphene with sulfonic acid group, which improved the dispersion of graphene and thus enhanced the adsorption capacity of graphene. The results showed that the adsorption capacity of functionalized graphene on naphthalene and naphthol reached 2.4mmol/g. Compared with two-dimensional materials which have low load rate and may cause secondary pollution, three-dimensional materials can be easily recycled in water purification. Research shows that the independent three-dimensional structure is easy to be recycled, which is conducive to the recycling of materials. This not only simple operation, but also greatly reduce the cost of recovery, for the actual industrial operation has great value. Three-dimensional graphene not only has good adsorption for organic dyes, but also has a high adsorption capacity for various types of oil, which has a high utilization value for future oil pollution in sea water.

Professor Ruoff et al. prepared spongy graphene by hydrothermal forming process; The material is then used to remove many commercial petroleum products (including kerosene, pump oil, grease, and organic solvents) from artificial seawater to test their oil-absorbing properties. The results showed that the graphene sponge material absorbed 86 times more oil than its own weight, exceeding the oil absorption capacity of any other common absorbent. The hydrocarbons are then simply heated back up to 99%. Through this process, the graphene sponge can be regenerated and reused more than 10 times without any loss of performance.

Professor li haitao selected graphene fragments with particle size of 500nm, and through a special physical technology, the graphene fragments were sent to the pores of activated carbon and attached to the inner surface, so as to increase the specific surface area of the filtration material and improve the filtration effect, and developed a functional material -- graphene carbon molecular sieve. The material has super high adsorption, and is light, stable, heat resistant, large specific surface area and harmless to human body. Compared with activated carbon of the same weight, the adsorption capacity is 20 times higher. This graphene carbon molecular sieve material can be synthesized into a composite membrane, which can effectively remove antibiotics in drinking water.

Professor liu zhaoping used formamide as the driving agent to prepare nitrogen-doped 3d graphene materials with nanocoil structure by ordinary heating. The removal efficiency of methylene blue and rhodamine B reached 96.8% and 94.6%, respectively. The helical channel brought by nitrogen doping is the key to the continuous and efficient diffusion of organic matter into the graphene inner layer.

The role of graphene in the treatment of heavy metal pollution

The unrestricted development of science and technology has caused great damage to the environment. For example, improper disposal of electronic waste products has caused heavy metal pollution to water bodies. Due to its large specific surface area, graphene has great potential in the field of adsorbents.

Using hydrazine to reduce graphite oxide, Leng et al. prepared rGO that can spontaneously absorb metallic antimony with a maximum adsorption of 7.463mg/g. Pan et al. reduced GO with the commonly used industrial urea disulfide, and the rGO produced could spontaneously adsorb the radioactivespecial fuel thorium with a maximum adsorption of 0.21mg/g. Due to the small number of functional groups on the surface of graphene, many researchers modified and modified graphene and applied it to the adsorption of heavy metals, achieving good adsorption effect and even simultaneous adsorption and removal of a variety of heavy metal ions.

Wu et al. modified graphene with cetyl trimethylammonium (CTAB) and found that the modified graphene could spontaneously adsorb metal Cr with a maximum adsorption capacity of 21.57mg/g. Mishra, such As using hydrogen making GO flake to obtain graphene layers, and then treated with nitric acid, rich in graphene surface functional groups, functionalization of graphene will be used for super capacitor electrode, not only to achieve the removal of the As and Na in water, at the same time, the sea water desalination, the largest adsorption quantity respectively As 142 mg/g (V), As (III) 139 mg/g, significantly higher than that of multi-walled carbon nanotubes and magnetic reductive graphite oxide.

Compared with graphene, GO is easier to be modified because it has more oxygen-containing functional groups and strong hydrophilicity, which also makes it easier to form complexes with metals, so it is more conducive to the adsorption of heavy metals in water. In addition to good adsorption to heavy metals, GO also has good adsorption to radioactive element uranium (VI).

Li et al. adsorbed uranium (VI) with GO nanosheet layer, and the results showed that the maximum adsorption capacity of GO for uranium (VI) was 299mg/g, and the adsorption effect was significantly better than that of rGO(47mg/g). Similar to graphene, modification of GO can further increase the adsorption effect.

Madadrang et al modified GO with EDTA. EDTA can form a stable chelating agent with metal, so it has a good adsorption effect. According to Langmuir model, the maximum adsorption capacity of go-edta on Pd2+ reaches 525mg/g, nearly 2 times higher than GO.

In addition, three-dimensional graphene can not only solve the problem of solid-liquid separation as a macroscopic body, but also has great advantages in adsorption performance. Lei et al. prepared a three-dimensional independent GO foam with a specific surface area of 578.4mg/g, and its maximum adsorption capacity for Cd2+, Pd2+ and Fe3+ was 252.5, 381.3 and 57.6mg/g, respectively.

Gao et al. prepared a 3d graphene hydrogel by reducing graphene with dopamine. While reducing graphene, dopamine modified graphene surface. The prepared graphene not only has good adsorption for heavy metals, but also for synthetic fuels and aromatic pollutants. Graphene can not only absorb metal cations, but also anions. For example, graphene can adsorb PO4-, ClO4- and F- in water. Different from the fixed adsorption mechanism of heavy metal cations, anions are adsorbed by forming a structure on graphene.

Vilela et al. developed a graphene-based micro-robot that can be used to clean up toxic heavy metals in polluted water. Tests show that the micro-robot can recover 95% of lead in polluted waters within an hour, reducing the lead concentration in the waters from 1.0 10-6 to 0.5 10-7. After the lead is chemically separated, the robot can be used again.

Although graphene has excellent performance in the absorption of heavy metal ions, it is difficult to put it into large-scale application in the field of heavy metal adsorption due to its high price and immature process conditions. With the decrease of the preparation price of graphene and the improvement of its adsorption effect, graphene is bound to have a great application prospect in the field of heavy metal ion adsorption in sewage.

Research on the degradation of pollutants using graphene as catalyst carrier

Although adsorption can remove pollutants in water, this technology can only adsorb pollutants, and to achieve true purification of water requires further treatment of the adsorbed materials. Total sedimentation or removal of contaminants can be achieved by means of catalytic degradation. Catalytic pollution control has low cost, high efficiency and good application prospect.

The huge specific surface area and abundant functional groups on the surface of graphene endow it with excellent properties, which makes it have great potential as a catalyst carrier and other aspects.

The results showed that the rate of degradation of rhodamine B and methylene blue by the photocatalyst prepared from titanium dioxide/graphene with graphene as carrier was significantly higher than that of pure TiO2 under ultraviolet light. Metal oxide/graphene composites have excellent photocatalytic efficiency, which can catalyze degradation of organic compounds, reduction of heavy metal cations and purification of water. Graphene catalysts also kill bacteria in sewage. Graphene and tungsten oxide nanocomposites exhibit good photocatalytic phage killing properties. TiO2/ graphene composite catalyst also showed adverse effects on nematodes and e. coli under light.

Market prospect analysis

In the past decade, there have been great breakthroughs in graphene and how graphene-based materials can be used for environmental protection. The unique structure and excellent properties of graphene will significantly improve the performance of environment-friendly materials. Because graphene is chemically similar to carbon nanotubes and fullerenes, it will have similar applications in environmental protection. Therefore, the choice of which carbon materials to use in the field of environmental protection mainly depends on their price, processability and impact on the environment.

In this regard, since GO is relatively cheap compared with raw graphene, it is likely to be the first to be applied in the field of environmental protection. The price of GO is comparable to that of multi-walled carbon nanotubes, which is more expensive than activated carbon, but much cheaper than graphene synthesized by single-walled carbon nanotubes and CVD. However, due to increased production capacity and process optimization, the cost of graphene-based materials is bound to decrease over time. Lab graphene production costs have fallen to about a quarter of what they were in 2012. Thanks to the reduction of graphene cost, the downstream application of graphene will achieve a breakthrough in industrialization as well as research.

Although graphene has achieved fruitful research results in the field of water treatment, due to its high price, its application in many fields will remain in the research stage for a long time.

The specific surface area of graphene is huge, and it can absorb heavy metals and dyes in water very well. The application of graphene as an adsorbent in the field of environmental protection will start the industrialization process. At present, although graphene has excellent adsorption performance, compared with activated carbon, it has low cost performance and no economic benefit. The theoretical value of graphene specific surface area is 2630m2/g, but it is difficult to exceed 1000m2/g for graphene of ordinary quality. However, the specific surface area of commercial activated carbon is generally between 800 ~ 1000m2/g, with developed microporous structure and excellent adsorption performance. Therefore, it seems unnecessary to replace activated carbon with expensive graphene for the adsorption of various pollutants.

However, graphene has incomparable advantages over activated carbon and other materials. Due to its special two-dimensional structure and pore size distribution, it has a higher adsorption efficiency for pollutants. In addition, the synergistic effects of various adsorption mechanisms (electrostatic, hydrogen bonding, pion-pi bonding, and hydrophobic) of functional groups on the GO and rGO surfaces also bring about better adsorption effects.

At present, many studies focus on the adsorption of graphene on various heavy metal ions in water. Although the adsorption of heavy metals by activated carbon is worse than that of graphene, this deficiency can be made up by reliable quantity. In recent years, many domestic graphene raw material manufacturers have set up mass production lines. With the development of downstream application products, it is only a matter of time before the production cost of graphene is continuously reduced, which will lead the market of sewage treatment in the future.

conclusion

Graphene, especially go, not only has a large specific surface area, but also has a large number of active functional groups on its surface, such as carboxyl, hydroxyl, carbonyl, epoxy, and a large number of pore defects. Therefore, it has a strong adsorption capacity and a good ability to remove heavy metal ions and organic pollutants in the environment. At the same time, when graphene is used as an adsorbent, its quality requirements are not very high, which reduces the difficulty of practical application and will have very important applications in the field of sewage treatment in the future. Countries around the world have also issued policies to guide the development of graphene industry, especially in the face of the world's top environmental problem, will continue to carry out in-depth research, give play to the huge advantages of graphene. Defective graphene is not perfect, and the scientific research personnel is dig out its internal potential, make its luxuriant turned, in improving the preparation process to lower production costs, on the basis of graphene graphene is bound to lead in environmental decontamination, purifying environment, will gradually occupy the dominant position in the future market competition.

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