Carbon-based nanomaterials for remediation of organic and inorganic pollutants from wastewater. A review

Abstract

The deterioration of water quality by pollutants is a major health issue. Actual remediation methods are limited, and, as a consequence, there is a need for new remediation technologies. In particular, nanomaterials of unique properties have been recently developed for remediation. Here, we review mechanisms and applications of carbon-based nanomaterials for the adsorption and photocatalytic removal of organic and inorganic pollutants in wastewaters. Nanomaterials allow enhanced adsorption due to strong interactions between pollutants and adsorption sites. In photocatalysis, enhanced efficiency is attributed to the improved light harvesting and reduced recombination of photo-induced electrons and holes.

Introduction

An increase in the world’s population together with the rapid growth of factories and industries has led to the energy crisis and environmental pollution. Environmental pollution has become a critical issue across the globe, threatening both industrialized and developing countries. Amongst the world’s top environmental concerns, water pollution is a major threat to the human race and aquatic life. Water is one of the most abundant natural resources on Earth, covering about 70% of the Earth’s surface, but only less than 3% of water is available and safe for human consumption (Grey et al. 2013; Adeleye et al. 2016). According to the study carried out by the World Health Organization (WHO), about 1.1 billion people worldwide are living without adequate water supply (Organization World Health (WHO) 2001; WHO 2017). The major concern in supplying freshwater lies in the contamination of water resources by a variety of organic and inorganic pollutants (Anderson et al. 2006; Gopinath and Kadirvelu 2018).

Inorganic pollutants include nutrients, sediments, heavy metals, and industrial waste (Zhang et al. 2020). Most of the inorganic pollutants exist naturally within the environment, but due to the increase in anthropogenic activities such as mining, industrial, and agricultural activities, their concentration within the environment has increased (Holt 2000). Most of the inorganic pollutants are non-biodegradable, and their accumulation into the human body can lead to serious diseases (Lu and Astruc 2018). Meanwhile, organic pollutants such as pesticides, pharmaceuticals waste, dyes, and many other forms of industrial waste that arise from the discharge of sewage, surface water runoff, agricultural activities, and industrial discharges cause significant environmental issues due to their persistence and bio-accumulation (Smith and Rodrigues 2015). Therefore, the pollution of water by inorganic and organic pollutants has become the most significant environmental problem threatening human life throughout the world (Feng et al. 2018). In light of this, attention to specific methods and technologies for their remediation is desired.

From the past decade, various conventional methods such as oxidation, precipitation, solvent extraction, distillation, bio-remediation, filtration, reverse osmosis, adsorption, coagulation, membrane technology, photocatalysis, etc., have been widely used for the remediation of both organic and inorganic pollutants in wastewater (Anjum et al. 2016; Thines et al. 2017; Wang et al. 2018; Feng et al. 2019; Shi et al. 2019). Nevertheless, some of those methods have limitations such as high operational cost, production of a high amount of sludge which results in the generation of secondary pollutants, and economically not viable (Gunatilake 2015; Oguz 2017; Boikanyo et al. 2018; Crini and Lichtfouse 2020). Among the above-mentioned conventional methods, nano-based technology like adsorption, and photocatalysis has received tremendous attention due to their higher removal efficiency, lower operational cost, simplicity in design, and environmentally friendly (Santhosh et al. 2016; Sadegh et al. 2017; Thines et al. 2017; Wang et al. 2018; Shi et al. 2019).

Nano-based technology involves the manipulation of nanomaterials at atomic levels. NMs have at least one of their dimensions in the range of 1–100 nm and are characterized by unique physical and chemical properties such as large surface area, specific affinity, and surface activity. With increasing interest in the field of nano-based technology, carbon-based nanomaterials and their derivatives have emerged. Carbon-based nanomaterials have obtained significant attention in the field of wastewater remediation due to their interesting properties discussed in "Carbon-based nanomaterials" section  (Ali et al. 2012; Smith and Rodrigues 2015; Thines et al. 2017; Lu and Astruc 2018). In light of this, this review highlights the deep insight of carbon-based nanomaterials and their potential applications in wastewater remediation. The emphasis on their roles in adsorption and photocatalysis removal of organic and inorganic pollutants is discussed. The upcoming section gives general information on various types of carbon-based nanomaterials.

Carbon-based nanomaterials

Carbon-based nanomaterials have received tremendous attention in the field of water and wastewater remediation (Shan et al. 2017). These nanomaterials are receiving much attention by researchers due to their unique properties such as small size, high surface area to volume ratio, high reactivity, high thermal and chemical stability, vast availability, and catalytic potential at the nanoscale (Madhura et al. 2019). The large surface area gives more active sites for interaction of the material with different chemical species from wastewater. They exist in various allotropic forms such as graphite, diamond, graphitic carbon nitrate, fullerenes, carbon nanotubes, graphene, etc., as shown in Fig. 1 (Jayaraman et al. 2018). Due to their unique properties, these carbon-based nanomaterials have made a greater contribution to the generation of clean, renewable, and viable forms of energy from light-based water splitting and pollutant removal. In this review, carbon nanotubes (CNTs), graphitic carbon nitrate (g-C₃N₄), graphene, and their derivatives are discussed with respect to their properties toward remediation of organic and inorganic pollutants in wastewater and some previously reported studies are given.

Fig. 1

Reproduced with permission from Jayaraman et al. (2018). Copyright 2018, Elsevier

Allotropic forms of carbon-based nanomaterials.

Carbon nanotubes

Carbon nanotubes are cylindrical carbon molecules rolled up in a tube-like structure (Cha et al. 2013). They are divided into two groups, i.e., single-walled carbon nanotube (SWCNT) and multi-walled carbon nanotube (MWCNT) depending on the number of carbon layers as shown in Fig. 2 (Ihsanullah et al. 2016). Single-walled carbon nanotubes are made up of single layer of carbon atoms, while multi-walled carbon nanotubes are made up of several layers of carbon atoms.

Fig. 2

Reproduced with permission from Ihsanullah et al. (2016). Copyright 2016, Elsevier

Multi-wall carbon nanotubes (a) and single-wall carbon nanotubes (b), representing number of layers in each type of carbon nanotubes.

Carbon nanotubes have been widely used in the field of wastewater remediation as adsorbents, sensors, membranes, and catalysts owing to their unique properties such as large specific area, high porosity, hallow structure, light-weight, layered structure, and also their strong interaction with pollutants (Cha et al. 2013; Shan et al. 2017; Das et al. 2018). Varieties of toxic pollutants have been removed from wastewater using carbon nanotubes due to those interesting properties (Sarkar et al. 2018).

In addition, carbon nanotubes can also be coupled with other carbon-based materials to increase their effectiveness in removing a wide range of pollutants in wastewater. Although carbon nanotubes have been widely used in the field of wastewater remediation, this carbon-based nanomaterials lack the optimum performance due to the presence of impurities that could be carbonaceous species and residues utilized during preparation. The presence of those impurities results in the alteration of their surface properties by blocking the active sites for the interaction of this nanomaterial with pollutants (Thines et al. 2017). However, most of the studies have shown that the alteration can be prevented by functionalizing carbon nanotubes in the presence of acid and alkali solution which also results in the additional of the new functional group on the surface of carbon nanotubes which can be of particular interest in removing pollutants (Mubarak et al. 2017; Thines et al. 2017). The presence of new functional groups onto the surface of carbon nanotubes together with the nature of adsorbent is of particular interest in measuring the adsorption capacity of this carbon-based nanomaterial. This carbon-based nanomaterial has successfully proved to be an outstanding adsorbent for the removal of pollutants from wastewater, leading to a clean environment due to their exceptional adsorption capability and excellent regeneration and recovery abilities.

Graphene-based nanomaterials

Graphene, as one of the advanced carbon nano-materials, has received tremendous attention since its discovery in 2004 as a two-dimensional (2D) monolayer carbon nanomaterial consisting of sp²-hybridized carbon atoms arranged in a hexagonal crystalline structure. They have gained significant attention as novel materials for environmental applications due to their high specific surface area (theoretically ~ 2630 m²/g), high thermal conductivity (~ 5000 W m⁻¹ K⁻¹), and rapid heterogeneous electron transfer (Perreault et al. 2015; Xu and Wang 2017; Jilani et al. 2018). In addition, their outstanding mechanical strength and some exceptional performance (such as abundant functional groups, high negative charge density, and extremely hydrophilic properties) make them exceptional nanomaterials in wastewater remediation (Pumera 2010; Wang et al. 2013; Qiu et al. 2018).

These nanomaterials are classified as graphene, graphene oxide, and reduced graphene oxide as shown in Fig. 3. Graphene (G) and reduced graphene oxide (RGO) are easily obtained through the reduction in graphene oxide (GO) using a simple chemical exfoliation method. In addition, graphene can also be functionalized into graphene oxide through Hummers method which shows a high density of oxygen functional groups (i.e., carboxyl, hydroxyl, carbonyl, and epoxy) in the carbon lattice which is of particular interest in removing a wide range of pollutants in wastewater (Perreault et al. 2015). This graphene oxide can be synthesized from simple and low-cost methods such as chemical oxidation of graphite to graphene oxide followed by exfoliation in ultrasonication (Lü et al. 2012). Due to its hydrophilic nature, high specific surface area, and high functional group density, it can be used as adsorbent and as a catalyst for the removal of organic and inorganic pollutants in wastewater (Perreault et al. 2015).

Fig. 3

Graphene-based nanomaterials: graphene sheets (a), graphene oxide (b), and reduced graphene oxide (c). Graphene oxide is obtained through functionalization of graphene using Hummer method and its chemical reduction yield reduced graphene oxide

Application of graphene-based nanomaterials in the field of wastewater remediation is currently hindered by poor reusability and separation, which can be overcome through surface modification and hybridization ((Jun et al. 2018). In addition, this material is usually coupled with other nanomaterials for suitable applications due to its zero bandgap and susceptible to the oxidation reaction. When coupled with other nanomaterials, it can enhance the photocatalytic activities of other nanomaterials by acting as an electron acceptor and transporter (Kumar et al. 2017).

Graphitic carbon nitrate

Graphitic carbon nitrate (g-C₃N₄) is considered the most stable allotrope of five carbon nitride, i.e., α-C₃N₄, β-C₃N₄, g-C₃N₄, cubic-C₃N₄, and pseudocubic-C₃N₄ at ambient conditions with a stacked 2D structure (Sun et al. 2016). It has received much attention due to its unique properties such as metal-free composition, extraordinary chemical stability, non-toxic, graphite-like 2D structure, visible light response with medium energy bandgap (~ 2.7 eV), and tunable electronic structure (Yang et al. 2013). This polymeric semiconductor material consists of two low-cost earth-abundant elements (carbon and nitrogen) as shown in Fig. 4 and can easily be prepared from cheap feedstocks like urea, thiourea, melamine, dicyandiamide, and cyanamide, through thermal condensation (Zhao et al. 2015; Zhang et al. 2019). Its unique properties make it a promising candidate for solar energy conversion and environmental remediation (Wen et al. 2017). This nanomaterial has widely used in the field of photocatalysis due to its medium energy bandgap. However, its industrial application is still limited by poor light absorption, low charge separation, and a small surface area. A significant number of studies have been undertaken to modify this polymeric metal-free to overcome such drawbacks.

Fig. 4

The structure of graphitic carbon nitrate consists of two low-cost earth-abundant elements: carbon and nitrogen

The upcoming section will highlight the application of discussed carbon-based nanomaterials, i.e., carbon nanotubes, graphene-based, graphitic carbon nitrate, and their derivatives in adsorption of organic and inorganic pollutants as well as in photocatalysis of organic pollutants in wastewater.

Application of carbon-based nanomaterials in water remediation

A variety of methods such as ion-exchange, coagulation, membrane filtration, electrochemistry, physical and chemical precipitation, chemical oxidation, adsorption, and photocatalysis have been commonly used for the removal of both organic and inorganic pollutants in wastewater (Demirbas 2008; Gupta et al. 2013). However, some of those methods suffer from drawbacks such as high operational cost, transforming the pollutants from one phase to another which results in secondary pollution being produced, difficult in operation, incomplete pollutants removal and are economically not viable (Oguz 2017). Amongst the methods listed above, adsorption and photocatalysis have received significant attention in the field of wastewater remediation. Hence, the upcoming section highlights the application of different types of carbon-based nanomaterials used in the adsorption and photocatalysis remediation of various pollutants in wastewater.

Adsorption

Adsorption is defined as a process in which gas or liquid solutes accumulate (adsorbate) on the surface of a solid (adsorbent) and forms a molecular or atomic film. This technique is regarded as a green, clean, and versatile method for the removal of organic and inorganic pollutants in wastewater due to its high efficiency, easy operation, and low cost (Radaei et al. 2017; Crini et al. 2019; Varghese et al. 2019). It usually occurs in two ways depending on the interaction between adsorbent and adsorbate: chemisorption and physisorption. Physisorption that is also known as physical adsorption is a reversible exothermic reaction, which occurs due to weak van der Waals forces of attraction between adsorbate and adsorbent. This process takes place with the formation of a multilayer of adsorbate on adsorbent (Demirkıran et al. 2017; Access 2018). On the other hand, chemisorption which is also known as chemical adsorption is an irreversible exothermic reaction that occurs due to electrons exchange and strong chemical bonds formed between adsorbate and adsorbent (Yagub et al. 2014; Demirkıran et al. 2017). Chemisorption takes place with the formation of a uni-layer of adsorbate on the adsorbent. Both chemisorption and physisorption are usually affected by various factors such as temperature, pressure, nature of the adsorbate, and surface area of adsorbent (Access 2018). The performance of the adsorption process is measured based on the removal efficiency of the pollutants and is quite related to the adsorption capacity of the adsorbents being used (Thines et al. 2017). Surface properties such as specific surface area, active sites available on the surface, and the affinity toward the contaminants play a crucial role in the adsorption capacity of pollutants. Adsorbent having a large surface area and proper functionalities are the most commonly used.

Various factors such as temperature, nature of adsorbates, and adsorbents, the presence of other pollutants and other atmospheric and experimental conditions such as pH, temperature, the concentration of pollutants, contact time, and particle size are key factors playing an important role in the adsorption process (Ali 2010). The adsorption studies are usually explained based on various models such as Langmuir, Freundlich, Halsey, Henderson, Smith, Elovich liquid film diffusion, intraparticle diffusion, and Lagergren (Ali 2012). These models operate in a similar version but with a little difference in their approach. Furthermore, those models give a clear understanding of the surface interaction of an adsorbent and the adsorbates. Langmuir and Freundlich are the two commonly used models to predict the interaction between the pollutants and the adsorbent in the field of wastewater remediation (Mubarak et al. 2017). Langmuir adsorption isotherm model is usually used for monolayer coverage, while Freundlich is usually used for heterogeneous surfaces. The nature of adsorption can be studied by calculating parameters such as changes in the free energy of adsorption (ΔG_θ), enthalpy (ΔH_θ), and entropy (ΔS_θ) (Anastopoulos et al. 2019; Sahmoune 2019). To study the surface reaction, the kinetics of the adsorption is studied using pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion kinetic models (Mubarak et al. 2017; Jun et al. 2018). Adsorption isotherms, kinetics, and thermodynamics are summarized in Table 1 together with their formulas and description (Kumar et al. 2018).

Table 1 Adsorption isotherm, kinetics models, and thermodynamics constant parameters and correlation coefficients calculated for various adsorptions onto the various adsorbent

Adsorption mechanism involves the adsorption of adsorbate molecules on the surface of adsorbent through molecular interactions, the diffusion of adsorbate molecules from the surface into the interior of the adsorbent to the interior; and the steps can be summarized as follows (Kannan and Sundaram 2001):

• Migration of pollutants from the bulk of the solution to the surface of the adsorbent,

• Diffusion of pollutants through the boundary layer to the surface of the adsorbent,

• Adsorption of pollutants within the active site on the surface of adsorbent and

• Intra-particle diffusion of pollutants into the interior pores of the adsorbent particle.

The adsorption capacity is estimated from the difference between the initial concentration (C₀) and the final concentration (C) of polluted water or wastewater as given by Eq. 1.

$${\text{Adsorption}} = \frac{{C_{0} - C}}{{C_{0} }} \times 100\%$$

(1)

The coming section highlights the application of carbon-based nanomaterials for the adsorption of organic and inorganic pollutants in wastewater.

Adsorption of organic pollutants

Recently, contaminations of water sources by a variety of organic pollutants such as dyes, pesticides, pharmaceutical waste, etc., are posing serious environmental issues across the globe. The discharge of organic pollutants into our water system without pre-treatment can cause harmful effects on living organisms and the ecosystem (Jarde et al. 2001). Most of the organic pollutants are reported to possess toxicity, mutagenic, and a carcinogenic property, i.e., dyes exhibit high toxicity levels even at low concentration (Yang 2011; Shanker 2017). In addition, most of the organic pollutants contain a complex structure that makes them highly resistant to biodegradation and recalcitrant to conventional biological and physical oxidation treatments. More recently, carbon-based nanomaterials have received tremendous attention as adsorbents for the removal of organic pollutants due to their incredibly unique properties. This section reviews the use of carbon nanotubes-based and graphene-based nanomaterials in the remediation of organic pollutants.

Carbon nanotubes for adsorption of organic pollutants

Amongst carbon-based nanomaterials, carbon nanotubes have been reported to have strong adsorption ability for polar organic compounds due to their characteristic structure which permits strong interaction of carbon nanotubes with organic compounds through non-covalent forces, such as hydrogen bonding, π–π stacking, electrostatic forces, van der Waals forces, and hydrophobic interactions (Gupta et al. 2013; Rong et al. 2017). Its mechanisms depend on the properties of the compound of interest (e.g., polar vs. nonpolar). The prediction of adsorption of organic pollutants on carbon nanotubes is not straightforward since it depends upon the nature of the interaction between pollutants and carbon nanotubes (Yu et al. 2014; Apul and Karanfil 2015). Properties such as adsorption site, surface area, purity, and surface functional groups presence onto the surface play an important role in the adsorption of organic pollutants by carbon nanotubes. Carbon nanotubes consist of controlled pore size distribution and high surface active sites, results in exceptional sorption efficiency (Madhura et al. 2019).

In addition, carbon nanotubes turn to aggregate in an aqueous solution resulting in the creation of a lot of interstitial space and grooves, which result in high adsorption sites and contribute to an increase in adsorption capacity of organic pollutants. Multi-walled carbon nanotubes have been reported to be more effective in the removal of organic pollutants such as methyl orange, arsenazo(III), and methyl red from wastewater. Meanwhile, single-walled carbon nanotubes have also been reported to have high adsorption properties of organic pollutants due to their high surface area, and large micro-pore volume (Yu et al. 2014). The key factor in determining the cost-effectiveness of carbon nanotubes is regeneration. It suggested that carbon nanotubes can be regenerated by reducing the solution pH value using an acid such as nitric acid (Rong et al. 2017). Hence, this carbon-based nanomaterial has been used as adsorbents for a wide range of organic pollutants.

Most studies have demonstrated that the adsorption capacities of carbon nanotubes can also be increased through the functionalization of this carbon-based nanomaterial, which results in increasing the number of adsorption sites. Zhang et al. (2011) studied the adsorption of sulfamethoxazole on functionalized carbon nanotubes and found that the adsorption was due to hydrophobic, electrostatic, π–π interactions, and hydrogen bonds. They also predict that the adsorption capacity can be improved by surface functionalization of multi-walled carbon nanotubes, which offers adsorption sites and functional groups. Li et al. (2015) also synthesize recyclable carbon-nanotubes/Fe₃O₄ magnetic nanocomposites using the facile hydrothermal method. The adsorption behaviors of synthesized nanocomposites were evaluated for the removal of bisphenol A (BPA) in aqueous solution. It was found that the dominant mechanisms were due to the π–π interactions between the carbon nanotubes and the benzene rings in bisphenol A molecules. Researchers have investigated the capability of various carbon nanotubes to remove a wide range of organic pollutants from wastewaters, and some examples are given in Table 2. In addition, Thines et al. (2017) review quite a number of adsorption studies of organic pollutants using carbon nanotubes-based nanomaterials. Much details and some examples of adsorption of organic pollutants using carbon nanotubes-based nanomaterials can be found on Thines et al. (2017) and Madhura et al. (2019).

Table 2 Removal of organic pollutants using various types of carbon nanotubes-based absorbents.

Graphene-based in adsorption of organic pollutants

Also, graphene and graphene-based nanomaterials have been used for the adsorption of organic pollutants. Graphene is considered an ideal material and substitutes for carbon nanotubes in the field of water and wastewater remediation due to its production cost-effectiveness (Madhura et al. 2019). It is reported that the adsorption capacity of graphene-based nanomaterials depends on the nature of adsorbate (ionic or hydrophobic). Graphene as a carbon-based material has a high specific surface area and good chemical stability, which makes it a good material for the adsorption of organic pollutants in wastewater. However, due to van der Waals interaction between neighboring sheets of graphene in water, graphene turns to aggregate which results in reduction in surface area which is not beneficial for the adsorption of pollutants; therefore, proper modification of graphene is required to overcome such drawback (Zhao et al. 2012; Perreault et al. 2015). Graphene that poses a high specific surface area has a large enough area for pollutant removal and functionalization.

The modification of graphene surface area with specific functional groups can be of significant importance since it increases the area of interaction between graphene and the pollutants, resulting in high adsorption capacity. Pristine graphene is reported to be hydrophobic which makes it difficult to disperse in water, which can be improved through chemical modification by adding functional groups onto the surface (Wang et al. 2013), i.e., graphene functionalized into graphene oxide using Hummer methods shows a disrupted conjugation in the graphene plane and abundant functional groups, such as epoxide, hydroxyl, carboxyl, and carbonyl, on its surfaces (Lü et al. 2012). The presence of those oxygen-containing groups allows graphene oxide to act as adsorbents for a wide range of pollutants.

Graphene oxide is reported to be more effective in the removal of cationic organic pollutants than anionic pollutants due to the strong electrostatic repulsion between graphene oxide and anionic organic compounds (Ramesha et al. 2011). Graphene oxide exhibits high adsorption for basic compounds due to its several functional groups and strong acidity, while graphene possesses hydrophobic surface and presents high adsorption to organic pollutants due to strong π–π interaction (Chowdhury 2014). Large surface area and electron-rich environment make graphene-based materials to serve as an efficient adsorbent for organic pollutants.

The adsorption mechanisms of graphene-based nanomaterials can also be explained based on the type of interaction (i.e., π–π interaction, hydrophobic effect, H-bonding, and electrostatic interaction) between graphene and organic pollutants (Wang and Chen 2015). i.e., for graphene oxide, the presence of oxygen-containing functional groups on the edge can weaken the hydrophobic effect with nonpolar organics and suppress the adsorption on it. Table 3 shows examples of some reported results on adsorption studies of organic pollutants using graphene-based nanomaterials. The table only gives examples of few adsorption studies carried out on adsorption of various pollutants using graphene-based nanomaterials at different conditions; hence, it cannot be used for comparison of the efficiency of different graphene-based nanomaterials for adsorption of organic pollutants. Other parameters responsible for the adsorption of organic pollutants listed in Table 3 can be found in the cited references.

Table 3 Adsorption studies on the removal of organic pollutants from water using graphene-based nanomaterials.

Adsorption of inorganic pollutants

The presence of inorganic pollutants in the water system has become one of the most worldwide environmental issues threatening both developing and developed countries. Heavy metals (i.e., Lead, cadmium, nickel, cobalt, chromium, arsenic, iron, and zinc) as the major part of inorganic pollutants are non-degradable and have a tendency to bio-accumulate, and they pose serious health threats even at very lower concentrations (Baruah et al. 2016; Ahmad et al. 2019; Bashir et al. 2019). Those metals come from different industries such as tannery, electroplating, textile, fertilizer, pesticide, and metal processing industries as well as mining sectors. Most of them are non-biodegradable, highly toxic, and carcinogenic in nature (Malik et al. 2016; Talaiekhozani and Rezania 2017; Abdi et al. 2018). Therefore, the removal of heavy metals from water before releasing them into the environment is of greater importance. The maximum accepted concentration levels of some heavy metals are shown in Table 4 together with their corresponding effects on human health once the limits are exceeded.

Table 4 Maximum concentration levels of heavy metals in drinking water with their corresponding health effects.

Many technologies have been employed for the removal of heavy metals from wastewater; however, adsorption has proven to be an economical and efficient method for removing heavy metals (Karnib et al. 2014). Recently, the uses of carbon-based nanomaterials as adsorbents for the removal of heavy metals have received tremendous attention owing to their large specific surface area and concentrated pore-size distribution (Yu et al. 2018). Graphene-based and carbon nanotubes-based nanomaterials have been widely used in the removal of heavy metals from wastewater owing to their large surface area, scalable production, tunable surface chemistry, non-corrosive property, and the presence of a surface oxygen-containing functional groups (Xu et al. 2018b). In this section, graphene-based and carbon nanotubes-based nanomaterials used in the removal of heavy metals from wastewater are discussed with respect to their properties, and examples of some previously reported studies are given.

Graphene-based nanomaterials in adsorption of inorganic pollutants

Graphene-based nanomaterials have been widely used for the removal of toxic metals ions in wastewater owing to their large surface area. Graphene is reported to be insoluble and hard to disperse in all solvents due to strong van der Waals interactions that can hamper adsorption of metal ions; hence, a significant number of studies have been undertaken on functionalizing graphene to overcome such limitations. Moreover, graphene oxide that is the derivatives of graphene is much more hydrophilic than graphene; hence, it is the commonly used graphene-based nanomaterials for the removal of heavy metals in wastewater. In addition to large quantities of oxygen-containing atoms on the surface of graphene oxide in the forms of epoxy, hydroxyl, and carboxyl groups, the large surface area of graphene oxide enables it to have a large adsorption capacity toward heavy metals (Sitko et al. 2013).

The adsorption capacities of various heavy metals against graphene-based nanomaterials are shown in Table 5. Sitko et al. (2013) conducted studies on adsorption of Zn(II), Cu(II), Cd(II), Pb(II), Cr(VI), and U(VI) using graphene oxide, and their adsorption efficiency are shown in Table 5. The adsorption models were best fitted by Langmuir, suggesting that the adsorption of those heavy metals on graphene oxide is monolayer coverage. The type of adsorption was investigated by conducting kinetics studies, and it was found to be chemisorption involving the strong surface complexation of metal ions with the oxygen-containing groups on the surface of graphene oxide. The review conducted by Chowdhury also reported the results of batch studies on the use of graphene, cetyltrimethylammonium bromide (CTAB)-modified graphene, graphene/multi-walled carbon nanotubes amongst others for the removal of metals ions in wastewater and some parameters can be found on the cited reference (Chowdhury 2014).

Table 5 Adsorption of various inorganic pollutants using graphene-based nanomaterials

Carbon nanotubes in adsorption of inorganic pollutants

Carbon nanotubes have also been reported to possess excellent adsorption capacity for removal of heavy metal ions from large volumes of aqueous solutions since its discovery in 1991 by Iijima. However, although a significant number of studies have been undertaken on adsorption of heavy metals using carbon nanotubes, its real work is hindered by the lack of surface functional groups such as carboxylate, hydroxyl, sulfate, phosphate, amide, and amino groups (Shao et al. 2010). Recently, it has been reported that the oxidizing of carbon nanotubes using HNO₃, NaClO, and KMnO₄ solutions can help to overcome such drawbacks (Lata and Samadder 2016). To achieve better adsorption, the modification of carbon nanotubes must be selected based on the contaminant of interest. Carbon nanotubes surface modification can enhance the adsorption capacities of metals ions through the introduction of new functional groups that can increase the number of surface adsorption active sites.

The surface modifications include methods such as oxidization, chemical grafting, and physical modifications such as impregnating or coating (Zhang et al. 2016). Table 6 shows the maximum adsorption capacities of heavy metals using carbon nanotubes and their derivatives. Other parameters on the adsorption of heavy metals using carbon nanotubes-based nanomaterials can be found in cited references and some examples are shown in Table 6. Several researchers have modified carbon nanotubes to evaluate the efficiency of the former with the unmodified carbon nanotubes for the removal of various contaminants. From Table 6, it can be concluded that the modified carbon nanotubes show high adsorption capacity as compared to unmodified carbon nanotubes. This can be attributed to the presence of new functional groups onto the surface of carbon nanotubes, which increase the number of adsorption sites on the surface.

Table 6 Maximum adsorption capacities of selected heavy metals using carbon nanotubes-based nanomaterials.

The surface functional group of graphene oxide and oxidized carbon nanotubes can provide major adsorption sites to heavy metals removal, due to the presence of electrostatic attraction and chemical bonding such as ion exchange between adsorbent and adsorbate. In addition, the presence of a highly accessible adsorption site and short intra-particle diffusion distance can result in fast adsorption kinetics. Nevertheless, the adsorption of organic and inorganic pollutants using carbon-based nanomaterials is not only limited to the discussed nanomaterials and other nanomaterials such as activated carbon, fullerenes, etc., have also been used. Most of the carbon-based nanomaterials display similar properties toward the removal of both organic and inorganic pollutants through the adsorption process. The choice on which carbon-based nanomaterial to use is usually governed by the cost of production in terms of synthesis of nanomaterials, and the environmental implication in terms of toxicity.

3.1.3 Summary of adsorption mechanisms of organic and inorganic pollutants

The adsorption process is a very useful strategy for the remediation of organic and inorganic pollutants in water. Adsorbents having large adsorption capacity and adsorption rates are the most preferable. To achieve high adsorption efficiency of pollutants removal, a better understanding of adsorption mechanisms between the adsorbent and adsorbate is the key factor. Different physical forces usually govern their adsorption mechanisms. The main forces controlling adsorption removal are hydrophobic interactions, π–π bonds, electrostatic interactions, and hydrogen bonds (Crini et al. 2019; Tong et al. 2019). Different adsorbent interacts with adsorbate in different manners, depending on their properties such as structure and nature of pollutants and the associated functional groups available onto the surface of the adsorbent. When dealing with the element in adsorption that contains functional groups such as amine, hydroxyl, and carboxyl groups, hydrogen-bonding interaction plays an important role. Meanwhile, for adsorbent with charged functional groups, electrostatic interaction is predominant and hydrophobic interaction is usually dominant for adsorbent with non-polar hydrocarbons.

Carbon-based nanomaterials have shown a significant efficiency for the removal of both organic and inorganic pollutants in water. Carbon nanotubes show high adsorption capacity for the removal of organic pollutants due to their large surface area and high interactions between organic pollutants. The adsorption mechanisms between carbon nanotubes and organic pollutants usually occur through forces such as hydrophobic interactions, π–π stacking interactions, van der Waals forces, electrostatic interactions, and hydrogen bonding interactions, which might act individually or simultaneously (Gupta et al. 2013; Yu et al. 2014; Mashkoor et al. 2020). In addition, surface oxidation of carbon nanotubes can significantly enhance the adsorption efficiency of inorganic pollutants due to the introduction of carboxyl, hydroxyl, and phenol functional groups at the surface of carbon nanotubes.

The adsorption mechanisms of untreated carbon nanotubes and surface oxidation carbon nanotubes are shown in Fig. 5, where for untreated carbon nanotubes, adsorption occurs at the surface of the material, while for oxidative carbon nanotubes adsorption occur at the surface and also through the functional groups available at the surface. Those surface functional groups are the major adsorption site of inorganic pollutants through electrostatic attraction and chemical bonding (Ihsanullah et al. 2016; Madhura et al. 2019). In the case of adsorption of organic pollutants by graphene-based nanomaterials, the adsorption is determined by physisorption between the pollutants and the graphene surface, as such the larger the surface area the greater the adsorption capacity.

Fig. 5

Adsorption mechanisms of water pollutants by carbon nanotubes and oxidized carbon nanotubes. Oxidation treatment results in introduction of functional groups on the surface of carbon nanotubes

Figure 6 represents the general mechanisms of adsorption–desorption removal of organic and inorganic pollutants using graphene-based nanomaterials in water. Desorption mechanisms usually occur in the presence of either acid or base, depending on the nature and compatibility of the materials. The most dominant interaction between organic pollutants and graphene-based nanomaterials is π–π interaction (Chowdhury 2014; Khan et al. 2018). For the removal of inorganic pollutants, the availability of surface functional groups onto adsorbent is key factors determining the adsorption efficiency. Therefore, graphene derivatives such as graphene oxide and modified graphene show high adsorption capacities toward inorganic pollutants than pristine graphene (Lü et al. 2012). The most common dominant interaction in the adsorption mechanisms of inorganic pollutants by graphene derivatives is electrostatic attractions that are usually between positively charged ions and negatively charged functional groups on graphene derivatives surfaces are the predominant adsorption mechanism (Kemp et al. 2013).

Fig. 6

Mechanism of adsorption–desorption removal of organic and inorganic water pollutants using a graphene-based nanomaterials. Desorption mechanisms usually occur in the presence of either acid or base

Photocatalysis removal of organic pollutants

Recently, the removal of pollutants by advanced oxidation processes such as Fenton-like oxidation, photocatalysis, sulfate radical-based oxidation, ozonation have emerged as a promising techniques for mineralization of organic pollutants and toxic metal ions (Awfa et al. 2018; Ganiyu et al. 2018; Miklos et al. 2018; Pang et al. 2019). Amongst various advanced oxidation processes methods, photocatalysis has received tremendous attention in wastewater due to its stable, low set-up cost, simplicity of design, and complete mineralization of pollutants into unharmful by-products (Ahmed and Haider 2018; Bishoge et al. 2018). This principle was first applied by to reduce cyanide (CN⁻) in water (Acharya et al. 2018). This method has been reported to be an environmentally friendly approach and promising method to remove a wide number of pollutants in water due to its potential to completely mineralize harmful pollutants into un-harmful by-products using solar light irradiation. It has been widely applied for the degradation of organic pollutants; since most of the organic pollutants are highly resistant to biodegradation and recalcitrant to conventional biological and physical oxidation treatments due to their complex structure. In addition, it has also been applied for the reduction in metals iron such as highly toxic chromium(VI) to less toxic chromium(III). The general mechanisms behind photocatalytic degradation and photocatalytic reduction in organic pollutants and chromium(VI), respectively, are shown in Fig. 7. Photocatalytic reduction in chromium(VI) occurs when chromium(VI) interacts with the electrons in the conduction band and undergoes reduction process to give chromium(III) as shown by equation in Fig. 7, while photocatalytic degradation occurs when organic pollutants interact with holes in the valence band and undergo oxidation process.

Fig. 7

Photocatalytic reduction of chromium(VI) to chromium(III) and photocatalytic degradation of organic dyes. Both degradation (oxidation) and reduction processes commonly take place on the surface of the photo-excited semiconductor photocatalyst

Generally, based on the electronic structure and energy bandgap (E_g) of the materials, photocatalysts use ultraviolet (UV) or visible light to activate catalysis. This technique does not require any secondary disposal, and also no trace of secondary pollutants are generated (Gunti et al. 2018). Its mechanism is based on the oxidative and reductive reactions which take place at the surface of the semiconductor materials (SMs) as shown by Eq. 2–10 (Ajmal et al. 2014). The graphical representation of mechanisms behind the photocatalytic degradation of organic pollutants is also shown in Fig. 8. This process is initialized when the light energy-carrying photon energy greater or equal to the energy bandgap of the semiconductor material is illuminated onto the surface of the photocatalysts, then the electrons (e⁻) from the valence band (VB) get excited and promoted to the conduction band (CB), leaving the holes (h⁺) in valence band. The exciting of an electron from valence band to conduction band results in the generation of electron–hole pairs.

$${\text{Semiconductor material}} + h\nu \left( {\text{UV/Visible}} \right) \to {\text{Semiconductor material}}\left( {e^{ - } \left( {{\text{CB}}} \right) \, + \, h^{ + } \left( {{\text{VB}}} \right)} \right)$$

(2)

Fig. 8

Photocatalytic mechanism of photocatalytic degradation of organic pollutants presents in water. The most common degradation by-products is water and carbon dioxide

The photo-generated holes at the valence band then react with water to produce OH^· radical:

$${\text{H}}_{2} {\text{O}}\left( {{\text{ads}}} \right) \, + \, h^{ + } \left( {{\text{VB}}} \right) \to {\text{OH}}^{ \cdot } \left( {{\text{ads}}} \right) + {\text{H}}^{ + } \left( {{\text{ads}}} \right)$$

(3)

Formed OH^· radical on irradiated semiconductor surface is strong oxidizing agents and can attack adsorbed organic molecules or those that are near to the catalyst surface, causing them to mineralize to an extent depending upon their structure and stability level. Meanwhile, electrons in the conduction band react with oxygen to generate anionic superoxide radical (O₂⁻):

$${\text{O}}_{2} + e^{ - } \left( {{\text{CB}}} \right) \to {\text{O}}_{2}^{ - \cdot } \left( {{\text{ads}}} \right)$$

(4)

The produced superoxide ions then take part in the oxidization process, and they also help in suppressing electron–hole recombination. Then, superoxide (O₂^−·) gets protonated forming hydroperoxyl radical (HO₂^·) and then subsequently H₂O₂ which further dissociates into highly reactive hydroxyl radicals (OH^·). Then, the hydroxyl-radical species attack vital organic components and decompose them through oxidation reactions:

$${\text{O}}_{2}^{ - } \left( {{\text{ads}}} \right) + {\text{H}}^{ + } \leftrightarrow {\text{HOO}}^{ \cdot } \, \left( {{\text{ads}}} \right)$$

(5)

$$2{\text{HOO}}^{ \cdot } \left( {{\text{ads}}} \right) \to {\text{H}}_{2} {\text{O}}_{2} \left( {{\text{ads}}} \right) + {\text{O}}_{2}$$

(6)

$${\text{H}}_{2} {\text{O}}_{2} \left( {{\text{ads}}} \right) \to 2{\text{OH}}^{ \cdot } \left( {{\text{ads}}} \right)$$

(7)

$${\text{Pollutants}} + {\text{OH}}^{ \cdot } \to {\text{pollutants}}\;{\text{intermediates}}$$

(8)

$${\text{Pollutants}} + h^{ + } \left( {{\text{VB}}} \right) \to {\text{oxidation}}\;{\text{products}}$$

(9)

$${\text{Pollutants}} + e^{ - } \left( {{\text{CB}}} \right) \to {\text{reduction}}\;{\text{products}}$$

(10)

Both oxidation and reduction processes commonly take place on the surface of the photo-excited semiconductor photocatalyst. Since the principle of photocatalytic removal of pollutants from water is a heterogeneous reaction at the liquid–solid interface, the numbers of active sites, as well as the surface area, are the key factors for the catalytic reaction.

As far as photocatalysis is a concern, carbon-based nanomaterials have received tremendous attention as photocatalysis for environmental pollution remediation. Amongst various carbon-based nanomaterials, graphene, carbon nanotubes, and graphitic carbon nitrate are commonly used nanomaterials for this field due to their unique properties (Kumar et al. 2017). In addition, the utilization of carbon-based nanomaterials in synergy with catalyst nanoparticles has been widely explored. Many studies have been carried out on the incorporation of catalytic nanoparticles within the metric of carbon-based nanomaterials. The incorporation results in improved photocatalysis performance of both catalytic and carbon-based nanomaterials. Carbon-based nanomaterials enhanced the photocatalytic degradation of catalyst nanoparticles through (1) increasing the adsorption site, (2) energy bandgap reduction and (3) suppressing the recombination rate of electron and hole generated which are major drawbacks of current used catalytic nanoparticles (Shan et al. 2017; Kumar et al. 2019).

Those carbon-based nanomaterials can also act as supporting material for other semiconductor nanomaterials that can ease the separation process of catalyst from the solution after degradation (increase the reusability of the catalyst). More recently, significant numbers of studies have been conducted on the use of carbon-based nanomaterials and their derivatives together in synergy with catalytic nanoparticles for the photodegradation of organic pollutants in water using solar light irradiation (Melchionna et al. 2016; Piovesana et al. 2016). This section will discuss the use of carbon-based nanomaterials together with their derivatives in the photocatalytic degradation of organic pollutants in water.

Graphene-based nanomaterials in photocatalysis

Graphene-based nanomaterials possess some unique properties such as ultrafast electron transfer, high adsorption capacity, high thermal conductivity (~ 5000 W m⁻¹ K⁻¹), extended light absorption range, and enhanced charge separation and charge transport properties which are of great importance in field of photocatalytic degradation of organic pollutants (Kemp et al. 2013). Those superior properties make graphene-based nanomaterials and their derivatives to be promising candidates for photocatalytic degradation of organic pollutants in wastewater. Since graphene is a zero bandgap material, it is usually coupled with other semiconductors catalytic materials. When coupled with other materials, it can enhance the photocatalytic performance by acting as an electron acceptor and transfer which lowers the recombination rate of electron and hole generated and can also increase the light absorption range owing to its exceptional electronic, optical, and mechanical properties (Kumar et al. 2017; Qiu et al. 2018). As represented in Fig. 9, the electrons generated from semiconductor nanomaterials upon light irradiation are transferred to the graphene matrix, which lower the recombination rate of electron–hole generated. Then, those electrons interact with oxygen molecules to form superoxide (O₂^−·) which then gets protonated to form hydroperoxyl radical (HO₂^·) and then subsequently H₂O₂ which further dissociates into highly reactive hydroxyl radicals (OH^·) which are responsible for photodegradation of pollutants. The availability of hydroxyl radicals from both graphene and semiconductor nanomaterials results in enhanced photocatalytic activities.

Fig. 9

Photocatalytic mechanisms representing the electron transfer between graphene and other semiconductors (SMC) nanomaterials and the generation of hydroxyl radicals responsible for degradation of various pollutants. Electrons transfer helps in reducing recombination of the photo-induced electrons and holes

Since the overall photocatalytic efficiency is significantly hindered by the fast electron–hole recombination and low light absorption, the coupling of graphene or its derivatives with other semiconductor materials can expand the light absorption range from ultraviolet region to visible region, changing bandgap of other material, and suppressing the electron–hole recombination (Li et al. 2018). Graphene modification and the coupling with other materials contribute to the formation of efficient visible active photocatalysts. In this regard, a significant number of studies have been undertaken on the synthesis of graphene-based nanomaterials and graphene-based-semiconductor-based nanomaterials for environmental remediation.

Du and Deng synthesize N-doped TiO₂ nanoparticles deposited on reduced graphene oxide (rGO) for the visible-light degradation of tetracycline hydrochloride (TC). The synthesized N-TiO₂/rGO shows an enhanced photocatalytic activity as compared to pure TiO₂ and N-TiO₂. The enhanced visible-light photodegradation activity of TC over N-TiO₂/rGO was ascribed to the synergistic effects of enhanced adsorption of tetracycline hydrochloride on reduced graphene oxide, expanded visible light absorption, and efficient charge transport and separation (Du and Deng 2015). CdS/rGO composite was prepared by Gao et al. using a simple hydrothermal process. The synthesized composite exhibits high adsorption and degradation efficiency of Rhodamine B (RhB) dye under visible light irradiation. The enhancement in photocatalytic activity was caused by a decrease in charge recombination, as well as the effective adsorption of Rhodamine B on reduced graphene oxide (Gao et al. 2018). Zhao et al. synthesize BaTiO₃/GO nanocomposites via the freeze-drying method for the photocatalytic degradation of methylene blue. It was founded that BaTiO₃/GO nanocomposites exhibit higher photocatalytic activities toward the degradation of methylene blue (MB), which was mainly ascribed to the appropriate energy band structure and effective adsorption function due to the presence of graphene oxide (Zhao et al. 2018). Other graphene-based nanomaterials which have been studied for the photocatalytic degradation of various pollutants under the visible light are GO/TiO₂, GR/TiO₂, N, S/TiO₂/RGO, Ag/TiO₂/GO, Ag/TiO₂/RGO, TiO₂/NiO/RGO, RGO/Fe₃O₄/Ag₃PO₄, and Cu₂O/RGO (Nikokavoura and Trapalis 2018).

Carbon nanotubes-based nanomaterials in photocatalysis

Carbon nanotubes have been used as a promising co-catalyst in photocatalysis due to their ability to accept, transport, and store electrons and thus suppress the recombination of electrons with the holes (Ye et al. 2012); which arise as results of high electrical conductivity, excellent electrocatalytic activity, and large specific surface area (Liu et al. 2018). These interesting properties make carbon nanotubes to be the superior catalyst for supporting materials. Meanwhile, the application of carbon nanotubes as a catalyst in large-scale operation is hindered by its high production cost. Carbon nanotubes have been widely coupled with the well-known photocatalyst semiconductors such as ZnO and TiO₂ for better photocatalytic activities as shown in Fig. 10 (Liu et al. 2014; Ahmadi et al. 2017). The photocatalytic activities of those metal-oxide nanomaterials are hindered by some drawbacks such as high energy bandgap and low charge carrier separation, which can be overcome by coupling metal-oxides with carbon-based nanomaterials such as carbon nanotubes (Mallakpour and Khadem 2016). When carbon nanotubes are coupled with other metals oxide, it can help to lower the recombination rate of electron–hole generated as displayed in Fig. 10 and also increase the number of hydroxyl radicals available for degradation of pollutants. Hence, coupling carbon nanotubes with semiconductor materials shows the synergistic influence that causes better photocatalytic efficiency.

Fig. 10

Photocatalytic mechanisms of carbon nanotubes-based nanomaterials representing the transfer of electron between semiconductor nanomaterial (SMC) and carbon nanotubes matrix for degradation of pollutants

Various carbon nanotubes-based photocatalysts for the removal of organic pollutants are shown in Table 7 together with their synthetic methods, targeted organic pollutants, removal efficiency, and degradation time. Shafei and Sheibani synthesized Cu–TiO₂–CNT using a sol–gel method for removal of the methylene blue in water. 81.5% removal of methylene blue was achieved in 60 min under visible light irradiation (Shafei and Sheibani 2019). TiO₂ absorbs in the ultraviolet region of the electromagnetic spectrum, but to the introduction of carbon-based matrix-like carbon nanotubes, the photocatalyst was able to degrade methylene blue under visible region, showing that the presence of carbon nanotubes helps in reduction in the energy bandgap of TiO₂. Lin et al. manage to photo degrade malachite green (MG) using Ag₃PO₄@MWCNT@Cr:SrTiO₃ in 10 min (Lin et al. 2018). The high efficiency was attributed to the change in surface properties and morphology of Ag₃PO₄ due to the addition of multi-walled carbon nanotubes that introduce more surface-active site for degradation.

Table 7 Results on various carbon nanotubes-based nanomaterials photocatalysts used for the photocatalytic degradation of organic pollutants together with their synthesis methods, removal efficiency, and irradiation time

Yazdani and Mehrizad also synthesized Ag–ZnS–MWCNT using ultrasonic probe methods for the degradation of Rhodamine B under visible light irradiation and achieve 87.53% removal in 116 min (Yazdani and Mehrizad 2018). The high degradation efficiency was attributed to being due to the excellent electronic property of multi-walled carbon nanotubes that help in suppressing the recombination of electron and hole generated. Chidambaram et al. used ZnO–Au/MWCNT, which was synthesized using the wet-chemical method for the degradation of methylene blue and achieved 98% removal in 40 min under visible light irradiation. The efficiency was attributed to the incorporation of multi-walled carbon nanotubes in ZnO–Au, which enhanced the visible light absorbance and effective separation of electron–hole pairs, leading to higher catalytic efficiency (Zhang and Jaroniec 2018).

Chen et al. (2014) used the solvothermal process to synthesized novel flower-like MWCNT/BiOBr photocatalyst that exhibited a conspicuous improved photocatalytic performance for the degradation of Rhodamine B under visible light irradiation. The improved photocatalytic performance was due to the reduction in electron–hole pair recombination as a result of the presence of multi-walled carbon nanotubes. Hosseini et al. (2018), aiming to shift the band edge toward longer wavelength, developed a novel Ag-doped ZnO nanoparticle incorporated onto multi-walled carbon nanotubes. The nanomaterial was used for the photocatalytic degradation of phenol, and 81% removal was achieved in 240 min. The results were explained based on shifting of band edge toward longer wavelength due to the presence of Ag dopant and enhancement of visible light absorbance and effective separation of electron–hole pairs due to the presence of multi-walled carbon nanotubes.

Graphitic carbon nitrate in photocatalysis

Recently, a fascinating π-conjugated polymeric graphitic carbon nitrate semiconductor with a suitable electronic structure has been discovered and widely applied in the field of catalysis (Liu et al. 2017). This metal-free polymeric photocatalyst has been a promising candidate in photocatalytic application due to its interesting properties such as medium energy bandgap with the good visible light response (2.7 eV), low production cost, easy availability, high stability, non-toxic as well as easily tailorable structure (Jiang et al. 2017; Liu et al. 2017). It exhibits good photocatalytic activity for the water splitting and the degradation of environmental pollutants under visible light irradiation. Nevertheless, its practical application is restricted by small specific surface area which leads to a fast recombination rate of the photo-induced electrons–holes pair and insufficient solar-light absorption which results in low photocatalytic activities (Cai et al. 2017). In this regards, a significant number of studies such as doping with nonmetal and metals species, morphology control, combination with conductive materials, nanocomposite structure construction with other semiconductors and dye sensitization have been done to enhance the photocatalytic activities of pristine graphitic carbon nitrate (Jiang et al. 2017; Asadzadeh-khaneghah et al. 2018; Mousavi and Habibi-yangjeh 2018).

A variety of organic pollutants such as dyes, phenol, atrazine, and humic acid have been degraded using graphitic carbon nitrate and its derivatives, indicating that graphitic carbon nitrate has the photocatalytic ability to degrade different kinds of organic contaminants from water and wastewater (Xu et al. 2018a). Due to the drawbacks of pristine graphitic carbon nitrate, it usually used in synergistic with other semiconductor nanomaterials to achieve enhanced photocatalytic activities. Figure 11 represents the photocatalytic mechanisms of graphitic carbon nitrate coupled with titanium oxide, where the electrons and holes are transferred between the two semiconductors nanomaterials, which is of significance to overcome the fast recombination rate of electron–hole pairs and also critical for high photocatalytic efficiency.

Fig. 11

Photocatalytic mechanisms of graphitic carbon nitrate (g-C₃N₄) coupled with titanium oxide (TiO₂) for enhanced photocatalytic activities through lowering recombination of electron–hole generated

Meanwhile, some examples of graphitic carbon nitrate-semiconductor-based nanomaterials used for photodegradation of organic pollutants are given in this section. Hao et al. (2017) synthesize g-C₃N₄/TiO₂ using the hydrothermal method for the removal of Rhodamine B. The synthesized g-C₃N₄/TiO₂ shows high photocatalytic degradation of Rhodamine B which was due to the increase in surface area and synergistic heterojunction structure between g-C₃N₄ and TiO₂. Cobalt-doped g-C₃N₄ synthesized by Wang et al. (2019) shows the complete mineralization of Rhodamine B in 25 min. The complete mineralization was explained to be due to the effective charge separation of photo-generated electrons and holes and also due to the sufficient availability of accessible active sites. Shi et al. (2018) synthesize g-C₃N₄/MoS₂ composite using ultrasonic-assisted exfoliation for the removal of Methyl orange. The synthesized g-C₃N₄/MoS₂ photodegradation 96.5% of methyl orange, which was ascribed to the availability of active sites, aligned energy levels of g-C₃N₄ and MoS₂ and also the efficient charge transfer. Ismael et al. (2018) also manage to remove 80% of methyl orange in 5 h using yFeO₃/g-C₃N₄. The enhanced photodegradation was due to the improved charge separation of photo-generated electrons and holes.

Xiao et al. (2018) synthesized WO₃/g-C₃N₄ using hydrolysis and polymerization process. The synthesized WO₃/g-C₃N₄ hollow microsphere was tested for the photodegradation of tetracycline hydrochloride (TC-HCl) and ceftiofur sodium (CFS). The removal efficiencies were reported to be 82% and 70% for tetracycline hydrochloride and ceftiofur sodium in 2 h, respectively. The mechanisms suggest that enhanced photodegradation was due to the trapping of incident light within the hollow structure and also the higher specific surface area. 80.2% removal of tetracycline was achieved by Jiang et al. (2018) using WO₃/g-C₃N₄/Bi₂O₃ in 60 min. The photocatalytic mechanisms predict that the enhanced degradation can be due to improved visible light irradiation, increased surface area and enhanced charge separation of photo-generated electrons and holes pair. Liu et al. (2019) synthesized g-C₃N₄/Ti₃C₃ using the evaporation-induced self-assembly method. The synthesized g-C₃N₄/Ti₃C₃ shows high degradation efficiency of Ciprofloxacin which was reported to be due to the suppressed recombination of electrons and holes pair and better photo-generated charge separation and migration performance. With respect to the above-reviewed article, it can be concluded that graphitic carbon nitrate and its derivatives play a crucial role in photocatalytic degradation of a wide number of organic pollutants; hence, it has received tremendous attention. The use of carbon-based nanomaterials in photocatalytic removal of organic pollutants is not only limited to the above-missioned literature. Awfa et al (2018) have conducted a critical review on the use of carbonaceous-TiO₂ nanomaterials in photocatalytic removal of emerging pollutants such as personal care products and pharmaceuticals (Awfa et al. 2018).

Conclusion

The presence of organic and inorganic pollutants in our water system is an issue of greater concern across the globe. Those pollutants pose a server threat to both living organisms and ecosystems; hence, their removal from the water system is of greater importance. Although a significant number of processes have been employed for the remediation of pollutants from wastewater, the use of nano-based technology such as adsorption and photocatalysis have been gaining popularity in this field. Adsorption and photocatalysis as nano-based technology hold a substantial promise for overcoming the limitations faced by other conventional methods. The uses of carbon-based nanomaterials have received significant attention as catalysts and as adsorbents for the remediation of organic and inorganic pollutants in wastewater owing to their excellent properties and their availability.

This review highlighted the use of carbon-based nanomaterials and their derivatives as adsorbents as well as catalysts for the remediation of organic and inorganic pollutants from wastewater. Only three types of carbon-based nanomaterials are discussed in this review. However, the application of carbon-based nanomaterials for wastewater remediation is not only limited to the discussed carbonaceous nanomaterials, but other carbonaceous nanomaterials have also been used. The uses of these nanomaterials as adsorbent and catalyst have proved astonishing properties in both laboratory scale and real wastewater treatment. The availability of active sites on the surface of material and strong interactions between the active adsorption sites and the pollutants results in the enhanced adsorption capacity of the materials. In the photocatalytic processes, enhanced efficiency is attributed to the improved light harvesting and reduced recombination of the photo-induced electrons and holes. However, for successful commercialization, further modification on these nanomaterials is needed in order to improve their properties, scaling-up production and reduce the cost-effectiveness.