Recent advances in the environmental application of graphene-based composites

Abstract

Graphene-based composites have been widely applied in environmental remediation owing to their high removal capacity. However, systematic reviews concerning the application of graphene-based composites have not been available for the last five years. In this review, the modification of graphene-based materials (e.g., various nanoarchitectures and hybrids (element-doped as well as metal oxides/ and polymer/graphene composites)) in recent years is primarily summarized. Thereafter, recent advances in the environmental remediation of graphene-based composites (i.e., removal of organics (dyes, EDCs, antibiotics and others), heavy metals (Cr(VI), Pb(II), Cd(II) and others) and radionuclides (U(VI), Sr(II), Cs(I) and others)) are described in detail. In addition, the interaction mechanism between environmental pollutants and graphene-based materials (i.e., physical/chemical sorption, surface complexation and redox) is demonstrated. Moreover, the current challenge and perspectives of graphene-based composites are proposed briefly. The novelty of this review lies in the comparison of the removal performance of different graphene-based materials towards various environmental pollutants. This review provides significant reference for readers to apply graphene-based composites in actual environmental cleanup situations.

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Environmental significance

Although there are many reviews on the environmental application of graphene-based composites, recent advances concerning the removal of various pollutants using graphene-based composites are not available. In this review, the removal of organics, heavy metals and radionuclides using various graphene-based composites was reviewed in detail, which is crucial for environmental scientists and engineers for applying graphene-based composites in actual environmental remediation.

1 Introduction

As a new allotrope of carbon, graphene is a single-layered hexagonal lattice made of sp²-hybridized carbon atoms from the delocalized network of π orbitals, giving a fully π–π long-range conjugated 2D structure.^1–3 Thus, the inherent properties (e.g., large specific surface area, extremely high electrical conductivity, super high strength, and better optical properties) of graphene have great potential for multiple applications in materials science, chemistry, physics, engineering and science technology (Fig. 1A). The large number of published articles in 2019–2022 concerning graphene (Fig. 1B) indicate that graphene is currently a research hotspot in various fields. In recent decades, graphene-bearing composites have also been widely used in environmental remediation.^4–8 For instance, Hu et al. reviewed the adsorption of various emerging contaminants on graphene and its derivatives (e.g., doped, modified, and composited) in detail.⁹ However, a systematic investigation on the recent advances concerning the removal of organics, heavy metals and radionuclides using graphene-bearing composites is not available.

Fig. 1 The numbers of published articles concerning graphene (as a keyword in Web of Science) in different fields (A) and years (2019–2023, B).

In this review, the functionalization of graphene and its derivatives (e.g., graphene oxides (GOs), reduced graphene oxides (rGOs)) and graphene-based composites (e.g., metal (hydr)oxide-, inorganic- and polymer–graphene) is primarily summarized in section 2. Thereafter, the environmental application (i.e., the removal of organics (dyes, EDCs, antibiotics and others), heavy metals (Cr(VI), Pb(II), Cd(II) and others) and radionuclides (U(VI), Sr(II), Cs(I) and others)) of graphene-based composites and their interaction mechanism is detailed in section 3. Finally, the current challenges and further perspectives of graphene-based composites are proposed briefly in section 4. This review could be helpful for environmental researchers to apply graphene-based composites in actual environmental cleanup situations. The uniqueness of this review is the investigation of the recent advances of graphene-based materials in environmental remediation, broadening the application field of graphene-based materials. The advantage of this review is the comparison of the removal performance of different graphene-based materials. The novelty of this review is the demonstration of the interaction mechanism of graphene-based composites with organics, heavy metals and radionuclides.

2 The functionalization of graphene-based composites

The versatile properties of graphene (i.e., large specific surface area, controlled pore size, strong electrostatic interaction and preferable dispersion) can be doped and modified by introducing various oxygen-containing functional groups (i.e., hydroxyl, carboxyl, and ketone), which significantly increase the adsorption rates and adsorption efficiency in actual environmental remediation applications.^10,11 Generally, graphene-based composites were mainly modified using bottom-up (e.g., liquid-phase, surface-assisted and chemical vapor deposition) and top-down approaches (e.g., chemical oxidization, electrochemical exfoliation, mechanical exfoliation).^12,13

2.1 The design of the nanoarchitecture

Graphene with different nanoarchitectures (e.g., graphene nanoribbons (GNRs), graphene quantum dots (GQDs), graphene nano meshes (GNMs) and 3D graphene sponges) are created by changing its composition or micromorphology. The current design methods of graphene with different nanoarchitectures in large-scale production involve exfoliation¹⁴ and chemical vapor deposition.^15,16 Vinchon et al. found that the damage of graphene at grain boundaries was slower than the interlayer with self-healing ability by local defect migration and structural recovery (Fig. 2A).¹⁷ GNRs have widely attracted great interest owing to their highly tunable electronic, optical and transport properties.^18,19 Recently, Kolmer et al. directly presented atomically precise GNRs via using the semiconducting metal oxide.²⁰ Niu et al. synthesized periodic sub-porous GNR via Scholl reaction of the polyphenylene precursor with preloaded hexagonal nanopores, which greatly reduced the π-conjugation and mitigated the interband π–π interactions.²¹ Based on an iterative synthesis strategy, Yin et al. designed GNRs by incorporating oligomeric chains in each chemical cycle. This culminated in the surface-assisted generation of nanobands through a protecting group, which precisely controlled the sequences, lengths and shapes.²² The appropriate precursor molecular design of the GNRs nanostructure exhibited superior physicochemical properties. GQDs as flat 0D nanomaterials have attracted increasing interest in environmental applications due to their exceptional chemicophysical properties. The synthetic strategies of GQDs include top-down (cutting the larger graphitized carbon materials) and bottom-up methods (fusion of small precursor molecules).^4,23 GNMs as 2D nanoporous materials have attracted much attention for molecular nanofiltration due to their large surface area and mechanical strength.^1,24 3D graphene sponges can be used as potential adsorbents due to their remarkable uniform pore size and high adsorption performance, which were generally synthesized using a hard template method.^25,26

Fig. 2 Morphologies of various graphene-based composites. A: Original graphene,¹⁷ reproduced with permission from ref. 17, Copyright, 2021, Springer Nature. B: GO,²⁷ reproduced with permission from ref. 27, Copyright, 2018, Springer Nature. C: N-doped graphene,¹⁹ reproduced with permission from ref. 19, Copyright, 2021, Springer Nature. D: GO/polymers,²⁸ reproduced with permission from ref. 28, Copyright, 2023, American Chemical Society.

Recently, graphene with different morphologies was obtained using different carbon resources such as coal, biochar, carbon black, discarded food and mixed plastic wastes.²⁹ Luong et al. synthesized the gram-scale yield (80–90%) of highly pure graphene (99%) by using the flash Joule heating (FJH) of a carbon source, such as high-carbon source, discarded food, rubber tires and mixed plastic waste.²⁹ The design of graphene using gas-phase microwave and plasma-assisted technique is a promising method to obtain flake-like graphene with excellent quality. Musikhin et al. investigated the gas-phase graphene synthesis under low carbon and high hydrogen concentrations, whereas soot-like graphene was formed at high carbon and low hydrogen concentrations.³⁰ Dong et al. reported on the highly dispersed graphene aerogels using the non-dispersion strategy, which can be directly used for 3D printing.³¹

2.2 The construction of hybrids

To meet the economic and technological developments of industrial applications, the functionalization of high-quality graphene-based hybrids has been widely investigated using low-cost and efficient synthetic strategies. The several empty spaces in graphene offer opportunities for the construction of graphene-based hybrids, such as nucleophilic, cycloaddition, free radical, and substitution.³²

2.2.1 The functionalization of graphene oxides. Graphene oxide (GO) is generally synthesized by the strong oxidation of graphite using Hummers or Brodie oxidation methods.^27,33 However, these methods suffer from explosion risk, serious environmental pollution and long reaction times (up to hundreds of hours). Recently, the fast, scalable, safe and green synthesis of GO has been reported. For instance, Pei et al. synthesized GO within a few seconds by the water electrolytic oxidation of graphite (Fig. 2B).²⁷ GO has been widely investigated in environmental remediation due to the various advantages, such as low cost, large-scale, easy processing and massive oxygenated functional groups (e.g., carboxyl (O–C=O), hydroxyl (–OH) and epoxy (C–O–C) groups), which allow GO to have excellent dispersibility in water and provide reactive sites. The excellent mechanical properties (e.g., amphiphilicity) of GO was conveniently frequently used as an adsorbent in environmental remediation due to its reusability and environmental friendliness.^34,35 Some economical and environmentally friendly approaches were applied to synthesize rGO to reduce poisonous byproducts. Guo et al. designed GO by epoxide ring-opening reaction and covalent conjugation, which still retained the structure and properties of GO.³⁶ Fan et al. proposed the hierarchical structure of GNRs/amorphous carbon using the one-pot “double-catalysts” strategy, which displayed excellent oxygen reduction reaction (ORR) electrocatalytic performance due to its high onset potential, half-wave potential, low Tafel slope, and long-term stability.³⁷

2.2.2 The functionalization of element-doped graphene. At present, the functionalization of graphene with element doping (e.g., nitrogen, sulfur, boron and metal atoms) is an efficient modulating strategy via physical and chemical methods.^18,19,38 Blackwell et al. investigated the spin splitting of the dopant edge state in zigzag graphene nanoribbons by replacing every sixth C–H with a N atom (N-6-ZGNRs, Fig. 2C).¹⁹ Chen et al. directly synthesized dispersed sulfur-doped graphene with excellent optical, electrical (e.g., electrochemical oxygen reduction reaction) and mechanical properties.³⁹ Su et al. prepared N-doped graphene by adjusting the pyrolysis temperature, featuring an effective electrochemical activity.⁴⁰

2.2.3 The functionalization of metal oxides/graphene composites. Various graphene derivatives (e.g., GO/metal oxides, rGO/metal oxides) have been widely functionalized in recent years.⁴¹ Wu et al. synthesized N-doped rGO/cerium oxide with low bulk density and remarkable net structure through hydrothermal route.⁴² Recently, graphene-based hybrids with multiple metal/metal (hdry)oxide nanoparticles have been successfully generated using a variety of methods. The performance of the graphene hybrid is preferable to those of the individual components, which is promising for applications such as batteries, adsorption and catalysis. Ma et al. prepared 3D ZnO/graphene by combining the calcination of the metal–organic precursor zinc methacrylate and self-assembly homogeneous anchoring of ZnO on graphene. The as-prepared architectures displayed the enhanced electrochemical properties in capacitors due to the prevention of graphene from restacking and increase of ZnO conductivity.⁴³ Mady et al. synthesized 3D γ-MnO₂@ZnFe₂O₄/rGO nanohybrids via one-pot hydrothermal self-assembly method.⁴⁴

2.2.4 The functionalization of polymer/graphene composites. The significance increase of the mechanical, thermal and durability properties of polymer/graphene composites depends on the functionalization method, binding locations (top, edge and top-edge) and surface area.⁴⁵ Generally, the functionalization method of polymer/graphene includes grafting-from (growth of polymer from surface) and grafting-to (end-tethering to the surface) methods.⁴⁶ A variety of different types of polymerizations (e.g., wrapping, layering, sandwich-style assembly, anchoring, mixing and encapsulation) have been employed by using different carriers such as atom transfer free radical polymerization, reversible addition–fragmentation chain transfer, Ziegler–Natta, ring-opening and electro-polymerization.⁴⁷

MOF/graphene composites exhibited the smaller particle size (100–200 nm) and enhanced MOF dispersion.⁴⁸ Ngo et al. developed a GO/polymer via coalescence of GO-stabilized Pickering emulsions around a porosity-generating polymer (as the spacer) via the intersheet spacing (Fig. 2D).²⁸ Li et al. presented a thermoplastic strategy of graphene by polymer intercalation from the GO precursor, which expanded the forming capability of GO materials and promised versatile structural designs for more broad applications.⁴⁹

The Ho-ion-polymer/graphene heterojunctions were fabricated under room temperature by the structural restriction and charge exchange of the Ho ion, which showed excellent high saturation magnetization (3.36 emu g⁻¹) and unprecedented sustainable ferromagnetism.⁵⁰ Compared to the melt technique and solution blending, the emulsion technique enhanced the interaction between the filler and polymer phases with a more homogeneous distribution of fillers, which increased the mechanical properties and lowered the permeation thresholds.

It is a challenge to prepare graphene-based composites with rapid, efficient and multichannel self-healing capability. Li et al. designed polymer/graphene with a cross-linking network structure via Diels–Alder reaction, showing the healing efficiencies (e.g., after heat (90%), infrared light (106%) and microwave (133%)), average photo-thermal conversion efficiency (59.8%) and formation of cleavage by Diels–Alder bonds.⁵¹ Thus, various graphene/polymer composites have recently received more attention in the energy and environment fields. The advantages and disadvantages of various graphene-based materials are summarized in Table 1.

Table 1 Comparison of the advantages and disadvantages of various graphene-based composites

Materials		Advantages	Disadvantages
Nanoarchitecture	GNRs	i) Various micromorphologies	i) Complex synthetic process
	GQDs	ii) Extensive carbon resources	ii) High cost
	GNMs	iii) Excellent removal performance
	3D graphene sponge
Hybrids	GO	i) Various functional groups	i) Serious environmental pollution
	Element-doped graphene	ii) Enhanced optical and stability	ii) Long reaction time
	Metal oxides/graphene	iii) Versatile structural design
	Polymer/graphene	iv) High removal/photocatalytic activities

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3 The environmental application of graphene-based composites

3.1 Removal of organic pollutants

Various organic wastes and/or effluents are mainly released from industries, municipalities and other sources, which causes serious harm to the eco-environment (i.e., flora and fauna) and human health. Graphene-based materials are promising sorbents for decontaminating organic pollutants due to the coexistence of sp²-aromatic and sp³-oxygenated domains, defects and nano-asperities in their matrices.⁵² Herein, we review the removal of dyes, environmental endocrine disruptors (EDCs), antibiotics and other organics on graphene-based composites in recent years.

3.1.1 Dyes. Various dyes (i.e., methylene blue (MB), crystal violet (CV), Congo red (CR) and methyl orange (MO)) have been widely used in various industries, such as textile manufacturing, staining biological and biochemical substances, food processing, cosmetics, leather, paint and pigments. Dye pollution leads to the eutrophication of aquatic environments, which reduces the transparency of water bodies, increases the COD and BOD₅, and elevates the biological toxicity (i.e., carcinogenic, teratogenic and mutagenic).^34,53 Herein, we reviewed the recent advances of dye removal on graphene-based composites.

The removal of dye on graphene-based composites include adsorption and photo-degradation. It is reported that the fantastic adsorption capacity of MB (679.55 mg g⁻¹) on nitrogen-doped graphene⁵⁴ was higher than that of graphene-based composites, such as lauryl sulfate@magnetic GO (624.42 mg g⁻¹)⁵⁵ and ionic liquid-grafted GO (448 mg g⁻¹).⁵⁶ In addition, the adsorption performance of graphene-based composites was significantly influenced by environmental factors, such as the high water permeance and underwater stability, which increased the channel sizes and decreased the diffusion path.⁵⁷ The excellent photo-degradation efficiency of positive CV, MB and MO (100% in 1 min) on nZVI@GO was due to the ·OH radicals and N, S-doping facilitated electron transfer.⁵⁸ Apart from adsorption and photo-degradation, the membrane separation of dyes on graphene-based composites has been widely investigated in recent years due to its excellent hydrophilicity, water flux, as well as retainment. Gao et al. fabricated polyacrylic acid/NH₂-MIL-88B(Fe)/GO membrane (68.21 L m⁻² h⁻¹ per bar) with the high separation of MB (98.79%).⁵⁹ The nanofiltration of MB on the ultrafine metal oxide/rGO nanocomposite with high selectivity of up to 98% (Table 2) was due to the utilization of oxygen functional groups on the GO surface as preferential active sites (Fig. 3A),⁶⁰ which was higher than the other GO composites such as loose GO nanofiltration membranes (92.9% of MB),⁶¹ N-PGO (92% of Reactive Red 195),⁶² and the GO membrane (90% of disperse blue 3).⁶³

Table 2 Removal of organics on various graphene-based composites

Composite	Target	Time	Mechanism	Efficiency%	Ref.
Co/graphene/polypyrrole	MB	400 min	Complexation, electrostatic	92.8	53
	CR			92.2
GO-ImOH	MB	—	Electrostatic, π–cation, π–π	~100	56
hPAN@GO-SF	Basic blue 26	24 h	Cation exchange, electrostatic, π–π, hydrogen bond	84	64
	Basic green 4			90
NGO-MnO	BPA	24 h	Catalytic oxidation	90	65
TAML/GO-PEI/PE	BPA	80 min	Ultrafiltration	100	66
RGO-PEG-ZnO	BPA	1 h	Adsorption	69.38	67
RGO-PEG-ZnO	Phenol		Adsorption	74.52	67
N-GE	Phenol	3 h	Catalysis	93.58	40
	Phenol	0.5 h	AOPs	85	44
Fe3O4@GO@g-C3N4	Phenol	9 h	Photocatalysis	~97	68
rGO/PPy	Phenol	1 h	Photocatalysis	94.8	69
PSSF-Gr	Phenol	0.5 h	Oxidation	99	70
CoFe2O4-rGO	Ofloxacin (OFX)	—	Radical-based oxidation	100	71
	CFZ		PMS-induced direct oxidation	100
Co3O4/R-rGO/CNTs	CTC	12 h	Reactive oxygen	99.3	72
BRGO/NRGO	TC		Reactive oxygen	95.6	73
	NOR		π–π interactions, O3, H2O2, ·OH	95.4
	SMX	—		82
	OFX	—		85
	ERT			74.7
	TMP			46

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Fig. 3 The removal of organics on graphene-bearing composites, A: dyes + metal oxide/rGO,⁶⁰ reproduced with permission from ref. 60, Copyright, 2022, Springer Nature. B: Dyes + hPAN@GO-SF membrane,⁶⁴ reproduced with permission from ref. 64, Copyright, 2022, Elsevier. C: BPA + GO/Fe-MOF.⁷⁴ Reproduced with permission from ref. 74, Copyright, 2020, Elsevier. D: Phenol + GR/PPy-30,⁶⁹ reproduced with permission from ref. 69, Copyright, 2022, Elsevier.

The removal mechanism of dyes on graphene-based composites includes ion exchange, surface complexation, electrostatic interaction, hydrogen bonding and π–π interactions. For instance, the highly efficient removal of BZC, SF, PRO and MB on hPAN@GO-SF membranes was attributed to π–π interaction, hydrogen bond and electrostatic interactions (Fig. 3B).⁶⁴ Kurniawan et al. demonstrated the RBBR removal on nitrogen-doped graphene/gold composites through π–π interaction and complexation of RBBR with hydroxy, carboxyl and imine groups.⁷⁵ The adsorption of RhB and MB on nitrogen-doped graphene included pore filling, electrostatic interaction, π–π interactions and hydrogen bonding.⁵⁴ The improved photodegradation of rhodamine B on g-C₃N₄/rGO@black phosphorus was due to the n–n type high-low junctions, as well as the internal electric field, which promoted the spatial separation of photogenerated carriers.⁷⁶

3.1.2 Environmental endocrine disruptors (EDCs). EDCs are widely used in personal care products, diet, agricultural pesticides, thermal receipts, antimicrobial soaps and cleaning products. The production and transport of EDCs interfere with the metabolism of endogenous hormone regulation in humans and wild animals. As one of the typical EDCs, bisphenol A (BPA, an intermediate of polyester resin) is widely used in various plastic containers.⁷⁷ Thus, the release of EDCs has become a serious environmental problem due to the serious health consequence. Herein, we summarize the removal of BPA and phenol pollutants on graphene-based composites in recent years.

To date, 90% BPA has been rapidly removed by β-cyclodextrin-modified GO membranes within 10 s.⁷⁸ The high BPA removal efficiency (98.1%) on the Co₃O₄/graphene sand composite was due to the excellent dispersion of Co₃O₄ and the sufficient surface area.⁷⁹ The high photodegradation of BPA on various graphene-based materials was also reported, such as NiZn@N-G-900/PMS (90%).⁸⁰ The BPA removal capacity on FeO_x@GC-NBC was 23.16 and 8.65-fold times higher than that of original BC and N-BC, respectively, due to the large specific surface area (1691.81 m² g⁻¹), massive active sites (e.g., Fe–N_x, graphitic N), high efficiency of electron–hole separation, and N retention ability, durability.⁸¹ The high degradation efficiency for BPA from real textile wastewater (97.27%) on the GO/Fe-MOF membrane due to the excellent separation of the photogenerated carrier, high chemical stability and photo-Fenton activity (Fig. 3C).⁷⁴ Apart from BPA, phenol removal on graphene-based composites has been widely reported in recent years. Owing to the enhancement of the dispersion of active sites and electron transfer, the high phenol removal (almost 100%) on rGO/Co/CoO_x@NC/PMS⁸² was significantly higher than that of other graphene-based composites, such as graphene/sintered stainless steel fibers (99%),⁷⁰ Fe₃O₄/GO@g-C₃N₄ (∼97%, Table 2),⁶⁸ rGO/polypyrrole (IR/PPy-30, 94.8%, Table 2),⁶⁹ Fe₃O₄@GO (∼75%),⁶⁸ Fe₃O₄ (∼62%)⁶⁸ and BiPO₄/graphene aerogel (50.67%).⁸³ The large surface area, various functional groups (e.g., amino, oxygenated and PMS activation) and synergistic effect significantly increased the removal performance, such as phenol removal (150%) on SiO₂/N-rGO.⁸⁴ These graphene-based materials exhibited good regeneration in the successive operation of phenol.⁴⁴

The interaction mechanism of EDCs on graphene-based composites included hydrogen bonding, electrostatic interactions and π–π interactions.⁸⁵ For instance, Wang et al. demonstrated that the interaction of BPA on FeO_x@GC-NBC included pore filling and π–π interaction.⁸¹ Rout et al. demonstrated the high removal of phenol and BPA on hydrophilic polyethylene glycol/rGO/ZnO by hydrogen bonds, surface complexation of oxygenated groups (e.g., hydroxyl, carboxyl) and electron donor–acceptor interaction (i.e., aromatic ring of phenolic compound).⁶⁷ The highly efficient photodegradation of IR/PPy-30 towards phenol was due to the abundant porous structure of porous graphene nanosheets and large specific surface area after the introduction of PPy (Fig. 3D), which increased the adsorptive sites and charge transfer.⁶⁹

3.1.3 Antibiotics. Various antibiotics (e.g., tetracyclines (TC), sulfonamides and floxacines) have been widely used in stock farming. Thus, the highly efficient removal of antibiotics.^71,73,86,87 Herein, we mainly summarized the recent advances on the photocatalytic removal of antibiotics on graphene-based composites.

The poor visible light absorption and rapid carrier recombination of single graphene-based materials limited the actual application.⁸⁸ Recently, various graphene-based composites exhibited the high photodegradation capacity (e.g., TC photodegradation on magnetic GO/ZnO (1590.28 mg g⁻¹)⁸⁶), and fast photodegradation efficiency (e.g., 98.7% of TC on rGO/Bi₄O₅Br₂,⁸⁷ 96.54% of TC on Fe–Ce/GO alginate hydrogels at pH 4.0,⁸⁹ 80.7% TC on Bi₂O₃/TiO₂@rGO⁹⁰). The high TC removal on graphene-based composites was due to the small energy gap, high charge conductivity and enhanced separation of e⁻/h⁺ pairs and massive radicals (e.g., ·OH and ·O₂⁻ radicals).⁹¹ Guan et al. reported that h⁺ played an important role in the superior photocatalytic degradation of TC on AgBr/GO/Bi₂WO₆ due to the broadened visible light response range and high carrier separation (Fig. 4A).⁹² Apart from TC, various graphene-based composites exhibited high removal capacities towards the other antibiotics, such as 99.5% ofloxacin on Bi₂MoO₆/MoS₂/GQDs,⁹³ 97.6% ciprofloxacin, 80.7% norfloxacin on rGO/Bi₄O₅Br₂,⁸⁷ and 86.3% NOR on P-BiVO₄/GQDs/P-C₃N₄.⁹⁴ The high ofloxacin degradation efficiency was due to the various photogenerated radicals, such as superoxide (·O₂⁻) and photo-generated (h⁺), where GO was considered as electron transmission media to enhance the catalytic activity (Fig. 4B).⁹⁵ The possible pathway of photodegradation included hydroxylation, dehydrogenation, dichlorination and the elimination of water.⁹⁶

Fig. 4 The removal of organics on graphene-bearing composites, A: TC + AgBr/GO/Bi₂WO₆,⁹² reproduced with permission from ref. 92, Copyright, 2021, Elsevier; B: ofloxacin + GO/UiO-67/CdS.⁹⁵ Reproduced with permission from ref. 95, Copyright, 2020, Elsevier; C: pesticides + heteroatom-doped graphene,⁹⁷ reproduced with permission from ref. 97, Copyright, 2020, American Chemical Society. D: Crude oil + MOF/graphene,⁹⁸ reproduced with permission from ref. 98, Copyright, 2023, Elsevier.

3.1.4 Other organics. Apart from the aforementioned organics, graphene-based composites also have been widely used to remove organics such as organic pesticides and crude oils. The multilayer adsorption of organo-thiophosphate pesticides on Ni@SiO₂-graphene nanocomposites occurred on heterogeneous surfaces by π–π, electrostatic action, and hydrogen boding.²⁴ Based on DFT calculation, Zhu et al. also demonstrated the adsorption of various pesticides (i.e., lactofen, fluroxypyrmeptyl, bensulfuronmethyl and fomesafen) on nitrogen-doped graphene through π–π stacking and hydrogen bonds (Fig. 4C).⁹⁷ Hu et al. reported that 3D layered graphene with hydrophobic polydimethylsiloxanes displayed high adsorbed capacity (118 times its own weight, Fig. 4D), shortened the adsorption time (from 5 h to 40 s), and exhibited good regeneration (retained 90% adsorption after 10 cycles) for crude oil due to the high photothermal conversion property (reached up to 80 °C in 100 s).⁹⁸ Generally, the interaction mechanism of organics on graphene-based materials included hydrogen bond, complexation and π–π conjunction.

3.2 Removal of heavy metals

In recent years, the removal of various toxic metal irons (e.g., Cd(II), Cr(VI), and Pb(II)) on graphene-based composites and their derivatives (e.g., GO, rGO) has been widely investigated under different conditions.^99–101 Herein, the recent advances and interaction mechanism of various heavy metals on graphene-based materials were reviewed in detail.

3.2.1 Cr(VI). Chromium as the typical heavy metal has been widely used in manufacturing. Two main oxidation states (i.e., Cr(III), Cr(VI)) occur in water and groundwater. The discharge of Cr(VI)-bearing wastewater in environments has attracted public concern due to the toxic effects on human health, such as DNA damages, carcinogenicity and teratogenicity. This work critically reviewed the recent advances of Cr(VI) removal on graphene-based composites to select the best and available recovering technologies.

In the recent studies, the high adsorption efficiency of Cr(VI) on various graphene-based composites such as GO/ZnO (96%),¹⁰² polyethersulfone functionalized GO (PES-GO membrane, 99.9%) has been reported.¹⁰³ Yaseen et al. reported the high removal capacity of polyvinyl acetate/rGO/CuO (99.9%) towards real industrial wastewater samples.¹⁰⁴ The maximum adsorption capacity of Cr(VI) on hydroxypropyl-β-cyclodextrin-polyurethane magnetic nanoconjugates/GO (987 mg g⁻¹, Table 3)¹⁰⁵ was significantly higher than that of other graphene-based composites, such as N-LEGO (416.97 mg g⁻¹),¹⁰⁶ GO-NH₂/cellulose acetate (410.21 mg g⁻¹),¹⁰⁷ PAN/GO (382.5 mg g⁻¹)¹⁰⁸ and PAH-ASGO (373.1 mg g⁻¹).¹⁰⁹ In addition, these graphene-based composites exhibited excellent regeneration efficiency towards Cr(VI), such as CoS₂/g-C₃N₄-rGO (98% after 5 cycles),¹¹⁰ and poly(allylamine hydrochloride)/amino-GO (PAH-ASGO, 91.8% after 10 cycles).¹⁰⁹ The regenerable adsorption of Cr(VI) (0.158 mmol g⁻¹) on the MOF@rGO electrode was significantly higher than that of As(III) (0.091 mmol g⁻¹, Fig. 5A) due to the high selectivity of the amine groups of the hybrids and CrO₄²⁻.¹¹¹ The adsorption of Cr(VI) on these graphene-based composites was also influenced by Cr(VI) speciation. Geng et al. reported that the adsorption of Cr₂O₇²⁻ on nitrogen-enriched lignosulfonate exfoliated graphene oxide (N-LEGO) at pH 2.0 was easier than that of HCrO₄⁻.¹⁰⁶ Apart from the adsorption, the photo-reduction of Cr(VI) into Cr(III) on various photocatalysts was recently reported due to the full-spectrum light-absorption, outstanding photo-stability, efficient charge-transfer capability, and high separation of photogenerated carriers.^112,113

Table 3 Removal of heavy metals on graphene-based composites

Heavy metals	Composites	Time	q e (mg g^−1)	Mechanism	Ref.
Cr	HPMNPU/GO	4.2 h	987	Chemical adsorption, electrostatic attraction	105
	N-Lignosulfonate exfoliated GO	—	416.97	Inner complex reduction, surface chemical adsorption	106
	Aminated GO@cellulose acetate	60 min	410.21	Surface adsorption, Coulombic interaction	107
	PAH-ASGO	4 h	373.1	Chemical adsorption	109
	GO/C60(OH)22	3 h	1307	Surface complexation	114
Pb	Alginate/GO	1 min	887.21	Cationic exchange	115
	COPYGO	400 min	780.36	Chemical adsorption	53
	GQDs/Ba(OH)2	15 s	748.55	—	116
Cd	GO/calcium alginate	12 h	602	Film diffusion, intra-particle diffusion	117
	CS/APSGO	60 min	566.2	Hydrogen bonding	118
	Bifunctionalized GO/MnFe2O4	90 min	366.4	Electrostatic attraction, ion exchange, complexation	119
	Magnetic titanate/GO	6 h	322.7	Ion exchange and surface complexation	120
	UiO-66-NDC/GO	2.5 h	254.45	Chemisorption	121
	COPYGO	400 min	794.18	Chemical adsorption	53
	Manganese ferrite/GO	45 min	232.56	Chemical and electrostatic interactions	122
	Thiourea rGO/NaA zeolite	12 h	196	Ion exchange and coordination	123
	GO/calcium alginate	12 h	181	Film diffusion, intra-particle diffusion	117
	Mg–Fe LDH/GO	2 h	174.83	Surface adsorption	124

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Fig. 5 The removal of Cr(VI) and Pb(II) on various graphene-based composites, A: Cr(VI) + MOF@rGO,¹¹¹ reproduced with permission from ref. 111, Copyright, 2020, American Chemical Society. B: Cr(VI) + PAH-ASGO,¹⁰⁹ reproduced with permission from ref. 109, Copyright, 2021, Elsevier. C: Pb(II) + GDC,¹²⁵ reproduced with permission from ref. 125, Copyright, 2024, American Chemical Society. D: Pb(II) + Mn₃O₄/rGO,¹²⁶ reproduced with permission from ref. 126, Copyright, 2022, Elsevier.

The interaction mechanism of Cr(VI) on various graphene-based composites included the surface complexation, adsorption and redox. For instance, Cr(VI) on the ammonium thiocyanates-functionalized nZVI/GO was due to the complexation of Cr(VI) with sulfuryl groups, and then the reduction of adsorbed Cr(VI) into Cr(III) by nZVI.⁹⁹ The electrostatic attraction and reduction of Cr(VI) into Cr(III) dominated Cr(VI) on PAH-ASGO (Fig. 5B).¹⁰⁹ For the photocatalytic removal of Cr(VI), it is demonstrated that the photo-generated e⁻ mainly participated in the photo-reduction of Cr(VI).¹¹³ It is anticipated that various graphene-based composites can be designed for the selective adsorption of Cr(VI)-bearing wastewater.

3.2.2 Pb(II). Lead (Pb) as a common heavy metal has been widely used in industrial processes. The release of Pb into the environment causes serious threat for human health due to endogenous lead poisoning and affecting children's development, among other issues through the food chain. Herein, the removal of Pb(II) on graphene-based composites was reviewed in detail.

To date, it is reported that the adsorption capacity of Pb(II) on GO/sodium alginate/tetrachlorosilicate (887.21 mg g⁻¹, Table 3)¹¹⁵ was higher than that of GO-based composites, such as Fe₃O₄/SiO₂-GO (385.1 mg g⁻¹),¹²⁷ GO/alginate (327.9 mg g⁻¹),¹²⁸ SA-PAM/GO (240.69 mg g⁻¹),¹²⁹ mGO/MgAl-LDH (192.31 mg g⁻¹),¹³⁰ GO/sodium alginate/1,2-propanediamine (189.25 mg g⁻¹),¹³¹ rGO-Mn₃O₄ (130.28 mg g⁻¹)¹²⁶ and SH-graphene (101 mg g⁻¹).¹³² In addition, the removal performance of graphene-based materials was not influenced by environmental factors, such as pH, ionic strength and temperature. Bao et al. demonstrated that Fe₃O₄/SiO₂-GO exhibited excellent tolerance to low pH (∼170 mg g⁻¹ at pH 4) and high Na⁺ (no change at 120 mg L⁻¹).¹²⁷ The adsorption capacity was also slightly increased (from 132 to 199 mg g⁻¹) with increasing temperature from 30 to 70 °C.¹³³ Moreover, these graphene-based materials exhibited appreciable reusability for Pb(II) adsorption.¹³⁴

The removal mechanism of Pb(II) on various graphene-based materials was demonstrated via adsorption, cation exchange, surface complexation and co-precipitation. Gou et al. demonstrated the excellent adsorption selectivity of Pb(II) on GO/DTPA/CMC (GDC, Fig. 5C) due to the coordination of a more distorted structure between the DTPA chelates and Pb(II).¹²⁵ In addition, the synergistic effects of GO and other supports also play an important role in Pb(II) removal.¹³⁵ The complexation of Pb(II) on GO-based composites was due to the large number of oxygenated groups (e.g., hydroxyl, carboxyl and carbonyl groups, Fig. 5D).¹²⁶ Zhang et al. demonstrated that the interaction mechanism of Pb(II) on S. putrefaciens/GO included two complexes (i.e., extracellular polymer substance and oxidative debris-Pb(II) complexes) and the precipitates (Pb₁₀(PO₄)₆(OH)₂), which may promote potential risks of Pb(II) in aquatic or soil environments.¹³⁶

3.2.3 Cd(II). As a typical toxic heavy metal, the high levels of Cd(II) exposure lead to diverse health problems, such as various cancers, and harm to the lungs, kidneys and blood composition.¹³⁷ Herein, we reviewed the removal of Cd(II) on various graphene-based composites.

It is reported that the maximum adsorption capacity of Cd(II) on S-graphene/layered double oxide (473.11 mg g⁻¹)¹³⁸ was significantly higher than that of other graphene-based composites, such as silica/GO (333.33 mg g⁻¹),¹³⁹ manganese ferrite/GO (232.56 mg g⁻¹, Table 3),¹²² and bio-reduced rGO (23.32 mg g⁻¹).¹⁴⁰ The high adsorption capacity and fast equilibrium rate of graphene-based composites (e.g., Mg–Fe LDH/GO (99%,¹²⁴), hexamethylenediamine/GO-HMDA (95.19%,¹⁴¹) and chitosan/GO/iron(III) oxide hydroxide (84%,¹⁴²)) were attributed to various functional groups, such as –OH, –COOH, –C=O, and silane-based functional groups.¹³⁹ In addition, these graphene-based materials exhibited excellent regeneration and bacterial inhibition efficacy.^122,124 These graphene-based materials still exhibited the high adsorption capacity and regeneration of Cd(II) even under a tertiary system (i.e., Pb(II), Cd(II) and Ni(II), Fig. 6A).¹⁴³ Recently, various graphene-bearing electrodes were fabricated to detect trace amounts of Cd(II).^144–147 Liu et al. fabricated a bimetal/laser-induced graphene electrode (i.e., SnO₂/CeO₂/LIG) with a low Cd(II) detection limit (0.01 μg L⁻¹).¹⁴⁸

Fig. 6 The removal of Cd(II) and other heavy metals on various graphene-based composites. A: Cd(II) + T-SGO-CS,¹⁴³ reproduced with permission from ref. 143, Copyright, 2024, Elsevier. B: Cd(II) + CN/IGO/Cel,¹⁴⁹ reproduced with permission from ref. 149, Copyright, 2024, Elsevier; C: Cu(II), Ni(II) and Co(II) + GO,¹⁵⁰ reproduced with permission from ref. 150, Copyright, 2020, Elsevier; D: Hg(II) + cysteamine/rGO (Cyst-prGO),¹⁵¹ reproduced with permission from ref. 151, Copyright, 2019, American Chemical Society.

The removal mechanism of Cd(II) on various graphene-based composites included the electrostatic interaction, surface complexation, hydrogen bonds and chelation/complexation.^{35,122,140,152} The adsorption of Cd(II) on magnetic GO/cellulose/cyclodextrin included physical forces, electrostatic interaction, coordination bond and cation–π interaction (Fig. 6B).¹⁴⁹

3.2.4 Other heavy metals. Apart from the above heavy metals, there are many research studies concerning the removal of other heavy metals (e.g., Hg(II),^151,153 Ni(II),^150,154 Cu(II)^155–157) on graphene-based composites in the recent years. The adsorption capacity of GO towards various metal ions followed the order of Cu(II) > Ni(II) > Co(II) with excellent regeneration capacity (Fig. 6C).¹⁵⁰ Ma et al. reported on the high Hg⁰ removal (92% at 100 °C) on Ag–Fe₃O₄@rGO due to the formation of the Ag–Hg amalgam,¹⁵³ in addition to the large surface area, simple thermal-desorption and thermo-stability, and electron transfer ability. Yap et al. investigated that cysteamine/rGO (Cyst-prGO) displayed high adsorption capacity (169 mg g⁻¹), high selectivity and high regeneration ability (>97% after five consecutive adsorption–desorption cycles) towards the removal of Hg(II) (Fig. 6D).¹⁵¹ Mashkoor et al. reported that the WO₃0.5H₂O/rGO nanocomposite presented high removal efficiency (86.45%) for Ni(II).¹⁵⁴ The high adsorption capacity of Cu(II) (132.57 mg g⁻¹) on GO-zirconium oxide/sodium alginate was due to the strong electrostatic force, ion exchange and complexation.¹⁵⁵ Chang et al. investigated the real industrial wastewater treatment of Cu(II) (>98%) using bentonite/GO composites.¹⁵⁸ Briefly, two samples of real Cu-bearing industrial wastewater were pre-cleaned with HNO₃ and then rinsed thoroughly with DI water. The samples were diluted with DI water (1 : 3) in the laboratory and filtered to remove solid impurities, and finally underwent acid digestion. The excellent adsorption of various heavy metals from aqueous solution and real wastewater on graphene-based composites demonstrated their significant potential for pollution mitigations in practical application.

3.3 Removal of radionuclides

With the development of advanced technologies, nuclear energy has emerged as a significant alternative renewable energy due to its clean property, high density of energy and economic competitiveness. These radionuclides (e.g., uranium (U), strontium (Sr), and cesium (Cs)) will be charged into the environment after the mining of radioactive ores, processing of nuclear power plants, and utilization in medical research. The pollutants of radionuclides exhibit a more hazardous effect to humans and the environment. Herein, the recent advances of adsorption of U(VI), Sr(II) and Cs(I) on graphene-based composites are described in detailed.

3.3.1 U(VI). The core of spent fuel was dissolved in concentrated HNO₃ to separate uranium and plutonium from fission products. Thus, the investigation of U(VI) removal at low pH is of great important for the safe disposal of radioactive liquid waste. Ma et al. reported excellent stability at pH 3–10 and the high removal of U(VI) (271.7 mg g⁻¹, pH 5.0) on 3D MnO₂/GO composites from the simple ultra-sonication process due to the coordination binding of oxygen-containing functional groups.¹⁵⁹ These findings are essential for the application of functionalized GO in the preconcentration of radionuclides in nuclear waste management at low pH.

Demand for highly selective materials for U(VI) enrichment competition from other cations (e.g., Ca(II), Sr(II), Cs(I), Pb(II), Fe(III), As(III) etc.) is the development direction of the adsorbent. Yang et al. reported the promoted adsorption selectivity of amidoxime-functionalized diaminomaleonitrile towards U(VI) (935 mg g⁻¹) due to the coordination or chelation of uranium with the organic groups.¹⁶⁰ Wang et al. reported that the maximum adsorption capacity of U(VI) on graphene aerogel/phytic acid (3550 mg g⁻¹, Table 4)¹⁶¹ was significantly higher than oxygen-defected GO (2250 mg g⁻¹),³³ amidoxime/mGO (435 mg g⁻¹),¹⁶² and 3D porous amidoxime functionalized GO nanoribbons (589.5 mg g⁻¹).¹⁶³ The high sorption capacity of U(VI) on GO was due to the defects of flake edges and defects of edge atoms of holes (Fig. 7A).³³ Chen et al. found that adsorption of U(VI) on g-C₃N₄/GO nanosheets was not affected by the coexisting metal cations due to the photoelectron transfer process.¹⁶⁴ Hu et al. also demonstrated the high enrichment performance of U(VI) on amidoxime/mGO, owing to the selective enrichment of U(VI) on the amidoxime group, inner-sphere surface complexation and reduced precipitation (Fig. 7B).¹⁶² However, the adsorption efficiency was observably reduced in the presence of coexisting ions (i.e., Al(III), Cr(III), Fe(III) and CO₃²⁻) owing to the high electron cloud density of the cation and/or high U–CO₃²⁻.¹⁶⁵ Wang et al. reported the excellent adsorption capacity and selectivity (65.23%, at pH 3.0) of U(VI) on the ultralight 3D porous amidoxime-functionalized GO nanoribbons aerogel (PAO/GONRs-A) due to the plentiful nitrogen content (13.5%), low density (8.5 mg cm⁻³), high specific surface area (494.9 m² g⁻¹) and strong coordination of amidoxime and uranium.¹⁶³

Table 4 Radionuclide removal on various graphene-based materials

	Materials	Mechanism	Time	Q e (mg g^−1)	Ref.
U(VI)	Graphene aerogel/phytic acid	Precipitation, crystallization	40 min	3550	161
	rGH-polysaccharides xanthan gum/rGH-chitosan	Electrochemical, physicochemical adsorption	4 h	1413	166
	APTES-functionalized GO intercalated Ni–Al-LDH	Interaction between U and COOH and NH2	24 h	1162	167
	GO-polydopamine–polyethylene imine	Coordination and intramolecular diffusion	6 h	530.6	168
	GO/hydroxyapatite	Multilayer adsorption	2 h	373	169
	Chitosan/GO	Electrical double-layer	90 min	271.2	170
	Phytic acid (PA)/GO	Hydrogen bonding, complexation	1 h	266.7	171
	CNQDs/rGO/ZIF-67	Electrostatic interaction and complexation	12 h	260.59	172
	Manganese ferrite/GO	Chemical and electrostatic interactions	30 min	201.65	122
Cs(I)	rGO/PBAs	Ion-exchange, ion trapping, complexation	48 h	204.9	173
	NiHCF/RGO	Ion exchange	—	320	174

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Fig. 7 The removal of radionuclides on various graphene-containing composites, A: U(VI) + GO,³³ reproduced with permission from ref. 33, Copyright, 2020, American Chemical Society. B: U(VI) + AO-GO,¹⁶² reproduced with permission from ref. 162, Copyright, 2019, Elsevier. C: Sr(II) + polyvinyl alcohol/GO/MnO₂,¹⁷⁵ reproduced with permission from ref. 175, Copyright, 2021, Elsevier; D: Sr(II) + GO,¹⁷⁶ reproduced with permission from ref. 176, Copyright, 2022, Elsevier.

Apart from adsorption, the graphene aerogel exhibited high photocatalytic performance of U(VI) (1050 mg g⁻¹) under visible light in air due to high photocurrent response and low band gap energy to generate and transfer electrons/holes.¹⁷⁷ Furthermore, the addition of an electron donor prevented the recombination of electrons and holes. Zhang et al. demonstrated that the 3D porous rGO-based covalent organic framework hydrogel achieved exceptional U(VI) adsorption capacity (521.6 mg g⁻¹) under sun irradiation through photothermal desalination.¹⁷⁸ Liao et al. reported the high removal efficiency of 99.9% and high adsorption capacity (1340 mg g⁻¹) on layered 2D/2D niobium phosphate/holey graphene due to electro-adsorption and electrocatalytic reduction.¹⁷⁹ Guo et al. found that chitosan-GO/ZIF exhibited high extraction of U(VI) (∼70%) from real seawater (Bohai Sea, China). These findings further confirmed that graphene-based materials displayed great potential in the practical application for the recovery of U(VI) from aqueous solution and extraction of U(VI) from natural seawater.

3.3.2 Sr(II). The removal of Sr(II) on various graphene-based composites (e.g., rGO,¹⁸⁰ magnetite/GO,¹⁸¹ alginate/GO/LDH,¹⁸² graphene/MnO₂ (ref. 183)) have been widely investigated in recent years. Huo et al. reported the improved adsorption capacity of Sr(II) (26.8 mg g⁻¹, at pH 4.0–9.0) on the polyvinyl alcohol/GO/MnO₂ composite owing to the hydrogen bonds, electrostatic attraction and complexation of Sr(II) with the oxygen-containing groups (–COOH of GO and –OH of MnO₂).¹⁷⁵ In addition, Mg²⁺/Ca²⁺ showed a severe interfering effect on Sr²⁺ adsorption (Fig. 7C).¹⁷⁵ Zhou et al. demonstrated that the adsorption of Sr²⁺ with oxygen-containing functional groups of GO (as crystal regulator) in the presence of S. pasteurii (produced CO₃²⁻ in microbial mineralization) formed SrCO₃ complexes.¹⁸⁴ Carr et al. also demonstrated that the Sr(II) removal on GO was higher than that of Cs(I) removal, according to vibrational sum frequency generation spectroscopy (SFG, Fig. 7D).¹⁷⁶ The interaction mechanism of Sr(II) on GO was attributed to the coordination of Sr(II) with oxygenated functional groups (O–C=O and C–O–C groups).^185,186 Vishwakarma et al. established the N-functionalization of GO membranes using NH₃ vapour to develop the OH–N bond sp²-hybrid, generated NH₄⁺ ions through carboxylic acid/adsorbed water interactions, featuring high Sr(II) recovery percentages (35–60%).¹⁸⁷ Vishwakarma et al. reported that the maximum adsorption capacity of Sr(II) on GO/crown ether (890 mg g⁻¹)¹⁸⁸ was significantly higher than the original GO (131.4 mg g⁻¹).¹⁸⁹

3.3.3 Cs(I). The removal of Cs(I) on various graphene-based composites (e.g., GO,^190,191 rGO,¹⁹² AC/GO,¹⁹³ and crown ether/GO¹⁹⁴) was investigated in recent years. Graphene oxide membranes (GOMs) separation as a new treatment technology is of great interest in the field of water treatment. Zhao et al. systematically investigated the HNO₃ concentration and membrane thickness on the permeation behaviors of typical radioactive ions through GOMs: Cs(I) > Sr(II) > Eu(III) > UO₂²⁺ > Th(IV), which could be well explained by size exclusion and chemical interaction, and proposed a novel technical route with the merits of a simplified process.¹⁹⁵ Ye et al. prepared a novel nanofiltration membrane (Prussian blue/GO/polyethylene glycol/polysulfone) with the highly efficient rejection of Cs(I) (99.5%) and Sr(II) (97.5%) due to the promoted Donnan effect of GO and the polyamide layer by enhancing the negative charge.¹⁹⁶ Zhang et al. also reported the high Cs⁺ removal efficiency (99.6%) on the polyvinylidene fluoride/Prussian blue/GO membrane.¹⁹⁷ Kuzenkova et al. reported that the removal of Cs(I) on HGO (obtained from the Hummer's method, Fig. 8A) and BGO (derived from Brodie's method) was significantly higher than that of TGO (derived from Tour's method).¹⁹¹ The selective electrochemical removal of Cs(I) on reduced GO-supported nickel hexacyanoferrate (NiHCF/RGO, 320 mg g⁻¹, Table 4)¹⁷⁴ was significantly higher than that of graphene-bearing materials, such as rGO/PBAs (204.9 mg g⁻¹),¹⁷³ GO (95.46 mg g⁻¹),¹⁹⁸ and AC/GO (22.9 mg g⁻¹).¹⁹³ The adsorption process included the following steps: i) Cs(I) was diffused and mainly adsorbed by the membrane channel of the GO/PB surface; ii) it gradually diffused into the inside of channels with the lower rate (Fig. 8B).¹⁹⁶ Deng et al. investigated the high removal of the radioactive Cs(I) (99.71%) and Sr(II) (99.99%) waste on rGOPGC under the simulated high salt liquid. Meanwhile, there was no aggregation of salt on the surface of the membrane device under solar intensity and 35 °C ambient temperature due to the high evaporation rate (1.75 kg m⁻² h⁻¹) and the evaporation efficiency (98.51%).¹⁹⁹ Novikau et al. demonstrated that the adsorption mechanism of Cs(I) on the muscovite mica clay-GO-γ-Fe₂O₃–Fe₃O₄ composite included surface complexation, electrostatic interaction and ion exchange.²⁰⁰

Fig. 8 The removal of Cs(I) and other radionuclides on various graphene-containing composites: A: Cs(I) + T/H/B-GO,¹⁹¹ reproduced with permission from ref. 191, Copyright, 2020, Elsevier; B: Cs(I) + GO/PB membrane,¹⁹⁶ reproduced with permission from ref. 196, Copyright, 2022, Elsevier; C: Th(IV) + GOCS, inset SEM of GOCS,²⁰¹ reproduced with permission from ref. 201, Copyright, 2019, Elsevier; D: Am(III) + defect-GO,³³ reproduced with permission from ref. 33, Copyright, 2020, American Chemical Society.

3.3.4 Other radionuclides. Apart from the above radionuclides, various graphene-bearing composites have recently been used to remove other radionuclides, such as Th(IV),^202–206 Am(III),³³ and Eu(III).^33,201 For instance, Huang et al. reported on the high adsorption capacity (220 mg g⁻¹ at pH 3.0) and selectivity (∼100%, Fig. 8C) of Th(IV) on GO/chitosan (GOCS) due to the strong inner-sphere surface complexation by –COO–, –OH and –NH₂ groups.²⁰¹ Gao et al. demonstrated that the adsorption performance of Th(IV) on GO was influenced by the oxygen content and types of oxygenated functional groups.²⁰⁷ Boulanger et al. reported that Am(III) sorption on defect-rich GO (dGO) at pH < 2.0 was significantly higher than that of GO using Hummers oxidation (HGO) due to the attachment of abundant carboxylic groups on the hole edge atoms of GO flake (Fig. 8D).³³ GQDs/Ba(OH)₂ achieved high adsorption capacity (1.5 mmol g⁻¹ at pH 5 at 15 s) and removal efficiency (94.6–96.2%) for La(III).²⁰⁸ Gao et al. reported that the high Th(VI) adsorption on GO was attributed to the different oxygen-containing groups.²⁰⁷ In summary, various graphene-based composites also exhibited high adsorption performance for other radionuclides (e.g., Th(IV), Am(III) and Eu(III)) due to the massive functional groups, which have important guiding significance for the application of graphene-based nanomaterials to treat the pollution of radionuclides in actual environmental cleanup situations.

4 Conclusions and perspectives

Various graphene-based materials as potential adsorbents exhibited excellent removal capacity towards the different environmental pollutants due to the large surface area, surface functional groups and controlled porous structure, which offer new opportunities for environmental cleanup. In this review, we firstly summarized the functionalization of graphene (i.e., design of nano-architecture, the construction of hybrids (i.e., GO, element-doped graphene and polymer/graphene composite)) in brief. Then, we reviewed the recent advances of the removal of organics (i.e., dyes, EDCs, antibiotics and other organics), heavy metals (i.e., Cr(VI), Pb(II), Cd(II) and other radionuclides) and radionuclides (i.e., U(VI), Sr(II), Cs(I) and other radionuclides) in detail. The interaction mechanism of the graphene-bearing composite and various environmental pollutants (e.g., physical adsorption (H-bond, van der Waals' force and hydrophobic effect), chemical adsorption, ion exchange, precipitation, surface complexation and redox) were demonstrated by using various advanced spectroscopic, modeling, and theoretical techniques at the atomic level. Although graphene-based materials have good adsorption performance in environmental remediation application, most of the research studies are still in the laboratory. In actual wastewater, intricate components (e.g., organics, heavy metals and pathogenic microorganisms) affect the removal performance of graphene-based materials, such as material poisoning and biological fouling. Thus, some unavoidable drawbacks and/or current problems need further refinement. The current problems of the application of graphene-based materials include the following: 1) the synthesis of graphene-based materials with versatile properties remains a challenge owing to the initial stage research. To date, most synthesis processes require high temperature and complex conditions, and are expensive, time-consuming and result in secondary pollution, which hinder their actual application in environmental remediation; 2) most graphene-based composites display limited adsorption capacity, low selectivity and poor cycling ability; 3) few research studies have been done on the environmental safety of these graphene-based materials. The feasibility of graphene-based materials in actual environmental remediation should be evaluated to avoid new environmental issues. Thus, the future direction of graphene-based materials in the environmental application are i) to fabricate the graphene-based materials in large-scale using green, environmentally friendly, low cost and convenient methods; increasing the production of graphene-based materials is an effective strategy to improve the application range; ii) to investigate the optimized conditions for graphene-based materials with high removal capacity, fast equilibration rate and good regeneration; by creating more reactive sites and massive porous structures, the removal performance of graphene-based materials can be significantly enhanced. In addition, the regeneration experiments could lead to the leaching or occupying of active sites. For instance, polymer/graphene composites can efficiently trap heavy metals due to high chemical stability and massive oxygenated functional groups. Thus, the selection of suitable support materials also can increase the removal performance; iii) thorough investigation of biocompatibility and toxicology of graphene-based materials in actual environmental remediation. The biocompatibility and toxicology of graphene-based materials are not clear nowadays due to the lack of reliable characterization techniques. In conclusion, the application of graphene-based materials in environmental remediation is still in its infancy stage, and numerous obstacles should be overcome. It is anticipated that this review will be crucial for environmental engineers and material scientists to fabricate efficient graphene-based materials, which facilitate their actual application in environmental remediation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported by the State Key Laboratory Open Foundation of Environmental-friendly Energy Materials of Southwest University of Science and Technology (No. 23kfhg08).

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