Graphene Oxide : A Key Solution for Future: Recent Achievements as A New Adsorbent for Water Treatment Applications: Review

Polluted water has been considered a critical issue nowadays, threatening the environment and lives of living creatures. Because of technological and industrial advancements, as well as increased social activities of humans in various countries, pollution sources have multiplied. To reduce the impact of this problem, many techniques have been developed in order to reach zero discharge pollution. In the last decade, graphene oxide (GO) - a member of the graphene nanomaterials family, has been the focus of many research efforts in the water treatment sector because of its extraordinary properties. This review highlights the research efforts conducted to investigate GO as a novel adsorbent for water treatment applications and recent fulfilments in the last 3 years. The synthesis techniques, properties, and efficiency of this material in water treatment will be explained. All results confirm the future role of GO as an efficient absorbent to solve wastewater purification challenges, but the big challenge is to reduce time and simplify the complicated extent of synthesis stages, besides reducing the high cost of production methods. According to the review, Iraqi researchers' efforts to use this nanomaterial in water purification are still in their early stages.


‫المياه‬ ‫معالجة‬ ‫تطبيقات‬ ‫في‬ ‫جديد‬ ‫كممتز‬ ‫ات‬ ‫االنجاز‬ ‫اخر‬ ‫المستقبلي:‬ ‫الحل‬ ‫افين‬ ‫الجر‬ ‫أوكسيد‬
The first nanomaterial was a carbon nanotube material that was discovered by chance when Japanese scientist Sumio Iijima saw a thin layer precipitated on the electric discharge electrodes of an electric arc discharge in 1991 [1].This discovery triggered the nanotechnology revolution, which has continued up to this time.As a result, the properties of carbon nanotubes have widely opened the door for scientists to synthesize other types of carbon nanomaterials in particular and to produce other nanomaterials in general.Since that discovery, nano-carbonous materials have observed a very rapid expansion.The new types are still appearing.The main motivation for this huge development in the synthesis and manufacturing of carbon nanomaterials is their extraordinary characteristics, which allow for their use in different applications with high performance.Figure 1 shows the main types of carbon nanomaterials.In return, each main type is divided into several secondary types.Graphene is one of these types, which has rapidly expanded to include a variety of types with common basic properties [2].

Graphene, structure classifications and properties
Graphene can be defined as a dual-dimensional hexagonal honeycomb structure in which carbon atoms are arranged and bonded with each other by covalent bonds at the sp 2 orbital.Each carbon atom is bonded with three neighboring atoms, but in general it tends to connect with a fourth to form the benzoic hexagonal structure.The main difference between graphene and graphite is that the latter consists of 3D stacks arranged together [3].In 2004, Geim and coworkers could isolate a single layer of carbon atom film from graphite using adhesive tape for the first time, and then this material attracted serious attention as a new material [4].This note was the real start of graphene synthesis and characterization [5].Figures 2 and 3 show the structures of graphene and graphite.The unique properties of graphene have contributed to making it an effective alternative for a variety of substances in a wide range of applications and industries.The first of these benefits is that it is a super electrical and thermal conductor.Besides, it possesses high mechanical properties, such as high elasticity; its E-modulus is about 1.1 terapascal.The strength resistance of graphene is approximately 200 times that of steel.So, it resembles a diamond but is lighter in weight.It also has antibacterial properties and, due to its high density, acts as a good barrier to water passing through.As well, it is characterized by a high surface area of about 2630 m 2 /g [8][9][10].Moreover, graphene has extraordinary optical and electronic properties, so it is suitable for the solar and electronic industries [11].The graphene family consists of several types because of its rapid expansion in recent years.Figure 4 clarifies the graphene derivatives.Graphene oxide (GO) is considered the most prominent example in the graphene group, which has been a main topic for a wide range of applications.

Graphene oxide (GO) 2.1. History background
The history of graphene oxide dates back to 163 years ago.The first GO synthesis was done by Brody in 1859 [12].At that date, no one had investigated its unique properties, But it acquired a new importance after the nanotechnology revolution because the properties of GO were reassessed according to new concepts and implications.As a result, the tests have approved that GO is classified as a 2D nanomaterial, in which its thickness is at the nanoscale while other dimensions are in the microscale range with outstanding characteristics [10,13 ].

Structure and properties of GO
GO is classified as a non-stoichiometric chemical material in which the ratios of carbon, oxygen, and hydrogen are variable depending on the synthesis methods and manufacturing conditions.Using various theories and explanations, the scientists created a punctual description model for GO structure.The early explanations, which had been fulfilled by Hofmann, Rudolf, Ruess, Scholz, and Boehm [14] were not successful because the first two researchers supposed that GO consists of epoxy groups distributed randomly on the graphite basal layer.In response, others attempted to modify this model by adding different groups to the basal carbon, such as hydroxyl and carboxyl, in addition to epoxy groups, and they proposed a less ordered structure for GO containing C-C with C-C bonds [15].The novel and most convincing models were formulated by Lerf-Klinowski [16] and Denkany [17].According to two models, GO, as a member of the graphene family, has a similar structure to graphene.Both of these compounds have a hexagonal carbon lattice, but the GO sheet is usually distorted where it is bonded to the oxygen groups.GO has a two-dimensional carbon sheet with various oxygenated functional groups on its basal planes and peripheries.As usual, the thickness between its layers is about 1 nm, while the lateral dimensions fall in a range between a few nanometers and several microns.The main groups on the basal plane are hydroxyl and epoxide groups.In the outermost layer, groups such as aldehydes, carboxylic acids, and ketones exist.Furthermore, reduced graphene (rGO) can be synthesized from GO via different methods with the same structure except that the number of oxygen atoms will be reduced, leading to differences in the characteristics of rGO [17][18][19][20].Figure 5 shows the structures of graphite or graphene, GO, and rGO.

Graphene oxide characteristics
Because of the oxygen present in its lattice interlayers, GO has distinct properties that differ from those of graphene.These differences have contributed to its use in new applications where graphene cannot be used.The important properties of GO can be outlined in the following [5,12,22, and 23]. 1. Hydrophilic material.2. Low electrical conductor.3. The functionality: It is easy to modify GO with different function groups.4. It can easily come back to graphene at an effective cost compared with CVD and mechanical exfoliation.5. High mechanical properties, but not as good as graphene.6.Low thermal conductivity.7. Unusual electronic properties.

Synthesis methods
The main idea of GO synthesis methods is the strong oxidation process of graphite.Following the synthesis of Broody GO, manufacturing efforts have continued to seek the most effective method with the lowest cost and the least negative impact on the environment, as well as the shortest possible synthesis time [15].So, many methods have appeared using different techniques.However, the chemical oxidation method is still the most effective method for now.
Despite this, all synthesis methods have resulted in the production of GO, albeit with significant differences in the properties of each product.Table 1 gives details about the chemical oxidation methods [16,[24][25][26][27][28][29][30].The modified and improved methods are the traditional and popular techniques that are used to produce GO in the present time.Furthermore, there are novel attempts to synthesize GO using novel technologies such as, electrochemical [31], microwave [32], ultrasonic [33], and laser ablation [34].

GO characterization methods
Different characterization methods have been used to specify all the properties of GO.Table 2 shows the details of these tests and their results.

FT-IR
Bonding structure and types 1620 cm -1 for water 3300-3400 cm -1 for C-OH 3600 wide shoulder for OH

Zeta potential
Suspension stability of GO The value of stable suspension is greater than 30 mV

Surface area
Porosity of material and number of active sites From 10 m 2 g -1 to 2000 m 2 g -1

UV-Visible
Wavelength of GO Wide peak 190-900 nm but the maximum at 230 nm Thermal and electrical conductivity

Showing the thermal and electrical conductivity
Thermal conductivity is no more than 2000 W/mK 4.07 10 -7 S/cm

GO applications in water treatment
Water treatment is an essential concern nowadays because of the population increase and the lack of surface water on the whole earth, besides the diverse chemical pollutants discharged as a result of industrial, medical, agricultural, and domestic activities [4].The purification of wastewater to obtain high-quality water is done with different techniques, such as mechanical, chemical, biological, and physical methods.Nowadays, one of the effective topics occupying considerable ranking in international scientific research is the adsorption technique, which can be defined as the surface process that leads to the movement of polluted molecules from liquid media to the surfaces of solid particles [24,44].This method has gained more attention because it is inexpensive and simple.Additionally, a wide range of materials, either nanomaterials or micromaterials, can be used as adsorbents.Lately, many papers have focused on investigating GO as a new absorbent to purify wastewater in general and to treat medical wastewater in particular, due to its structural characteristics and its special surface properties.Table 3 lists some scientific advancements achieved during the previous three years involving the use of GO as a novel water treatment adsorbent.

Pollutant removal type Important results
Ref.The maximum adsorption capacities of GO and Grafted GO are 833.33 and 1000 mg/g, respectively.

Mechanism of GO adsorbent
The adsorption mechanism of GO is unclear until now, so it needs more research before a clear interpretation can indeed be made [58].At the moment, the proposed mechanism of GO adsorption is based on hydrogen hydrophilic bonding and interactions between the adsorbate and the adsorbent.This interaction is responsible for the efficiency of the adsorption process [24].Furthermore, the pH role in the adsorption process is critical in determining the efficiency of using GO as an adsorbent to remove various pollutants from water [59,60].

Challenges and future solutions
Many difficulties and limitations remain in the way of using GO as an effective adsorbent.The first is that the production of GO is still at the laboratory scale due to the complex and long stages that must be reduced to reach bulk production.Another important limitation is related to defects in the GO structure because of various manufacturing methods, which are still in use and have impressive effects on the properties of GO.These need to be improved in order to easily use GO with polymers or other nanomaterials to produce composites, especially in the membrane industry.Subsequently, bulk production of GO is still very limited because of these reasons, which contribute to making the synthesis process expensive.To address these issues, efforts should be focused on developing synthesis methods that yield high purity products as well as a cost-effective technology that yields an uncomplicated bulk production method that yields a cost-effective product.

Future developments and projects
The main limitations of using GO as an adsorbent are its high price and the complicated successive steps of the manufacturing process, as shown previously.However, several ambitious scientific projects have been launched to use GO as an adsorbent in water treatment plants while also identifying simple and cost-effective methods to reduce the complexity and high cost of production.One of these projects is a Monash University project (the CRC project) in cooperation with two Australian private companies in 2017.The project intends to use GO as a filtration medium, either as GO membrane composites or as traditional adsorbents coated with GO, on a commercial scale to purify wastewater while reducing electricity consumption.Up until now, initial results showed the potential merits of using GO in the water treatment sector [61].Khalifa University in the UAE recently announced the launch of the RIC-2D project in 2022.This project is aimed at funding and supporting global scientific research on graphene and its derivatives, as well as 2D carbon nanomaterials, for water and energy applications [62].In the UK, the University of Manchester could secure sufficient funding in 2022 for its project, which is aiming to produce graphene-based membranes in order to recover lithium and other materials from brines and wastewater produced from battery manufacturing for reuse in the same industry again [63].In addition, in 2022, GO commercial scale membranes were synthesized by NematiQ, an Australian developer company, where GO flat sheets of 1000 m in length and 1 m in width were achieved as a result of five years of hard scientific research and development, a milestone success in the GO membrane industry [64].These projects assure the importance of GO in future applications, especially in the water treatment sector.

GO ranking in Iraqi scientific research
The importance of GO was explained in detail in previous sections showing the international efforts of scientists throughout the entire world.In Iraq, the scientific efforts to utilize GO as an adsorbent are still in the beginning for many reasons, but the most effective reason is the time consumption of preparation.Nonetheless, the activity of Iraqi researchers has begun to increase in the recent five years.For example, the published papers that were published in academic journals (IAJS) inside Iraq are about 31 papers from 2015 to July 12, 2022 according to the Iraqi academic journals database [65].However, only 5 papers dealt with GO as an adsorbent.In other words, 16.161% of GO papers published in national Iraqi journals dealt with the topic of water treatment.Iraqi researchers, on the other hand, have published 52 papers in Science Direct International journals for various applications since 2015, as shown in Figure 6.Nonetheless, the number of papers using GO as an adsorbent is still limited, accounting for only 5 (9.62%) of the total number of published papers.
. Figure 6: Iraqi researchers published papers in peer-reviewed journals

Conclusions
Novel materials for water treatment, particularly adsorbents, are currently regarded as a critical topic in scientific research.So, the properties and characteristics of GO may make it a promising future water treatment material.Different synthesis methods have been used for 165 years.The results, which are recorded in international papers, indicate its effective ability to treat different water contaminants with high adsorbent capacity.GO has begun to be used in the industrial-scale production of water purification materials and tools containing GO as a novel nanomaterial absorbent that can play an important role in water treatment in the near future, particularly in large projects.The main obstacle to using GO extensively is the complex successive oxidation stages of synthesis methods that keep production of GO in the laboratory or small-scale industrial range, which contributes to making the price of GO expensive compared with other types of adsorbent.In Iraq, scientific efforts to apply GO adsorbent in water treatment are still limited, as evidenced by publications in national and international journals.

Table 1 :
Chemical oxidation methods