Carbon-based sustainable nanomaterials for water treatment: State-of-art and future perspectives

Abstract

The supply of safe drinking and clean water is becoming increasingly challenging proposition throughout the world. The deployment of environmentally sustainable nanomaterials with unique advantages namely high efficiency and selectivity, earth-abundance, recyclability, low-cost of production processes, and stability, has been a priority although several important challenges and constraints still remained unresolved. Carbon nanomaterials namely activated carbon, multi-walled- and single-walled carbon nanotubes, have been developed and applied as adsorbents for wastewater treatment and purification; graphene and graphene oxide-based nanomaterials as well as carbon and graphene quantum dots-derived nanomaterials have shown significant promise for water and wastewater treatment and purification, especially, for industrial- and pharmaceutical-laden wastes. This review encompasses advanced carbonaceous nanomaterials and methodologies that are deployed for the elimination of contaminants and ionic metals in aqueous media, and as novel nanosorbents for wastewater, drinking and ground water treatment. Additionally, recent trends and challenges pertaining to the sustainable carbon and graphene quantum dots-derived nanomaterials and their appliances for treating and purifying wastewater are highlighted.

GRAPHICAL ABSTRACT

Advanced nanomaterials for water treatment comprising carbon nanotubes, carbon- and graphene quantum dots and graphene-based nanomaterials, are highlighted.

1. Introduction

Water is one of the foremost environmental stress issues as resources of water are limited, and the population which depends on these constrained supplies is predictably growing. Conventional technologies for treating wastewater include ion-exchange, reverse osmosis, oxidation, adsorption, flocculation, ultra-filtration, sedimentation, membrane, and advanced oxidation processes (AOPs). Wastewater treatment approaches, including bio-processes, UV photolysis/photocatalysis, activated carbon adsorption, and ozonation, have been deployed for removing organic contaminants and pollutants from waters (Cai et al., 2018; Colmenares et al., 2016; Khan et al., 2020a; Nasrollahzadeh et al., 2020a; Padhye et al., 2014; Pakzad et al., 2019).

Nanomaterials can be employed for removing organic pollutants and pharmaceutically active compounds via processes such as adsorption, photocatalysis, AOPs, filtration, etc. (Cai et al., 2018; Nasrollahzadeh et al., 2020b, 2020c). There are numerous emerging contaminants in wastewater discharge which have adverse health effects namely pesticides, textile dyes, plasticizers, disinfection byproducts, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), various endocrine disrupting materials, pharmaceutical and personal care products (Bousselmi et al., 2004; Mozia et al., 2007; Rizzo et al., 2009). Per- and polyfluoroalkyl substances (PFASs) contaminants can be removed from aqueous environments by carbonaceous nanomaterials as adsorbents; electrostatic and hydrophobic interactions, hydrogen bonding and ligand exchange being the main mechanisms for the adsorption of PFOS and PFOA (Liu et al., 2019; Saleh et al., 2019). In this regard, innovative engineered nanomaterials are very promising for removing these hazardous contaminants, as they have high surface areas and remarkable reactivities (Zhang et al., 2019).

The development of “greener” and environmentally sustainable techniques namely using less hazardous and toxic solvents, the deployment of less chemical reagents, precursors and catalysts with fewer steps in the production pathways have been the core emphasis lately. Green chemistry and nanotechnology can potentially be applied to address emerging and critical challenges posed by various contaminants and microorganisms; novel and suitable approaches for safer destruction of hazardous contaminants and pollutants is imperative. Green nanotechnology may enhance the environmental sustainability of underlying procedures producing negative externalities, as attested by the application of renewable energy and hydrogen for water treatment. It has a significant potential for decreasing the costs and enhancing the efficiency in prevention and treatment of wastewater contaminants as exemplified by nanoscale filtration techniques, adsorption of pollutants on nanoparticles (NPs), and breakdown of contaminants by nanocatalysts (Carpenter et al., 2015; Chorawalaa and Mehta, 2015; Cloete et al., 2010; Ma et al., 2011; Meng et al., 2011; Nasrollahzadeh et al., 2020a; Sajjadi et al., 2020; Shatkin et al., 2013; Xie et al., 2005; Zhang et al., 2014, 2016). The need for potable fresh water is significantly increasing in both, developing and industrialized world and assorted micropollutants are entering in the water resources (Ali et al., 2017; Das et al., 2017, 2018; Khin et al., 2012; Simeonidis et al., 2016). Contemporarily, the conventional decontamination methods (e.g., chlorination and ozonation) utilize excessive amounts of chemicals, and can generate hazardous and toxic by-products (Gehrke et al., 2015; Sajjadi et al., 2020; Varma, 2016; Westerhoff et al., 2016).

Nanomaterials with their high surface area, significant chemical reactivity, mechanical properties, cost-effectiveness, and low power consumption, can meaningfully be applied for water treatment and remediation. These materials, endowed with well-defined and controllable morphologies of proper size and porosity, can be potential good adsorbents (Perez-Page et al., 2016; Yan et al., 2020).

Indeed, advanced nanomaterials are being applied in the development of economical and high-performance water treatment systems for ‘real-time’ and continuous monitoring of water quality; nanomaterials have been extensively utilized for the remediation, prevention and treatment of effluence. These recyclable materials can preserve the long-term water quality, accessibility and feasibility of water supply while constantly detecting the biological and chemical contamination from man-made, municipal or industrial waste. The next section discusses nanomaterials comprising graphene-based nanomaterials, carbon-based nanotubes, carbon- and graphene-derived quantum dots for the treatment of wastewater and removal of pharmaceutical contaminants (Fig. 1).

Fig. 1.

Specific advantages of diverse carbon-based nanomaterials.

It appears that these nanomaterials with their unique advantages can be considered as promising alternatives for wastewater treatment, although some significant challenges still persist namely potential health risks, higher cost of production, specific selectivity, sustainability, and recyclability (Lu and Astruc, 2020). Moreover, additional detailed studies should be focused on standardized analytical methodologies, removal kinetics, simulation models, and evaluation of environmental behaviors of hazardous contaminants, toxicity issues and the environmental effects of applied nanomaterials. Consequently, researchers have focused on the production of highly efficient and environmentally sustainable nanomaterials at an affordable cost for eco-friendly treatment of waters and wastewaters, especially comprising pharmaceuticals, endocrine disrupting chemical materials, common industrial wastes, pesticides, hazardous organic dyes, personal care products, detergents, and other emerging and recalcitrant pollutants (Lu and Astruc, 2020; Mukherjee et al., 2020).

This review underscores the advanced sustainable nanomaterials for the elimination of aqueous pollutants and contaminants using carbon-based nanomaterials, namely carbon nanotubes, graphene and its oxide, including carbon- and graphene quantum dots.

2. Carbon nanotubes (CNTs)

2.1. Chemistry and properties

High aqueous solubility of drugs can lead to their adsorption in target tissues/cells in humans and animals or plants; often they are extremely resistant to biodegradability and are stable under normal conditions. Each year, more than thousand tons of active drug and pharmaceutical components enter the environment, a large proportion being the unconsumed pharmaceuticals. Further, they are converted to active components anew during metabolism processes (Carballa et al., 2004) and urban sewage is especially contaminated by diverse pharmaceutical contaminants. Carbon-based (nano)materials have interestingly large surface-to-area ratio, superior chemical stability, low cost, and reduced chemical impact on the environment, and these properties render them favorable for the treatment (Hu et al., 2011; Konicki et al., 2012; Wei et al., 2013; Yang et al., 2011). Carbon nanotubes (CNTs), and carbon allotropes with a cylindrical nanostructure, can be applied for eliminating the pharmaceutical-derived contaminants (Cong et al., 2013; Das et al., 2014a; Ji et al., 2009; Qu et al., 2013) or their metabolites (Table S1); they are superior nanosorbents for the removal of organic/inorganic pollutants in comparison to common sorbents e.g. zeolite, clay, activated carbon and diatomite (Upadhyayula et al., 2009). CNTs as good adsorbents have well-defined cylindrical hollow structures, stronger physicochemical interactions, high aspect ratios, large surface area, high sorbent capacity, hydrophobic side and easily modifiable exteriors (Das et al., 2014b).

2.2. Applications for water treatment

CNTs and CNT-based composites exhibit the capacity for dye adsorption from aqueous systems which include multi-walled nanotubes (MWCNTs) and single-walled nanotubes (SWCNTs); later demonstrated superior adsorption capacity than MWCNTs (Gupta et al., 2013; Liu et al., 2013a, 2013b). Though, CNTs have interesting edge over activated carbon, their large-scale applications for sizeable wastewater treatment is not favorable due to the excessive manufacturing costs (De Volder et al., 2013). In addition to high surface area, CNTs have modifiable surfaces and are endowed with significantly measurable adsorption sites located on their outer and/or inner layer surfaces; theoretically a favorable third generation of carbon-based adsorbents. CNT-based composites exhibit remarkable wastewater remediation capabilities for multiple organic/inorganic and biological water contaminants (Upadhyayula et al., 2009); CNTs function as scaffolds in water purification as illustrated in Fig. 2. Because of their hydrophobic surface, however, they should be stabilized in aqueous suspensions for avoiding aggregation which can reduce their active surface area (De Volder et al., 2013).

Fig. 2.

Functions of CNTs as scaffolds in wastewater remediation technologies.

Fugetsu et al. (2004) reported the first application of the encapsulated MWCNTs for removing ionic dyes (Fig. S1), wherein Ba²⁺-alginate background constituting an enclosure that preserves the entombed MWCNTs. Apparently, cages carry negative charge on their surface which restricts the capture of the large molecular weight anions e.g. humic acids, due to electrostatic repulsion. Thus, Ba²⁺-ALG/MWCNT composite could be used for the elimination of eosin bluish, acridine orange, orange G, and ethidium bromide with the adsorption efficiency of 0.44, 0.31, 0.33 and 0.43 μmol mg⁻¹, respectively. Ghaedi et al. (2011) studied the adsorption efficiency of activated carbon and MWCNTs for Eriochrome Cyanine R at diverse pH, electrolyte, temperature, initial dye concentration and adsorbent amounts, where pH is a significant factor controlling adsorption. Higher adsorption capacities are attainable expeditiously by MWCNTs (95.2 mg g⁻¹) with respect to activated carbon (40.6 mg g⁻¹). In another study, alkali-activated high surface area CNTs, with large number of mesoporous, were applied for the elimination of methyl blue and MO with high adsorption capacities of 399 and 149 mg g⁻¹, respectively (Ma et al., 2012); several studies have ensued for the removal of dyes using CNTs from the textile industries wastewaters (Mishra et al., 2010). Qu et al. (2008) reported a wet chemical process for combination of MWCNTs with Fe₂O₃ NPs for decontamination of neutral red and MB from polluted water (Qu et al., 2008); adsorption capacities being 77.5 and 42.3 mg g⁻¹, respectively. Similarly, Gong et al. (2009) researched the elimination of neutral red, MB and brilliant cresyl blue deploying magnetic MWCNT nanocomposite which could effectively degrade cationic dye pollutants in water. Gao et al. (2013) offered a magnetic polymer MWCNT nanocomposite for remediation of anionic azo dyes. Several CNT-based nanosorbents have been reported for the adsorption of dyes e.g. CNTs-cellulose, CNTs-graphene, CNTs-Fe₃O₄, CNT-chitosan and CNT-activated carbon fiber, among others (Ai and Jiang, 2012; Gupta et al., 2013; Rajabi et al., 2017).

CNT sheets can be utilized to remove toxic heavy metals e.g. cadmium (AlSaadi et al., 2016), lead (Kabbashi et al., 2009), copper (Jeon et al., 2010; Ouni et al., 2019), zinc (Pyrzynska and Stafiej, 2012; Zhao et al., 2015), arsenic (Naghizadeh et al., 2012) and cobalt (Ouni et al., 2019; Wang et al., 2011); the efficiencies achieved were 94 for Cd²⁺, 85 for Pb²⁺ and more than 50 percent for Co²⁺ from contaminated water besides being effective for the removal of Cu and As. The low adsorption capacities for toxic heavy metals using raw CNTs could be considerably increased up on oxidation by KMnO₄, HNO₃ and NaOCl as shown by Kuo and Lin (2009) for the efficient adsorption of aqueous Cd²⁺ pollutants onto modified MWCNTs; microwave (MW)-assisted oxidation by oxidants and/or acids demonstrated that the Cd²⁺ adsorption capacity on MW/KMnO4/H2SO4-modified MWCNTs was increased in comparison with MW/H₂SO₄-modified CNTs. In another study with CNT sheets, exclusion of various heavy divalent metals from aqueous solutions was reported (Tofighy and Mohammadi, 2011); sheets were prepared by chemical vapor deposition of ferrocene and cyclohexanol under N₂ environment at 750 °C followed by room temperature oxidation with conc. nitric acid. The adsorption of Cr(VI) pollutants from drinking water could be achieved by immobilizing ceria NPs on aligned CNTs (CeO₂/ACNTs) as a novel adsorbent (Di et al., 2006). The chemical transformation of CeCl₃, with aqueous NaOH in aligned CNT tailed by thermal treatment, afforded the adsorbent whose maximum adsorption capacity, as investigated by Langmuir model at pH 7.0, was found to be 30.2 mg g⁻¹. Furthermore, oxidized MWCNTs and SWCNTs using a NaClO solution were deployed for Ni²⁺ adsorption from aqueous solutions (Lu and Liu, 2006) including HNO₃ oxidized MWCNTs (Chen and Wang, 2006). A detailed investigation on the Pb²⁺ adsorption was undertaken using MWCNTs (Yu et al., 2013); apparently, MWCNTs with a higher O2 content and smaller diameter were shown to have greater lead removal ability. In contrast to conventional materials, CNT-based nanosorbents have grown as excellent materials for the remediation of diverse pollutants as shown in Table S2 (Kumar et al., 2014); an array of contaminants or heavy metals has been reported with diverse surface chemistry and physical structures.

Metal ions are adsorbed by CNTs via chemical bonding or electrostatic attraction (Rao et al., 2007) and additionally, they show antimicrobial characteristics by triggering oxidative stress in bacteria and abolishing the cell membranes. Interestingly, oxidative chemistry occurs with no generation of hazardous or toxic byproducts, which is a salient advantageous feature over established disinfection approaches besides easy renewal of CNTs via suitable regulations of conditions (e.g., pH) (Li et al., 2013b). Indeed, conventionally applied desalination approaches are high energy-demanding and technically challenging, while adsorption-centered approaches can find appliances at point-of-use water decontamination gadgets, although their competence for removing salts is slightly restricted. Plasma-altered ultra-long CNTs have been produced with very high adsorption capacity (>400% by weight) specifically for salt (Xing et al., 2013) relative to conventional carbon-based water treatment systems; they can be applied in multimodal membranes for removing salt, organics, and metal contaminants thus comprising an integral part of next-generation potable water purification strategies for desalination, disinfection, and filtration (Hashim et al., 2012).

Due to exceptional electrical, optical and mechanical properties, graphitic carbon-based nanocomposites (particularly CNTs) can serve as a promising building block for hybrid nanocatalysts thus increasing their performances in wastewater treatment. Semiconductors or metallic NPs has been discussed for the photoreduction of diverse organic/inorganic contaminants (Alwash et al., 2018; Gan and Qiao, 2012; Hoffmann et al., 1995); although several semiconductors e.g. TiO₂, CdS and ZnO, possess low quantum efficiency, large band gap and slow ultraviolet photoresponse. CNTs have high electron-storage capacity (Kongkanand and Kamat, 2007) and can extend the light sorption region and increase the quantum ability of hybrid nanocatalysts, due to their notable chemical stability, hollow and high specific surface area, unique electronic structure and high absorbability (Liu et al., 2018; Woan et al., 2009; Xu et al., 2010, 2015; Zhang et al., 2018). Thus, novel CNT-based photo (nano)catalysts can be effectively deployed as ideal catalyst supports in wastewater remediation. Lee et al. (2011) fabricated the N-doped CNTs/TiO₂ nanowires of core-shell configuration for light-mediated biomineralization of MB by TiO₂ nanoshells (~5 nm) at a surface of graphitic carbon; direct contact of TiO₂ nanoshell and NCNTs surface with no adhesive interlayer offered a novel carbon energy level for the band gap of TiO₂, initiating an effective visible light photocatalytic degradation of MB. Besides, this ideal NCNTs/TiO₂ core-shell hybrid nanostructures can facilitate a diversity of pragmatic applications in (nano)catalysts, sensors, energy conversion and/or storage. Zhang et al. (2010) described the fabrication of CNT-pillared rGO (reduced graphene oxide) platelets by chemical vapor deposition processes for photocatalytical degradation of RhB (Fig. S2). Li et al. (2011) described the synthesis of Au NPs@POM (polyoxometalate)-CNT nanohybrids for the degradation of RhB where POMs serve as encapsulating, bridging molecules and reducing agents; high electron-conducting capability of MWCNTs render these nanohybrids as efficient photocatalysts with a selective overview for their use in water remediation being summarized in Table S3.

3. Graphene and graphene oxide (GO)

3.1. Chemistry and properties

Graphene nanomaterials have been applied for treating wastewater (Perreault et al., 2015) as they show significant relevant physicochemical characteristics that include high surface area, simple functionalization, and hydrophobicity. Graphene nanocomposites can be utilized as nanosorbents for eliminating hazardous and toxic mix of multiple pollutants, and for their subsequent catalytic degradation using catalytic, photocatalytic, electrocatalytic and photo-electrocatalytic oxidation, and reduction processes (Al-Wafi et al., 2020; Gandhi et al., 2016; Khan et al., 2020b; Nasrollahzadeh et al., 2019a, 2020b; Wani et al., 2020) via their commercial production, and use is still a formidable challenge.

Uniform nanoporous graphene sheets can be applied for filtrating and desalinating water by varying their pore size and hence the pressure (Aghigh et al., 2015), the main drawback being their mechanical stability after increasing the pore number. Yan et al. (2018), produced N-doped sandwich-type graphene composites, and applied them as capacitive deionization electrodes via a self-assembly method. This sandwich composite offered substantial accessible surface area and lower electronic resistivity and displayed considerable salt adsorption capability with superior modification ability coupled with recycling operations. It has been shown that the GO nanosheets could be applied as suitable selective barriers for water permeability (Goh et al., 2015); super-hydrophobic and superoleophilic porous graphene has been generated as absorbent for various materials, demonstrating extreme selectivity, suitable recyclability, and significant absorption capacities of more than 90% (Tabish et al., 2018). Additionally, GO-based nanofiltration membrane with an enormously porous polyacrylonitrile nanofibrous mat has been fabricated for treating water; graphene oxide could generate a barricade on a polyacrylonitrile nanofibrous mat with well-regulated thickness (Wang et al., 2016a).

Unlike CNT-based nanomaterials, the aqueous removal of toxic dyes by graphene-based nanoadsorbents have varied advantages wherein single-layered graphene or GO nanosheets have two basal planes accessible for contaminant adsorption (Nasrollahzadeh et al., 2019a; Zhao et al., 2014); CNTs inner walls are not available to adsorbates (Sitko et al., 2013b). Moreover, graphene and its analogues could be simply fabricated via chemical/mechanical exfoliation of graphite without employing metallic catalysts (e.g. nickel) and complex apparatus (Nasrollahzadeh et al., 2019a; Sitko et al., 2013b). Scheme 1 illustrates the accepted common mechanisms for adsorption and pros and cons of utilizing graphene-based nanosorbents to eliminate metallic pollutants (Perreault et al., 2015) Notably, the resulting graphene-based nanomaterials are completely devoid of catalytic residues, thus not necessitating additional purification steps. Concerning GO, the nanosorbents contain a vast quantity of O2-bearing functionalities that precludes additional acid treatments to enhance reactivity and hydrophilicity character to graphene oxide (Zhao et al., 2011). It could positively interact with a diversity of organic/inorganic contaminants in ionic and/or molecular forms through the mechanisms, commonly known as the electrostatic π-π and hydrophobic interactions, among others. Thus, GO has drawn considerable attention as an effective nanosorbent for contaminated water/wastewater (Nasrollahzadeh et al., 2019a; Perreault et al., 2015; Wang et al.,2019).

Scheme 1.

Specific interaction mechanisms and probable advantages/disadvantages for using graphene-based nanosorbents for aqueous degradation of pollutants.

3.2. Applications for water treatment

Graphene-based objects are deployed as efficient adsorbents for treating pharmaceuticals-laden wastewaters; their adsorption capacity can be varied by changing the experimental conditions (Carmalin Sophia et al., 2016). Additionally, they can be harnessed for removing radionuclides and heavy metals from wastewater; various key parameters, embracing the initial dosage and characteristics of graphene materials and metal ions pH, temperature, contact time, acidic organic ligands, and coexisting ions, can affect their adsorption capacity (Xu and Wang, 2017) as is evident from the recent data on remediation of toxic dyes and heavy metals in water (Tables S4 and S5) (Huang et al., 2011; Sitko et al., 2013a; Zhao et al., 2011). Significance of O₂-bearing functional groups has been illustrated (Huang et al., 2011); GNS-500 and GNS-700 (heated at 500 or 700 °C after exfoliation) exhibited a greater adsorption capabilities for Pb²⁺ compared with pristine graphene thus signifying the importance of carboxyl/hydroxyl groups in the Pb²⁺ adsorption. Ghaedi et al. (2014) reported an efficient removal technology for mixed adsorption of brilliant green and MB as cationic toxic dyes by GO fabricated from modified Hummers process as calculated by Langmuir model at pH 7.0 were 416.67 and 476.19 mg g⁻¹, respectively; removal was apparently via a pseudo-second-order (PSO) kinetic process based on a monolayer adsorption manner, mechanism attributed to π-π interactions or electrostatic attraction (Jiang and Fan, 2014; Shen et al., 2014; Sui et al., 2013; Wu et al., 2013) between positively charged dyes and negatively charged GO. Sabzevari et al. (2018) researched the fabrication of GO-chitosan without sonication for the MB removal to find that high adsorption ability for MB (402.6 mg g⁻¹) could be obtained in comparison with other GO from original Hummers means and/or via sonication.

Graphene can be applied as a suitable separation membrane, by forming nanoscale pores in its layer (Song et al., 2018; Surwade et al., 2015); it has exclusive characteristics such as flexibility, mechanical and chemical stability, with essentially thickness of one-atom. Nanometer-sized pores in monolayer of graphene have been prepared by applying oxygen plasma engraving procedure, enabling the fine tuning of the pore size; ensuing membranes showed a salt refusal rate of nearly quantitative and speedy water transference. Particularly, water fluidities of up to 106 gm⁻²s⁻¹ were attained by using powerful force via pressure alteration, while water fluxes evaluated at osmotic pressure did not surpass 70 gm⁻²s⁻¹atm⁻¹ (Surwade et al., 2015). It has been reported that single-layered self-supporting graphene with nanometer-scale pores could efficiently sieve NaCl salt from water (Cohen-Tanugi and Grossman, 2012) which varied significantly by changing pore diameter. Additionally, by analyzing the function of chemical groups attached to the graphene pore’s edges, it was revealed that commonly present hydroxyl groups could nearly double the water fluidity owing to their hydrophilic nature (Cohen-Tanugi and Grossman, 2012). A recyclable nanocomposite was prepared by growing both iron oxide and silver NPs on the GO’s surface; relative to silver NPs, the produced nanocomposite showed much superior antibacterial efficiency towards the Gram-positive and -negative bacteria. Moreover, magnetic iron oxide NPs facilitated repeated applications by enabling easy recycling of the nanocomposite via magnetic separation (Tian et al., 2014). Blue luminescent sulfur-doped graphene quantum dots (~1–5 nm) have been synthesized via a single-step pyrolysis method in presence of 3-mercaptosuccinic and citric acid (Anh et al., 2019) where 4-NP can function as quencher of fluorescence by π-π interaction with sulfur-doped graphene quantum dots, as shown by the linear reduction in fluorescence intensity after adding varying amounts of 4-NP (10–200 μM). Additionally, the generated quantum dots served as sensors to accelerate the analytic implementation of 4-NP detection, the limiting values being 0.7 and 3.5 nM in deionized water and wastewater, respectively; sulfur-doped graphene quantum dots-based paper sensor could quickly monitor 4-NP in wastewater within a minute (Anh et al., 2019).

Assorted (nano)catalysts have been fabricated via physicochemical and/or biological methods comprising a series of metals such as Pd, Co, Au, Ni, Cu, etc. (Nasrollahzadeh et al., 2019b, 2019c). Graphene-based (photo)nanocatalysts (G, GO or rGO-based) has been recently explored and employed for the alteration of photocatalysts or nanocomposites because of their exceptional features, namely, prominent surface area (~2630 m² g⁻¹), high electron conductivity (~7200 Sm⁻¹), high thermal/chemical stability, and excellent capacity for the immobilization of metals (Ak and Novoselov, 2007; Nasrollahzadeh et al., 2019a, 2019c; Neto et al., 2009; Qiu et al., 2015) or via their decoration with numerous NPs e.g. metal oxides, noble metals and magnetic NPs (Julkapli and Bagheri, 2015; Naghdi et al., 2018; Nasrollahzadeh et al., 2019a).

GO decorated with highly stable lanthanum-substituted manganese ferrites (LMF) have been synthesized for the removal of PFOA from aqueous environments (Fig. S3) (Elanchezhiyan et al., 2020) After the substitution of lanthanum on MF, the surface charge density and the crystalline nature changed remarkably; electrostatic interaction and hydrogen bonding had significant roles in PFOA adsorption. Additionally, it has been revealed that the coexisting pollutants, including anions, cations, and other organic compounds had no significant effects on the adsorption of PFOA deploying these nanohybrids (Elanchezhiyan et al., 2020).

Multifunctional chitosan-attached plasmonic Au NPs conjugated GO architecture-based 3D porous membrane, chitosan-Au NPs-GO (Fig. 3A), was designed and fabricated by Jones et al. (2017) for the supply of clean and safe drinking water. The 3D porous membrane was used as a novel nanocatalyst in efficient separation and label-free Surface Enhanced Raman spectroscopy (SERS) documentation of pharmaceutical pollutants and killing of methicillin-resistant Staphylococcus aureus (MRSA) in polluted water. The HRSEM image in Fig. 3B illustrates the morphology of chitosan-attached Au NPs on the GO sheet and Fig. 3C depicts MRSA bacteria being easily trapped using highly porous chitosan-Au NPs-GO membrane. Besides, the amount of colonies of live MRSA on the extremely porous 3D membrane was determined (Fig. 3D and E), affirming that ~100% of MRSA superbug was killed using as-prepared porous membrane.

Fig. 3.

Fabrication of (A) chitosan-attached Au NPs conjugated GO architecture-based multifunctional 3D porous membrane. (B) SEM image of chitosan-attached plasmonic Au NPs conjugated with hybrid 3D GO membrane. (C) SEM image showing the MRSA being captured using the 3D membrane. (D & E) Colonies of bacteria displaying the amount of live MRSA (D) after filtration by only a GO-based 3D membrane, (E) after filtration by using the chitosan-attached plasmonic Au NPs-based 3D GO membrane. Reproduced with permission from Ref. (Jones et al., 2017).

4. Carbon and graphene quantum dots

4.1. Chemistry and properties

The research community is optimistic about the pervasive potential utilization of carbon nanostructured materials to ameliorate sustainable energy conversion and wastewater treatment. Among the nanocarbon materials, carbon- and graphene quantum dots are a fascinating category of the nanomaterials that can find widespread applications. Quantum-sized carbon dots (or carbon NPs) were initially discovered by Xu’s group in 2004 (Xu et al., 2004), (i.e., a few months before graphene analogues), and the common name “CQDs” was assigned by Sun’s group in 2006 (Sun et al., 2006), although their utilization as a heterogeneous photocatalysts only started recently. Carbon quantum dots refer to carbon-based sub-ten-nanometer NPs (at least one dimension below 10 nm), and flat or quasi-spherical shape and comprise sp³ and/or sp² hybridized carbon atoms (i.e., diamond-like and/or graphene/GO sheets), with composition-dependent fluorescence and possess diverse surface functional groups (Lim et al., 2015; Yu et al., 2016a). Graphene quantum dots comprise graphene of single and/or a few layers (<2 nm thick) with lateral dimensions (<10 nm) being considerably larger than their height. Carbon nanodots (CNDs), CQDs, GQDs, N-doped CDs/GQDs, carbon nitride dots, and CDs/GQDs (nano)composites and (nano)hybrids represent the same (nano)carbon family (Garg and Bisht, 2016; Mahmoud et al., 2020; Zhu et al., 2015).

These carbon-based nanostructured materials have garnered significant interest as they could be applied in non-biomedical areas, such as optical devices (Jiang et al., 2015) and in photocatalysis (Ke et al., 2017; Su et al., 2020), because of their unique features e.g. strongly size-dependent electronic, electrochemical, and optical, and quantum size effects (Cai et al., 2019; Cao et al., 2012), easy surface functionalization, adjustable composition, ability to harness long-wavelength light, and easy synthesis (bottom-up and/or top-down approaches) (Cao et al., 2012; Fernando et al., 2015; Hisatomi et al., 2014; Zhao et al., 2016). Besides, CQDs/GQDs are promising candidates for biomedical appliances, such as drug delivery (Tang et al., 2013; Zheng et al., 2014), biosensing (Dai et al., 2014; Ding et al., 2013; Tajik et al., 2020), gene delivery (Hu et al., 2014; Zemmouri et al., 2013), and bioimaging (Ding et al., 2013; Luo et al., 2013; Zhang and Ding, 2018), because of their distinctive attributes e.g. nontoxicity, high photostability, excellent mechanical/thermal stability in vivo, and high biocompatibility, electrochemiluminescence (Dong et al., 2013), nonlinear optical response (Bourlinos et al., 2013), and intense and tunable photoluminescence (PL) by using surface functional groups and dot size (Bourlinos et al., 2008, 2011; Hola et al., 2014; Li et al., 2010). Based on DFT (density functional theory) and time-dependent DFT calculations, Chen et al. (Sk et al., 2014) determined that the GQDs properties can be adjusted from deep UV-to-NIR (near infrared) via alteration of morphology, particle size, surface functional groups, surface defects, edge configuration and heteroatom doping.

Nanocomposites with carbon-based nanomaterials are known mostly for graphene (i.e., 0D CQDs onto 2D graphene sheets), and to a lesser extent for nanodiamonds and CNTs. Moreover, GQDs could self-assemble to fabricate honeycomb nanostructures (Fan et al., 2012) and/or hollow microspheres (Ding et al., 2012). Compared to their 2D counterpart, GQDs and their functionalized analogues can offer novel application opportunities and unique merits, namely, better tunability in physicochemical features, high dispersibility, bandgap opening owing to quantum confinement, comparable sizes to biomolecules, and also more abundant active sites (edges, and functional moieties) (Anastopoulos et al., 2019; Li et al., 2013; Wang et al., 2016b). Because of these intriguing properties and their possible fabrication from biomass, CQDs and/or GQDs have potential to substitute conventional semiconductor/quantum which have severe disadvantages of high cost and toxicity; they could ameliorate the performance of several nano/photocatalysts via band gap alignment, morphology change, enhancement of light adsorption, synergic cooperation, and/or the charge transferal promotion. Thus, exploiting good light absorption, fully available surface, excellent photostability, high dispersibility, tunable bandgap structure, and their capability to fabricate heterojunction or hybrid with other nanostructures, CQDs/GQDs have been often applied as efficient nano/photocatalysts for (photo)catalytic energy generation (Chen et al., 2016a; Yeh et al., 2014; Yu et al., 2016b) and (photo)degradation of pollutants/contaminants (Chen and Bai, 2020; Ebrahimi et al., 2016; Pan et al., 2015; Yan et al., 2017).

4.2. Applications for water treatment

CQDs (1.2–3.8 nm) with high up-conversion PL properties were first prepared by Li et al. via a single step alkali-assisted electrochemical process (Li et al., 2010); the preparation of TiO₂/CQDs or SiO₂/CQDs semiconductor nanocomposites was reported with a strong and stable visible-light response for the decomposition of an organic dye, MB. The maximum photodegradation abilities of SiO2/CQDs (TiO₂/CQDs) nanocomposites towards aqueous MB pollutants were found to be ~100% after 25 min irradiation with halogen lamp (300 W), whereas control experiments with pure CQDs and/or without either the metal oxide component (SiO2 or TiO₂) demonstrated a minimal MB (0%) photodegradation. A CQDs/ZnO nanocomposite (~20–30 nm) was fabricated via a hydrothermal one-step route (Yu et al., 2012), which was then utilized for the photodegradation of toxic methanol and benzene (>80% degradation efficiency after 24 h) in air under visible-light conditions at room temperature.

The fabrication of nanocomplex-based photocatalysts (e.g. WO₃/CQDs, N-ZnO/CQDs, CQDs/BiOI) has been revealed with substantial proficiency towards the photodegradation of a variety of organic contaminants (Chen et al., 2016b; Muthulingam et al., 2016; Yan et al., 2016a). Zhao et al. (Qian et al., 2016) established the preparation and photocatalytic prowess of CQDs decorated nanocomposites of Bi₂WO₆ for the photocatalytic removal of gaseous VOCs under both, the UV irradiation and visible-light; it could broaden the absorption visible-light and improve the photoexcited charge recombination/separation thus revealing an enhanced photo-oxidation performances and stabilities towards toluene and acetone. Similarly, Mendes et al. reported on the performance of N-CQDs/TiO₂ (P25) nanocomposite for NO pollutant’s photo-oxidation under both, UV and visible-light irradiation (Martins et al., 2016) wherein the NO conversion augmented from ~10% (for P25) to ~27% (for N-CQDs/TiO₂) and from 58.4% to 79.6% under both, UV-Vis light, respectively.

One of the candidates for the improvement of CQDs-based photocatalysts is the utilization of metal sulfides (e.g. CdS) with their superior properties, namely narrow band gap, good transport, and high chemical/thermal stability. In this context, CQDs/CdS photocatalysts were prepared by Zhang and co-workers via a hydrothermal route and their application towards the photodegradation of RhB was evaluated (Li et al., 2013b) where CQDs could efficiently trap electrons and slow down the recombination of the photoexcited carriers (holes or electrons); ~50% RhB degradation efficiency was attained by flower-shaped CdS while 1% CQDs/CdS could enhance the degradation efficiency to 90%. The photodegradation of Alizarin red S (ARS) dye was accomplished using CQDs/ZnS photo/nanocatalyst by Kansal et al. (Kaur et al., 2016), where it demonstrated the superior photocatalytic activity for ARS degradation (~89% after 250 min), 1.4 times better than pristine ZnS (~63%) under comparable visible-light irradiation conditions.

Polymer dots (PDs) have been fabricated from polymers such as polyvinyl alcohol with core comprising CDs (Zhu et al., 2015) as exemplified by hydrothermal treatment of PVA precursor (Li et al., 2018). CDs/PDs-TiO₂ nanocomposites with Ti-O-C bands were made by grafting CDs/PDs with TiO₂ NPs via facile hydrothermal thermal treatment, and the activities of ensuing semiconductor/CD nanohybrids were evaluated for degradation of MO dye degradation in aqueous solutions (Fig. 4A). Synergy between CDs/PDs and TiO₂ could effectively improve the removal of toxic MO, and the nano/photocatalytic rate constant of PDs/TiO₂ is 9.5 and 3.6 times more than commercially available pure P25 and TiO₂, respectively (Fig. 4B and C).

Fig. 4.

(A) Mechanistic photocatalytic illustration for CDs/PDs-TiO₂ under vis-light irradiation. (B) MO degradation under UV/Visible-light irradiation (220–800 nm), (C) rate constant k of CDs/PDs-TiO₂ nanohybrids, pure TiO₂, and P25 under vis-light treatment. Reproduced with permission from Ref. (Li et al., 2018).

The fabrication of new hybrid nanocomposites comprising CQDs and inorganic NPs namely zinc oxide, iron oxide, titania, and silica have been accomplished. Zhang et al. (2011) synthesized, via hydrothermal process, Fe₂O₃/CQDs nanocomposites as effective photocatalysts for the degradation of noxious gases namely benzene or methanol in vis-light conditions; pathway for Fe₂O₃/CQDs nanohybrid with cubic morphology of magnetic architecture and their photochemical application, are shown in Fig. 5. Owing to the CQDs nature, magnetic nanohybrids can combine magnetic response with fluorescence on a single platform. CQD has high-electron storage where photon-excited electron from Fe₂O₃ NPs can easily be transported in the CQD conducting system, as exhibited in Fig. 6. The adsorbed reductants/oxidants (such as O₂ and/or OH) interact with the electron-hole pairs, forming an enhanced quantity of active oxygen radicals with a strong oxidation prowess for removing toxic gases. Besides, the resulting nanohybrids could effectively combine the fluorescence features of CQDs with magnetic, mechanical or optical properties/features of the oxide’s core culminating in a core-shell fluorescent/magnetic photocatalyst (Li et al., 2010; Markova et al., 2012; Ruan et al., 2009; Yu et al., 2012) where an iron oxide core is attached to CQDs either through a covalent bond (by utilization of surface functional groups present on the nanohybrid components) and/or electrostatic interactions (via an electrostatically charged shells of the nanohybrid elements). In this context, a series of core-shell fluorescent photocatalysts using shells-derived from CQDs and a variety of NP-cores (biogenic magnetite, CNTs and silver) were prepared by Markova et al. (2012) by a novel and effectual protocol for the modification of negatively charged NP-cores using CQDs and betaine hydrochloride via electrostatic interactions (Fig. S4) which illustrates a core-shell architecture attained by coating magnetite and CNTs using CQDs.

Fig. 5.

Up: Fabrication of Fe₂O₃/CQDs nanocomposites via the hydrothermal process, which photographs illustrate; (i) aqueous CQDs dispersion, (ii) solution comprising CQDs, (NH₂)₂CO, and, FeCl₃, and (iii) final product. Down: (a) SEM image of Fe₂O₃ NPs, (b) (HR) TEM images of CQDs, (c & d) SEM and TEM images of Fe₂O₃/CQDs nanocomposites. Reproduced with permission from Ref. (Zhang et al., 2011).

Fig. 6.

Schematic model of the mutual interactions between CQDs and Fe₂O₃ NPs describing the enhanced photocatalytic ability of nanocomposite under vis-light (Zhang et al., 2011).

GQDs have been similarly deployed in photocatalytic environmental remediation by elimination of organic pollutants such as azo dyes, and VOCs. Several procedures have emerged to ameliorate photocatalytic efficiency of semiconductor GQDs-based photocatalysts, including loading of noble metal and/or metal oxides, structure design, and the preparation of a semiconductor nanocomposites/hybrids (Kaur et al., 2018; Kemp et al., 2013; Putri et al., 2015; Yan et al., 2019). A series of newly designed GQDs-based photo/nanocatalysts, including GQDs coupled with TiO₂ (Bian et al., 2017; Qu et al., 2013), Fe₃O₄ (Wu et al., 2014), zinc porphyrin (Lu et al., 2015), MnNb₂O₆ (Yan et al., 2017), ZnO (Ebrahimi et al.,2016), g-C₃N₄ (graphitic carbon nitrides) (Bian et al., 2017; Liu et al., 2017), and/or N-Bi₂O₂CO₃ (Yu et al., 2017), have been designed for the visible-light- and/or NIR-driven photodegradation. In this context, GQDs/modified mesoporous-C₃N₄ heterojunction can be produced via an electrostatic interaction (Yu et al., 2017) wherein the nanocomposite can form more superoxide radical species and small holes towards an efficient photodegradation of RhB and tetracycline (TC), as displayed in Fig. S5. Similarly, Liu et al. (Lu et al., 2015) have reported GQD/zinc porphyrin heterojunction, and evaluated its performance for the faster decomposition of aqueous MB visible-light-driven photodegradation reactions.

GQDs have been known for a more intact aryl-basal flat structure and rich periphery carboxylic groups than the micro-sized GO layers (Zhang et al., 2013; Zhou et al., 2012). More attractively, GQDs demonstrated much better inherent peroxidase-like prowess compared to micro-sized GO, thereby improving efficiency and stability in detection of H₂O₂. Zhang et al. (2013) reported that Au NPs/GQDs nanohybrids exhibited a synergetic and switchable peroxidase-like ability for H₂O₂ decomposition and/or H₂O₂ detection in physiology or pathology. Similarly, nanocomposites comprising GQDs and Fe₃O₄ NPs, as encouraging peroxidase-mimic nanocatalytic systems, have been reported (Wu et al., 2014), and they could be fabricated via a single-step co-precipitation route to remove phenolic compounds. The as-prepared GQDs/Fe₃O₄ nanocomposite bearing a weight ratio of 1:1 for H₂O₂- facilitated the oxidation of 3,3′,5,5′-tetramethylbenzidine demonstrated comparable or superior catalytic performances, compared to individual Fe₃O₄ NPs (~22 fold higher), GQDs (~25 fold higher), and also GO/Fe₃O₄ NPs (~9 fold higher). In fact, the combination of Fe₃O₄ NPs via Fe-O chemical links led to electron transfer from electron-rich GQDs to magnetic NPs, preserving a greater number of Fe(II) ions which play a main role in the peroxidase-like nanocatalytic activities of Fe₃O₄ NPs.

The doping of nitrogen is one of the novel strategies to modify and modulate the structural functionalities, chemical and electronic properties/features of graphene and GQDs (Li et al., 2012; Subramanian et al., 2017) as shown for the preparation of N-GQDs/TiO₂ nanocomposite via a hydrothermal procedure and its deployment as a photo/nanocatalyst for the photodegradation of aqueous MB (Safardoust-Hojaghan and Salavati-Niasari, 2017); N-GQDs displayed an eminent effect on the photocatalytic efficiency of TiO₂ NPs from 40% to 85% for MB degradation under UV-light. A recent study (Yan et al., 2016b) revealed that photocatalytic activity of N-GQDs-BiVO₄@C₃N₄-Z scheme heterojunctions to (photo) degrade the antibiotics pollutants such as oxytetracycline, tetracycline, and ciprofloxacin was higher than BiVO₄@C₃N₄; as-prepared heterojunction exhibited superior photocatalytic efficiency of 91.5% tetracycline degradation in 30 min. Besides, the combined S, N co-doped GQDs with g-C₃N₄ was fabricated as an ideal photo/nanocatalyst for pollutant degradations (Cai et al., 2017); as-prepared S, N-GQD/g-C₃N₄ (6–10 nm) composites (Fig. S6) revealed higher photocatalytic activities in remediation of RhB dye in vis-light condition than by pure g-C₃N₄ under identical conditions. Indeed, enhancement in degradation was because of the appropriate separation of photogenerated holes and electrons in S,N-GQD/g-C₃N₄ nanocomposite.

A variety of CQDs- and GQDs-based photocatalysts, deployed in eliminating highly toxic pollutants namely dyes and heavy metals, are shown in Table S6, where diverse preparations of CQDs- and GQDs-based photocatalysts via hydrothermal-, solvothermal-, ultrasonic treatment and microwave-assisted routes are documented. Among these methods, the hydrothermal route is a facile, greener and an effective approach (Chu et al., 2019; Pirsaheb et al., 2018; Yu et al., 2016a; Zeng et al., 2018). Natural bio-wastes/biosources have also been used as environmentally-friendly, low-cost and abundant carbon sources for the synthesis of these photocatalysts.

5. Conclusion and future prospects

Treatment of water and wastewater including industrial water is very critical and essential for safeguarding the environment and human health, and it is a major apprehension for the public health. As is obvious from this review, tremendous progresses have been achieved in recent years pertaining to the environmental pollution abatement. Among advanced technologies, greener nanotechnology and applications of sustainable nanomaterials can be exploited for the treatment of wastewater, and especially in the handling of previously untreatable hazardous contaminants. However, there are still several hurdles to endure, mainly the lack of infrastructure and resource utilization that need to be addressed besides the obvious concern about the potential toxicity of the nanomaterials themselves where carbon-based materials may have an edge. The consequence of releasing nanomaterials into the aquatic system is more nuanced, especially if the nanomaterials are part of a composite material.

Nanotechnology has the potential to make industrial water treatment more efficient where the contaminants can be removed by using relatively greener nanotechnology, but this field needs extensive explorations. Eventually, carbon nanotubes, carbon and graphene quantum dots and graphene-based nanomaterials may emerge as effectual, cost-effective, and environmentally-friendly substitutes for the prevailing treatment supplies, from the viewpoints of both environmental cleanup and resource conservation. For instance, the modification of CNTs such as, oxidation, and applying electrochemical assistance can meaningfully improve their adsorption rate and capacities for the removal of contaminants, and may classify as an innovative strategy in environmental remediation, but the toxicity issues need to be evaluated, comprehensively.

CNTs and some earth-abundant nanometals are naturally green and extraordinarily applicable nanoadsorbents for removing the heavy metals and hazardous contaminants; major benefit for CNTs is their strong adsorption capacity for polar organic compounds due to their assorted interactions with pollutants. Unfortunately, CNTs are not cost-competitive relative to adsorbents like activated carbon. The manufacture of CNTs is rather an expensive proposition, and supplementary technical devices, for example, plants equipped with membrane filtration, need to be incorporated to ensure that no NPs are emitted into the aqueous streams or air. Impending appliances may emphasize very particular adsorption activities, where only miniscule amounts of CNTs are needed or unique situations where CNTs are the exclusive adsorbent. Commercialization of natural nano-engineered materials for wastewater technology is greatly contingent on their effect on the aquatic ecosystem (Dwivedi et al., 2015). Several investigations embracing toxicity analyses, life cycle assessments, strategy evaluations, and dispersion of NPs in water systems have to be conducted to assess the risks of nanomaterials to human health (Bakand and Hayes, 2016; Borm et al., 2006; Nasrollahzadeh et al., 2018). These informative findings would lead to better awareness of the conduct of NPs (like CNTs) in aqueous systems (Asghari et al., 2012; Clemente et al., 2012; Jackson et al., 2013; Petersen et al., 2011). Besides CNTs-based nanoadsorbents, other allotropes of nanocarbon, such as graphene, GO, CDs or GQDs, have shown promise to enhance the (photo)catalytic activities of conventional semiconductors, rendering them widely applicable for the (photo) catalytic degradation of contaminants.

In spite of the remarkable advances in fabrication and the demonstrated catalytic prowess of sustainable nanomaterials such as carbon nanotubes, carbon and graphene quantum dots and graphene-based nanomaterials, more attention need to be focused on the following issues in future investigations:

• Novel and innovative synthetic methods to reduce the cost of production of nanomaterials and improve their catalytic prowess.

• Improvement and reuse of the magnetic nanomaterials for water treatment.

• Advancement and use of mild, water soluble and low-cost reductants for the degradation of various pollutants.

• Utilization of biowaste-derived nanomaterials for wastewater treatment.

• Exploitation of natural materials such as clays, montmorillonite, bentonite, zeolites as nontoxic and cost-effective and abundant supports for the synthesis of nanomaterials and application in water treatment.

• Manipulation of animal residues such as bone, eggshell and bristles for the synthesis of nanomaterials and application in water treatment.

• Toxicity evaluations should be accomplished for all the aforementioned proposals; comprehensive assessments of the risks and the potential impact of nanomaterials to human health and ecosystems, mainly potential cellular toxicities, must be evaluated and documented.