Over 20 years ago, the concept of “green chemistry” emerged as a result of a shift in emphasis on material research with the move towards the use of waste materials and waste-reduction strategies. The chemical industry as a whole and scientists are directed to follow and promote sustainable and environmentally friendly pathways governed by the twelve principles of green chemistry (DeVierno Kreuder et al., 2017). The seventh principle buttresses the need for designing synthetic procedures with minimal negative impacts on the environment and without the use of hazardous reagents (Kharissova et al., 2019; Forbes, 2021). Thus, using renewable feedstocks and biodegradable raw or waste materials that are typically rich in carbon, and are available from natural or biological sources, is a more acceptable approach for the environmentally friendly preparation of carbon-based materials from the bulk to the nanoscale.

The synthesis of carbon-based nanomaterials has been facilitated by enabling technologies such as hydrothermal, solvothermal, chemical vapor deposition, microwave-assisted synthesis, and other top-down/bottom-up techniques (Yan et al., 2016; Macairan et al., 2019; Adeola and Forbes, 2021a; Gulati et al., 2023). In comparison to other conventional methods, microwave-assisted synthesis of carbon-based nanomaterials is a non-invasive, easy, quick, clean, and eco-friendly process (Oladipo et al., 2018; Dutta et al., 2022a). The reaction is facilitated and accelerated by the microwave, frequently increasing relative yields. According to reports, microwave-assisted synthesis can circumvent synthesis-related challenges through energy efficiency, simplicity of experimental setup, adjustable process conditions, speedy reactions, and uniform heating/thermal processing. Microwave-assisted synthesis of carbon-based nanomaterials requires low growth temperatures and offers high surface areas, adsorption capacities, and purity while providing energy savings. As such, it presents a promising approach to addressing the challenges for industrial and municipal water treatment (Schwenke et al., 2015; Baghel et al., 2022).

Environmental pollution is unquestionably one of the major challenges facing civilization today. Every day, thousands of tons of dangerous chemicals are released into water bodies (Nassar et al., 2015; Dutta et al., 2019; Tong et al., 2022). Some dangerous contaminants that can be found in the aquatic environment include heavy metals, textile dyes, herbicides, surfactants, microplastics, hydrocarbons, pharmaceuticals, and personal care products, to name a few. To get rid of the contaminants in polluted water, new methods are constantly being researched. While developing carbon-based nanomaterials for environmental remediation, some major factors for consideration include target-specific identification, simplicity of design, affordable production, the toxicity of nanomaterial, biocompatibility, reusability, and the capacity for regeneration after usage. Therefore, new technologies are particularly intriguing because they may hold the key to addressing these challenges. As a result, numerous studies have concentrated on applying green chemistry concepts with the physicochemical surface modification of carbon-based nanomaterials to produce nanomaterials that can mitigate challenges faced during contaminant cleanup (Das et al., 2020; Dutta et al., 2022b; Mohapatra et al., 2023; Osterberg et al., 2023).

Scopus search revealed that over 62% of published data on carbon-based materials in the last decade have focused on design and application in water treatment, and well over 10,000 articles have been published on the subject (Dutta et al., 2022a). Thus, there is a need to constantly review the advances in this field of research, particularly the recent integration of microwave-assisted synthesis in carbon nanomaterials development. The current review assesses the development of advanced carbon nanomaterials (graphene, carbon nanotubes, and carbon dots) for water pollution remediation using microwave-assisted synthesis. The significant aspect of the review includes green nano-synthesis from waste/biobased resources using the microwave method and applications in water treatment, highlighting the merits, challenges, and future prospects.

Carbon-based nanomaterials have become an essential component of modern technology, with applications in fields such as electronics, environment, catalysis, energy storage, and drug delivery due to their unique qualities, such as high surface area, exceptional mechanical strength, and electrical conductivity (Fathy et al., 2020; Zakaria et al., 2022). Numerous methods are utilized to synthesize these nanomaterials, which can be classified into top-down and bottom-up approaches. The top-down approach consists in breaking down large carbon structures such as graphite, into smaller carbon nanostructures, while in the bottom-up approach, the carbon nanostructures are formed from atoms and molecules (Supplementary Figure S1–S3) (Baig et al., 2021; Abid et al., 2022). Depending on the nature of the precursor and process conditions, some conventional and microwave-assisted techniques can carry out bottom-up/top-down synthesis of carbon nanomaterials.

Arc discharge and laser ablation are among the most used top-down approaches for the synthesis of carbon nanomaterials. These were the earliest methods used for synthesizing fullerenes and carbon nanotubes (CNTs) (Kroto et al., 1985; Iijima, 1991; Arora and Sharma, 2014) In the arc discharge method, a high-voltage electric current is applied between two graphite electrodes in an inert environment/system. An electric arc is formed, generating a high temperature that is responsible for vaporizing the graphite electrodes. Then, the vapor condenses into CNTs or fullerenes (Arora and Sharma, 2014; Deng et al., 2016). On the other hand, the laser ablation technique utilizes a high-power laser to vaporize graphite under an inert atmosphere. The vapor is then carried by the inert gas and condensed into carbon nanomaterials (Deng et al., 2016; Aravind Kumar et al., 2020). Nonetheless, these techniques show several drawbacks including low yield for laser ablation, presence of impurities such as amorphous carbon and residual catalyst, limited control over size and shape, requirement of high temperatures, and consequently high energy consumption, in addition to costly setups. Thus, bottom-up approaches, including chemical vapor deposition (CVD), hydro/solvothermal, and microwave-assisted techniques are considered more promising for the production of carbon nanomaterials on a large scale due to their low cost, high control over size and shape as well as high material yields (Saputri et al., 2020; El-Khawaga et al., 2023).

CVD has been one of the conventional methods most explored for synthesizing different carbon nanomaterials owing to its simplicity and ability to produce high-quality materials (Wang et al., 2018; Fathy et al., 2020; Zakaria et al., 2022). It consists of the thermal decomposition of a carbon source under an inert atmosphere at a high temperature, in the presence of a metallic catalyst. As the carbon source decomposes, the carbon atoms deposit on the catalyst surface, and due to its low solubility in these metallic particles at high temperatures, precipitates are obtained and then carbon nanomaterials are formed (Deng et al., 2016). Although this conventional method is indeed capable of producing high-quality carbon-based nanomaterials, it relies mainly on carbon sources derived from fossil fuels for synthesizing these materials, such as ethylene, acetylene, benzene, methane, ethane, toluene, and xylene (Goswami et al., 2021; Shi et al., 2023a). In this regard, there has been a rise in interest in a more sustainable and greener synthesis of carbon nanomaterials that focuses on the development of environmentally friendly and sustainable alternatives (Deng et al., 2016; Vivekanandhan et al., 2017; Fathy et al., 2020; Shi et al., 2023a). These alternatives focus on the use of non-toxic, renewable, biodegradable, and abundant precursors, such as honey (Mandani et al., 2017) and juices (Wang et al., 2022), biomass (Zhao et al., 2020), vegetable and animal oils (Duarte et al., 2022), industrial residues (Hu et al., 2019), and agricultural waste (Somanathan et al., 2015). Supplementary Table S1 provides a summary of biobased resources that have been utilized in the synthesis of carbon nanomaterials through conventional techniques.

Owing to its high yield, high purity, and the possibility of morphological and structural control, researchers have focused on developing a more sustainable CVD process. Thus, greener solvents and more environmentally friendly solid carbon sources have been employed as precursors to the synthesis of these nanomaterials (Manawi et al., 2018; Fathy et al., 2020). However, the evolution of toxic gaseous by-products during CVD remains a drawback (Manawi et al., 2018; Ijaz et al., 2020). In this way, pyrolysis, hydro/solvothermal and microwave-assisted techniques have attracted attention due to the non-necessity of the presence of a substrate for the growth of the nanomaterial, in addition to the possibility of using different carbon sources derived from biomass sources, renewable materials, and wastes. As a result, these approaches have been widely utilized in many parts of the world, including developing countries (Wang et al., 2018; Wang et al., 2020a; Devi et al., 2021).

The pyrolysis method consists of the thermal decomposition of a carbon source at a high temperature under an inert atmosphere and has been used extensively for synthesizing carbon dots (CDs) and graphene-based materials (Devi et al., 2021; Asif and Saha, 2023). In the hydro/solvothermal methods, the carbon source is converted into nanostructured carbon materials under high pressure and mild temperatures (<350°C) in a sealed vessel (Byrappa and Adschiri, 2007; Heidari et al., 2019; Gong et al., 2023). In the hydrothermal method, the reaction occurs in an aqueous medium, while in the solvothermal approach, different solvents such as ethanol, acetone, and DMF, are used as the reaction medium. Both methods can produce carbon-based nanomaterials with controlled size and morphology, depending on the synthesis parameters, such as temperature, time, and concentration of the carbon source and solvent (Wang et al., 2020a; Huo et al., 2023).

These methods with the inclusion of microwave technology have proven to be excellent to obtain carbon nanomaterials from renewable and natural sources and residues, rendering this process more sustainable, green, and economically viable. In addition, they provide an alternative to recycling and reusing some residues, not only transforming them into value-added products but also addressing some environmental issues associated with the disposal of these materials. More information about various top-down and bottom-up synthesis approaches for carbon-based nanomaterials, including their merits and demerits are available in recent reviews by Abid et al. (Abid et al., 2022), El-Khawaga et al. (El-Khawaga et al., 2023), Gong et al. (Gong et al., 2023) and Huo et al. (Huo et al., 2023). According to these reports, these methods present some limitations such as the requirements for high temperatures and pressures, long reaction times, and difficulty in measuring, understanding, and controlling the synthesis/growth process of the nanomaterials (Abid et al., 2022; Huo et al., 2023). Hence, researchers have sought the development of microwave-assisted synthesis methods to address these shortcomings while achieving faster results through better-controlled reactions and product formation (Omoriyekomwan et al., 2019; Omoriyekomwan et al., 2021; Vignesh et al., 2022).

Gedye et al., in 1986, and Mingos and Baghurst in 1991, reported an unprecedentedly quick organic reaction under microwave irradiation (Gedye et al., 1986; Mingos and Baghurst, 1991). These were two of the earliest reports on the use of microwaves in chemical synthesis. Prior to these reports, Randall and Booth invented the magnetron, a device that generated fixed microwave frequencies, ushering in the era of microwave-assisted chemical processes (Cole and Cole, 1941; Kaul, 1989). As soon as scientists realized the magnetron’s amazing capacity for rapidly heating water, they began to investigate the mechanism at play, and subsequently, its potential for domestic, laboratory, and commercial applications (de Medeiros et al., 2019; Singh et al., 2019).

Domestic microwaves were used during the development phase of microwave-assisted chemical synthesis in research laboratories and unfortunately are still in use in developing countries across the globe (Monte-Filho et al., 2019). Although advanced microwave reactors currently exist (Figure 1), the use of more affordable kitchen microwaves for research has not been completely phased out. Some prestigious publishers reject manuscripts that include household ovens as heating sources. The reason why more advanced microwave equipment has been developed includes safety features offered by modern microwave reactors for chemical synthesis. Furthermore, the main benefit of using microwaves for chemical synthesis is superheating in an air-tight reaction vessel that can withstand high temperatures and pressures, since the Arrhenius rule allows for significantly faster reaction times.

Advanced microwave reactors function as highly practical autoclaves that can swiftly and effectively heat reaction mixtures to 300°C and 80 bar while allowing the reaction medium to stay sealed throughout the duration of the reaction. In contrast, domestic microwave ovens do not offer reaction parameter control and it is almost impossible to monitor the temperature and pressure of the reaction in real-time, with IR sensors for reaction temperature control, pressure sensors to keep track of the reaction pressure in the closed vessels, and a magnetic stirrer to facilitate agitation (Figure 2). Electromagnetic radiation between 0.3 and 300 GHz is referred to as microwaves. To prevent interference with telecommunications equipment, most typical microwave equipment emits electromagnetic radiation at 2.45 GHz.

The type of material being heated by microwaves and the amount of interaction between the sample and the radiation are key determining factors in how a reaction proceeds since conductors, dielectric materials, and insulators all have different characteristics. Following exposure of a material to microwave radiation, three different types of mechanisms have been described in the literature (Mishra and Sharma, 2016; Kahraman et al., 2020), namely reflection, absorption, and transmittance. Metals and other electrical conductors act as microwave radiation reflectors (Rafael Zamorano et al., 2019) while PTFE and quartz are two substances that permit microwave radiation transmission. Because insulators do not absorb microwave radiation, they are unable to be heated via this technology (Motasemi and Ani, 2012). In what concerns the reaction medium, or the solvent, for these reactions, polar liquids are excellent at absorbing microwaves. As such, they heat up quickly following microwave irradiation because of dipolar interactions and ionic conduction (Hu et al., 2021).

Polar molecules’ capacity to absorb microwave radiation and transform it into heat through a process known as dielectric heating is the key to the effectiveness and success of microwave-assisted chemical reactions (de Medeiros et al., 2019). When exposed to microwave radiation, polar molecules with electrical dipole moments rotate continuously in an effort to align themselves with the electric field. This phenomenon is caused by dielectric polarization and conduction loss. Through dielectric loss and molecular friction, the molecules reverse direction when the field alternates, thereby generating heat. The loss factor (tan δ) is a measure of a solvent’s capacity to transform electromagnetic energy from microwaves into heat. According to Table 1, solvents are frequently categorized as strong, medium, or weak microwave absorbers. A solvent’s microwave heating process is quicker and more effective with a higher value of tan δ. A low tan δ value, on the other hand, suggests ineffective and slow heating operation under microwave irradiation (de Medeiros et al., 2019).

Figure 3A demonstrates how microwave heating differs from traditional heating in that the center of the material is heated to a greater temperature than the medium allowing for a quick and efficient reaction. In contrast, under conventional heating, the vessel must be heated first followed by heating of the medium and eventually the heating of the sample. Under microwave heating, the molecules’ dipoles attempt to align with the applied electric field during irradiation in microwave-assisted synthesis (Figure 3B). Higher frequency field oscillations cause the dipoles to continually realign with the alternating electric field, which generates heat through molecular friction and dielectric loss. No heating occurs if the dipole doesn’t have time to realign (at much higher frequencies), or if it reorients too quickly (during low-frequency irradiation) (Sun et al., 2016; Jiang et al., 2023). Ionic conduction can also contribute to the heating mechanism. Under the influence of the microwave energy field, charged particles that are dissolved into the sample, in ionic form, bounce back and forth and subsequently produce heat by interacting with nearby molecules or atoms (Gabriel et al., 1998; Singh et al., 2019). This technique have been used to prepare different types of nanomaterials including metallic nanoparticles (Zhu and Chen, 2014; Tsuji, 2017), semiconductor quantum dots (Xuan et al., 2015), metal-organic nanomaterials (Thi Dang et al., 2020), polymeric nanomaterials (An et al., 2006), carbon-based nanomaterials (de Medeiros et al., 2019; Thakur et al., 2023), and many others.

Attention has been drawn to the synthesis of next-generation nano-carbon materials, including heterostructures and nanocomposites with improved performance and multifunctionality (Speranza, 2021). To determine the feasibility of prolonged utilization of microwave technology for developing advanced carbon-based nanomaterials like CDs, CNTs, and graphene-based materials (GBMs), the availability of carbon sources is crucial. These nanomaterials have so far been produced using carbon sources including citric acid, acetylene, benzene, methane, toluene, xylene, and other natural hydrocarbon resources (Liu et al., 2011; de Medeiros et al., 2019; Macairan et al., 2019; Sajid et al., 2022; Gulati et al., 2023; Liu et al., 2023). These hydrocarbons contribute to the release of CO₂ which may leak into the atmosphere, leading to global warming. Methane and other volatile hydrocarbons are greenhouse gases that act as more powerful heat-traps than CO₂ and contribute more to climate change than CO₂, which they release following complete combustion. Furthermore, these hydrocarbons are fast depleting and valuable carbon-based nanomaterials may experience a shortage issue in the future due to their reliance on these non-renewable resources. Hence, a more practical, affordable, and environmentally benign method and resources are required to sustainably prepare advanced carbon-based nanomaterials.

Microwave irradiation, which causes polymerization and carbonization, has been used to prepare carbon-based nanomaterials (Alarfaj et al., 2018). Carbon-based nanomaterials absorb electromagnetic energy during the microwave irradiation process and transform it into heat energy (Yang et al., 2018a). The molecular rotation of polar solvent molecules, caused by the interaction of electrical dipole moment with microwave energy, produces thermal energy. During microwave-assisted heating, electromagnetic radiation transfers thermal energy to the carbon precursor (Singh et al., 2019).

The preparation of carbon-based nanomaterials from biobased resources is essentially a top-down approach, which involves the break down of large carbonaceous materials, such as agricultural wastes, into nano-sized carbon structures (de Medeiros et al., 2019; Thakur et al., 2023). The microwave-assisted synthesis is effective for producing carbon nanomaterials and has the added benefits of rapid volumetric heating, high reaction rates, precise control over size and shape of nanomaterials by reaction parameter adjustment, and energy efficiency. Additionally, the homogeneous nucleation and growth conditions created by the homogenous heating of the precursors during microwave synthesis reduce thermal gradients and result in the development of nanomaterials with a uniform size distribution (Bacon et al., 2014).

Significant advancements in microwave instrumentation have aided in the synthesis of materials with improved reaction conditions control, producing materials with more desirable physicochemical characteristics. Commercial microwave reactors are now equipped with fiber-optic probes, magnetic stirrers, and sensors for more precise monitoring of the synthesis conditions, resulting in an enhanced reaction yield with higher reproducibility (Favretto, 2003; Ferguson, 2003; Luo et al., 2013b; Dudley et al., 2015), lower energy requirements, and shorter reaction times, among others (Figure 12). However, pressure sensitivity and thresholds (typically lower than 400 psi), and penetration depth limitations are one of the main drawbacks of microwave-assisted synthesis (Antonio De La et al., 2011; de Medeiros et al., 2019). The volume needed for commercial scale-up is also a challenge as conventional microwave reactors cannot accommodate large-scale synthesis to this day. There are also some restrictions on the size of the reaction sample for the microwave-assisted synthesis of carbon nanomaterials due to the short penetration depth in particular materials. In-situ measurements of the reaction conditions, particularly with metallic substrates, are the other major restriction of microwave-assisted synthesis. Metallic substrates can’t be used for material synthesis or processing since they interfere with the electromagnetic field (Bilecka and Niederberger, 2010; Schwenke et al., 2015).

The use of microwave reactors for chemical synthesis and industrial scale-up applications still faces several fundamental difficulties and there are knowledge gaps. Important features like heating homogeneity and rate may change during scale-up, and may significantly impact operating efficiency and product yield, yet these characteristic changes may occur via vaguely understood mechanisms (Sturm et al., 2014; Buttress et al., 2017). Some theoretical tools and predictive models have been developed to model large-scale applications of microwaves; however, the intricacy of the electromagnetic field distribution inside microwave reactors supports only qualitative results from these models (Robinson et al., 2010a; Robinson et al., 2010b; Li et al., 2023b). Despite all of this, these models provide valuable insights in addition to experimental results. A promising method of creating quantitative numerical models is now available thanks to recent developments in commercial software and computational resources (Yang and Chen, 2021). These models can guide the design of advanced reactors by predicting their performance and validating those predictions with experiments and clarifying scale-up behavior to increase industrial capacity. Techno-economic analysis such as a bottom-up or top-down cost method, can be used to estimate the budget for industrial scale-up, in terms of plant set-up, workers, electricity, waste generation, etc., to assess the technological and economic viability of carbon-based nanomaterials for water treatment (Ghosh et al., 2022; Mpongwana and Rathilal, 2022).

Carbon-based nanomaterials have demonstrated extremely high efficacy in water treatment, particularly when it comes to the elimination of both organic and inorganic pollutants. However, some carbon nanomaterials may not be the most appropriate choice for water treatment, if intended for direct use in membranes (Cao et al., 2022; Guo et al., 2022). Due to the presence of negatively charged groups, most carbon-based nanomaterials rapidly disperse in water, requiring high-speed centrifugation for their recovery (Almanassra et al., 2021; Abboud et al., 2022; Ge et al., 2023; Singh et al., 2023). Membrane fouling remains the main issue in the water treatment industry that can be reduced by utilizing carbon-based nanomaterials (Adeola and Forbes, 2021a; Singh et al., 2023). There is little techno-economic research on nanomaterials made of carbon and thus, challenging to determine the full potential of this class of nanomaterials in water treatment procedures, relying solely on adsorption performance.

In summary, microwave radiation has proven to be a very efficient heating source. It has the following benefits: 1) an effective heating source (superheating) that increases reaction rates and facilitates faster synthesis, 2) the ability to readily adjust instrumental or reaction parameters, 3) size and form control, 4) selective heating based on the idea that various materials react to microwaves differently, 5) due to homogeneous heating and improved process parameter control, chemical reactions are more reproducible under microwave heating than under traditional heating, 6) microwave chemistry may be combined with other established techniques, such as solvothermal and sonochemical techniques. On the other hand, the drawbacks of microwave-assisted synthesis include its lack of scalability (batch sizes are normally limited to a few grams), restricted applications (to materials that absorb microwaves), safety and health risks associated with the use of microwave-heating apparatus and cost of purchase of advanced microwave apparatus. Prior to industrial-scale utilization of advanced carbon nanomaterials synthesized with microwaves, commercial readiness, manufacturing scalability, availability at an economically viable cost, and post-treatment toxicity assessments are required. Furthermore, it will be useful to compare laboratory-scale and industrial site findings that will facilitate the development of carbon-based prototype filters and/or membranes for long-term wastewater treatment.

Advanced carbon-based nanomaterials, such as CNTs, CDs, graphene, and their composites along with other carbonaceous and non-carbonaceous materials, have been widely researched for the adsorptive removal of organic and inorganic pollutants in water. The development of effective methods for the separation and recycling of these contaminants is vital from environmental safety and process sustainability points of view. However, to facilitate the future design of carbon-based nanomaterials from bioresources via microwave technology and other techniques, it is important to have a thorough understanding of the bonding behaviors and intermolecular interactions using spectroscopic and microscopic characterization. The need for the techno-economic feasibility study for field-based/industrial-scaled applications of microwave-synthesized carbon nanomaterials from biobased materials cannot be over-emphasized.

The compendium in this comprehensive review (Tables 1–5), reveals that numerous carbon-based nanomaterials have been modified using functional groups, containing fluorine, nitrogen, oxygen, and sulfur, and via composite formation with other nanoparticles such as polymers, chitosan, magnetite, and biobased materials. These modifications create binding sites, large surface areas, and/or magnetic properties for optimal pollutant removal. In the mechanistic study of interactions between contaminants and carbon-based nanomaterials, a variety of approaches have been used, including zeta potential measurement, spectroscopic characterization, imaging techniques, theoretical calculations, and computational modeling. Electrostatics, complexation, π-π interactions, hydrogen bonds, hydrophobic interactions, covalent chemisorption, and van der Waals interactions are a few examples of potential sorbate-sorbent interaction processes that may occur. Future research can focus on several areas to address some lingering challenges or gray areas. The selective adsorption of chemical contaminants in aqueous matrices has not been thoroughly studied, and the principles underlying the selectivity are still not entirely understood.

In conclusion, due to the myriad of functional groups on the nanomaterials/nanocomposite, multifunctional carbon-based sorbents can be used to remove multiple contaminants simultaneously. It is possible to regenerate carbon-based nanomaterials depending on the nature of the sorbates, either by washing with suitable solvents, photodegrading, mechanical discharge, and/or combusting at appropriate temperatures, without inducing any significant damage to the nanomaterial. Physically adsorbed pollutants can be released more easily than chemically adsorbed ones since desorption requires less energy, thus preserving the carbon nanomaterials’ capacity for re-use. Recent developments demonstrate that under reasonably benign conditions, microwave technology would be an efficient method for producing pristine and functionalized carbon-based nanomaterials from biobased resources, with several benefits and prospective applications including the treatment of wastewater.