Synthesis of carbon-based nanomaterials and their application in pollution management

With the fast development of industry, large amounts of organic and inorganic pollutants are inevitably released into the natural environment, which results in the pollution of the environment and are thereby dangerous to human health. The efficient elimination of these pollutants is crucial to environment protection and human health. The high sorption capacity of carbon-based materials and high photocatalytic ability of carbon-based composites result in the application of carbon-based materials in environmental pollution cleanup. In this review article, we summarized recent studies on the synthesis of carbon-based materials, and their application in the sorption of organic and inorganic pollutants, the photocatalytic degradation of organic pollutants, and the in situ photocatalytic reduction–solidification of heavy metal ions. The sorption method is useful to remove pollutants from aqueous solutions. The sorption-photocatalytic degradation of organic pollutants is applicable, especially at low concentrations, whereas the catalytic reduction of metal ions is the best method for the in situ immobilization of high valent metal ions under complicated conditions. The interaction mechanism is discussed using advanced spectroscopy analysis and theoretical calculations, and at the end the challenges in the future are described.


Introduction
The development of modern society, the acceleration of urbanization, the growth of industrial production and the modernization of transportation have brought serious environmental pollution issues. The disturbance of the natural environment caused by human productivity and daily living activities is increasing. Harmful pollutants such as antibiotics, pesticides, dyes, heavy metals, greenhouse gases, endocrinedisrupting chemicals and volatile organic compounds are discharged into water, soil and air, posing a serious threat to the environment and human health. 1,2 According to the "2018 UN World Water Development Report", about 80% of wastewater from industrial and municipal activities are released into nature without pretreatment, directly or indirectly causing the deterioration of water quality. 3 Most wastewater contains toxic levels of metal ions, organic molecules, dyes and other carcinogenic chemicals, and should be pretreated before releasing into the natural environment. 4 Scientists have made great efforts to nd many effective materials to remove pollutants from wastewater, such as clay minerals, carbon materials, and natural/man-made nanomaterials. 5,6 Different kinds of these materials have been reviewed in the elimination of pollutants, and showed advantages/disadvantages in real applications. [7][8][9][10] Carbon-based nanomaterials have large specic surface areas, excellent acid stability, and thermal resistance, which have drawn much attention in the eld of pollution treatment because of their excellent physicochemical properties. 11 It is found that carbon materials can effectively remove pollutants such as nitric oxide, hydrogen sulde, heavy metals, dyes and drug compounds from the environment, with adsorption efficiency > 80% or photocatalytic degradation efficiency > 98%. 1 There has been a steady increase in reports of carbon-based nanomaterials in pollutant treatment over the last decade (2012-2021) (Fig. 1, retrieved 2012-2021 publications from the Web of Science Core Collection database using keywords of carbon based or carbon-based, nanomaterials or nanomaterial, and pollutants or environment or environmental). It turned out that more than 2000 records matched the selected keywords. Comparing the annual publications and citations from 2012 to 2021, it shows the research interest increase of carbon-based nanomaterials in the environmental pollution treatment eld.
From the bibliometric analysis of keywords co-found by Cite Space it is seen that several keywords (such as nanoparticle, carbon nanotubes (CNTs), nanomaterial, water, aqueous solution, graphene oxides (GOs) and multi-walled CNTs (MWCNTs)) were frequently used in the published papers, indicating that these carbon nanomaterials have received more attention in current research on carbon-based nanomaterials (Fig. 2). According to their shape, size and dimension, carbon nanomaterials 12,13 can be divided into zero-dimensional nanomaterials (buckminsterfullerene 14 and carbon dots (CDs) 15 ), one-dimensional nanomaterials (CNTs 11 and carbon nanobers (CNFs) 16 ), two-dimensional nanomaterials (graphene 17,18 ) and three-dimensional nanomaterials such as carbon sponges 1,19,20 (Fig. 3).
As a kind of zero-dimensional nanomaterial, C 60 has gained much attention owing to its unique structure and performance since its discovery in 1985. Aer several years of in-depth research, C 60 was found to have great potential as a cornerstone of molecular engineering, new material synthesis, supramolecular chemistry and pharmaceutical chemistry. 21 CDs and their composites are easy to obtain raw materials, and convenient to synthesize and function, and have unique optical properties and rich functional groups, which make them widely applied in electronic equipment, biomedicine, environmental pollution treatment and many other elds. 22 One-dimensional carbon nanomaterials mainly consist of CNTs and CNFs. CNTs are tubular structures formed from round graphene sheets and can be divided into single-walled CNTs, double-walled CNTs or Fig. 2 The analysis of keywords co-found. MWCNTs. MWCNTs are more easy to synthesize and can effectively remove metal ions such as Cr(III). 23 CNTs have high mechanical strength and chemical stability and have broad application prospects. As a typical two-dimensional nanomaterial, graphene is composed of a single layer or several layers of graphite, and has broad application prospects in environmental remediation. GOs are widely used for the treatment of wastewater, heavy metal ions, and organic compounds. 24 CNT sponge is a classic three-dimensional nanomaterial, which is a kind of high porous material with many different properties such as exible deformation and shape recovery. CNT sponges provide a common platform for the design and manufacture of functional composites at various levels by introducing polymer or inorganic guests. 18 In this review article, we mainly summarized the recent studies on the synthesis of carbon-based nanomaterials, the adsorption mechanism of organic pollutants and heavy metal ions, and the photocatalytic degradation of organic pollutants and in situ (photo)catalytic reduction-solidication of high valent heavy metal ions to low valent metal ions. The interaction mechanism was discussed from advanced spectroscopy analysis and theoretical calculations in detail.

Synthesis of carbon-based nanomaterials
2.1 Synthesis of low dimensional carbon nanomaterials 2.1.1 Carbon dots. The synthesis methods of CDs include hydrothermal/solvothermal, microwave, electrochemical, microplasma and chemical oxidation methods. The properties of CDs synthesized by different methods are different. 22 These methods can be generally divided into "top-down" and "bottomup" approaches, i.e., one is to use physical or chemical methods to cut a large number of carbonaceous materials into CDs, including electrochemical synthesis, chemical oxidation and solvent heat treatment; the other is to synthesize CDs from a precursor, using the method of hydrothermal treatment, microwave assisted synthesis, thermal decomposition or carbonization ( Fig. 4 and 5). 25,26 According to the structures, properties and preparation methods, CDs are mainly divided into carbon quantum dots (CQDs), graphene quantum dots (GQDs) and carbonized polymer dots (CPDs). 27 Specic CDs were prepared by optimizing the synthesis conditions such as time, temperature and voltage. To obtain independent puried and monodisperse CDs, the resulting products are usually further puried using either simple treatment steps (e.g. centrifugation, dialysis and ltration) or complex and precise treatment processes (e.g. electrophoresis, silica gel column chromatography, high performance liquid chromatography, etc.). 28 Hydrothermal/solvothermal synthesis is the most commonly used method for CD synthesis. The usual processes are as follows: the raw material is dissolved in water or solvent, poured into a Teon-lined stainless autoclave, placed in an oven and heated at a certain temperature for a period of time to obtain CDs. This method has many advantages, such as a simple process and easy operation, uniform synthesis of CDs with high yield, and easy to add other elements and adjust the diversity of CDs. 29 Su et al. 30 synthesized multifunctional CDs using one-step hydrothermal synthesis of nanotoners. By uorescence changes, the proposed CDs exhibited multiple functions of sensing temperature (induced uorescence enhancement, achieving a wide linear range from 25 C to 95 C), Ni(II) (specic uorescence quenching, detection range of 80-6000 mM) and doxycycline (uorescence from blue to green, 10 to 1000 mM can be visually observed) by virtue of three different routes (Fig. 6). 30 The preparation of CQDs by electrochemical carbonization was rarely reported. Zhang and co-workers 31 prepared CQDs by electrochemically carbonizing alcohols of low molecular weight, with two platinum plates as working and auxiliary electrodes and a calomel electrode mounted on a freely adjustable Luggin capillary as a reference electrode. Using electrochemical carbonization, alcohols were converted to CQDs. The size and graphitization of CQDs increased with the increase of application potential. 25 The CDs showed high sorption capacity of pollutants because of their high specic surface area, stability and abundant functional  groups, and therefore attract much attention in environmental pollution management.

Carbon bers & CNTs.
In the 1960s, carbon bers (CNFs) became one of the most important industrial materials of modern science and technology. CNFs were produced by melt spinning processes from carbon precursors. Polyacrylonitrile (PAN) was used as the main precursor for various modications, such as adding additives, oxidation stabilization at low temperature, and stretching during stabilization and carbonization. A catalytic chemical vapor deposition (CVD) process was applied for vapor grown carbon bers (VGCFs). Thin tubes consisting of straight carbon layers had been found in the center of VGCFs, which were named CNTs. It was reported to be formed via arc-discharging. CNTs are considered as important materials for the development of modern nanotechnology in the 21st century. 32 There are two main methods to synthesized CNFs, i.e., (1) catalytic decomposition of carbon precursors; and (2) carbonization and electrospinning of polymers. The most suitable method is selected according to the desired synthetic CNF properties. 33 CNFs are synthesized by CVD through decomposition of carbon containing gas catalyzed by metal particles. Common gases are acetylene, ethylene, methane, and propylene. Activated by dissolved KOH, the surface area of the CNFs increased 4-7 times (i.e., from 300-400 to 1700 m 2 g À1 ) (Fig. 7). 34 The most commonly used metal catalysts for CNF growth are Fe, Co and Ni. Cr, V and Mo have also been studied. 35,36 Compared to the production of other similar carbon nanostructures such as single-walled CNTs, the cost effectiveness of mass production of CNFs is signicantly improved. This is a particular advantage. 33 Electrospinning is an effective technique for the production of polymer nanobers. It is used to produce nanobers in different forms from tens of  nanometers to several microns. A strong electrostatic eld is applied to a capillary, which is connected by a polymer solution reservoir. 33 In electrospinning, polymer ber morphologies are signicantly affected by the polymer solution concentration, working distance, spinning voltage and feeding speed. The ber size decreases with the increase of carbonization temperature. 37 Arc discharge, laser evaporation and CVD are three main methods for preparation of single-walled CNTs (SWCNTs). 38 Arc discharge involves a closed reaction chamber, which is lled with H 2 , He and other gases. A thick graphite rod is used as the cathode, which is lled with Fe, Co, Ni and other metal catalysts as the anode, using the glow between the graphite electrodes to generate electricity. High purity SWCNTs can be synthesized in the inner wall of the reaction vessel. CNTs prepared by the arc discharge method have high yield, high crystallinity and a short preparation time. The layer number of CNTs can be adjusted by the selection of catalyst. The laser evaporation method is similar to arc discharge. The transition metal catalyst and graphite composite rod are vaporized by a laser in a high temperature quartz tube, and the product is deposited on a water-cooled copper column by owing inert gas to obtain CNTs. This method is not suitable for large-scale production because of its high energy consumption, high preparation cost and difficult popularization. CVD is used to catalyze the pyrolysis of ethanol, ethylene, ethane and other hydrogen compounds to produce CNTs at a high temperature. 11 CVD can produce CNTs in a large scale, including xed bed CVD, uidized bed CVD, plasma enhanced CVD and aerosol-assisted CVD. 39

Synthesis of high dimensional carbon nanomaterials
Graphene nanosheets are made up of a single-atom-thick hexagonal lattice of carbon atoms, each covalently bound to three others, whereas graphite consists of many graphene nanosheets with van der Waals bonding. In 2004, Geim and Novoselov 40 prepared graphene using Scotch tape and graphite, and thereby they were awarded the Nobel Prize for their work. Since the discovery, research on graphene had proliferated. Graphene has high strength and superconductivity. 41,42 It has many advantages for water treatment because of its thinness, light weight, photoactivity, high surface area and chemical stability. With these advantages, graphene has attracted multidisciplinary interest for many areas. 17 Graphene functionalization for environmental applications is mainly performed by chemical methods, including pure chemical processes (i.e., chemical oxidation and deposition) and extended chemical processes (i.e., electrochemistry, sol-gel, microemulsion and hydrothermal methods). These methods can introduce functional groups to carbon nanomaterials to achieve a variety of functions. 43 GOs have received great attention in the eld of environmental science because they can be prepared by simple chemical methods and have strong hydrophilic ability to form a stable GO sol suspension. In 1958, Hummers et al. 44 prepared high-quality GOs by treating graphite with a mixture of KMnO 4 , NaNO 3 and concentrated H 2 SO 4 ( Fig. 8a and b). The application of Hummers method for the preparation of GOs has the advantages of simple reaction, a relatively short time and high safety. Aer decades of development and improvement, the Hummers method has become a relatively mature method for GO preparation. On this basis, several methods for GO preparation by stripping graphite have been derived, as shown in Fig. 8c. 45,46 There are a variety of methods for synthesizing 3D carbon nanostructures. CNT sponge is a high-performance porous material with a variety of different properties such as exible deformation and shape recovery. It provides a common platform for the design and manufacture of functional composites at all levels by introducing polymers or inorganic guests. 19 Gui et al. 48 synthesized CNTs by CVD and controlled the synthesis of sponge CNT blocks with isotropy and a exible independent structure. CNT sponge consists of overlapping CNTs which interweave themselves into a highly porous interconnected 3D framework (Fig. 9). 20,49 From the abovementioned methods for the synthesis of carbon-based nanomaterials, one can see that it is still difficult to synthesize carbon-based nanomaterials with high quality on a large scale and at a low price. However, for the application of carbon-based nanomaterials in the efficient elimination of pollutants from wastewater, the nanomaterials should have a high specic surface area with large amounts of functional groups and abundant binding sites. The quality of the nanomaterial is not essential for the materials to have high sorption capacity.

Adsorption of pollutants
The functionalized derivatives of carbon-based nanomaterials have high specic surface areas and abundant adsorption sites, which can form strong complexes with heavy metals and organic molecules on solid surfaces and thereby can remove pollutants efficiently from wastewater. 22,50,51

Adsorption of metal ions
In terms of the adsorption of metal ions from aqueous solutions, physical adsorption, precipitation, sorption/reduction, ion exchange and electrostatic interactions dominate the interaction between heavy metal ions and surface functional groups, which contribute mainly to the uptake of metal ions by carbon nanomaterials, depending on the surface properties of the nanomaterials and functional groups (Fig. 10). 43,52 In general, the interaction between metal ions and surface oxygen-containing groups plays an important role in the adsorption capacity of carbon-based nanomaterials for heavy metals. CNT raw materials typically require acid treatment or oxidation treatment to increase their surface oxygen-containing functional groups, whereas GOs do not require additional acid treatment or oxidation treatment because a large number of oxygen-containing functional groups have been introduced to GOs in the preparation process. 18, 53 Wang's group rst reported the surface modication of CNTs with a plasma technique, and their application in the removal of metal ions. 54,55 In 2011, the earliest results of GOs for the removal of heavy metal ions (i.e., Co(II), Cd(II)) 18 suggested that GOs are very suitable materials for the elimination of pollutants in pollution cleanup. The high dispersion properties of GOs in solutions can greatly increase the interactions of GOs with metal ions, thereby increasing the sorption ability of metal ions to GOs. The oxygen-containing functional groups could avoid the aggregation of GOs and improved the available sorption sites for the binding of metal ions, which increased the sorption capacity of metal ions. 56 In addition, GOs can be functionalized with metal oxides and organic chemicals, which also could improve the sorption capacity. 47 The surface electrokinetic potential was dependent on pH values, and also changes the surface properties of carbon nanomaterials and thereby affects the sorption of metal ions. 23 The kinetic sorption of Cr(III) on CNTs increased quickly at the initial contact time (<10 minutes), then the sorption equilibration was reached (contact time from 10 to 60 minutes), and at last part of surface adsorbed Cr(III) was desorbed from CNT surfaces.
Heavy metal ion sorption on GOs is mainly dominated by ion exchange, electrostatic attraction, and surface complexation 46,56,57 ( Fig. 11a and b). However, in the real polluted environment, multiple pollutants always coexist. The simultaneous removal of two or more metal ions from wastewater by functionalized carbon nanomaterials is most important for wastewater treatment (Fig. 11c). Most research studies on the application of carbon nanomaterials for the elimination of metal ions are still at the laboratory level. There is still a lack of research on the removal of combined pollutants by carbon nanomaterials, but these studies are very important. 43 The application of CDs in the adsorption of U(VI), Eu(III), Sr(II) and other radionuclides is reviewed, and the contents indicated that CDs could efficiently remove radionuclides from solutions. 22 Wang et al. 58 prepared CDs by a microplasma-assisted in situ method and introduced them into functionalized ordered mesoporous silica (SBA-NH 2 ). The composite retained the high specic surface area of mesoporous silica and the uorescence properties of CDs. It has high adsorption efficiency for uranium and the adsorption process can be monitored simultaneously (Fig. 12).
Based on the abovementioned results, one can see that carbon-based nanomaterials have high sorption capacity, which is most important for their application in environmental pollution cleanup. With the development of technology, the price for the synthesis of carbon-based nanomaterials on a large scale will decrease. Although the purity of carbon-based nanomaterials is not high enough when they are prepared on a large scale, the high sorption ability ensures their application in the preconcentration of heavy metal ions from large volumes of wastewater. The high surface areas, abundant functional groups and available free sites for the binding of metal ions are most important for the preconcentration of metal ions from

Review
Nanoscale Advances aqueous solutions. However, for the highly selective removal of target metal ions from complicated solutions, the surface graing of special functional groups, which could form selective surface complexes with target metal ions, could improve the selectivity of metal ions binding on carbon-based materials. Another method is the synthesis of nanomaterials with special structures and functional groups, which could selectively adsorb target metal ions.

Adsorption of organic pollutants
Different types of carbon-based nanomaterials have different surface functional groups, which indicate that the interaction mechanism of organic pollutants with carbon-based nanomaterials is relatively different. However, physical adsorption, electrostatic action, p-p action, Fenton/Fenton-like processes and photocatalytic degradation are generally considered the main reactions for the elimination of organic pollutants. 5 Zhao et al. 59 rst reported the application of GOs for the removal of persistent aromatic pollutants (i.e., naphthalene and 1-naphthol) and the results showed an adsorption capacity of 2.3-2.4 mmol g À1 for 1-naphthol and naphthalene, which was the highest adsorption capacity of today's nanomaterials, suggesting the great potential of GOs in environmental pollution cleanup. Jeong et al. 60 applied carbon-based nanomaterials including single-  walled CNTs (SWCNTs), multi-walled CNTs (MWCNTs) and expanded graphite (EG) in wastewater treatment, and the results showed that MWCNTs had much higher adsorption capacity than EG. The EEM images of Suwannee River Natural Organic Matter (SRNOM) have the typical fulvic acid-like and humic acidlike peaks, at Ex/Em of 220-250/400-450 and 300-350/400-450, respectively. The EEM images of residual SRNOM do not show a notable peak shi. However, the uorescence intensity decreased. The decrease was more signicant for the humic acidlike peak than the fulvic-like peak. Humic acid-like, fulvic acidlike, and SMP-like (Ex/Em 250-300/330-380 nm) peaks were found in EEM images. Aer the adsorption of SRNOM, the peak intensity of SWCNT greatly decreased. The SMP-like peak disappeared. For MWCNTs, the fulvic acid-like peak decreased more signicantly than other peaks similar to the SRNOM results. For EG, the SMP-like peak was slightly decreased, while a uniform decrease of the three peaks was observed. The uorescence intensity spectra were in good agreement of the EEM images. 60 Chen et al. 50 studied three kinds of carbon nanomaterials including SWCNTs, oxidized SWCNTs and non-porous graphite powder for the removal of six compounds (thiophene, pyrimidine, benzene, aniline, 2-aminopyrimidine and 4,6-diaminopyrimidine) from aqueous solutions. They found that the affinity of different organic molecules on the three adsorbents varied greatly. The heterocyclic compounds contained N and S sites, which was helpful to improve the adsorption through non-hydrophobic action. The oxidation degree of the adsorbent and the solution pH affected the adsorption of N-containing heterocyclic compounds to the solid phases. The adsorption of organic molecules on carbon-based nanomaterials depends on the properties of organic molecules and the surface properties of nanomaterials. It is well known that the surface properties of materials and the organic molecule properties are dependent on solution conditions, such as pH, ionic strength, temperature etc. The properties of nanomaterials are related to the structures, surface functional groups, the synthesis conditions and the precursor used for the preparation of nanomaterials. The competition among different organic molecules in complicated solutions also affects the binding of target organic molecules on nanomaterials.
For the efficient elimination of target organic molecules from wastewater, it is necessary to consider all the above-mentioned parameters to evaluate the suitable materials and best conditions for the selectively and highly efficient elimination of organic pollutants from solutions. The surface properties of carbon nanomaterials are dependent on solution conditions. The properties of organic molecules are also affected by solution conditions. For the application of carbon-based nanomaterials in the elimination of organic pollutants, solution conditions, material properties such as structures and functional groups and organic molecule properties should be taken into consideration.

Photocatalytic removal of pollutants
Photocatalytic degradation of organic pollutants and sorption/ photocatalytic reduction of metal ions under visible light irradiation are useful methods for the efficient degradation of organic molecules or in situ solidication of metal ions, especially at low concentrations. In this section, we mainly discussed the photocatalytic degradation of organic pollutants and photocatalytic degradation of heavy metal ions. 60,61

Photocatalytic degradation of organic pollutants
Although adsorption has been widely used in the treatment of organic pollutants, it also has its disadvantages, such as the rerelease of pollutants aer the aging of materials. Photocatalytic degradation provides a more efficient solution. Organic molecules can be degraded to carbon dioxide and water under the action of a photocatalyst. In photocatalysis, the absorption of UV/visible light to generate e À -h + pairs, the excited charge separation, the transfer of holes and electrons to the catalyst surface, and the generation of active species are necessary for organic molecule degradation. For the effective photocatalytic degradation of organic pollutants, the sorption of organic pollutants on the catalyst is crucial for the degradation reaction of organic molecules by the active species. Carbon-based nanomaterials themselves can be used as photocatalysts. For example, carbon points can degrade rhodamine B (RhB) directly, and benzyl alcohol can be selectively converted to benzaldehyde. Aer being modied by ZnO, TiO 2 , SiO 2 , CdS, AgNPs, ammonia and sulfuryl chloride, carbon-based nanomaterials can degrade dyes in printing and dyeing wastewater. 62 When used for pollutant degradation, CDs were mainly used for photocatalytic degradation of organic pollutants. 63 CNTs were attached to the surface of porous carbon sponge in a carpet-like vertical arrangement, and used to adsorb methylene blue in wastewater. It was found that the number of CNTs on the sponge determined the adsorption performance of the dyes. 64 The effect of xenon lamp irradiation on RhB was studied, and it was found that the removal rate of CDs to RhB reaches almost 100% within 120 min under xenon lamp irradiation, and the photocatalytic efficiency gradually improved with the increase of CD content. In the study carried out by Cheng et al., 63 high quantum yield uorescent carbon quantum dots (CQDs) were prepared by a hydrothermal method using aqueous mesophase pitch (AMP) as the carbon source. The CQDs were modied by ammonia and thionyl chloride, respectively. The CQDs showed good photocatalytic degradation performance of RhB, methyl blue (MB) and indigo carmine (IC) (Fig. 13a). The degradation rate of N-CQDs to RhB reached 97% in 4 h under natural light irradiation with the degradation rate constant of 0.02463 min À1 . Aer repeated use for 5 times, the degradation rate still remained 93%. 63 Li et al. 65 prepared up-conversion photoluminescence CDs by an alkali-assisted electrochemical method and used it as an efficient nearinfrared photocatalyst for selective oxidation of benzyl alcohol to benzaldehyde with a high conversion rate (92%) and high selectivity (100%). Huang et al. 66 prepared a CDs/ZnFe 2 O 4 photocatalyst by a simple hydrothermal method, whose transient photocurrent response was about 8 times higher than that of the original photocatalyst. Under visible light irradiation, CDs/ ZnFe 2 O 4 nanoparticles can selectively remove gaseous NO x and form nitrate (Fig. 13b). The introduction of CDs into metal-free photocatalysts could greatly improve their catalytic performance. Recently, Liu's group prepared a metal-free CDs/g-C 3 N 4 photocatalyst for the degradation of diclofenac. 67 The modication of CDs did not have an obvious effect on the morphology and structure of g-C 3 N 4 , while it drastically inuenced the optical properties so that the CDs/g-C 3 N 4 composite could be applied to the photocatalytic degradation of diclofenac. 26 In the treatment of printing or dyeing wastewater, photocatalytic degradation of dyes is an effective economical technique for the elimination of dyes, especially at low concentrations. 68 Different kinds of catalysts such as TiO 2 , ZnO, CdS and Ag nanoparticles have been extensively investigated for the photocatalytic degradation of dyes. [69][70][71][72] Zhang and Lu 73 treated wastewater containing reactive brilliant blue KN-R using a TiO 2 /BC photocatalyst. The decolorization efficiency (81.09%) of KN-R under a UV Xe lamp (300 W) was higher than that of KN-R by a TiO 2 /BC composite without a xenon lamp. Under the condition of no light source, the decolorization rate of active brilliant blue KN-R by original BC reached 65.48%. The original BC showed excellent performance in the decolorization of reactive brilliant blue KN-R, which was due to the appearance of neat large pores with a diameter of 15-20 mm during the adsorption process. When TiO 2 absorbs enough photon energy equal to or higher than the energy band (E gap of TiO 2 is 3.2 eV), TiO 2 NPs can generate H + holes and e-electrons. The photogenic hole H + reacted with H 2 O and OH À to generate a hydroxyl radical, and the photogenic electron is captured by O 2 to generate a superoxide radical (Fig. 13c). The decolorization rate reached 99.7% and 97.0% within 60 min, respectively, under strong acid and strong base conditions. In addition to TiO 2 , Khataee et al. 74 prepared a novel cerium oxide nanoparticle doped BC (CeO 2 -H@BC) composite and applied it for the degradation of active red 84 (RR84) with the degradation rate of 98.5%. The synergistic effect between cerium oxide nanoparticles and BC improved the photocatalytic degradation capacity. 5 The carbon-based photocatalysts are stable enough and could be applied for the photocatalytic degradation of organic pollutants under visible light conditions, which is critical for the application of carbon-based catalysts in the photocatalytic degradation of dyes.
For organic pollutants at low concentrations, photocatalytic degradation of organic molecules to form the nal products (theoretically the best results are CO 2 and H 2 O) is the best method. The absorption of visible light is most important to improve the photocatalytic ability of the catalyst under visible irradiation conditions. The doping of elements and change of catalyst structure with other materials are good methods to change the catalyst properties such the band gap, and band energy level. The relative contribution of reactive species, the intermediate products in the photocatalytic process, the fast carrier recombination and charge separation/transfer are still unclear now. The stability and reusability of catalysts are the fundamental properties for the application of carbon-based nanomaterials in real applications to avoid possible pollution by the catalysts.

Photocatalytic reduction of metal ions
It is well known that the sorption method is the most efficient approach to remove heavy metal ions from wastewater.
However, the sorption technique is not useful in some cases, such as for soil, river, lake, and mine pollution. The solidication and immobilization of metal ions is suitable to decrease the heavy metal ions' pollution. Photocatalytic reduction of a high valent element to a low valent element under visible light irradiation is one of the best methods for the precipitation and immobilization of high valent metal ions such as Cr(VI) to Cr(III), and U(VI) to U(IV). [75][76][77] The sorption-reduction of As(V) by biochar-nZVI showed that As(V) could be reduced by nZVI and then formed precipitates on biochar-nZVI. 78 The sorption-reduction of Cr(VI) in soil by carboxymethyl cellulose-supported nanoscale iron sulde biochar composites was studied and the results suggested that Cr(VI) was reduced to Cr(III) to form Cr 2 O 3 or Cr(OH) 3 . The composites could efficiently immobilize Cr(VI) and converted the more toxic Cr(VI) to less toxic Cr(III), thereby reducing the bioavailability of toxic Cr(VI). 79 Wang et al. 80 synthesized C 3 N 4 and applied it for the sorption-photocatalytic reduction of Cr(VI) and U(VI) in the presence of bisphenol A. The results showed that the presence of bisphenol A and Cr(VI) could enhance the photocatalytic reduction of U(VI) to U(IV). The electron spin resonance analysis and radical scavenger effects showed that the conduction band edge and photogenerated electrons mainly dominated U(VI) photocatalytic reduction, whereas H 2 O 2 contributed to Cr(VI) photocatalytic reduction (Fig. 14). Li et al. 81 found a new eco-friendly method for the selective and rapid photocatalytic reduction-precipitation of U(VI) without a catalyst, which is useful for the extraction of U(VI) from complicated solutions. In the presence of alcohols and under visible light irradiation, U(VI) could be photocatalytically reduced to U(V) and then transformed into U(IV) through a disproportionation reaction. This nding provided an excellent selective extraction of U(VI) in the presence of other kinds of metal ions and lanthanides. Yang et al. 82 synthesized an ironnitrogen-carbon catalyst and applied it for efficient extraction of U(VI) from seawater. The amidomine functional groups are specic binding sites for the capture of U(VI), and then the surface adsorbed U(VI) was reduced to U(V) by the FeN x centres. The unstable U(V) was oxidized to form solid Na 2 O(UO 3 $H 2 O) x , which can easily be collected from seawater (Fig. 15).
The sorption-(photo)catalytic reduction-solidication of high valent metal ions to low valent metal ions is an efficient method for the extraction or in situ precipitation of high valent metal ions. In the natural environment, it is not necessary to extract metal ions from sediments of rivers or lakes, polluted soils or mines because it is nearly impossible to separate the nanomaterials from the systems. The in situ immobilization/ solidication of heavy metal ions is one of the best methods to decrease the mobilization of active free metal ions in the environment, and thereby decreases the possible transfer of toxicity of metal ions from the environment to plants or other biological forms. In future, the in situ reduction-precipitation of active high valent metal ions is expected to be one of the best techniques to decrease the toxicity of metal ions.

Interaction mechanism discussion
An understanding of the interaction mechanism of metal ions and organic molecules with carbon-based nanomaterials is helpful to evaluate the potential behavior of pollutants in the environment and also useful for the synthesis of nanomaterials. The sorption mechanism of pollutants from batch techniques is generally evaluated considering the effect of pH and ionic strength. The pH-dependent sorption is mainly dominated by surface complexation, whereas ionic strength-dependent sorption is mainly dominated by ion exchange. At the molecule level, time resolved laser uorescence spectroscopy analysis could provide the species of metal ions such as the coordination number and H 2 O number in the rst coordination sphere, which is helpful to evaluate the formation of inner-sphere or outer-sphere surface complexes. XAFS spectroscopy could provide the unique structure and composition information of metal ions on nanomaterials such as the bond distance, and coordination number, which are helpful to discern the sorption mechanism such as inner-sphere or outer-sphere surface complexation, (co)precipitation, oxidation/reduction etc. 83  XPS analysis could provide some information about the interaction of pollutants with functional groups. From the red/blue shi of peak position or appearance of new peaks, one can draw a conclusion about the complexation of special functional groups with pollutant molecules. 85 Advanced spectroscopy analysis is useful to provide some information about the interaction of pollutants with nanomaterials at the molecule level. More importantly, the combination of several spectroscopy analyses is more important to understand the interaction mechanism.
Besides the advanced spectroscopy analysis, density functional theory (DFT) is an indispensable method to simulate the whole reaction processes. For some reactions, it is difficult/ impossible to measure the intermediate products or the contribution of active species from experimental analysis. The DFT calculation could simulate the intermediates, transition states, structures and species of the organic pollutant degradation processes. The interaction energies, bond distances, cluster sizes and radial distribution functions in the possible pathway could also be calculated to understand the possible mechanism. From the results of the batch technique, advanced spectroscopy analysis and DFT calculations, one can draw a conclusion regarding the pollutant molecule interaction mechanism with carbon-based nanomaterials.

Conclusion and perspective
With a deep understanding of the environmental remediation of pollutants using carbon-based nanomaterials, scientists are paying more and more attention to this eld. The synthesis methods and removal mechanism of carbon-based nanomaterials are reviewed herein. According to the material dimension, carbon-based nanomaterials can be roughly divided into four types: zero dimensional materials, one dimensional materials, two dimensional materials and three dimensional materials.
The application of carbon-based nanomaterials in environmental pollution management is mainly realized through adsorption of porous structures and functional groups. For organic pollutants, photocatalytic degradation is an efficient technique for elimination of organic molecules, especially at low concentrations. The nal products of CO 2 and H 2 O are the best results in the photocatalytic degradation of organic molecules. For heavy metal ions, besides the adsorption of metal ions on carbon-based nanomaterials, the photocatalytic reduction of high valent elements to low valent elements, and then the formation of in situ precipitates could immobilize the metal ions in the natural environment.
Carbon-based nanomaterials still face the problems of high production cost and difficulty in mass production, leading to the development of most nanomaterials still being in the laboratory stage, which is difficult for large-scale production and use. Nowadays, more and more attention has been paid to environmental pollution control in order to protect the human homeland. It can be predicted that more attention should be paid to cost reduction and mass production in the future research and development of carbon-based nanomaterials.
With the development of techniques, carbon-based nanomaterials could be produced on a large scale at a low price. The toxicity of carbon-based nanomaterials in the natural environment should also be considered because they are inevitably released into the environment.
When carbon-based nanomaterials are used in environmental pollution cleanup, it is inevitable that some carbonbased nanomaterials are released into the environment during their synthesis processes and nanomaterial residue in the environment should also be considered. The toxicity of carbonbased nanomaterials should be considered although the research of nanomaterial toxicity in the environment is still at the preliminary stage.
The photocatalytic degradation of organic molecules is still a "black-box" process, especially the analysis of reactive species contribution, and the measurement of intermediate products, which are helpful to evaluate the catalytic properties of nanomaterials. The on-line measurements of intermediates are still very difficult quantitatively measured. Quick on-line analysis may be developed in future to understand the photocatalytic degradation mechanism of organic pollutants.

Conflicts of interest
The authors declare that there is no conict interesting.