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GRAPHENE-BASED MATERİALS FOR WASTEWATER TREATMENT

Wastewater treatment is an important topic concerning the sustainability of clean water resources and human health. Research shows that graphene and GO are promising adsorbent materials for the removal of organic and inorganic water pollutants. Graphene-based wastewater treatment systems should be developed considering the properties of adsorbent, contaminant, background solution, and adsorbent regeneration.

Water is the most fundamental material on Earth essential for survival, any industrial process, or day to day activity. Up until recently, water was considered to be an abundant material in nature. However, long gone the days we could carelessly waste the precious water sources of Earth. As industrialization and consumer based economy became prominent in modern times, the natural water cycle has become inefficient and failed to keep up with the pace of human activities. Water pollution needs to be controlled to avoid further effects on the entire biodiversity leading to the destruction of living and non-living things on Earth. For this purpose, contaminating sources should be identified and controlled. Proper wastewater treatment processes need to be installed at the output of these contaminating sources in order to remove toxic and hazardous components from the sludge. Cleaning the discharge water of industrial plants, sewage lines, and mining activities ensures suitability for household purposes, natural usages, groundwater recycling, and many other purposes. This is why water pollution and wastewater treatment have become one of the most important topics of science and engineering. Even though some innovative solutions are suggested and utilized in wastewater treatment plants, the current treatment methods are still not efficient enough and often require a lot of space. In order to obtain desirable solutions to these problems in wastewater treatment processes, scientists have recently turned to nanotechnology and the use of nanomaterials.

Why Nanomaterials?

Nanomaterials are advantageous in wastewater treatment processes due to their wide surface area, better chemical properties, lower cost, and high reusability. Attributing to their desirable properties several different nanomaterials and nanostructures have been suggested to be used in wastewater treatment systems. In the search for better wastewater treatment methods and materials, graphene and graphene-based materials have recently attracted attention. In particular, graphene oxide (GO) holds great potential for effective wastewater treatment methods.

Graphene is a 2D material purely composed of carbon atoms arranged in a hexagonal structure. The structure and bond type of graphene endows unique properties such as high mechanical strength, porous structure, electrical and thermal conductivity, chemical resilience, optical activity, and high surface area. Graphene-based materials include graphene oxide, and reduced graphene oxide (rGO). These materials are the products obtained during graphene production trials. Graphene oxide is obtained through the oxidative treatment of graphite while reduced graphene oxide is obtained through the reduction of GO sheets. As much as they seem like undesired results of a specific process, th

ey are proven to be equally useful in several different application areas. GO is defined as a graphene layer with various oxygen containing functional groups attached to its surface. Such functionalities can include epoxide, carbonyl, carboxyl, and hydroxyl groups. Reduced graphene oxide is obtained by removing most of these functional groups on the GO surface. GO and rGO show different properties than pristine graphene sheets. The chemical, optical and electrical properties of GO and rGO are considered to be significantly different. Rather than decreasing their value, these differences give GO and rGO distinct benefits.

What is Wastewater Treatment?

Manufacturing processes and most of the day to day activities involve the use of highly toxic and hazardous materials. Types of pollutants to be removed from wastewater are as important as the material used for the treatment process. Water contaminants can be categorized as organic and inorganic pollutants. Organic pollutants include dyes, polycyclic aromatic hydrocarbons (PAHs), pesticides, fertilizers, herbicides, phenols, hydrocarbons, biphenyls, greases, oils, proteins, carbohydrates, and pharmaceuticals. Organic pollutants can be divided into two categories owing to their biotic degradation ability. The contaminants with modest structure and hydrophilic properties can break down in the water and show severe toxicity at high concentrations. Some components such as methanol and polysaccharides are degraded by fungal or algal bacteria. Other types of organic contaminants are referred to as persistent organic pollutants (POPs). This group includes polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and dichloro-diphenyl-trichloroethane (DDT). These pollutants degrade slowly and cause great anxiety owing to their persistence, toxicity, long-distance transport, and bioaccumulation ability. Most of the POPs are considered teratogenic, carcinogenic, and neurotoxic. On the other hand, inorganic materials mainly include heavy metal ions and rare earth elements. Heavy metals such as mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), thallium (Tl), and lead (Pb) are released into the water through natural or anthropogenic acti

vities. Fuel combustion, mining activities, mineral processing, manufacturing activities, and sewage discharge release a significant amount of heavy metal ions to water resources which create a variety of health and environmental problems. Rare earth elements are chemically alike elements including La, Pr Ce, Pm, Nd, Dy, Sm, Tb, Gd, Er, Tm, Ho, Yb, Sc, Lu, Eu, and Y. these elements are commonly used as a catalyst or in fertilizers. Their active chemistry and resistivity make these contaminants highly toxic to the human body and the environment.

Wastewater treatment processes aim to remove or reduce the contaminants to provide the required safe water supply. These processes are usually composed of three stages. The first stage is the mechanical stage removing solids through physical methods such as filtration and coagulation. The second stage gets rid of remaining microorganisms while the third stage removes any impurities before the water can be accepted for everyday usage. The wastewater treatment methods for the removal of organic and inorganic impurities include membrane filtration, chemical precipitation, solvent extraction, and adsorption. While all of these methods have their advantages and disadvantages, adsorption is considered to be relatively easy, low-cost, and effective. Graphene and graphene-based materials are utilized as adsorbents for the removal of organic and inorganic pollutants from wastewater.

Removal of Organic Contaminants with Graphene-Based Adsorbents

The bioaccumulation of toxic organic materials we have mentioned before cause severe problems for human health and the environment due to their toxic nature. The removal of these contaminants is affected by a number of factors including the adsorption behavior of the system and the properties of materials involved. This is why it is crucial to understand these effects. To understand the adsorption behavior, the adsorption affinities, and the interaction between graphene and organic contaminants should be assessed. Hydrophobic, π-π electron donor-acceptor, van der Waals, electrostatic, Lewis acid-base, hydrogen bonding interactions hold an important place in graphene wastewater treatment methods. Furthermore, the properties of graphene-based materials, organic contaminants, and the background solution are also extremely important for these systems.

The important physical properties of graphene-based materials include surface area, pore volume, and pore size distribution. On the other hand, the important chemical properties of these materials are surface charge and polarity, surface chemistry, and purity. Understandably, the higher surface area is advantageous for adsorptio

n systems. On the other hand, pore volume and pore size distribution of the graphene sheets should comply with the properties of the contaminant. If the pore size is too small for the particular contaminant, the size exclusion will inhibit the effective adsorption of the toxic molecules. In various studies, graphene is reported to have meso- or macro-pores while GO is reported to have meso- and micro-pores. It is important to take these differences into consideration for obtaining better wastewater treatment methods based on graphene-like materials. The surface chemistry of graphene-based materials plays a crucial role in adsorption behavior. The oxygen containing functional groups on the surface of GO show two opposing effects on the adsorption capacity. While increased surface charge increases adsorption due to improved water solubility, the water clusters formed on the material surface decrease the active adsorption sites. On the other hand, the oxygen content on GO assists the adsorption of amino and hydroxyl containing contaminants due to strong hydrogen or Lewis interactions. The purity of graphene-based materials is crucial for accurate estimation of treatment performance.

The most important organic pollutant properties include size, geometry, hydrophobicity, functional groups, and aromaticity. The hydrophobicity of organic contaminants is one of the major factors in adsorption systems. Hydrophobic or polar organic pollutants are compatible with graphene while ionic or acidic/basic pollutants are compatible with the charged surface of GO. Additionally, the number of aromatic rings and their spatial arrangement influences the adsorption of the organic pollutant. Studies show that with the increased number of aromatic rings in the molecule structure, the adsorption affinity of the pollutant increases.

Besides the properties of graphene-based materials and the organic contaminants, the properties of the background solution are also important for wastewater treatment. The most important factors affecting the adsorption are reported as pH, temperature, ionic strength, and presence of NOM. These factors affect the chemical and physical interactions between graphene-based adsorbents and organic pollutants.

GO is found to be considerably effective in the removal of dye pollutants such as methyl orange (MO), methyl green (MG), methyl blue (MB), rhodamine B, basic red 12, etc. An endothermic adsorption process is observed during the removal of dye molecules. The degree of oxidation of GO is one of the important factors affecting the adsorption. As the degree of oxidation increases the adsorption of dye molecules from the aqueous phase also increases. Graphene oxide composites are also utilized for dye adsorption to obtain suitable wastewater treatment systems. These composites might be hydrogel/GO, polymer/GO, or magnetic graphene/GO systems.

Graphene and GO are also found to be effective for the removal of pharmaceutical contaminants such as antibiotics and endocrine disrupting chemicals. The adsorption process involving GO is mainly dominated by π-π electron donor/acceptor interactions and hydrophobic interactions. Oxygen containing groups of GO assists the effective adsorption of endocrine disruptive chemicals because of the strong hydrogen bonding between these two materials. Hydrogen/GO composites are also advantageous for the removal of pharmaceutical contaminants.

Polyaromatic hydrocarbons (PAHs) are a group of organic chemicals including naphthalene, pyrene, phenanthrene, etc. The removal of these contaminants is crucial because of their toxic nature and human health. Graphene is found to be more effective in removing PAHs than GO due to its slightly larger pores. Derivatives of naphthalene such as naphthol are also suitable for adsorption processes utilizing graphene and GO.

Overall, graphene and graphene-based products are great candidates for the removal of toxic organic compounds and the improvement of these adsorption systems is the key to obtaining better wastewater solutions.

Removal of Inorganic Contaminants with Graphene-Based Adsorbents

Heavy metals and rare earth materials raise great concerns in terms of clean water supply. The persistence of toxic inorganic materials leads to accumulation in the human body and environment causing various diseases. Hence, the removal of these materials holds an important place in wastewater treatment processes. The pH of water, temperature, presence of background ions, and NOM greatly affects the elimination of these contaminants. Adsorption of inorganic contaminants is highly sensitive to changes in the pH of the environment. Since each material has a different response in varying pH conditions, wastewater treatment systems should be analyzed accordingly. In general, lower pH values are favorable for anionic contaminants while higher pH values are favorable for cationic contaminants. The presence of background ions can interfere with the activity and mobility of inorganic contaminants. Furthermore, an increase in the ionic strength of the water reduces the amount of available binding sites on graphene and GO surface. NOM molecules affect the adsorption process by changing the electrostatic properties of the wastewater. The adsorption process of inorganic contaminants is endothermic and spontaneous which makes graphene-based wastewater treatment systems desirable.

Studies show that graphene is a promising adsorbent for the removal of cobalt (Co(II)), fluoride, and iron (Fe(II)). GO is another effective adsorbent for cationic pollutants such as copper, nickel, zinc, palladium, etc. The oxygen groups on GO sheets act as anchors in the adsorption process. While some studies focused on graphene and GO sheets solely, the others show that graphene and GO-based composites and rGO also have great adsorption capacities. For the removal of inorganic contaminants the surface of graphene and GO is often decorated with different nanoparticles or oxygen containing groups to increase the adsorption affinity. Decorating graphene and GO with magnetic nanoparticles such as magnetite is suggested as an effective method for the separation of inorganic contaminants.

Rare earth metal ions are noxious and known to create severe water pollution. Various studies incorporate graphene-based adsorption systems for the removal of these contaminants. Most of these studies focus on GO due to its charged surface and oxygen containing groups. GO is found to be effective for the elimination of La, Gd, Y, and Nd from wastewater. GO nanocomposites with polyaniline (PANI), GO functionalize with magnetite and titanium phosphate are also suggested as strong candidates for the job.

Regeneration and Toxicology of Graphene-Based Adsorbents

The same properties that make graphene-based materials great players in various applications also make these materials possible threats. High chemical and biological activity of graphene-based materials may cause toxic effects on the human body and eco-system. The negative effects of graphene on the human body include lung injury, kidney failure, cell membrane, and DNA damage, etc. At the moment there is no definitive guide to handle toxicity induced by graphene-based materials. This is why the control of graphene wastewater systems and regeneration of graphene-based materials are important topics.

Regeneration of graphene-based adsorbers is not only important for avoiding toxic effects but also the sustainability of wastewater treatment systems. Any successful adsorber should show great desorption behavior in addition to adsorption properties. Regeneration is an essential part of effective wastewater treatment, reduces the cost of the treatment system, and enables the recycling of valuable industrial materials such as organic and inorganic molecules. Furthermore, Spent and regenerated adsorbents may be employed in the manufacturing of steel, cement, brick, and other construction materials. Establishing a successful adsorption-desorption cycle is a tedious process requiring fine adjustment of process parameters. Several different methods are suggested for the effective regeneration of graphene-based materials. These methods include a mixture of physical and chemical procedures including centrifugation, cross-flow filtration, field-flow fractionation, solvent treatments, and electric field. Due to the different nature of each contaminant different strategies should be considered for each wastewater treatment process. Alcohols such as ethanol, basic materials such as NaOH, and acidic materials such as HCl and HNO3 are the commonly used ingredients for the regeneration of graphene-based adsorbents.

 

Conclusion

Water is arguably the most important material on Earth crucial for the survival of living species and the continuum of daily life. Preserving scarce water resources is an important issue as the water polluting activities gain acceleration and water consumption increases with the increasing population. One of the most important aspects of water sustainability is wastewater treatment systems. Waste treatment ensures that effluent water from industrial plants and sewage lines is cleaned from toxic and harmful materials. Preventing the bioaccumulation of these materials is crucial for human health and environmental protection. Such materials are divided into two categories as organic and inorganic. Toxic organic materials include pharmaceutical contaminants, dyes, PAHs, pesticides, fertilizers, herbicides, phenols, hydrocarbons, biphenyls, greases, oils, proteins, and carbohydrates. Organic contaminants interfere with the environmental systems and functions of the human body. On the other hand, toxic inorganic materials are heavy metal ions and rare earth metal ions. These inorganic materials are highly resistant and can accumulate in plants and the human body causing various diseases. Current wastewater treatment methods attempt to eliminate these contaminants; however, they are not efficient enough for today’s requirements and require a lot of space. Thus, researchers have turned to nanotechnology for more satisfying treatment methods. In this context, graphene and graphene-based materials offer promising wastewater treatment solutions. Graphene and GO are especially at the center of attention of adsorption based water treatment systems. Their high surface area, lower cost, reusability, and chemical properties are highly desirable. Furthermore, the oxygen containing groups on the GO surface provide anchoring sites for charged materials. For the removal of both organic and inorganic contaminants, the physical and chemical properties of adsorbent material, properties of contaminants, and properties of background solution are all important. Size, geometry, hydrophobicity, functional groups, and aromaticity of toxic organic materials greatly affect the adsorption efficiency. On the other hand, the charge of heavy metal and rare earth ions affect the adsorption efficiency. The regeneration of graphene-based adsorbents is as important as the adsorption of contaminants from wastewater. Regeneration is important for the system efficiency, reusability, and recycling of valuable materials. Common regeneration methods suggested for graphene-based wastewater treatment systems are centrifugation, cross-flow filtration, field-flow fractionation, solvent treatments, and electric field. Regeneration is also important in terms of controlling the toxic effects of graphene on the environment and human health. Graphene and graphene-based materials are reported to show toxicity because of their chemical and biological activity. Hence, proper separation of these adsorbents is crucial.

Overall, graphene and graphene-based materials show great potential as adsorbents and can be applied to develop better wastewater treatment systems. It is important to note that, these systems still require further improvement and optimization for widespread applications.

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