Graphene and Graphene Oxide-Based Composites for Removal of

Review Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Graphene and Graphene Oxide-Based Composites for Removal of Organic Pollutants: A Review Kirti Thakur† and Balasubramanian Kandasubramanian*,‡ †

Department of Atomic and Molecular Physics, Manipal Academy of Higher Education (MAHE), Manipal, 576104, India Nano Surface Texturing Lab, Department of Metallurgical and Materials Engineering, Defence Institute of Advanced Technology (DU), Ministry of Defence, Girinagar, Pune 411025, India

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‡

ABSTRACT: Graphene, graphene oxide (GO), and their composites have been prominently utilized for wastewater purification because of their adsorption, oxidation, and catalytic properties. Graphene and GO and its composites naturally have significant pore volume, high conductivity, rich surface chemistry, and an exceptionally large aspect ratio which make it favorable for adsorption and catalysis of organic pollutants from wastewater. The sheet-like, resonating, polyaromatic π-system of graphene subsidiaries play a significant role in π−π interactions, hydrogen bonding, and/or electrostatic interactions with organic pollutants that include dyes, pharmaceutical waste, and agricultural and industrial effluents whose base structure consists of notably reactive unsaturated aromatic rings and oxygen-rich functional groups. The adsorption capacities of pollutants have been widely researched and catalogued by considering the adsorption isotherm (Langmuir, Freundlich, Temkin, DR model) they fit, the kinetic models (pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion) they follow, the parameters that affect the process (pH, temperature, etc.) and the reusability of the adsorbent. The photocatalytic efficiency has been anthologized with the viewpoint of the radicals being involved in photocatalysis and the light source used for the process. This review focuses on adsorption, advanced oxidation, and catalysis of various emerging organic pollutants using graphene subsidiaries, graphenebased composites, and hybrids; proves their efficacy as multifunctional materials for the expulsion of toxic aqueous phase pollutants; and presents new prospects for designing advanced water treatment strategies.

1. INTRODUCTION Graphene, an sp2 hybridized, hexagonally arranged, covalently bonded chain of polycyclic aromatic hydrocarbon arranged in a single sheet of atomic thickness in a honeycomb crystal lattice with a unit cell of two carbon atoms has unique properties owing to its structural and chemical morphology such as high surface to volume ratio,1 excellent transparency,2−5 unmatched conductivity,6,7 and high mechanical strength.8 Thus graphene attracted worldwide attention from various researchers and can be assessed by its application in varied fields of science and engineering such as biosensors,9,10 fuel cells,11,12 electromagnetic shielding,13 aerospace applications,14,15 energy storage devices,11,16 photonic17,18 and electronic devices as nanoantennas,19 sound transducers,20,21 organic light emitting diodes,22,23 protective coatings,24,25 integrated circuits,26,27 biomedical devices,28,29 and contamination purification in wastewater management.12,30,31 We discuss thoroughly the exceptional properties and wide applicability of graphene inadvertently ignoring that the mere existence of 2D crystals was considered implausible.32 Laudau and Peierls both argued the thermodynamic instability of 2D nanocrystals and individually reported their theory deeming 2D structures as non- existent and were further experimentally supported by Mermin.33−35 The unstable nature of the 2D © XXXX American Chemical Society

structures was based on the melting point depression experienced because of a large difference in the surface volume proportionality, and thus the structures struggle to exist in the nanometre regime.12,36,37 The intercalation and exfoliation of graphite with the help of nitric and sulfuric acid by German scientist Schafhaeutl in 184038 and the reduction of the intercalated layers using potassium chlorate by a British scientist, Brodie, resulted in the formation of first graphene oxide layers on graphite in 1962.39 Boehm and Clauss further reduced the graphene oxide layers with the help of hydrazine and hydroxyl amine to form reduced GO and coined the term “graphene”40−42 In 2004, Novoselov and Geim mechanically stripped graphene using a scotch tape method for electric field effect studies which led to them receiving the Nobel Prize in 2010.6,12,43 Historically, activated carbon has been a conventional adsorbent for waste water treatment (WWT)44,45 due to its versatility, benign nature, and wide applicability but falls short due to high regeneration cost and chemical rigidness.46 The need for a better carbon-based alternative is satisfied by Received: November 9, 2018 Accepted: February 1, 2019

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DOI: 10.1021/acs.jced.8b01057 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Figure 1. Structure of (a) graphene and (b) graphene oxide.

minerals, less toxic fragments of carbon-based pollutants, and neutral entities such as water and carbon dioxide.59 Graphenebased materials are known to generate hydroxyl and sulfate radicals that undergo oxidation to eliminate aromatic hydrocarbons successfully.60 Photocatalysis is also a part of the advanced oxidation process and has been effective in the degradation of various organic entities by graphene-based materials and their composites.61 Photoelectrocatalysis,62 ozonization,63 etc. of organic pollutants has also been reported for organic effluent elimination.64−67 Reduction by graphene is also reported and processes such as photoreduction and chemical reduction form important processes for enhancing wastewater remediation.68,69 This review provides a systematic anthology of graphenebased materials in effective wastewater treatment technology. In today’s scenario, efficient water purification systems are desired and hence many technologies are being developed after scrutinizing their practicality, affordability, and durability. To maximize the removal of pollutants from graphene, first we need to understand the morphology of the pollutants with which the graphene composite interacts and the intricacies of these interactions. Hence in the following sections an understanding of the structure of pollutants and graphene, their possible interaction mechanism, thermodynamics and kinetics of the said interactions, and catalytic efficiency of some of the designed composites have been discussed. The review focuses on graphene as a state of the art and establishes that its prominence in organic effluent eradication is colossal and cannot be overlooked for designing effective wastewater treatment strategies.

graphene because apart from being carbonaceous it has a vast surface area by virtue of its planar sheet-like morphology. The existence of few layered graphene (FLG) also enhances its adsorption capabilities due to interlayer adsorption.47 Graphene’s polyaromatic π-system which readily reacts with the aromatic rings of organic entities in water by either π−π stacking interactions48 or strong hydrophobic effect49 make them highly efficient adsorbent materials and have been successful in eradicating pollutants such as naphthalene, 1naphthyl-amine, aspirin, and dyes50 such as Rhodamine B (RhB),51 methylene blue (MB),52,53 etc. The hydrophobic nature of graphene makes it nondispersible in water and thus the applicability of graphene, in its pure form, is limited for decontamination purposes. The use of graphene is thus either supported by or in the form of composites that render it useable.48 The properties offered by the graphene skeleton have been further enhanced by oxidizing it to graphene oxide that contains substantial oxygen functional groups such as carboxyl, carbonyl, epoxy, and hydroxyl, which provide it with a negative charge and make it hydrophilic.54,55 Further functionalization of graphene and graphene oxide lead to surface modification and can be tuned to be reactant specific for various organic entities. The large number of oxygen-based functionalities affects its conductivity, and GO has an electrically insulating reputation.56 The reduction of graphene oxide does not form pristine graphene, instead, it forms reduced graphene oxide (rGO) that has a carbon to oxygen ratio to 246:1 when a comparison is made with GO(2:1).57 Thus, rGO can be considered as an intermediate of graphene and GO although its π−π interactions with organic effluents are augmented more than both of its associates and thus makes it advantageous for the removal of organic effluents efficiently from wastewater.58 The structure of graphene and graphene oxide can be visualized in Figure 1. The oxygen functional groups present on GO also participate in redox reactions and transform various pollutants into environment friendly and degradable form. This eliminates the issue of disposing of waste matter as is in the case of adsorption. The radical-based oxidation processes also termed as advanced oxidation processes (AOP) directly transform the organic entities to innocuous entities which are compatible with the environment and include various

2. NECESSITY OF GRAPHENE-BASED ORGANIC EFFLUENT ERADICATION Aromatic organic compounds are those compounds that have one or more than one unsaturated cyclic carbon chain in the whole molecule. Their negative biological impact is a source of concern in reference to their removal from the water system. The toxicity of these compounds can be established by studies that classify them as carcinogens, mutagens, teratogens, and render them genotoxic and immunotoxic.70−72 Hence they pose a great threat to aquatic life due to bioaccumulation and biomagnification and hamper the food chain because of their B



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characteristic lipophilic property.73,74 Because of their high affinity toward water owing to the hydrogen bonds and π ring structures, aromatic hydrocarbons are more toxic than aliphatic hydrocarbons with almost the same number of carbon atoms.75,76 Polycyclic aromatic hydrocarbons (PAHs) are a class of hydrocarbons that are made up of carbon and hydrogen arranged in repeated ring structures. They are noncharged and nonpolar molecules formed from various processes such as burning of petroleum products, incomplete combustion of biomass, coal mining, etc.77 PAHs have harmful effects on human health and are known to cause skin, blood, bladder, and liver cancer78,79 and cardiovascular diseases.80 Monocyclic aromatic hydrocarbons such as toluene, xylene, benzene, etc. are also harmful to the human population as they affect the central nervous system and hamper its activity.81 They are largely excreted from industry due to improper disposal of waste, leaks, and accidents, etc. Other organic compounds such as bisphenol A (BPA), phenols, benzene, biphenyls, toluene, ethylbenzene, xylenes, anthracene, pyrene, phenanthrene, and fluoranthene are a few of the organic pollutants that have been hampering the aquatic environment and pose a threat to human wellbeing.82,83 Pharmaceutical wastes are also organic pollutants that have adverse effects on the environment and on human health as they have been designed for biocompatibility. Drugs such as aspirin, acetaminophen, dorzalamide, ketoprofen, and ciprofloxacin can cause adverse effects in humans as they have been designed for uptake by the human body.84 These compounds are very hard to eliminate and biodegrade even at low concentrations. Another class of aromatic hydrocarbons widely found polluting the environment are dyes and pigments such as methylene blue (MB), methyl orange (MO), reactive black, etc. that are difficult to remove from the environment and cause disruption in the photosynthetic process of aquatic plants by changing the color of water, thereby blocking sunlight causing an imbalance in the whole aquatic ecosystem.85 Some of the structures of these pollutants have been shown in Figure 2 and their aromatic structures can be easily observed. Adsorption, advanced oxidation, and photocatalysis have been used to remove the organic pollutants from harming the environment and graphene-based materials have been proven to be highly efficient for wastewater remediation.86,87

Graphene-based remediation of organic pollutants from wastewater is extensively investigated and catalogued.76 The various combination of graphene with other metallic or nonmetallic entities create new composites or enhance the effectiveness of the existing materials for adsorption, oxidation, or catalysis of the pollutants such that they disintegrate into harmless constituents or are easily separated by techniques such as filtration, magnetic separation, or a simple solvent wash. The ability of graphene-based composites to individually separate a plethora of pollutants makes it a promising candidate for overcoming the drawbacks of existing wastewater technology.88,89 The inept existing WWT strategies and the need for novel ways to eliminate organic effluent has steered us toward graphene since it has been extensively studied that graphenebased materials show enhanced usability in adsorption,90,91 advanced oxidation52,92 and catalysis93,94 for the removal of a multitude of organic pollutants. These include polyaromatic hydrocarbons (PAH) and their derivatives, dyes,95,96 pesticides,97 and pharmaceutical waste such as antibiotics98 and other organic entities generated during the manufacturing processes of various industries. Graphene modified with functional groups or enhanced by forming a polymer and nanobased composites accentuate these properties of graphene materials and provide extra support and stability thus providing major advantages in effective WWT.99,100 Therefore, it is essential to understand the ways graphene interacts with other moieties. This is discussed in the next section.

3. INTERACTIONS OF GRAPHENE The remarkable properties of graphene are highly dependent on its availability as a single layer. If the layers are in close vicinity to each other graphene tends to agglomerate or restack like graphite owing to the π−π interactions between the graphene layers and its properties are somewhat impaired.101 When dispersed in water, graphene, due to its hydrophobic nature may or may not agglomerate but does not remain suspended due to the absence of any electronegativity difference in the planar carbon lamella. This agglomerating behavior of graphene can be reduced by attaching smaller molecules or polymers which are either hydrophilic and/or hydrophobic to the graphene sheets. These functional groups inhibit the agglomeration of graphene layers either by immensely polar interactions or by their bulky nature and also aid in the dispersion of graphene in water or any other polar or nonpolar solvent enhancing its usability. The reactions involved in functionalization not only signify the transformation of graphene sheets for applicability in water dispersion but also incorporate how graphene or its subsidiaries interact when aromatic molecules are in close proximity. Hence these interactions form the foundation of wastewater treatment technology and signify how organic pollutants react with functionalized or nonfunctionalized graphene systems. GO and rGO being hydrophilic also undergo functionalization to enhance the properties offered by the graphene backbone and tune them according to their use. The GO surface is rich in oxygen functional groups such as hydroxyl, epoxide, diol, carboxyl, etc. that modify the van der Waals interactions. Although the aromaticity of graphene is lost due to utilization of π electrons in covalent bonding of these oxy groups on graphene skeleton, the presence of carboxyl, carbonyl etc. groups at the edge make them highly soluble and dispersible in water.

Figure 2. Structure of some of the (a) pharmaceutical pollutants, (b) dye pollutants, (c) other organic entities. C



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such as amide and carbamate that functionalize via ester linkages.112 Chitosan-modified graphene is also formed by this mechanism for which the amine group of chitosan and carboxyl group of GO react. This has been used in the removal of several organic pollutants such as dorzalamide and hydroquinone, and dyes such as methylene blue, and has good aqueous solubility and biocompatibility. The esterification of carboxyl and hydroxyl groups of β-cyclodextrin (β-CD) is also used to functionalize graphene.113 β-CD forms hybrids with graphene and other entities such as Fe2O3 that enhance its magnetic character and helps in removal of dyes such as brilliant green and MB.114 Polyvinyl acetate (PVA) is also used for formation of ester linkages for surface modification of GO.115 Amide bond formation in amine functionalized porphyrin (TPP-NH2) enhances the solubility and constancy of graphene subsidiaries in organic and aqueous phases.116 An ion exchange material, sulfanilic acid functionalized GO has been prepared, and amide bond formation takes place between amine group of sulfanilic acid and carboxylic group of GO.117 3.1.3. Electrophilic Substitution (SE) Reaction. When the hydrogen atom is replaced by an electrophilic entity the reaction is called an electrophilic substitution reaction. Graphene has hydrogen atoms on its edges, and they can be easily replaced by other functional groups. GO decorated with oxidized functional groups is highly affected by the hydroxyl radical that possesses the dual nature of being able to oxidize and participate in electrophilic addition. The Fenton reaction is a method to degrade aromatic compound in water by a highly selective hydroxyl radical. The GO can either provide the hydroxyl radical or the radical can be utilized to breakdown the graphene sheets. Other entities such as para-nitro aniline,118 aryl diazonium salt,119,120 4bromo-aniline,121 ferrocene,122 methyl-2-pyrrolidone,123 and polystyrene124 have also been used to functionalize graphene composites via electrophilic substitution. 3.1.4. Addition Reaction. Addition reactions are responsible for forming larger molecules by a combination of two smaller molecules and are devoid of any byproducts. Khatri and group reported an aza Michael reaction125 that involves the addition of an amine to the β position of an α,β-unsaturated carbonyl and nitrile106,126 compound of graphene to give a β-amino carbonyls and nitriles127 as products with an aqueous dispersion of GO. This reaction had a high catalytic activity with reaction times as low as 5−10 min. Another example is the formation of a GO-based polymer nanocomposite incorporated with polydopamine and poly(sodium p-styrenesulfonate) hydrate (GO-PDA-PSPSH) in which the addition of PSPSH and GO-PDA took place via Michael addition. These nanocomposites showed great results for the removal of methylene blue dye from wastewater.128 3.2. Noncovalent Functionalization. Noncovalent functionalization refers to all the types of bonding that do not involve sharing electrons but are based solely on the interactions based on the charges of the moieties.29,129 π -π bonding, electrostatic attraction, and hydrogen bonding, etc. are few of the noncovalent interactions that graphene-based materials are involved in for functionalization or other applications.102 Usually π−π interactions happen with organic molecules and polymers with extensive π systems while van der Waals forces are formed between graphene and GO with the same type of characteristics as for π−π interactions but the functionalizing entities should have a hydrophobic character.29 The oxygen functionalities on the surface and edges of GO

The interactions of graphene with other moieties for its functionalization can either be covalent, noncovalent, or a combination of both. Depending on the type of interaction the molecules either disturb the hybridization of graphene from sp2 to sp3 disrupting its aromatic nature or follow a more versatile noncovalent approach preserving the aromaticity of the lamella. The apperception of these interactions is imperative as they alter the chemistry of graphene and its subsidiaries and assist us in designing better graphene-based materials for specific applications. Therefore, a detailed study of these interactions of graphene and its subsidiaries with other organic entities is essential to enhance their attributes and usability. 3.1. Covalent Functionalization. The functionalization that is associated with the transformation of some of the sp2 hybrid carbon of graphene to sp3 hybridization by sharing of the delocalized electrons is known as covalent functionalization and results in the loss of aromaticity.102 Graphene can be functionalized by nucleophilic substitution, electrophilic addition, condensation, and addition reactions. 3.1.1. Nucleophilic Substitution (SN) Reaction. The reactions in which an electron-rich nucleophile is attracted to an electrophile, that is, a positively charged atom and replaces a leaving group are known as nucleophilic substitution reactions.103,104 These kinds of reaction are possible in graphene that has defects or GO and rGO layers as they have electron-rich groups. A representative case is the functionalization of GO by substituting it with amine groups that attack the epoxy group on the GO surface. The lone nitrogen pair renders it as a nucleophile, and the reaction takes place at room temperature. Not only amine but also amino acids and amine terminated biomolecules, etc. have also been used to functionalize graphene.105−107 One of the methods of amine group substitution is by using ammonia and ethylene glycol as solvent and nitrogen as precursor. The formation of rGO-NH2 at relatively low temperature by the solvo-thermal method was achieved, and hydroxyl, epoxide, and carboxyl groups containing oxygen functionalities such as lactones, anhydrides, etc. were obtained on the graphene skeleton via nucleophilic substitution of carboxyl and epoxide group with ammonia radicals.108 The addition of amino groups provides graphene with excellent complexing properties that can be used in various applications. Tetrafluoroterephthalonitrile (TFT) or decafluorobiphenyl (DFB) were linked to GO sheets via an aromatic SN reaction and resulted in porous GO. The pore size and surface area were observed to have considerably increased and thus this material acted as adsorbents for specific pharmaceutical effluents such as carbamazepine, paracetamol, ibuprofen, sulfadiazine, sulfamethoxazole, and phenacetin.109 Ethylenediamine-N,N-disuccinic acid (EDDS) was used to reduce GO and was used to eradicate tetracycline from aqueous media. Epoxide was removed from GO by protonation by hydronium ion followed by nucleophilic substitution for ring opening that formed an intermediate with the hydroxyl group. The hydroxyl group was removed via dehydration followed by deprotonation, and sp2 hybridization was restored110 3.1.2. Condensation Reaction. In the condensation reaction, the combination of two molecules lowers the entropy102 and yields some other small molecule as byproducts such as water, ammonia, acetic acid, or hydrogen sulfide. Graphene undergoes this transformation with isocyanates, diisocyanates,111 and amine compounds and forms compounds D



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induce ionic interactions and hydrogen-bond formation. These noncovalent bonds do not interfere with the π system of the graphene-based materials, and hence the properties offered by unique sp2 hybrid planar structure is maintained.29 The π−π interactions are formed between entities that have extended π systems and a compatible geometry. The planar graphene readily interacts with other aromatic planar molecules. This ability is reduced in rGO and GO and the π−π interactions thus formed are weaker than that formed by graphene. Other interactions such as hydrogen-bond formation and electrostatic interactions take place due to electronegativity differences between reacting molecules and their proximity, respectively. These have been depicted in Figure 3

Figure 4. SMFS and DFT simulations to determine the interactions between (a) MB/graphene, (b) MB/epoxy graphene, (c) MB/unionized carbonyl-graphene, (d) MB/ionized carbonyl graphene.131 Symbols: gray, carbon; white, hydrogen; red, oxygen; blue, nitrogen; yellow, sulfur. Figure 3. Noncovalent interactions of GO.

(organic pollutants) and the adsorbent (graphene-based materials).

and can be well understood from these figures. The adsorption of organic compounds on carbon-based graphene materials depends on several interactions such as hydrophobic interaction, H-bonding, electrostatic interplay, pore-filling mechanism, or a combination of these.130 A study which explained the interactions with graphene and MB gives an example as to how the adsorption process takes place. SMFS and DFT simulations have been systematically and quantitatively investigated and the nanomechanical interactions have been understood. While most of the studies focus on electrostatic and π−π interactions, this study proves that the epoxy on the GO surface participates in most binding events. The activation Gibbs Energy of bond dissociation is (ΔG) −4.61 kcal mol−1.131 This also explains the unfavorable removal of anionic AOCs by GO. This can be visualized in Figure 4. These interactions play a major role in the eradication of pollutants using graphene. While the covalent bonds are mainly formed when the formation of the composites takes place, the weaker noncovalent bond forms the locus of separation of pollutants as they are mainly involved in adsorption following the mechanical separation of pollutants or their catalysis. The mechanical separation and catalysis require the pollutants to be separable with minimum amount of energy which is evident in the noncovalent bond breakage. A study of the interactions of graphene and their pollutants revealed that most of the organic pollutants interact via noncovalent bonds as evident in Tables 2 and 3. The study of interactions leads the way to analyze the adsorption process with a view of thermodynamics and kinetics that further establish the kind of interactions between the said adsorbate

4. GRAPHENE-BASED ADSORPTION OF ORGANIC POLLUTANTS In 1881, German physicist Heinrich Kayser conferred the term “adsorption”132 which according to IUPAC is defined as “increase in the concentration of a substance at the interface of a condensed surface and a liquid or gaseous layer owing to the operation of surface forces”, that is, the adherence of atoms or molecules onto a surface as a result of surface energy. The adsorption process mainly involves the atoms on the surface whose bonding efficiency is not fulfilled as they are not wholly surrounded by the constituents of the adsorbent and hence have the ability to attract the adsorbate. This involves a multitude of interactions in accordance with the adsorbate (the species getting adsorbed) and the adsorbent (the surface for adsorption) and includes attractive forces such as electrostatic interactions, van der Waals forces, or stronger covalent or hydrogen bonding.133 Adsorption is a surface phenomenon and that involves a superficial interaction of molecules and is used in various applications such as chromatographic techniques,134 purification of sugar,135 the cleaning mechanism of soaps and detergents,136 and removal of metallic and nonmetallic pollutants from water.74,137−139 In water treatment, adsorption has played a vital role, and activated carbon is still being used as a primitive adsorbent. Because pharmaceuticals were designed to be compatible with human anatomy,57,84 the waste generated from these materials is highly toxic. In controlled doses and for particular ailments the dosage of pharmaceutics is helpful but when present in the environment they pose a threat as their consumption can have adverse effects. Dyes and other organic pollutants such as herbicides E



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temperature. When the adsorbate and the adsorbent have been contacted long enough equilibrium is established and is described by the adsorption isotherms. The free energy of the adsorption process is related to the equilibrium constant by the Van’t Hoff equation:

and pesticides are removed by adsorption since antiquity and graphene-based materials are used to efficiently adsorb dyes and widely used in wastewater treatment technology.140,141 The need to understand the basic requirement for a material to be an adsorbent is extremely necessary to analyze the adsorption process. The materials generally used as adsorbents must have a large surface area, minute pore diameters, high thermal stability, and the ability to withstand abrasion.142 In industry, adsorbents are classified into three categories based on the constituents of the adsorbents. Oxygen-containing compounds such as silica gel143−145 are hydrophilic and polar; hence, the interactions between the water molecule and the pollutants are enhanced. Carbon-based materials such as activated carbon44,135 are hydrophobic and nonpolar and have innumerable pores that offer a larger surface area that ameliorates the adsorption process as pollutants get adsorbed in the pores. Another category of compounds utilized for adsorption are polymer-based compounds, and they have a mix of polar and nonpolar entities in a porous polymer matrix and have been used widely for organic pollutant adsorption.138,146 Graphene-based materials have a carbon base and are oxidized to form GO and rGO and form polymer composites,147 metal ceramics,148 hybrids,149 or are wrapped,150 encapsulated,151 sandwiched,148,152 or mixed with other materials to synthesize high performance adsorbents which can revolutionize existing water treatment technologies. Adsorption has been an effective tool in the removal of heavy metals, dyes, pharmaceutics, herbicides, pesticides, and other toxic elements from the aqueous media. For the study of adsorption dynamics, a detailed study of activation energy and parameters, change in Gibbs free energy, enthalpy, and entropy is essential. These parameters are critical to predict the performance and mechanism of the adsorption process and provide us with tools for characterization and optimization of the adsorption mechanism involved. The parameter that a thermodynamic isotherm conveys about a reaction is the spontaneity or the thermal requirement of a system, that is, the reaction is endothermic or exothermic. Considering an example in which GO and rGO separately form a complex with sodium alginate, the former graphene derivative displays an exothermic process while the latter displays an endothermic nature.153 The amount of adsorbate adsorbed as a function of temperature and adsorbent concentration is determined by a simple adsorption isotherm that is expressed by the equation qt =

(Co − C t)V m

ΔG 0 = −RT ln KD

(2) −1

−1

where R is the ideal gas constant (8.314 J mol K ), and T is temperature (K), ΔG0 is Gibb’s free energy change (kJ mol−1) and KD is the single point or linear sorption distribution coefficient and is expressed as KD =

Ca Ce

(3)

where Ca is the equilibrium adsorbate concentration on the adsorbent (mg L−1) and Ce is the equilibrium adsorbate concentration in the solution (mg L−1).155 So, we can conclude that if we can determine the adsorption isotherm of a process we can deduce other thermodynamic parameters that can enhance the adsorption capacity. The adsorption state describes the final state of the system but reaction dynamics is not governed only by the initial and the final state but also by the reaction pathway that is explained by kinetics. The understanding of the reaction timeline helps to control the reaction parameters and thus the overall thermodynamic considerations of the system can be influenced. A detailed study of the possibility of reactions between an organic pollutant (adsorbate) and graphene-based materials (adsorbent) has been described in this section, and to utilize that knowledge we need to further understand the thermodynamics and kinetics of the reaction mechanism. The need for an ideal adsorption system with which to compare the given adsorbent materials on the basis of adsorption parameters and quantitative analysis of their adsorption capacity for varied conditions is of utmost importance.156 Hence it is crucial to establish a standard for the adsorption equilibrium for analyzing the given adsorption system for their efficiency.157 These equilibrium correlations are established by adsorption isotherms that can be used to validate the interaction of pollutants with the adsorbent materials and help in establishing adsorption mechanisms, enhancing adsorption capacities, tuning the surface attribute and help in designing competent adsorption systems.5,158 The standard for adsorption was determined by mathematical expressions159 specified for adsorption of a pollutant from the aqueous phase to the adsorbent, that is, graphene-based materials and is governed by various parameters such as pH, temperature, and other entities present in the media, etc., and tools developed for this specific purpose have been used to determine the mechanism of adsorption, its affinity toward pollutants, and the effect of the change in environment of the pollutants while being adsorbed on graphene-based materials. Thermodynamics and kinetic models are some of these tools which augment the understanding of the adsorption process. When an adsorbate is in contact with an adsorbent for specific amount of time it undergoes a dynamic adsorption desorption process that defines a state of equilibrium.160 The physicochemical parameters explaining the thermodynamics are represented by mathematical models and can be expressed graphically.161 Several models have been proposed that consider different parameters for the adsorption process. The best fit of adsorption data in any of the models describes the

(1)

−1

where qt (mg g ) is the amount of adsorbate per mass unit of adsorbent at time t, V is the volume of the solution (L), Co and Ct (mg L−1) are the concentration of adsorbate at initial time and at time “t”, respectively, and m is the mass of adsorbent (g).154 But this was proposed without considering complex systems, that is, when more than one reaction parameter changes at the same time. In 1894, Freundlich isotherm was proposed that gave the best fit for a gaseous adsorption but lacked in several parameters.4 Then in 1916, Irving Langmuir proposed a model which came to be widely accepted although it did not consider all parameters affecting rate of adsorption.2 The establishment of a relationship between the different adsorption parameters and adsorption isotherm is thus essential to get any meaningful information. Gibbs free energy along with entropy and enthalpy give details about spontaneity of a given reaction at a certain F



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Table 1. Linear and Nonlinear Forms of Adsorption Isotherm adsorption isotherm Henry Adsorption constant

Langmuir

nonlinear form

linear form

reference

One-Parameter Isotherm qe = KHECe Two-Parameter Isotherms q KLCe qe = qo + e qe = qo KLCe 1 + KLCe

1

2−5

1 log Ce n

Freundlich

qe = KFC1/n e

log qe = log KF +

Dubinin−Radushkevich isotherm

qe = (qs) exp(− kadε 2)

ln qe = ln qs − kadε 2

Temkin

qe =

Flory−Huggins

θ = KFH(1 − θ)nFH Co

Redlich Peterson Isotherm

qe =

CeKR 1 + CepaR

Sips

qe =

(K sCe n) 1 + (asCe n)

Toth

qe =

Koble Karrigan

qe =

Khan

qe =

Radke Prausnitz

qe =

RT ln A TCe bT

qe =

RT RT ln a T + ln Ce bT bT

2,3,158 5,162,163

ij θ yz logjjjj zzzz = log KFH + nFH log(1 − θ) k Co { Three Parameter Isotherms ij C yz lnjjjjKR e zzzz = g ln Ce + ln aR j q z e{ k

a TCe1/ t

ij K yz n ln Ce = −lnjjjj S zzzz + ln(aS) jq z k e{ qe 1 ln = ln Ce − ln(a T + Ce) KT t

ACen 1 + BCen

1 1 B = + qe ACen A

K TCe

qsbK Ce

157,164,165 166

167

168

169 170 171

(1 + bK Ce)μ K qMRPKRPCe

172

(1 + KRPCe)MRP

the process of adsorption and can be exemplified by a cellulose/GO composite as increase in adsorption capacity from 245.7 to 451.2 mg g−1 was observed when the concentration of MB increased from 100 to 220 mg L−1 at 318 K.191 A pH increase from 3 to 7 resulted in the increase in adsorption capacity from 28.5 to 542 mg g−1 in the adsorption of MB by GO at 293 K. This was attributed to the greater affinity of H+ ions to the GO surface than the MB molecule. At pH 9, hydrolysis of GO took place hence the adsorption ceased to exist. MB was also affected by the high pH as a result of stepwise demethylation.192 The effect of temperature was evident in the case of cellulose/GO fibers that signify an endothermic process as the adsorption capacity increases from 419.09 to 451.18 mg g−1 when temperature is increased from 278 to 313 K.191 The polydopamine-coated layer of GO had a negative enthalpy value which concluded that the reaction was exothermic.193 The concentration of organic and inorganic compounds other than the pollutant in consideration strongly influences the adsorption process.194,195 A representative of this can be the adsorption of tetracycline from GO in which univalent sodium ions decrease the adsorption of the organic pollutant. When 100 mmol L−1 NaCl was present in the solution which contained 166.67 mg L−1, the adsorption decreased by 50% which shows that the univalent sodium ions minimized the adsorption of the tetracycline.98 An adsorbent is desired only when it is cost-effective, and the easiest way to achieve this is by recycling the adsorbent. If the efficiency of any adsorbate is retained after processing it with mechanical methods such as magnetic separation,

isotherm of the system. There have been many isotherm models established on different factors and utilized for removal of pollutants from aqueous media.138 Some of these isotherms have been stated in Table 1. The thermodynamic isotherms determine the final state of the system but to understand the changes in chemical properties with respect to time, the kinetics of the system needs to be studied. The kinetics helps us to determine the rate controlling mechanism such as mass transport and chemical reactivity.173−175 The pseudo-first-order,176,177 pseudo-secondorder,65,178 Elovich,179 and intraparticle diffusion are some of the kinetic models that have been determined for graphenebased systems. Adsorption is affected by the interfacial atmosphere, and hence surface area, nature, and initial concentration, pH,180,181 temperature,182,183 and interfering substances play an important part in the adsorption process. The extent of adsorption is directly proportional to the surface area and requires the adsorbent to be finely divided and porous as more adsorption is achieved per unit weight.184,185 Graphene has a sheet-like structure and both sides are available for adsorption,186,187 but due to other chemical properties graphene is more advanced than activated carbon for the adsorption process. The physicochemical nature of both adsorbate and adsorbent affect both the rate and capacity of adsorption.184,188 The solubility,189 molecular size of adsorbate with respect to adsorbent, 190 delocalized electrons, polar entities, and proximity of the adsorbate and adsorbent all contribute to G



Review

a wrinkled surface distributed unevenly that provided them with a larger surface area and better thermal stability than the graphene/alginate single network (GAS). The Langmuir model was a fit to the MB adsorption isotherm, and adsorption capacity was determined to be 7.2 and 5.6 mmol g−1, respectively, and the kinetics followed the pseudo-secondorder rate model. The desorption rate was 60.2% and 47% for GAD and GAS, respectively. While the porosity played a major role in the adsorption process the presence of a carboxyl group also enhanced the efficiency by making the material surface negatively charged.247 In another study the concentration of both alginate and GO was altered, and it was noticed that an increase in both of the components, in two separate processes, resulted in the increase of adsorption of MB. The increase of adsorption because of GO was attributed to the interactions of oxygen functional groups, while that of alginate was attributed to the porous structure. The highest concentration samples of both components were then dried by ethanol treatment and lyophilized which helped in determining the maximum adsorption capacity as 3.2 and 4.2 mmol g−1 by fitting the data in the Langmuir adsorption model.248 A layered 3D GO-sodium alginate (SA) and rGO-SA were other alginate nanocomposites that were used to adsorb MB, and the adsorption capacity was determined to be 833.3 and 192.2 mg g−1, respectively, at 303 K and contact time of 12 h and rendered it as a spontaneous process. While the GO nanocomposite displayed electrostatic interactions and the reaction was exothermic, the rGO composite interacted via π−π interactions and the reaction was found to be endothermic. 153 GO-calcium/alginate nanocomposite, strengthened the composite structure and acted as a good adsorbing agent for the eradication of MB dye. A decrease in adsorption capacity was observed when the temperature was increased from 298 to 328 K, and the adsorption intensity was recorded as 163.93 and 140.85 mg g−1, respectively. This proved that the reaction was exothermic222 Other polysaccharide/GO (PS/GO) nanocomposites consisting of inulin, xylan, and κ-carrageenan formed nanocages and a highly disordered structure to facilitate adsorption of MB, R6G, orange II, and acid fuchsin. MB had a maximum adsorption capacity of 769 mg g−1 and the reaction followed a pseudo-second-order kinetic model and Langmuir isotherm model. Also, the reaction was strongly affected by pH conditions, and electrostatic interactions dominated the adsorption process. Overall the reaction was spontaneous and exothermic.249 Another widely available natural polymer is chitin which is obtained from mainly two types of marine crustaceans, shrimp and crabs.250 Deacetylation of chitin with alkaline substance yields chitosan which is a natural polymer used extensively for many applications such as agriculture,251 medicine,252 etc. Its role in water purification cannot be undermined,188 and hence graphene and chitosan composites for organic pollutant eradication have also been reported. Chitosan and GO (CS− GO) have amphoteric characteristics and have been utilized to remove MB and MO from wastewater within 10 min217 especially at low pH levels. The GO−chitosan hydrogel composite was reported to adsorb MB and eosin Y dyes, and the interactions were mostly electrostatic. The composite could be used as a column packing and the dyes could be removed by filtration with it.235 The removal of chitosan was made effortless by developing magnetic chitosan−GO nanocomposites that were used for MB adsorption, and adsorption

chemical methods such as solvent wash, or even simple filtration then the use of that material is coveted. Graphenebased materials have shown exemplary performance in being recycled with an efficiency of more than 90% even after a repeated number of cycles. This is evident by the reusability column mentioned in Tables 2 and 3. The adsorption isotherms and kinetic models have been established for a large number of graphene-based materials and the environmental factors also play a vital role used for organic pollutant removal. Some of the examples of these have been discussed in the following section and have proved that the adsorption capability of graphene-based materials and their mechanisms have helped us to understand the role graphene and its composite play in enhancing this process. Graphene technology has contributed vastly to water remediation by the adsorption of organic pollutants. Nanosheets of graphene, GO, and rGO have been designed for adsorption and they have been incorporated in threedimensional structures such that the structure formed retains most properties of pure graphene as this is desirable for implementing it in large scale water treatment plants and recycling of these sorbents.237,238 Graphene, GO, and rGO have been used to prepare aerogels and hydrogels, or have been chemically doped to tune the properties of graphene and functionalize it by integrating heteroatoms that improve its physicochemical properties.239 The industrially used sorbents are generally polymer based,139,240 carbon based,163 or are oxygen functional group-containing compounds. As graphene is a carbon-based compound it forms composites with polymers and oxides to enhance its adsorption capacity. With this view the following examples of graphene, GO, rGO, and their polymer-based and oxide-based composites have been cited because they aid the understanding of the process of adsorption and the thermodynamic and kinetic prospect of their interactions with the organic pollutants in wastewater. Graphene-based materials are tested for their potentialities as sorbents by the batch assay method which provides fundamental information on the adsorption process. The results obtained from these dynamic experiments are crucial for the design and feasibility of a full-scale treatment unit.241 The obtained optimal conditions were achieved by studying parameters such as initial concentration of the contaminant, pH of the sample, dosage of adsorbent, temperature, and capacity of adsorption.242 4.1.1. Adsorption by Graphene-Based Polymer Nanocomposite. Graphene related nanocomposites have been widely investigated for dyes adsorption and pharmaceuticals as is evident from the adsorption capacities. To understand the batch processes better some of the studies have been compiled to support graphene-based materials as adsorbents and prove their efficacy.243 With increasing demand of green chemistry, biopolymers in a combination with graphene materials have also been investigated.244,245 Biocompatibility, biodegradability, availability, high reactivity, and easy isolation make a combination with graphene materials desirable for the adsorption of organic entities.246 The most easily available natural polymers are polysaccharides and they play many roles in nature, thus their use to clean water is inevitable. They were functionalized with graphene and GO, etc. to successfully remove organic entitites. Alginates, polysaccharide obtained from seaweed, were combined with porous graphene to obtain porous graphene/alginate double network nanocomposite beads (GAD). These beads exhibited H


I

17b-Estradiol (E2)

aspirin, caffeine, acetaminophen

Dorzolamide

CIP and NOR

Few layer-GO

G nanoplatelets

GO/CSA

reduced GO/magnetite composites (RGO-M),

diclofenac (DCF) and sulfamethoxazole (SMX) acrylonitrile, p-toluene sulfonic acid, 1-naphthalene sulfonic acid Pararosaniline

rhodamine 6G, chlorpyrifos,

rhodamine 6G (R6G), chloropyrifos (CP)

naphthalene and 1-naphthol

GO

super paramagnetic G/Fe3O4

G sand composite(GSC)

G sand composite (GSC)

GO (chemically reduced (CRG), annealed(ARG))

G

Atenolol (ATL) and propranolol (PRO)

GO

G and activated G(G-KOH)

ketoprofen (KEP), carbamazepine (CBZ), and bisphenol A (BPA) ciprofloxacin (CIP)

phthalic acid esters (DMP, DEP and DBP)

biochar-G nanosheets

rGO1, rGO2 and graphene

TBBPA

NaHSO3/rGO aerogels

Tetracyclines

Atrazine

biochar-supported rGO

GO nanoparticles (GO-NP)

BPA

pollutants

MAP/G-based monolith

composite

Langmuir

128.37 mg g−1

Langmuir

N.A.

N.A.

Langmuir

75.4 mg g−1 55, 48 mg g−1 21.4(CRG), 57.7(ARG) mg g−1

N.A.

∼1.43 g g−1G, ∼1.46 g g−1G and ∼1.52 g g−1G 198.23 mg g−1

Freundlich

Langmuir− Freundlich

Langmuir

Dubinin Ashtakhov

Langmuir

Langmuir and Temkin

43.9 and 1.19 mg g−1

67 and 116 mg g−1

KEP- 62.5, 54.1, and 15.3 mg g−1, CBZ- 115, 105, and 22.8 mg g−1, BPA- 152, 128, 26 mg g−1 145.9, 194.6 mg g−1

322.43 mg g−1

18.22 or 22.20 mgg−1

Langmuir− Freundlich

N.A.

13.02 mg g−1, 19.72 mg g−1, and 18.06 mg g−1, 334 mg g−1

Langmuir

149.4 mg g−1

Langmuir, Freundlich

Langmuir

67.55 mg g−1

45.65,31.78, and 25.43 mg g−1

Langmuir

adsorption isotherm

324 mg g−1

adsorption capacity

pseudosecondorder pseudosecondorder pseudosecondorder N.A.

N.A.

pseudosecondorder pseudosecondorder N.A.

pseudosecondorder pseudosecondorder pseudosecondorder pseudosecondorder pseudosecondorder pseudosecondorder pseudosecondorder pseudosecondorder pseudosecondorder N.A.

kinetics

Table 2. Graphene-Based Composites for Removal of Pharmaceuticals and Other Aromatic Pollutantsa interactions

n−π EDA interaction, cation− π interactions, π−π interaction and hydrogen bonding

π−π interaction.

electrostatic interaction

π−π interactions

N.A.

1.38 mg/g loss for third run of eluent in column by acetone

complete regeneration by acetone

∼92%, 5 cycles, ethanol

constant during first five cycles

N.A.

72%, 67% methanol

N.A.

hydrogen bonding, π−π electron donor− acceptor interactions, and electrostatic interactions). electrostatic forces, H-bonding or π−π interactions

N.A.

N.A.

90%, 10 cycles

N.A.

94.14%, 5 cycles, strong alkali

thermal treatment

93.17%, 5 cycles, methanol

up to 81%, 4 cycles, MW irradiation

N.A.

hydrophobic interactions and π−π electron donor−acceptor (EDA) interactions π−π interactions

recycling up to 88%, 5cycles, methanol

π−π EDA interaction, H bonding, electrostatic interactions

π−π interaction and cation-π bonding

π−π interaction and cation-π bonding

amide bond formation

van der Waals interactions.

π−π interactions and hydrogen bonds

π−π EDA (electron donor−acceptor) interaction, hydrophobicity, H bonding.

π−π interaction


hydrogen bonding, π−π interaction

ref

213

212

211

210

209

208

207

206

205

204

203

202

201

200

199

198

197

196

Journal of Chemical & Engineering Data Review


capacity was determined to be 95.31 mg g−1 at a pH of 5.3 and temperature of 303 K; the reaction was found to be spontaneous and exothermic. Desorption was done using 0.5 M NaOH and 90% was recycled.236 Cyclodextrin, a complex sugar was also complexed with GO and chitosan to form magnetic β-cyclodextrin chitosan−GO composite and was used to enahnce adsorption of MB dye at regular pH. The monolayer adsorption intensity was determined to be 84.32 mg g−1 at 293 K for which the hydrophobicity of GO and the amine and hydroxyl groups of chitosan together cooperated for the removal of dye.147 Even without chitosan, magnetic cyclodextrin−GO composite removed MB efficiently with an adsorption capacity of 261.78 mg g−1 at a pH 10 and temperature of 303 K, and the complex formed was efficiently removed by magnetic separation. The hydrophobicity of cyclodextrin and iron oxide was also overcome by forming the composite with GO which shows high hydrophilic properties.234 Polydopamine coated GO(PD/GO) composite which was based on the structure of mussels was used for effective removal of MB, and a range of PD/GO were synthesized with different concentrations of PD by a percentage that varied from 5%, 15%, 35%, and 70% and their adsorption capability were reported as 1.3, 1.89, 1.7, and 0.6 mg g−1, respectively.193 Synthetic polymers were also doped with graphene and its subsidiaries to enhance adsorption of dyes and pharmaceuticals. PVA (poly(vinyl alcohol)) and PAA (polyacrylamide), etc. have shown high adsorption capacities. GO/PVA were combined to form an aerogel whose structure stability depended on the incorporation of PVA that exerted a reinforcement effect. MB and Congo Red was efficiently removed. The adsorption capacity of GO/PVA composite for MB removal was 127.5 mg g−1 and for other cationic dyes the efficiency was up to 96%.253 GO and PAA were used to eradicate MB and R6G and a multitude of interactions such as hydrogen bonding, electrostatic interaction, van der Waals forces, π−π interactions, and hydrophobic interactions with the GO sheet. The behavior of the composite changed with the change in concentration of the GO and PVA and the maximum adsorption capacity was determined to be ∼293 and ∼288 mg g−1 in 20 and 60 min, respectively. Both Langmuir and Freundlich model fit the data and the reaction followed second order kinetics.254 Polystyrene/Fe3O4-GO nanocomposites arranged in a core− shell structure and bound by electrostatic interactions were used for the adsorption studies of RhB, and the adsorption capacity at room temperature for 24 h was determined to be 13.8 mg g−1.232 The magnetic character makes the composite easier to separate after adsorption. Photodecolorization was observed for Rose Bengal using a polymer-based nanocomposite. Polyaniline/graphene (PANI−G) nanocomposites were also used to degrade RhB and the photocatalytic activity was much higher than that of PANI. PANI is basically a polymer of aniline monomers and it was prepared while the photodegradation was taking place. Superoxide anion, hydroperoxyl, and hydroxyl radical were formed, and their scavenging led to enhanced photocatalytic activity.255 The natural and synthetic polymers were combined in some cases for enhanced adsorption. Dorzalamide, a biomedical waste in effluents was eradicated using GO/PAA grafted chitosan nanocomposite (GO/CSA).57 The adsorption intensity was found to be 334 mg g−1 at 25 °C, and the

216

216

215

215

214

214

ref

Review

N.A. hydrophobic interactions and π−π bonding N.A. Freundlich 174.6, 59.0 (mg g−1)/(mg L−1)n phenanthrene, biphenyl

N.A. = not available.

GNS A −208.3, 102.6 (mg g−1)/(mg L−1)n, GNS B- 163.6, 104.7 (mg g−1)/(mg L−1)n phenanthrene, biphenyl

G nanosheets(GNS A with higher surface area than GNS B) GO

a

N.A. hydrophobic interactions and π−π bonding N.A.

N.A. π−electron donor−acceptor interaction pyrene, anthracene, naphthalene rGO

4.842, 1.028, 46.132 mmol g−1

Polanyi− Dubinin-Ashtakhov Polanyi−Dubinin−Ashtakhov Freundlich 1.050 mmol g−1 pyrene GO

Langmuir naphthalene, phenanthrene, and pyrene GO

2.62, 5.90, and 6.12 mg g−1,

N.A.

N.A. π-electron donor−acceptor interaction

N.A. π−π interactions

N.A.

pseudosecondorder pseudosecondorder N.A. Langmuir naphthalene, phenanthrene, and pyrene

interactions kinetics

N.A. Langmuir

naphthalene-145(CRG), 52.4(ARG) mg g−1 1naphthol- 269(CRG), 282(ARG) mg g−1 127.7, 136.4, and 170.2 mg g−1 naphthalene and 1-naphthol

reduced G(CRG-chemical reduction, ARG-annealed) G nanosheets(GNS)

adsorption isotherm adsorption capacity pollutants composite

Table 2. continued

n−π EDA interaction, cation− π interactions, π−π interaction and hydrogen bonding π−π interactions

N.A.

recycling

213


J


Freundlich Langmuir Langmuir Langmuir

81.97 mg g−1, 97% 45.27 and 33.66 mg g−1, 43.82 mg g−1 1.939 mg mg−1

MB

MB, CR

MB

MB

MB

Acridine Orange BB41, BR18, and BR46

MB

MB,MG

cylindrical G−CNT hybrid magnetic Fe3O4@G (FGC) G/Fe3O4

GO

GO

GO GO nanosheet

GO

layered GO

K

Methyl Green

MB, RhB

rGO

rGO -based hydrogels

rGO−TiO2 hybrids (sheets, nanotubes) magnetite/rGO (MRGO)

83.26, 75.36 mg g−1, 90% 91%, 94%

RhB, Malachite Green

30, >90, 50, 88, 50, ∼46 mg g−1

35, 23 mg g−1, ∼100%, 92%

∼100%, 97%

77%

Rhodamine 6G (R6G), Acid Blue 92 (AB92), Orange (II) (OII), Malachite Green (MG), and new coc-cine (NC) MB

MB, RhB

Methyl Green

GO

rGO supported ferrite (Mn/Zn/Co/Ni) Fe2O4 rGO/Fe3O4 (for 40 mg GO)

Langmuir

∼3.92 × 10−4 mol/g

Safranin O

Freundlich

N.A.

N.A.

N.A.

pH 4 to 6-Toth > Sips > Dubinin − Radushkevich (D − R) > Scatchard > Langmuir > Temkin > Freundlich pH 4 to 6-Toth > Sips > Dubinin − Radushkevich (D − R) > Scatchard > Langmuir > Temkin > Freundlich Freundlich

Langmuir

17.862 mg g−1, >96%

MG

GO caged in cellulose microbeads (GOCB) MgO/MLG 93%

Langmuir

350 and 248 mg g−1

Freundlich to Langmuir as oxidation degree increases

Langmuir Langmuir

Langmuir

Langmuir

240.65 mg g−1, 94.8−98.8% 2158 mg g−1, 95% 1429, 1250, and 476 mg g−1, 89%, 94%, 76% (50 mg L−1) N.A.

adsorption isotherm Langmuir, Freundlich

43.82 mg g−1.

MB

G/Fe3O4

adsorption capacity 1.60,0.80 mmol g−1

N.A.

MB

amide functionalized MOF/GO

pollutants

97% in 15 min

MB, MO

chitosan/GO

composite

Table 3. Graphene-Based Composites for Removal of Dyesa kinetics

pseudo-secondorder pseudo-secondorder

pseudo-secondorder

pseudo-secondorder

pseudo-secondorder

pseudo-secondorder

pseudo-first-order and pseudo-second-order pseudo-secondorder pseudo-secondorder pseudo-secondorder pseudo-secondorder

pseudo-secondorder N.A. pseudo-secondorder

pseudo-secondorder pseudo-secondorder pseudo-secondorder pseudo-secondorder N.A.

pseudo-secondorder and pseudo-first-order pseudo-first-order

interactions

reusability

N.A. magnetic separation


annealing

∼100%, >80%, 3 cycles ethylene glycol magnetic separation

N.A.

N.A.

>90%, 5 cycles filteration ∼73.5% ethanol

N.A.


π−π stacking and anion−cation interactions electrostatic interaction

electrostatic interaction

π−π EDA interactions, electrostatic covalent bond (chemisorption) N.A.

>90.97 2% NaOH, ethanol

π−π interaction and electrostatic interaction electrostatic interaction

N.A.

N.A. N.A.

decreases with each cycle until 5 cycles. 37%, 30% acetic acid, ammonia in ethanol N.A.

magnetic separation

>90% after 5 cycles, acidic/alkaline eluent no significant change, 4 cycles, ethanol and DMF decreases with each cycle until 5 cycles acidic ethanol

hydrogen bond electrostatic interaction

π−π EDA interactions

electrostatic attraction, π−π interactions electrostatic interaction

π−π interactions, electrostatic or acid−base interactions, electrostatic attraction, π−π interactions hydrophobicity, π−π interaction external diffusion

electrostatic interaction, π−π-stacking

ref

231

149

230

229

228

227

227

152

151

226

225

223 224

222

221

219

141

220

219

218

217

Journal of Chemical & Engineering Data Review


193 N.A. 1,4-Michael addition

153 N.A. π−π stacking interactions

153 N.A.

interactions involved were a combination of electrostatic forces and hydrogen-bond formations, etc.202 Many graphene and GO composites resulted in the formation of hydrogels and aerogels which shows a great capacity for adsorption of dyes and pharmaceuticals. Nitrogen and sulfur codoped graphene hydrogels (N/S-GHs) were synthesized by a chemical route in which glutathione acted as a binder, modifier, source of N and S, and also as a reducing agent as it reduces hydroxyl radicals. Malachite Green was adsorbed the most because of the porous structure of the composite and accessibility of the composite for adsorption followed by MB and crystal violet. The highest adsorption was observed at 313 K, and the adsorption efficiency was determined to be 806.5 mg g−1. The Langmuir model was a proper fit to the experimental data while the reaction followed a pseudo-second-order kinetics. The recycling capacity was 100% even after six cycles of adsorption.256 3-D sodium bisulfite reduced-graphene aerogels (S-rGA) for the removal of tetrabromobisphenol A (TBBPA) showed a maximum adsorption capacity of 128.37 mg/g, and the reaction was found to be exothermic and spontaneous.198 Hydrothermally prepared graphitic carbon nitride−titanium dioxide−graphene aerogel (g-C3N4−TiO2−GA) composites showed 20.0 mg L−1 adsorptivity for rhodamine B (RhB) in an hour and were found to be 75.6% efficient even after four cycles.257 Tetracycline was adsorbed using GO as a result of π−π interaction and was evaluated using the Langmuir model, and the adsorption capacity was determined to be 313 mg g−1. The extent of adsorption was influenced by pH or the concentration of univalent sodium ions.98 Graphene CNT hybrid aerogels were used to adsorb RhB, MB, and fuchsine, and their adsorption capacities were found to be 150.2, 191.2, and 180.8 mg g−1, respectively. This composite was obtained by GO and CNT with Vitamin C without stirring which led to CO2 drying.258 MB and RhB were adsorbed using rGO-based hydrogels which were prepared by sodium ascorbate and showed 100% and 97% removal, respectively, at a pH of 6.4 and a temperature of 298 K and contact time of 2 h. Ethylene glycol was used for regeneration of the hydrogels.228 While polymers in the form of graphene and its composites have made their mark in water purification advancement, they also give a glimpse of the importance of metal oxide composites. 4.1.2. Adsorption by Graphene-Based Materials and Their Metal Oxide Composites. Adsorption by graphene-based material and/or combined with oxides has been an essential part of pollutant eradication and has been widely catalogued for pharmaceuticals, dyes, and other organic pollutants management and disposal. A study on the adsorption of antibiotics showed that the number of aromatic rings on antibiotics affected the adsorption rate, that is, the more aromatic rings the antibiotics have, the faster is the adsorption rate on the carbon-based materials. This was studied by the adsorption of ofloxacin (OFL), sulfadiazine (SD), sulfamethoxazole (SMX), sulfamethazine (SMZ), cefalexin (CFX), and tetracycline (TC) on graphene-based materials.259 The following are some examples of graphene-based materials adsorbing aromatic-based substances followed by dyes. Chemically reduced graphene and annealing reduced graphene showed adsorption capabilities because of the oxygen functional groups.214 Graphene nanosheets and GO were utilized to adsorb phenanthrene and pyrene where phenanthrene showed more affinity toward the carbon allotropes and graphene nanosheets due to the wrinkled surface of the

pseudo-secondorder, intraparticle diffusion N.A.

ionic interactions

Electrostatic interaction

147

236

90%, 4 cycles 0.5 M NaOH 60%, 5 cycles ethanol ionic interactions

pseudo-secondorder pseudo-secondorder N.A.

235 water filteration electrostatic interaction

233 234 magnetic separation magnetic separation

N.A. pseudo-secondorder N.A.

ref reusability

magnetic separation

π−π interaction, hydrogen bonding π−π interaction N.A.

Review

Langmuir

N.A. = not available.

MB

2.18 g g−1

Langmuir 833.3 mg g−1 MB

MB PDA/GO

a


Langmuir


GO−Fe3O4 hybrid GO/magnetic cyclodextrin GO−chitosan hydrogels (GO−CS)(GO− CS10−10:1 w/w) magnetic chitosan/GO (MCGO) magnetic cyclodextrin− chitosan/GO(MCCG) GO-sodium alginate (GO-SA) rGO-sodium alginate (rGO-SA)

192.2 mg g

Freundlich 387,326 mg g−1 MB and Eosin Y

−1

N.A. Langmuir 167.2 and 171.3 mg g−1 228.5 mg g−1 MB and Neutral Red (NR) MB

adsorption capacity

N.A. RhB composite

polystyrene/Fe3O4/GO

Table 3. continued

pollutants

13.8 mg g−1

adsorption isotherm

N.A.

kinetics

interactions

232


L



Review

isotherm established an adsorption intensity as 149.4 mg g−1 at 298 K and the process was found to be spontaneous.263 Biochar-graphene (BG) nanosheet composites were used to adsorb dimethyl phthalate (DMP), diethyl phthalate (DEP), and dibutyl phthalate (DBP) as model phthalic acid esters (PAEs) where the adsorption depended mainly on π−π interactions and DBP was adsorbed because of hydrophobicity.199 GO using magnesium ascorbyl phosphate (MAP) was used for Bisphenol A (BPA) removal in an endothermic process and the adsorption intensity of MAP-GBM (graphene-based monolith) was 324 mg g−1 for BPA and an 88% regeneration capacity was observed.196 Biochar-supported reduced GO composite (rGO-BC) was used for eradication of atrazine through π−π interactions and showed capacity of adsorption at 67.55 mg g−1 for atrazine.197 The adsorption of ciprofloxacin (CIP) and norfloxacin (NOR), by reduced GO (rGO)/magnetite composites has also been studied to facilitate the removal of graphene composites after adsorption. The batch equilibrium tests indicated that rGO/magnetite-based composite is strongly dependent on initial pH for the removal of CIP and NOR via adsorption. π−π interactions as well as electrostatic repulsions were responsible for adsorption.130 Apart from pharmaceutical wastes, self-assembled GO after combination with PEI acted as dye adsorbents for wastewater treatment.264 Other organic compounds such as toxic polyaromatic hydrocarbons expelled by industry also reacted with graphene-based materials for efficient eradication from wastewater. rGO−magnetite composite has been utilized for the removal of fluoroquinolone antibiotics such as ciprofloxacin (CIP) and norfloxacin (NOX) for which both electrostatic and π−π interactions helped in the adsorption process. The monolayer adsorption intensity was determined to be 18.22−22.20 mg g−1, respectively, using the Langmuir and Temkin models.203 Organic compounds such as bisphenyl, phenol, anthracene, pyrene, naphthalene, naphthyl amine, naphthol, benzene, sulfonic acid, and their derivatives have been successfully adsorbed on graphene, GO, rGO, and graphene-based composites. GO was used to adsorb naphthalene and 1naphthol in which the naphthalene showed more affinity toward adsorption while the adsorption intensity of 1-naphthol was influenced by pH and showed higher adsorption in acidic medium. The presence of bivalent cadmium ions increased the adsorption efficiency of 1-napthol by 120%.213 Activated graphene acted as an adsorbent and was efficient in removal of CIP and the adsorption capacity was determined to be 194.6 mg g−1.206 The Hummers method was used to prepare GO that adsorbed beta blockers such as atenolol and propranolol by varying pH, temperature, time of contact between adsorbate, and adsorbent and the dosage of GO. Surface charge and the ionizability played a crucial role in determining the adsorption capability of GO for adsorption of these beta blockers. Hydrogen bonding and electrostatic attractions between positively and negatively charged entities are involved in the adsorption. It was also observed that atenolol was more readily adsorbed than propranolol as a result of more favorable structure.207 The removal of diclofenac and sulfamethoxazole using GO was found to be favorable under acidic pH due to hydrophobic and π−π interactions. The extent of adsorption was determined to be 8.8 and 5.9 kcal mol−1 for diclofenac and sulfamethoxazole, respectively. The

nanosheets.214 For the adsorption of phenanthrene and biphenyls by the same composites, phenanthrene showed greater adsorption due to greater hydrophobicity. The larger the surface area was, the greater was the adsorption of phenanthrene, whereas no such observation was made for biphenyls. Organic matter did influence the adsorption of phenanthrene and biphenyls, but it still proved to be more efficient than the currently used materials such as activated carbon, proving the efficacy of the graphene in wastewater treatment.216 Phenol, which is responsible for local and systemic toxicity, was adsorbed by graphene, and the temperature and pH played an important role in its removal. Between pH 4 and 6 phenols were readily adsorbed, and the gradual increase of temperature from 12 to 60 led to an increase in the adsorption of phenols.260 Bisphenol A (BPA) was also affected by pH as a value more than 7 caused a sharp decrease in the adsorption of BPA which was due to the repulsive electrostatic interactions. The adsorption process for BPA was spontaneous and exothermic in nature and established graphene as the preferred adsorbent for BPA.261 p-Toluene sulfonic acid and 1naphthalenesulfonic acid were successfully removed, and their removal was attributed to the large size and presence of benzene rings in their structure.209 Other complex phenolic substances such as 1,2,4-trichlorobenzene (TCB), 2,4,6-trichlorophenol (TCP), naphthalene, and 2-naphthol were absorbed by graphene and GO and adsorption kinetics were studied by the Freundlich isotherm. The adsorption of TCP and 2-naphthol were also found to be dependent on the pH. TCP adsorbed more when the pH was low while the adsorption of 2-naphthol for graphene increased between pH 8 and 9.2. The study also points out that the adsorption of the four aromatic hydrocarbons also depended on the properties graphene and GO offered.262 Acetaminophen, aspirin, and caffeine are pharmaceutical wastes that have been eradicated using graphene nanoplatelets. The reaction of graphene with the three compounds was found to be spontaneous as the change in enthalpy was negative, while a detailed study with respect to the effect of pH and temperature was also studied, and the adsorption capacity for the three compounds was found to be 19.72, 13.02, and 18.76 mg g−1, respectively.201 Tylosin was adsorbed by graphene nanosheets and GO as a result of electron donor−acceptor interactions, and it was highlighted that a Lewis acid base-type of interaction could be the contributing factor in the adsorption of tylosin50 rGO was also used to adsorb anthracene, pyrene, and naphthalene, and around 95% was absorbed by the rGO surface which was significantly higher than that adsorbed by GO which adsorbed only 60% of the naphthalene. This is due to the surface structure of the rGO. The adsorption of pyrene depended on temperature. The size of the adsorbate also influenced the adsorption process.215 Another method of adsorption of tetracycline on the GO surface was reported. The extent of adsorption was determined to be 323 mg g−1, and the reaction was determined to be endothermic and spontaneous.204 Ketoprofen and carbamazepine were adsorbed by rGO and graphene where rGO showed better performance of adsorption than graphene. This can be attributed to the hydrogen bonding and π−π interactions between the molecules and oxygen functional groups of rGO.205 Few-layered GO nanosheets (FLG-GO) adsorbed 17β-estradiol from water with great efficiency. The Langmuir M



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thus reducing GO to nanoporous rGO which also adsorbs dyes such as methylene, congo red, and lemon yellow with an adsorption efficiency of 2.6, 7.6, and 3.2 g g−1 with an efficiency of 98.46%. The byproducts can be used for generating energy or in agricultural fields as fertilizers.272 The process of rGO formation and dye adsorption has been depicted in Figure 5.

Freundlich isotherm was used and 35% diclofenac and 12% sulfamethoxazole was found to be removed.208,265 Ethylbenzene, toluene, and xylene were adsorbed by GO which was modified by 4-aminodiphenylamine (GO-A). Several factors such as time, pH, and adsorbate concentration affected the adsorption process and at pH 4 the adsorption was found to be at maximum, and an increase in the adsorbate concentration increased the adsorption capacity. The regeneration of GO-A was also studied, for which even after three cycles the GO-A performed well as an adsorbent, and only after seven cycles did their adsorption intensity decline.266 Nanoporous graphene was used for the removal of benzene, toluene, and xylene and 119, 123, 125 g of benzene, toluene, and xylene were efficiently adsorbed by 1 g of nanoporous graphene. The time taken for this process was rapid and was completed in about a minute.267 Phenanthrene was removed using GO, rGO, and sulfonated graphene that provided additional nonaggregating properties due to the HSO3 groups present on the surface. An equilibrium was attained in 120 min, but compared to other adsorbents this process was faster.117 Sulfonated graphene was also used to remove 1-naphthol and naphthalene, and the adsorption capacity was influenced by pH. As the pH increased the adsorption decreased although at pH more than 7 the 1naphthol adsorption did not follow this trend. The adsorption efficiency of sulfonated graphene was as high as 95%. It was also reported that 1-naphthol adsorption increased with increasing temperature.268,269 1-Naphthylamine, 1-naphthol, and naphthalene were adsorbed by graphene−iron oxide nanocomposites. The mechanism of adsorption largely depended on electron donor−acceptor interaction between the π electrons of aromatic compounds and the GO-FeO· Fe2O3 nanocomposite as the increase in polarity led to greater capability of donating electrons. The removal of naphthalene was dependent on temperature. These magnetic GO nanocomposites were also used to eradicate 1-naphthol and 1naphthylamine and showed great efficiency owing to the removal of complexes formed using the magnetic character of the nanocomposites.270,271 All the data with a view on thermodynamics, kinetics and reusability is compiled in Tables 2 and 3. Graphene-based materials not only adsorb antibiotics and other phenolic substances but are efficient to remove dyes that change the color of water and hinder the growth of flora and fauna under water. Graphene was used to adsorb MB and showed promising results with an adsorption intensity of 153.85 to 204.08 mg g−1 when the temperature was increased from 293 to 333 K. The maximum percentage was 99.68% at a pH of 10, and the whole process was endothermic and spontaneous as was the adsorption of Cationic Red X-GRL on graphene with an adsorption capacity of 238.10 mg g−1 at 333 K. Another method of removing MB by graphene involved the determination of spontaneous and heat absorbing nature of the interaction through fluorescence spectroscopy studies and a variation of pH, temperature at 303 K, and a contact time on 1 h. The initial adsorption was 5 mg/L, and adsorption was found to be dependent on it. Graphene also eradicated ptoluenesulfonic acid (p-TA), methylene blue (MB), and 1naphthalenesulfonic acid (1-NA). The adsorption capacities of p-TA, MB, and 1-NA are 1.43, 1.52, and 1.46 g g−1, respectively.209 rGO was synthesized in situ with a concoction of GO, Zn, and NH4Cl in wastewater. The mixture was stirred for 10 min

Figure 5. (a) Schematic process for the treatment of polluted water, (b) photos showing adsorption of MB, CR, and LY, (c) graphs showing adsorption of MB.272 Reprinted with permission from ref 272. Copyright 2018, Elsevier B.V.

The adsorption of Acid Orange 8(AO 8) and Direct Red 23(DR23) (ionic azo dyes) from aqueous solutions were studied. The adsorption isotherms were represented by Langmuir and Redlich Peterson models and maximum adsorption capacities were determined as 29 mg g−1 and 15.3 mg g−1, respectively, while the reaction pathway followed a pseudo-second-order model. The reaction was spontaneous for both dyes although the reaction of AO 8 was classified as exothermic while that of the DR23 was endothermic. The adsorption occurred mainly because of electrostatic interactions due to a difference in charges of the dye and the GO at pH less than 7, but there was a possibility of hydrogen bonding and π−π stacking as well.273 MB was adsorbed by GO at pH 6 and a temperature of 298 K and adsorption intensity was reported to be 714 mg g−1 with an efficiency of 99%. The adsorption was dependent on pH and ionic strength and dissolved organic matter, and the data supported the Freundlich isotherm with preference to low temperatures and high pH.274 Another study showed MB absorbed by GO at pH 3 and a temperature of 298 K, and an absorption capacity of 1.939 mg g−1 by the Langmuir adsorption model and was found to be exothermic in nature. An increase in pH increased the adsorption of the dye onto GO.221 This has been depicted in Figure 6. Exfoliated GO was further analyzed for the adsorption of MB, RhB, methyl violet (MV), and orange G, and due to the oxygen functional groups showed excellent adsorption intensity of the cationic dyes (MB, RhB,MV). The observed adsorption capacities were 17.3, 2.47, and 1.24 mg g−1 for MB, MV (both at pH 6) and RhB (at pH 10). The RhB has both N



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Figure 6. Adsorption of methylene blue on GO221 where the EDA interaction between sulfur (yellow) of MB and oxygen (red) of GO and the π−π interaction between the aromatic rings are highlighted.

increase in adsorption intensity, and the Langmuir adsorption model showed that adsorption was classified as physisorption.227 While graphene, GO, and rGO can adsorb dyes on their own, they are combined with other metal oxides to induce characteristics that make the adsorbed species easier to extract from water or to degrade the organic pollutant after being adsorbed. The ease of removal of adsorbent paves the way to recycle the adsorbents. This can be done by combining graphene with magnetic metal oxides to enhance their removal from water by using a magnet. Magnetic ferric oxide−graphene composite (GO-Fe2O3) was used for eradication of MB and Congo Red (CR) for which the adsorption capacities were reported to be 45.27 mg g−1 and 33.66 mg g−1, respectively, at 298 K by the Langmuir adsorption isotherm.141 Graphene nanosheets/iron oxide nanocomposite synthesized in a single step removed MB from aqueous solution with an adsorption intensity of 43.82 mg g−1 at 298 K.219 Graphene/magnetite composite was used for eradicating Basic Red 9 and pararosaniline adsorption capacity was found to be 198.23 mg g−1 at a temperature of 298 K and pH of 6.6. An increase in pH led to the precipitation of the dye, and hence pH was important for efficient removal of the dye.210 Superparamagnetic GO-Fe3O4 nanocomposites were decorated with amino-functionalized Fe3O4 (NH2−Fe3O4) particles to remove MB and neutral red (NR). Their adsorption intensities were reported as 167.2 and 171.3 mg g−1. The composite kept the NH2−Fe3O4 particles from agglomerating and thus enhanced their efficiency for wastewater removal.233 Rhodamine 6G (R6G) was removed by a graphene sand composite where graphene was immobilized with the help of asphalt on sand. The extent of adsorption was determined to be 75.4 mg g−1 at 303 K and 6 h contact time. A pesticide, chlorpyrifos was also studied along with the dye removal.211 Another study shows the same composite with the adsorption intensity of R6G to be 55.5 mg g−1 for an 8-h contact time. The graphene sand composite nanocomposites can be easily regenerated by acetone.212 GO-wrapped magnetite nano-

negative and positive charges in its structure because the adsorption was carried out in the basic medium.95 At a temperature of 298 K and pH 6, GO showed excellent adsorption properties, and the extent of adsorption was 243.90 mg g−1 with a contact time of 5 h. The adsorption took place in a monolayer manner as shown by the Langmuir adsorption isotherm and is caused by the exchange of electrons.222 In situ reduction of GO by sodium hydrosulfide and the adsorption of acridine orange was reported, and this showed much better adsorption capabilities than GO; the adsorption capacities were reported to be 3333 mg g−1 and 1428 mg g−1, respectively. MG was removed by GO by varying the pH from 4 to 9 at 298 K, and the adsorption intensity was recorded to be 4.821 to 7.613 mmol g−1223. Basic Blue 41(BB41), Basic Red 18, and 46(BR18, BR 46) were adsorbed by GO, and the adsorption intensities were reported to be 1429, 1250, and 476 mg g−1, respectively, at 298 K and an initial concentration of 5 mg L−1 for an hour. The Langmuir adsorption isotherm helped to classify the reaction as chemisorption. As the degree of oxidation of GO was varied it led to different adsorption capacities. At pH 7 and temperature of 298 K and a contact time of just 15 min the adsorption capacities of six differently oxidized GO yielded adsorption capacities ranging from 40.6 to 570.4 mg g−1224. In another case GO with different oxidation degrees was synthesized and was found to be pH independent, and all the GO variations exhibited high affinity toward MB in water. Adsorption increased as the oxidation of GO increased, and the binding changed from parallel stacking to vertical stacking due to electrostatic interactions.225 The extent of adsorption was determined to be 351 mg g−1 for MB, and 248 mg g−1 for malachite green (MaG) for GO synthesized by the modified Hummers−Offeman method.226 MG was adsorbed by rGO and an outstanding adsorption intensity of 3.163 mmol/g was observed at pH 5 and temperature of 298 K with contact time of 1 hour. Electrostatic interactions increased by the increase in pH, and thus more MG was absorbed. The increase in temperature showed an O



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Figure 7. (a) Schematic representation of adsorption of RhB and MB using Fe3O4@GO and recycling of catalyst by magnetic separation and subsequent ethanol/water wash. Photographs of (b) MB and (c) RhB dye solution before and after adsorption process and separation of adsorbent. Reprinted with permission from ref 275. Copyright 2018, Elsevier B.V.

Figure 8. Binding mechanism of Rhb dye during adsorption of Bi2O3@GO composite. Reprinted with permission from ref 276. Copyright 2018, Elsevier B.V.

adsorbent could easily be separated by magnetic separation and showed excellent regeneration and stability.275 The structure can be visualized in Figure 7 rGO supported ferrite was efficient in 92% RhB and 100% MB removal. The metal in ferrite could be Mn, Zn, Co, or Ni and the ferrites were homogeneously distributed over rGO.

clusters (Fe3O4@GO) were prepared by utilizing the weak electrostatic attraction of the negatively charged GO to Fe3O4 to form a hybrid core−shell nanostructure. The adsorption capabilities were determined by taking MB and RhB as cationic dyes and MO as the anionic dye that were adsorbed at the rate of 131.10, 34.50, and 39.95 mg/g, respectively, at 303 K. The P



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Their magnetic nature suggests easy separation from the sample.229 rGO-Fe3O4 nanoparticles were used for removal of RB, R6G, and acid blue 92(AB 92), MG, etc. The dye showed the capability of simultaneous adsorption of multiple dyes without any effect of pH and temperature, and could be easily regenerated by annealing without any significant loss in regeneration capacity.230 Magnetic modified rGO was utilized for the adsorption of RB and MG dyes, and the adsorption scope was found to be 13.15 and 22 mg g−1 with pH 7 and temperature of 298 K and showed a 91 and 94% efficiency, respectively, for the dye removal.231 In another study, a comparison was made between GO and Bi2O3@GO for adsorption of RhB as shown in Figure 8.276 At 408 K, at pH 4 with 5 mg of adsorbent, the adsorption capacity increased from 64% to 80.7% for GO and Bi2O3@GO, respectively, in 65 min. It followed a Langmuir as well as Temkin isotherm and followed both pseudo-first-order as well as an intraparticle diffusion model, effectively. Graphene-carbon nanotube hybrids (G-CNT) that were cylindrical in shape and formed as a result of self-assembly efficiently removed MB, and the adsorption capacity was determined to be 81.97 mg/g after 3 h contact time. The Freundlich adsorption isotherm was used and an efficiency of 97% was achieved.220 MB and RhB were adsorbed onto Cu2O−graphene277 and Mg(OH)2−graphene.278 Cu2O−graphene was synthesized by heating graphene and copper ions in the presence of glucose while the Mg(OH)2−graphene generated by chemical deposition method had a mesoporous structure. Sonochemically synthesized GO-[Zn2(oba)2(bpfb)]· (DMF)5] metal−organic framework nanocomposite (GOTMU-23) for MB eradication showed a 90% efficiency of removal within 2 min.218 A temperature varying study was done of adsorption of Methyl Green (MG) onto CoFe2O4/graphene nanocomposite, and the adsorption intensities were found to be 203.51, 258.39, and 312.80 at 298, 313, and 323 K by Langmuir adsorption isotherm which revealed the adsorption to be endothermic and spontaneous and was classified as a physical adsorption process. MO was adsorbed by CoFe3O4-functionalized graphene nanosheets and a maximum adsorption capacity of 71.54 mg g−1 was reported for an initial concentration of 10 mg/L. N,P co-doped reduced GO (PA-rGO) was used for the removal of Rhodamine B (RhB) and various parameters such as different pH of initial solution, dosage, temperature, etc. were studied. The adsorption intensity of 149 mg g−1 was established, and the adsorption process followed pseudosecond-order kinetics.279 GO caged in cellulose bead synthesized by the sol−gel method also proved efficient for the removal of MaG, and the adsorption extent was calculated to be 30.091 mg g−1 at pH 7 and temperature of 298 K. As the cellulose is pH dependent, a pH of 6−8 was maintained.151 Fe3O4/SiO2−GO nanocomposites were used as an adsorbent for MB, and adsorption extent was reported as 97, 102.6, and 111.1 mg g−1 for temperatures 298, 318, and 333 K, respectively.274 MgO-multi layered graphene (MDMLG) synthesized by burning magnesium in dry ice was used to remove safrainin-O dye from water, and the adsorption capacity was reported to be 3.92 × 10−4 mol g−1 at pH 12 with a contact time of 2 h. MDMLG could also be recycled and it also was supported in the dye regeneration.152

rGO-Titanate hybrids synthesized by the solvo-thermal method were used to remove MB with an adsorption capacity of 83.26 mg g−1, which was higher than that of graphene and tubular TiO2. The preparation of the composite depended on the rise in the autoclave temperature that led to the formation of nanoribbons in the composite.149 The drawback of adsorption process is the agglomeration of large amount of waste. Most of this waste can be recycled but the cost is higher. The adsorption process is efficient in removal of organic pollutants but it will be more desirable if these organic wastes could be treated without generating any waste. This is efficiently done by oxidizing these products into smaller nontoxic components and graphene-based materials have shown their capability for these processes which are highlighted in the following sections.

5. ADVANCED OXIDATION PROCESSES Wastewater treatment processes-based on radicals are known as advanced oxidation processes. These processes are advantageous as the pollutants are converted into basic components such as carbon dioxide and water and other simple and biofriendly compounds.59 Dehydrogenation, redox reaction, and hydroxylation are some of the reactions that take place when a highly reactive hydroxyl radical is involved in the oxidation process that results in the degradation of persistent organic pollutants.280,281 Some of the oxidation processes have been described in this section with an aim to prove their efficacy as an important tool for wastewater treatment. Graphene is chemically stable and has a large surface area and high mobility for charge transport and thus is used extensively for these processes. 5.1. Fenton Process. The Fenton process deals with the generation of hydroxyl radicals in water with the help of ferrous ion and hydrogen peroxide. But the ferrous ions increase sludge production and are not environment friendly. Graphene with its exceptional properties was able to replace ferrous ions and proved highly efficient for the Fenton process and was found to generate hydroxyl radicals.282 Iron oxide combined with graphene further improved this process. The zerovalent cerium and iron ions on rGO resulted in a breakdown of sulfamethazine.283 The cerium and iron ions are oxidized and generate hydroxyl ions, turning into superoxide that enhances the degradation process. Graphene not only forms the hydroxyl radicals but also helps in converting ferric ions to ferrous ions for reuse which is generally done by hydrogen peroxide.284 β-FeOOH/rGO prevented the agglomeration of β-FeOOH and enhanced the degradation of 2chlorophenol.285 Fe3O4/GO resulted in complete dye removal and provided higher stability than Fe3O4. FeO/Fe3O4/rGO was also efficient in removing Methylene Blue(MB) within an hour with the Fenton process although the efficiency decreased after two cycles of dye degradation.52 Cobalt ferrite, another composite that degraded methylene blue did so in just 15 min but was found to have low metal leaching property.286 The hydrogen peroxide used in this reaction can be generated while the reaction is being carried out by the electro-Fenton process. Hydrogen production takes place at the cathode and so the material of cathode is very important. Graphene proved to be an excellent option, and the production of hydrogen peroxide happens at neutral pH.287 This removed the barrier of maintaining the pH at 3 which was a necessary condition for the conventional electro-Fenton process.280,281 rGO/carbon felt electrode which was produced by electroQ



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Figure 9. (a) GC−MS graphs of intermediates while degrading RhB under different irradiation time. (b) Photo-Fenton degradation: schematic representation of mechanism of RhB without heavy metal ions. Reprinted with permission from ref 290. Copyright 2018, Elsevier B.V.

the reaction between them results into degradation of phenol. The doping of nitrogen on the graphene enhances the ability to graphene to convert ozone into superoxide anion radicals.59 TiO2/UV/O3 and TiO2 rGO/UV/O3 degraded bisphenol A but the latter was 1.17 times more efficient than the former for the reaction.294 Graphitic carbon nitride/rGO oxidized oxalic acid in the presence of triethanolamine and tert-butanol as hole and hydroxyl radical scavengers, respectively, Yin et al. (2016), but the decrease in the efficiency from 90% to 27% and 40% hinted toward the importance of holes and hydroxyl radicals in the degradation process.295 5.3. Sulfate-Based AOP. Sulfate radicals are more reactive than hydroxyl radicals due the interaction of persulfates (PS) and permethoxysulfates (PMS) with water.60 The hydroxyl ions have a life span of 20 ns, whereas sulfate radicals have a life span of 30−40 μs and thus the longer lifespan contributes to greater efficiency.296,297 This provides sulfate radicals more time to collide with the pollutants and the larger is the number of collisions, the greater is the probability of reactions, and thus they have proved to be efficient at every pH condition. Graphene, GO, and rGO have been proven good catalysts for generating these sulfate radicals, and metal oxides of metals such as cobalt, manganese, and iron have shown great efficiency in generating sulfate radicals from PS and PMS. Co3O4/GO298 was reported to be 50% more efficient than Co3O4 for the removal of Orange II dye and complete dye removal was observed in 4 min. Co3O4/graphene displayed

deposition of rGO on carbon felt proved its efficiency for the mineralization of Acid Orange 7 from wastewater.288 The heterogeneous electro-Fenton process resulted in the degradation of Rhodamine B at the anthraquinone/rGO nanohybrid cathode.289 The Fe3O4/rGO/PAM hydrogel adsorbed heavy metal ions in its 3D structure, which accelerated the photo-Fenton reaction for organic effluent degradation. RhB dye degradation was studied to understand the mechanism of the Fenton reaction. Gas chromatography−mass spectrometry (GC−MS) was employed for the same, and m/z 479 (RhB) and 473 (Ndeethylated intermediates) were in the presence of heavy metal ions (Ce3+, Cu2+, Ag+, Cd2+) and in their absence the increase in light radiation showed extra peaks depicting further breakdown of the dye. The whole procedure took 50 min, and the breakdown of dye can be understood by Figure 9. A mass to charge ratio of 73 shows that the dye eventually breaks down to carbon dioxide and water.290 5.2. Ozonation. The use of ozone for degrading pollutants is known as ozonation. p-Hydroxybenzoic acid was removed from wastewater using rGO which was responsible for the activation of ozone and producing superoxide anions from the carbonyl groups on rGO.291 α-MnO2/rGO292 and Co3O4/N/ graphene293 also activated the ozone efficiently and degraded bisphenol A and phenol, respectively. The mechanism of phenol degradation involves the adsorption of pollutants, then of the ozonation of the surface of the graphene sheet, and then R



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Figure 10. (a) Setup for photocatalysis, (b) degradation of RR195 dye.

Figure 11. Mechanism of photocatalysis.

complete degradation in 10 min of the same dye, but the recycling of graphene composite was higher than it was for the GO part.299 MV, MO, MB, Orange II, and RhB were separated by using MnFe2O4/rGO which showed excellent reusability and was easily separable by the magnetic separation technique.140 Fe3O4/rGO showed almost complete removal of trichloroethylene within 5 min with the help of sulfate radicals that were generated by PMS.300 Nitrogen doped/ aminated graphene, a metal-free composite was efficient in generating sulfate radicals from PS and efficiently removed sulfamethoxazole in the presence of humic acid and carbonates and the pH of the solution.301 5.4. Light-Based Oxidation. Light-based oxidation or photocatalysis plays a major role in the removal of organic content especially dyes from wastewater. Graphene which acts as an adsorbent and enhances the photocatalytic activity by forming nanocomposites has been widely researched. Photoelectrocatalysis (PEC) is another process which is sought for the removal of organic pollutants. PEC is a combination of photocatalysis and electro-catalysis. The photons and electrons both simultaneously degrade the electron hole recombination efficiency. Graphene−Ti electrode experienced an improved

electron migration and the adsorption of dyes was significantly enhanced302 by this method. rGO/Pt/TiO2 nanotube arrays were used as anode and were tested for methanol removal and showed a 4-fold improvement303 for methylene blue removal. Photocatalysis thus has an important role in dye removal and has been explained in detail in the following section.

6. ERADICATION OF ORGANIC POLLUTANTS BY PHOTOCATALYSIS According to IUPAC a catalyst is “a substance that increases the rate of a reaction without modifying the overall standard Gibbs energy change in the reaction; the process is called catalysis”.304 When the reaction these catalysts alter is dependent on light energy then it is termed as photocatalysis and is carried out in a special setup as shown in Figure 10. There are two types of photocatalysts depending upon the phase of the catalysts. Catalysts in the same phase of the reactants are homogeneous while that in different phases are heterogeneous. Steps involved in heterogeneous photocatalysis form a well-known sequence of reactions and form a major part of removal of aqueous organic pollutants.306 Graphene having a zero-band gap and acting as a semimetal can absorb light S



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depicted in Figure 12. The hydroxyl radicals were found to be the reactive oxygen species that degraded 4-chlorophenol

over a large range of wavelengths and is helpful in light catalysis.307,308 Few-layered graphene can also act as semiconductors which are considered to be good photocatalysts. A simple mechanism of heterogeneous photocatalysis includes the excitation of electrons with the help of photon energy from the valence band (VB) to the conduction band (CB) while holes are formed in the valence bands where the energy of the incident light is greater than the band gap of semiconductors.309 Both electrons and holes participate in redox reactions and are responsible for photocatalysis. The reactions thus cause the generation of several types of radicals or intermediates such as hydroxyl radicals that act as powerful oxidizers around the surface of the photocatalyst and are widely studied for organic pollutant eradication from wastewater that include dyes and other organic entities.310 (Figure 11). It is also necessary to provide enough reactants so that the charge carriers are utilized and do not interfere with the aquatic ecosystem as the radicals generated by them are highly active. Photocatalysis also requires the presence of an adsorbent where the reaction can take place and thus graphene is suitable as it acts as a highly efficient adsorbent thus enhancing the photocatalytic activity.311 Titanium dioxide (TiO2) possesses photocatalytic properties that were first demonstrated and published by Akira Fujishima in 1972.312 TiO2/graphene nanocomposites show excellent photocatalytic property and have been widely researched and catalogued.30,313−317 Both UV radiation and visible radiation enhance the photocatalytic activity of TiO2−graphene composite.116 The increase in the catalytic activity was thus due to the transfer of electrons from the conduction band of TiO2 to graphene and enlarging the band gap and decreasing the chances of electron hole recombination in TiO2.318 Graphene alone proved to be more efficient in the eradication of RhB as this was thermodynamically favorable.319 Graphene behaved as an electron mediator in the composite formed with SnO2 but in the case of TiO2, electrons were free to move into the graphene sheet or interact with oxygen functionalities.320 A similar mechanism was realized with a gold−graphene (Au/ graphene) composite.61 2,4-Dicholorophenoxyacetic acid with TiO2/graphene resulted in the degradation which was four times more efficient,321 and that of dodecylbenzenesulfonate(DBS) increased three times in comparison with Degussa P25(commercial TiO2) when platinum was doped onto graphene in graphene/TiO2 nanocomposites.322 The enhancement of degradation of the organic entities can be attributed to the effective adsorption of graphene, the charge transfer from TiO2 to graphene and the bridged bond from titanium to carbon with the help of oxygen which results in the lessening of the bandgap value.323−326 P25-graphene was efficient in the removal of Reactive Black 5 using UV light radiation than the Degussa P25 alone. More than 90% degradation was observed for 100 min irradiation with UV owing to the high surface area and narrow bandgap.327 Flower-like G/titania nanosheets (G/ TNS) were synthesized by a hydrothermal process in which Ti cryastalline phases of both anatase and sodium titanate formed. Graphene acted as template for Ti growth and inhibited the growth of TiO2 and degraded 4-chlorophenol with efficiency more than 99.2% in 120 min.328 The use of graphene enhanced the titanium photocatalytic activity due to two reasons: first, it enhances the rate of transfer of photoexcited electrons therefore not allowing recombination of holes and electrons; and second, the narrowing the band gap energy leads to enhancement in visible light absorption. This has been

Figure 12. Schematic illustration of mechanisms for grapheneenhanced photocatalytic activity by G/TNS. Reprinted with permission from ref 328. Copyright 2018, Elsevier B.V.

and were verified by DFT calculations and maintained their efficiency even after five cycles.328 TiO2 and graphene-based composites have been widely studied for the removal of MO.329−336 GO/TiO2 composite showed higher efficiency toward the removal of MO than GO combined with carbon nanotubes or fullerenes under UV−vis light radiation. The self-assembly of the components led to the composite having properties of both electron donor and acceptor thus decreasing the charge recombination.337 The rGO-TiO2 hybrid was also utilized for one step photocatalytic reduction of dyes and other organic pollutants by capturing dyes and electrons generated during the photoreaction by rGO. TiO2 is widely used in environmental remediation338,339 but has limited optical absorbance.229 This drawback can easily be solved by doping TiO2 with any metal such as Cu, Cr, Fe, Mn, and Mo, etc. or nonmetals such as N, S, and C or stabilizing it on graphene or other carbon-based materials.340,341 Further enhancement can be achieved by doping it, and further immobilizing it on graphene. TiO2 nanoparticles doped with N, Fe, Cu, and Nd were used to degrade MO, and nitrogendoped TiO2 showed maximum efficiency among these.342 One of the examples of these composites are SnS2 quantum dots and nanodiscs supported on rGO sheets developed hydrothermally that degraded Remazol Brilliant Red (RBR) and Remazol Brilliant Blue (RBB) under visible light radiation.343 The BET surface area after including rGO increased to ∼103.55 m2/g from ∼71.09 m2/g and the photodegradation escalated to ∼99.7% and ∼97% for RBB and RBR dye. The comparison data of photocatalytic degradation of RBB and RBR dye by SnS2 and SnS2/rGO is shown in Figure 13. Hexagonally arranged mesopores of graphene−TiO2/SiO2 nanocomposite enhanced the degradation capacity up to a 100% in 30 min under sunlight. The reaction between graphene and titanium/silicon dioxide increased the quantum efficiency for which graphene as an acceptor decreased the charge carrier recombination possibility and increased the degradation of RhB. The mesoporous structure enhanced the adsorption thus enhancing the photocatalytic activity.33 T



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Figure 13. UV−visible spectra for the photocatalytic degradation of (a and b) RBB dye and (c and d) RBR dye by SnS2 and SnS2/rGO under visible light photodegradation. Reprinted with permission from ref 343. Copyright 2019, Elsevier B.V.

composite.352 The presence of graphene proved to be antiphotocorrosive and another study showed that the ZnO− graphene for which the percentage of graphene was 2% showed better degradation capability of MB than commercial titanium oxide and pure ZnO. The composite containing 2.5% graphene exhibits high activity and shows 80% efficiency after five times of recycling.353 ZnO−GO nanohybrids were researched for the degradation of Crystal Violet and 95% of the dye was found to degrade on UV irradiation in 80 min.354 ZnO/rGO hybrids under UV light for 3 h degraded 94% of MO dye.355 ZnO hollow microspheres/reduced graphene oxide nanocomposites (ZnO−rGO) synthesized at very low temperature by a sonochemical method displayed a crystalline structure and were used to degrade 2,4-dichlorophenoxy acetic acid (2,4-D) under sunlight irradiation. The hollow microspheres act as antennas for the transfer of photoelectrons and increasing the electron transport and the addition of rGO decreases the electron hole recombination.356 La/TiO2−graphene and graphene−Ag/ZnO composites also showed higher degradation capability and both were prepared by solvo-thermal methods. The latter showed an efficiency of 100% after 90 min of irradiation with visible light.332 ZnFe2O4/ZnO/G was studied by the Langmuir− Hinshelwood model and was efficient in the degradation of MB. Furthermore, its magnetic character was helpful in separating it from aqueous media. Graphene−Fe2O3/ZnO

The same mechanism worked for graphene−tourmaline− TiO2 nanocomposite and it also showed efficient RhB removal.345 For TiO2−dextran−rGO nanocomposites in which dextran reduced GO, the self-assembly of TiO2 nanoparticles took place on the reduced GO. The nanocomposite thus formed is ecofriendly and highly efficient for the removal of RhB.346 Bi2O3/TiO2/graphene,347 ZnO/Ag/ graphene,348 and Ti/Ce−GO,349 also show good photocatalytic activity for the removal of RhB. A removal efficiency of 94% of Acid Orange 7 was obtained after 30 min of UV irradiation using GO−TiO2 nanorod composites.350 Reactive Red 195 (RR 195) in aqueous media was treated with CeO2−TiO2−graphene (CT−G) nanocomposite along with other substitutes for graphene. The highest degree of photocatalysis was observed in the graphene-based nanocomposite owing to its unique structure and electronic properties. An important fact reported was that the graphene concentration was inversely proportional to the degradation of dye. 305 Pt or Pd−TiO 2 −graphene were successful in eliminating RR195 dye using both UV visible radiation. Pt showed better photocatalytic activity than Pd.351 ZnO/graphene nanocomposites showed excellent photocatalytic properties and have been studied for the removal of MB when irradiated with UV light, and it showed 4−5 fold enhancement as compared to ZnO due to interelectron transfer at the interface of the pollutants and the nanoU



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photocatalytic activity when compared to pure bismuth niobate and to commercially available titanium oxide and was still efficient after five cycles of dye degradation.379 Graphene−Bi2MoO6 nanocomposite showed 90.4% degradation potential for remediation of Reactive Brilliant Red X-3B dye as compared to the metal oxide part alone which had a degradation capacity of 63.8% and followed a first order kinetics.380 Hydrothermally prepared graphene/BiOBr was used to decompose sulforhodamine 640 (Srh 640) dye. A linear regressional analysis determined it follows first order kinetics with a rate constant of 0.00798 min−1, which is higher than the pure BiOBr component.381 Eosin was separated from aqueous media using graphene−Mn2O3 composites.382 ZnO− Bi2O3/GO was successful in eradicating Acid Blue, Acid Yellow, Acid Red, Reactive Blue, Reactive Yellow, and Reactive Red efficiently and proved helpful in managing textile waste.358 Metal sulfides342 such as CdS/graphene showed a 90% efficiency after 5 h visible light exposure.383 In2S3 prepared by solvo-thermal method had its efficiency increased 5 times than the sulfide on its own, and the maximum efficiency was seen at a temperature of 180 C.384 An efficiency of 90% was reported for CdS/Al2O3/GO and 99% efficiency for CdS/ZnO/GO for 60 min of visible light irradiation for degradation of MO.385 Calcined layered double hydroxides (CLDH) photocatalysts are extremely successful for photodecolorization of organic dyes. rGO sheets act as a base for LDH crystallites and NiFeLDH lamellar crystal and decolorization of methylene blue was observed with 1% rGO and heating at a temperature of 500 °C; the photocatalytic degradation efficiency was 93.0% in 5 h duration.386 High photo response, increased charge carrier separation, and high adsorption affinity resulted in the high performance of Au/rGO/TiO2 nanotube hybrids which showed an enhanced photocatalytic activity for the degradation of MO in the presence of sunlight and when irradiated by UV light.387 Ag/AgCl/rGO, Ag/AgBr/rGO, and Ag@Ag3PO4/rGO have shown excellent dye removal capabilities.388−390 A 95% efficiency was recorded after 11/2 hours of visible light irradiation of Cu2O/PA/rGO where Cu2O nanoparticles were decorated on n-propylamine and coordinated with GO nanosheets. N-propylamine improved the surface area in turn improving accessibility for adsorption and photocatalysis.391 The liquid phase deposition synthesis of Ag3PO4/grapheneoxide composite yielded superior catalytic efficiency, and an increase in graphene content further enhanced the degradation process. The oxidation mechanism was found to be dependent on holes for the degradation of dyes.392 Monodisperse cobalt ferrite (CoFe2O4)/reduced graphene oxide (rGO) displayed sonocatalytic activity solutions under ultrasonic irradiation. The sonocatalytic removal of AO7, and the effect of parameters such as pH, dosage of catalyst, H2O2 concentration, power, and reaction time displayed a photocatalytic efficiency of 90.5% at pH 3 under 350 W ultrasonic power in 120 min. The Langmuir−Hinshelwood (L-H) kinetic model fit the studies and showed that superoxide anion radicals dominate the reaction process.393 rGO−CoFe2O4 composite showed better photocatalysis properties for RhB removal than rGO−ZnFe2O4, rGO− MnFe2O4, and rGO−NiFe2O4 under visible light irradiation.229 rGO/KNbO3 intertwined nanosheets were developed and showed moderate efficiency of 68.3% after half an hour of UV radiation394 CoFe2O4−graphene was hydrothermally synthesized and used for degradation of Active Black BL-G

composites are also composites for dye degradation and possess magnetic character and were efficient even beyond five degradation cycles.357 ZnO−Bi2O3/GO showed an efficiency of 99.62% after 2 hours of visible light radiation.358 Bi2O3− rGO composites also gave similar results.359 The redox reactions on graphene that act as cocatalyst have been established as rate-determining steps in the degradation of organic pollutants. Gold,360,361 silver,362,363 and platinum364 form hybrids with graphene nanosheets enhancing the redox reaction due to excess electrons offered by graphene. Plasmonic Ag/Ag2CO3−rGO was used for oxidation of pollutants and was determined to be more efficient than the standalone Ag2CO3 or when it formed a composite with GO. The collaborative effect of Ag nanoparticles and graphene leads to an increase in redox activity and in turn enhances the photocatalytic efficiency.365 Plasmonic Ag-RGO/Bi 2WO6 results in the increase of efficiency of photocatalysis by three times than just the Bi2WO6 complex. The mechanism in effect is the same as that for the nonreactive noble elements and graphene nanosheets.362 GO−Ag3PO4 composites ensured complete decomposition of Acid Orange 7 dye under visible radiation. A mixture of silver acetate and disodium phosphate in the presence of GO resulted in the composite which showed excellent dye adsorption capability.366 GO−CdS also removed Acid Orange 7 dye and enhanced the capability of CdS by 80%. This was caused by the formation of hydroxyl radicals and confirmed by transient photocurrent detection and measurement of the scavenging of formed radicals.367 A mixture of oleic acid and oleyamine with zinc acetate and GO led to the formation of the ZnO−rGO composite that augmented the eradication of diamino-triphenylmethane and Malachite Green dyes showing a high degeneration rate constant for pseudo-first-order kinetics.368 Ag−ZnO/rGO composite degraded 80% of the RhB dye after 200 min of UV irradiation. Highly charged carrier species were formed that promoted the degradation of the given dye.369 Ag3VO4/TiO2/graphene composite showed higher activity than one or a combination of two of the components.370 WO3 nanorods@graphene utilized sunlight to degrade MO owing to the increase in charge separation.371 α-SnWO4/rGO nanocomposites also showed similar properties.372 The BiOI/rGO-1 (BG-1) obtained through a chemical mechanical method with microsphere structure and BiOI/ rGO-2 (BG-2) obtained through microwave hydrothermal method witha nanoplate-like structure made composites with graphene and rGO respectively; 40 mg/L Methylene Blue (MB) and 20 mg/L levofloxacin (LEV) was 11 and 3 times more efficient than that of P25, respectively, under visible light radiation. The species that reacted with the hole was found to be responsible for degradation.373 Other metal oxide such as BiVO4−graphene (BiVO4−G) showed a 99% degradation with 5 h of visible light exposure.374 BiOBr/graphene nanocomposites showed a greater efficiency than the commercially available titanium oxide.375 GO/BiOI and rGO/BiOI both showed photocatalytic activity, but the efficiency of rGO/BiOI was reported to be more than that of GO/BiOI.376,377 Bi2O3−rGO showed a 93% efficiency for degradation of dye under visible light in 4 h.359 BiOBr− graphene and Bi2WO6/graphene were found to show excellent photocatalytic activity for the removal of RhB dye under UV− vis radiation.378 Hydrothermally prepared Bi5Nb3O15/graphene nanocomposites showed a 2.8 times increase in the V



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Table 4. Graphene-Based Composites and Their Photocatalytic Degradation of Organic Pollutants composite

pollutant

percentage removal

reaction time (mins)

source

oxidizing species

ref

120 120

UV lamp Xe lamp

O2·−, ·OH O2·−

396 397

BPA paracetamol 4-NP, CIP and DEP

93% 79(2-CP), ∼88% (phenol, BPA), 70%(DP) ∼96% ∼95% 89%, 74%, 48.6%

240 420 120

UV lamp Xe Arc lamp 300 W Xe lamp

O2·−, ·OH O2·−, ·OH, H+ ·OH

398 399 400

RhB, CIP NO, acetone

∼100%,96%

Graphene and Graphene Oxide-Based Composites for Removal of

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