Surface functional groups of carbon-based adsorbents and their roles in the removal of heavy metals from aqueous solutions: A critical review

Abstract

Carbon-based adsorbents such as graphene and its derivatives, carbon nanotubes, activated carbon, and biochar are often used to remove heavy metals from aqueous solutions. One of the important aspects of effective carbon adsorbents for heavy metals is their tunable surface functional groups. To promote the applications of functionalized carbon adsorbents in heavy metal removal, a systematic documentation of their syntheses and interactions with metals in aqueous solution is crucial. This work provides a comprehensive review of recent research on various carbon adsorbents in terms of their surface functional groups and the associated removal behaviors and performances to heavy metals in aqueous solutions. The governing removal mechanisms of carbon adsorbents to aqueous heavy metals are first outlined with a special focus on the roles of surface functional groups. It then summarizes and categorizes various synthesis methods that are commonly used to introduce heteroatoms, primarily oxygen, nitrogen, and sulfur, onto carbon surfaces for enhanced surface functionalities and sorptive properties to heavy metals in aqueous solutions. After that, the effects of various functional groups on adsorption behaviors of heavy metals onto the functionalized carbon adsorbents are elucidated. A perspective of future work on functional carbon adsorbents for heavy metal removal as well as other potential applications is also presented at the end.

1. Introduction

Water pollution by heavy metals poses a serious threat to both human health and the environment [1, 2]. Most heavy metals are extremely toxic, seriously harming all forms of life including human beings when exceeding their tolerance levels [3, 4]. Heavy metals are also not biodegradable and tend to accumulate in living organisms [5]. Due to rapid global development over the last century, elevated amount of heavy metals enter into surface and ground water through discharges of wastewater produced from metallurgical, mining, chemical and battery manufacturing industries [6, 7].

Many treatment methods have been developed for the removal of detrimental heavy metal ions from aqueous solutions, including adsorption, chemical precipitation, electrostatic interaction, ion exchange, coagulation, and membrane filtration [8–14]. Adsorption using carbonaceous materials as adsorbents (carbon adsorbents) is perhaps one of the most cost-effective methods for removal of heavy metals from aqueous solutions [15–19].

Carbonaceous materials including activated carbon (AC), biochar, carbon nanotubes (CNTs), and graphene oxide (GO) have been widely studied for adsorption of various environmental contaminants [20–28]. Their performance for the removal of heavy metals from aqueous solution has been widely reported [29–35], and most studies in the literature focus on sorption characteristics of a specific carbon material. AC is the most widely used carbon adsorbent for water and wastewater treatment. A wide range of AC adsorbents can be prepared to suit for various environmental applications including the removal of heavy metals from aqueous solutions [22, 36]. Due to the high cost associated with production of coal-based AC, biochar has recently emerged as a low-cost alternative of AC with comparable or superior performance for heavy metal adsorption [19, 37–39]. A variety of woody biomass including agricultural wastes or byproducts such as peanut hull and dairy manure can be used to develop biochar [40–42]. Its multifunctionalities including carbon sequestration, soil fertility improvement, and environmental remediation are also well recognized [25, 43].

With the advent of nanotechnology, graphene-based materials including GO or reduced GO and CNTs have been studied intensively for their potential applications in removal of heavy metals [34, 44, 45]. Graphene is a single sheet of 2D hexagonal network of carbon and GO is the oxidized form of graphene synthesized through chemical or thermal reduction processes. GO has high specific surface area and rich surface functional groups, ideal for adsorption of heavy metals. CNTs are cylindrical carbon tubes rolled from one or multiple sheets of graphene. They feature well-defined cylindrical hollow structure, large surface area, hydrophobic wall, and easily modified surfaces. Both single-walled and multi-walled CNTs have been tested as adsorbent materials [35, 46, 47]. With excellent electrical, thermal and mechanical properties, CNTs and GO possess great potential in adsorption technology for removal of heavy metals.

While every carbon adsorbent has its unique structure characteristics and functionality, one common feature shared by all carbon adsorbents is that they all contain rich active surface functional groups, which are crucial for the surface chemistry of carbon materials and the adsorption of heavy metals [48, 49]. It is generally believed that the chemical/physical interactions between heavy metals and functional groups of adsorbents contribute significantly to the adsorption of heavy metals [37, 50]. Functional groups are typically bonded with heteroatoms on carbon surfaces, commonly oxygen, nitrogen, sulfur, phosphorus and halogens. Therefore, functional groups are conventionally classified according to heteroatoms bonded to the surface of carbon i.e., oxygen-containing functional groups, nitrogen-containing functional groups, and sulfur-containing functional groups [51]. The functionality and quantity of each type of functional groups can be enhanced by chemically and/or physically modifying the surface of carbon materials to introduce the desired heteroatoms on carbon surfaces [22, 52–57]. Thus, one key area of research deals with the modification of carbon materials to enhance their surface chemistry for selective adsorption of target heavy metals [34, 38, 40, 58, 59]. While it is generally known that such modifications may improve the surface properties of carbon adsorbents (specific surface area, pore-size distribution or pore volume), increase the presence of functional groups, and enhance their structural stability [23, 60, 61], a systematic documentation of how surface functional groups of carbon adsorbents interact with aqueous heavy metals is currently lacking.

The overarching objective of this work is to provide a comprehensive review of recent research on carbon adsorbents in terms of their surface functional groups and the associated removal behaviors and performances to heavy metals in aqueous solutions. The specific objectives are as follows: 1) Outline and clarify the governing removal mechanisms of carbon adsorbents to aqueous heavy metals with a focus on the interactions between functional groups and heavy metals; 2) summarize and categorize various modification methods for introduction of heteroatoms, primarily oxygen, nitrogen, and sulfur, onto carbon surfaces for enhanced surface functionalities; and 3) elucidate the effects of various functional groups on adsorption behaviors of heavy metals onto carbon adsorbents. Perspectives of future work on functional groups of carbon adsorbents for heavy metal removal is also presented.

2. Mechanisms of heavy metal adsorption

Despite a great number of reports in the literature about the adsorption of heavy metals by carbon adsorbents [62–65], few have conducted quantitative evaluation of the roles of function groups in heavy metal adsorption [66–69]. In general, the inter-molecular interactions between functional groups and heavy metals are complex, depending on the heterogeneity and chemistry of the carbon surface, ionic environments of the aqueous solution, and the nature of the adsorbate. The mechanisms by which heavy metals are adsorbed onto carbon adsorbents may involve physical adsorption, electrostatic interaction, ion exchange, surface complexation, and precipitation (Fig. 1). The specific role of each mechanism in heavy metal adsorption varies considerably, depending on the target metal ion, ionic environment of the solution, and the carbon adsorbent [70–72]. Harvey et al. [73] highlighted that soft Lewis base functional groups (e.g., carbonyl and aromatic structures) favor dipole-dipole interactions such as cation-π bonding for Cd (II) adsorption while hard Lewis base functional groups (e.g., deprotonated carboxylic acids and phenols) favor sorption via cation exchange. Another important factor is solution pH because it influences not only metal speciation but also surface charge of carbon absorbents and complexation behavior of functional groups. Table 1 lists selected studies on adsorption capacities and mechanism of major heavy metals onto selected carbon adsorbents. For each of the metals, only the carbon adsorbent with the highest adsorption capacity from the research papers reviewed in this study is listed in the table. In general, chemisorption (ion exchange, surface complexation, precipitation) plays a more significant role in the removal of heavy metals from aqueous solution than physisorption (electrostatic interaction and physical adsorption) [51, 74]. Also note that several mechanisms can be at work in a particular aqueous environment. For example, ion exchange, electrostatic interaction, and surface complexation are closely related to surface functional groups through electrostatic forces, formation of binding sites, and covalent bonding [75–80]. The following discussion attempts to elucidate the linkages between each adsorption mechanism and various functional groups along with supporting examples in the literature.

Fig. 1.

Schematics of adsorption mechanisms of heavy metals onto carbon adsorbents.

Table 1.

Heavy metal adsorption onto selected carbon adsorbents with the highest capacity.

Heavy Metal	Carbon Adsorbent	Capacity (mg/g)	Mechanism	Reference
Pb(II)	Polyrhodanine modified multi-walled CNTs	8118	Chemical adsorption	[193]
Cr(VI)	Graphene sand composite	2859	Chemical adsorption	[194]
Cr(III)	Nitrogen-doped magnetic CNTs	638	Chemical adsorption	[165]
Cd(II)	GO	530	Chemical adsorption	[195]
Zn(II)	GO	345	Chemical adsorption	[195]
Ni(II)	GO	180	Surface complexation	[196]
Cu(II)	Polyvinylpyrrolidone-reduced GO	1689	Physical adsorption	[89]
Co(II)	zero valent iron/graphene composites	134	Chemical adsorption	[197]
Fe(III)	chitosan/poly(ethylene-oxide)/AC	217.4	Surface complexation & ion exchange	[72]
U(VI)	Graphene oxides	1330	Surface complexation	[121]
As(V)	3-dimensional graphene	177.6	Electrostatic attraction & ion exchange	[98]

2.1. Physical adsorption

Physical adsorption is a weak process involves diffusional movement of heavy metals into the pores of carbon adsorbents and then deposition onto the carbon surface without forming chemical bonds. In general, physical adsorption is strong affected by pore size distribution and surface area of the adsorbents [81, 82]. Increases of micropores can dramatically increase surface area to promote physical adsorption, while increase of mesoporous can facilitate contaminant diffusion to accelerate adsorption kinetics [83, 84]. The type of feedstock materials and carbon synthesis methodology such as carbonization/pyrolysis temperature for AC and biochar and the graphitization process for GO and CNTs are common factors determining the pore structure of carbon adsorbents [42, 85]. Thus, the heterogeneity and polarity of carbon surface in association with functional groups can aid in physical adsorption, especially driving the physical movement of heavy metals onto carbon surface via electrostatic attraction and ion-dipole forces.

Typically, physical adsorption is dependent on the surface properties and pore structure of carbon materials as well as the nature of heavy metals. Physical adsorption is common but unlikely serving as a predominant adsorption mechanism for heavy metals. Nevertheless, some studies have pointed out the importance of physical adsorption to the adsorption of aqueous heavy metals on carbon materials. For example, Nayak et al. reported that the removal of Cd(II) and Ni(II) from aqueous solutions by two types of ACs is mainly controlled by both physical adsorption and pore diffusion processes [85]. Xie et al. found that [48] physical force plays a role in adsorption of Cu(II) onto walnut shell derived AC while ion-exchange is the dominant mechanism [48]. Biochars produced from switchgrass (at 300 °C) or pyrolytic pinewood (at 700 °C) can sorb U(VI) and Cu(II) partly through physical diffusion adsorption [86, 87]. Liu et al. [88] revealed that adsorption of Pb(II) onto biochars prepared from hydrothermal liquefaction of biomass is exactly a physical endothermic process. Zhang et al. [89] reported that Cu(II) ions are significantly attracted by carbon atoms in reduced polyvinylpyrrolidone- modified GO through physical adsorption processes.

2.2. Electrostatic interaction

Electrostatic interaction occurs between positively charged heavy metals and negatively charged carbon adsorbents, especially with the presence of functional groups [90, 91]. As a relative weak process, the contribution of electrostatic interaction to heavy metal adsorption onto carbon adsorbents is only secondary [92–94]. Because most carbon surfaces are variably charged, prevalence of electrostatic interaction in heavy metal adsorption was dependent on solution pH and the point of zero charge of the adsorbent [95, 96]. The charged interface between carbons and the solution depends strongly on ionization of surface groups.

A few studies have attributed electrostatic interaction as one of the mechanisms for heavy metal adsorption by carbon materials. For example, Lv et al. [91] found evidence of high efficiency for removal of Pb(II) and Cu(II) by EDTA-functionalized bamboo AC through synergetic interactions of strong complexation, electrostatic attraction, ion exchange, and physical adsorption. Liang et al. [97] indicated that MnO₂-biochar possesses superior adsorption performance (maximum capacities: Pb(II) 268.0 mg/g and Cd(II) 45.8 mg/g) due to its electrostatic attachment, specific complexation and ion exchange. Chen et al. [98] confirmed that synergetic contribution of electrostatic attraction and ion exchange between hydroxyl groups and metal ions plays a significant role in the adsorption of As(V) and Pb(II) ions onto 3D graphene foams consisting of few-layered vertically-aligned graphene sheets with highly graphite structure. Complexation, ion exchange, and electrostatic attraction have been found to be the main mechanisms of Pb(II) ions adsorbed by a magnetic AC incorporated with amino groups [90]. Lu and Chiu [99] investigated the adsorption of Zn(II) ions onto purified CNTs and indicated that electrostatic attraction and adsorption of Zn(II) onto CNTs increase with increasing solution pH (in the range of 1–8).

2.3. Ion exchange

Ion exchange between heavy metals and protons on oxygen containing functional groups such as carboxyl and hydroxyl groups is one of the major mechanisms for adsorption of heavy metals by carbon adsorbents [100–102]. The efficiency of ion exchange process in adsorbing heavy metals onto carbon surfaces depends largely on the ion size of the metal contaminant and surface functional group chemistry of the adsorbent [19, 102]. Cation exchange capacity (CEC) is an important indicator of heavy metal adsorption when ion exchange is the predominant mechanism.

Ion exchange occurs typically between divalent metals of M²⁺ and H⁺ on oxygen containing functional groups. The processes can be represented as follows:

$$-COOH+M^{2+}\to -COO{M}^{+}+H^{+}$$ (1)

$$-OH+M^{2+}\to -O{M}^{+}+H^{+}$$ (2)

$$-2COOH+M^{2+}\to -COOMOOC-+2H^{+}$$ (3)

$$-2OH+M^{2+}\to -OMO-+2H^{+}$$ (4)

$$-\mathrm{COOH}+M^{2+}+-\mathrm{OH}\to -\mathrm{COOMO}-+2H^{+}$$ (5)

Obviously, solution pH is a key factor influencing ion exchange [103]. At acidic pH, more protons (H⁺) are available to saturate metal-binding sites. The release of H⁺ from carbon surface where metals are adsorbed alters solution pH [104, 105]. El-Shafey [106] revealed that sulfuric acid treated carbonaceous sorbent derived from rice husk via (175–180 °C) exhibits strong adsorption of Zn(II) and Hg(II) through deprotonation of H⁺ and subsequent ion exchange. Thitame and Shukla [107] found that the efficient adsorption of Pb(II) onto AC is dominated by ion exchange with acidic functional groups, and the adsorption capacity reaches maximum at pH of 6. Wu et al. [108] indicated that cation exchange of hydrogen with Cd(II) is the main mechanism for Cd(II) removal by 3D sulfonated reduced graphene oxide aerogel. Adsorption of heavy metals can also involve with ion exchange with other cations including Na⁺, K⁺, Ca²⁺, and Mg²⁺. Rio et al. [109] reported that the removal of aqueous Cu(II) by sludge-derived biochar is mainly due to the exchange of the toxic heavy metal with the calcium ions on biochar surface. Ion exchange has also been found to dominate the adsorption of heavy metals onto Canna indica-derived biochar and titanate nanotubes [110, 111].

2.4. Surface complexation

Surface complexation (inner- and outer-sphere) forms multiatom structures (i.e., complexes) with unique metal-functional groups interactions [112], playing a predominant role in adsorption of heavy metals onto carbon adsorbents [113, 114]. For example, heavy metals can be effectively bound by complexation with the carboxyl, phenolic and lactonic functional groups in biochars prepared at low temperature [115]. Surface complexation can also enhance the removal of Cu(II) ions by calcium alginate immobilized CNTs, adsorption of As(V) ions onto iron(III) oxide loaded ethylenediamine functionalized multi-walled CNTs (MWCNTs), and Th(IV) immobilization by GOs [114, 116, 117]. Guo et al. [118] indicated that Cd(II) removal by carbon adsorbents derived from phragmites australis through urea phosphate activation involves ion exchange, electrostatic attraction, and surface complexation, whereby nitrogen containing functional groups are primarily responsible for Cd(II) update. Pourbeyram [119] revealed that adsorption of Pb(II), Cd(II), Cu(II), and Zn(II) onto GO-Zr-P nanocomposite is mainly through surface complexation. Meena et al. [120] reported that surface complexation and ion exchange are the major removal mechanisms of heavy metals from aqueous solutions by carbon aerogel adsorbent. Wang et al. [121] revealed that strong chemical sorption and inner-sphere surface complexation dominate the U(VI) sorption by graphene oxides at high pH. Wang et al. [122] found that electrostatic interaction, precipitation, and surface complexation play important roles in controlling the adsorption of both As(V) and Pb(II) onto biochar.

2.5. Precipitation

Precipitation involves the formation of solid products during the adsorption process, occurring either in solution or on a surface [12, 123, 124]. For examples, the removal of Pb(II) from aqueous solutions by biochars may be controlled by the precipitation mechanism that turns the lead ions into minerals such as cerrusite (PbCO₃) and hyddrocerrusite (Pb₃(CO₃)₂(OH)₂) on biochar surface [12, 123]. As one of the major mechanisms for heavy metal removal, precipitation often works synergistically with other mechanisms, such as ion exchange, electrostatic interaction, and surface complexation [19]. Precipitation of heavy metals onto carbon adsorbents may have faster kinetics than other processes [12, 123]. Surface functional groups may only have little effect on the precipitation of heavy metals onto carbon adsorbent, but they can indirectly promote the precipitation process by influencing other adsorption mechanisms [19]. For example, surface functional groups contribute to several mechanisms involved in Cr (III) adsorption onto biochar derived from municipal sludge and promote the surface precipitation process to form Cr(OH)₃ on biochar surface [125, 126]. Oxidized CNTs can adsorb more heavy metals such as Pb²⁺, Cu²⁺ and Cd²⁺ than the pristine ones due to the combination of adsorption-precipitation and electrostatic attraction [127]. The uptake of Cu(II) ions onto graphene oxide is dominated mainly by precipitation and inner-sphere surface complexation at high pH (pH > 7.0) [128, 129].

3. Functionalization of Carbon Materials

The surface chemistry of carbons is determined, to a large extent, by the number and the nature of surface functional groups on carbon surface [49, 130]. Several studies have provided evidences of the enhanced heavy metal adsorption capacity with modifications of carbon adsorbents with functional groups [53, 56, 131–133]. The driving mechanism of the modifications lies with the introduction of various functional groups into carbon materials through doping heteroatoms onto carbon surface [134]. Ancillary benefits of grafting functional groups onto carbon surface may include increased electrostatic stability for the exfoliation of CNT bundles due to the charges created. From a practical standpoint of view, modifying the surface chemistry of carbon materials is crucial for the development of novel adsorption technology.

The synthesis route to modify surface functional groups involves post-treatment of the carbon material with chemical agents to control the properties of the final product. Thus, the method of modification and the type of raw carbon material from which the final adsorbent is prepared influence the nature of these functional groups. Selection of modification methods depends on the intended application and the chemical characteristics of target contaminants. In generally, oxidation, nitrogenation, and sulfuration are the common modification techniques that are employed to introduce oxygen, nitrogen, and sulfur heteroatoms on carbon surface for generation of functional groups [135, 136]. Fig. 2 is a graphic exhibition of these modification techniques, depicting the chemical agents used in each technique and the desired functional groups generated. The following discussion pertains to the details of each modification treatment along with supporting studies to demonstrate the enhanced adsorption of heavy metals.

Fig. 2.

Modification techniques to functionalize carbon adsorbents with various functional groups.

3.1. Oxidation

Surface oxidation is the most common and the easiest technique to generate oxygen containing functional groups (e.g., -OH, -COOH, -C=O and -C-O) onto the surface of carbon adsorbents [137, 138]. Fig. 3 exhibits typical oxidation treatment methods to introduce oxygen containing functional groups into carbon materials for enhanced adsorption of heavy metals. Conventionally, oxidation is performed under a refluxing condition in the presence of a single or a mixture of inorganic acids (e.g., HNO₃ and H₂SO₄) and an oxidizing agent (e.g., H₂O₂, KMnO₄, and NaOCl) [37, 40, 139]. This acidic treatment is favorable for enhanced adsorption of heavy metals. Regulating the amount of oxygen onto carbons with the right dosage of oxidation agents and experimental conditions such as reaction time and temperature are essential for building desired functional structures to targeted heavy metals. While the wet oxidation treatment applies to all carbon materials, the refluxing acids can sometimes be too harsh or detrimental to the physical aspects of carbons. Observations of shortening of CNTs, damage of CNT sidewalls, and reduced the Brunauer–Emmett–Teller (BET) surface area and total pore volume of AC have been reported with wet oxidation treatment [140, 141]. For example, Maroto-Valer et al. [140] indicated that nitric acid oxidation of AC can reduce the BET surface area by 9.2% and total pore volume by 8.8%.

Fig. 3.

Typical oxidation synthesis routes to introduce oxygen containing functional groups into carbon adsorbents [34, 141, 146, 213].

To reduce the undesired physical damage induced by wet oxidation treatment, alternative treatment methods have been explored. Oxygen plasma treatment has been used to incorporate oxygen containing functional groups into the external surface of AC while maintaining the internal surface intact [142–144]. Examples of enhancement of oxygen plasma treatment include Lee et al. [145] treating AC with He/O₂ plasma generated in a dielectric-barrier discharge reactor and Chen et al. [146] functionalizing MWCNTs with microwave-excited Ar/O₂ surface-wave plasma treatment. While the external carbon surfaces become rougher with treatment, it should be noted that the outcome changes with experimental conditions. Chen et al. [146] noted that the efficiency of Ar/O₂ plasma treatment in enhancing functional groups increases with increasing plasma power and treatment time and Lee et al. [145] indicated a reduced BET surface area due to destruction of pore walls by the plasma treatment.

Another example of physical treatment to facilitate oxidation is the irradiation of graphite with a high-energy electron beam at an intensity of 6.25 mA to control oxygen content in the formed GO [34]. The authors found that the oxygen-containing groups of GO increase with increasing irradiation dosages, varying from 6.4 and 19.2 kGy obtained by changing the irradiation time [34]. In other words, oxygen-content-controllable synthesis of GO can be realized by changing irradiation dosages. They further noted that the maximum Pb(II) sorption capacity increases with irradiation dosages, confirming that oxygen-containing functional groups are responsible for sorption of Pb(II).

Several studies have reported the enhanced sorption capacities of carbon adsorbents after oxidation modifications [147–149]. For example, nitric acid, ozone, and electrochemical oxidation techniques can enhance Cd(II) sorption capacity of AC cloth, CNTs, carbon nanofibres, and AC [150, 151]. Cationic surfactant oxidation of AC can improve its adsorption rate and capacity to Cr(VI) in aqueous solution [152]. Chitosan-coated phosphoric acid treated coconut shell carbon also exhibits great effectiveness in the removal of aqueous Zn(II) [153].

3.2. Nitrogenation

Nitrogenation is a widely used technique to generate nitrogen containing functional groups (e.g., -NH₂, -NH, -C=N and -C-N) onto the surface of carbon materials [154]. The introduction of nitrogen onto carbon materials is conventionally accomplished through reactions of carbon with ammonia, urea or amines, and this can significantly enhance the polarity of its surface and thus increase its specific interaction with polar adsorbates [132]. Fig. 4 exhibits typical nitrogenation treatment methods to introduce nitrogen containing functional groups onto carbon surface for enhanced adsorption of heavy metals. Typically, wet chemistry treatment involves nitration of the carbon followed with reduction [52]. Nitration is carried out by refluxing carbons with the nitrating mixture consisting of concentrated H₂SO₄ and concentrated HNO₃ at a specified volume ratio. The active nitrating species, nitronium ion (NO₂⁺), is produced by protonation and dissociation of nitric acid:

$$HN{O}_3+2H_{2}S{O}_4\to N{O}_2^{+}+H_3{O}^{+}+2HS{O}_4^{-}$$ (6)

Fig. 4.

Typical nitrogenation synthesis routes to introduce nitrogen containing functional groups into carbon adsorbents [52, 159, 214].

The NO₂⁺ ion is very reactive and can attach to aromatic rings of carbon surface by electrophilic aromatic substitution to form nitroaromatic compounds. These compounds are then reduced by sodium dithionite (Na₂S₂O₄) of a specified concentration. Among the two steps, nitration is the slow process that controls the efficiency of the treatment. The mixed acid ratio has a significant effect on nitration efficiency. An increase of the ratio from 1:1 to 2:1 or 3:1 can shorten the reaction time to about 30 min. Like wet acid oxidation treatment, the nitration/reduction treatment may result in reduction of BET surface area and pore volume though negligible effects on porous properties of carbons were also reported [155].

Thermal treatment of carbons in the presence of nitrogen supplying agent such as ammonia, urea, melamine or nitrogen oxides may also produce nitrogen-enriched carbons. For example, nitrogenation of AC can be accomplished by activation of AC with KOH at 800–850 °C, followed by ammoxidation at 350 °C for 2–3 h, by a mixture of ammonia and air at the ratio of 1:3 [156]. Similarly, a post synthesis treatment with ammonia to introduce nitrogen in CNTs can be accomplished with first treating the CNTs in nitric acid at 100 °C for 2h followed with thermal treatment of the oxidized-CNTs in ammonia flow at 600 °C for 4h [157].

Producing amino (-NH₂) functional groups onto the surface of CNTs, or amino-functionalization of CNTs, has recently drawn a great deal of interest due to its potential for multiple applications including absorbents for heavy metal removal. Alireza and Ali [158] provided an excellent review on the synthesis of amino-functionalized CNTs. The most common route of amino-functionalization of CNTs is firstly to produce oxidized CNTs via various agents, such as HNO₃, H₂SO₄, and H₂O₂ or oxidative coupling to generate carboxylic acid (COOH) groups on the sidewalls of the CNTs, followed by converting carboxylated CNTs into amino-functionalized CNTs in the presence of molecules containing the free amino groups [159]. The amide formation process can be accomplished by direct coupling of ethylenediamine with carboxylic groups or reducing the carboxyl group to hydroxymethyl, followed by forming amino groups [160]. Another commonly used method to create active sites for incorporating amino-functional groups on the CNT surfaces is the NH₃ plasma treatment and the microwave-excited NH₃/Ar surface wave plasma [161, 162]. Similar to the oxygen plasma treatment, the power level and the gas flow rate are important factors influencing the number of amino groups created onto the surfaces of CNTs.

Many publications have reported the enhanced heavy metal sorption capacity through nitrogenation treatment of carbon adsorbents [163]. For example, nitrogenation of AC under NH₃ atmosphere after pre-oxidation with HNO₃ can improve the adsorption rate and capacity of Cu(II) ions due to the increased amount of nitrogen-containing functional groups on carbon surface [155]. Nitric acid with thionyl chloride treatment and ethylenediamine reaction can effectively incorporate N-, S-, and Cl-containing functional groups onto the surface of AC, resulting in great adsorption rate and capacity for Hg(II) ions [164]. Amino-modified biochar exhibits excellent adsorption performance for Cu(II) through strong chemical complexation [52]. Nitrogen-doped magnetic carbon nanoparticles exhibit 10-fold increase in adsorption capacity for Cr(III) ion compared to AC with naphthalenesulfonic acid [165].

3.3. Sulfuration

Sulfuration treatment is often used to introduce sulfur containing functional groups (e.g., C-S, C=S, or S=O) onto the surface of carbon adsorbents. The introduction of sulfur onto carbon materials is accomplished through reactions of carbon with sulfurizing agents such as elemental S, SO₂, H₂S, Na₂S, K₂S, and dimethyl disulfide. Such modifications have been typically done on AC [54, 132] and recently on GO and CNTs [166–169]. Fig. 5 exhibits typical sulfuration treatment methods to introduce sulfur containing functional groups onto carbon surface for various applications. Similar to the nitrogenation treatment, sulfuration of carbon surface can also be realized through different chemical reaction routes that often require heat or acid treatments (Fig. 5). A large amount of sulfur (up to >30% wt) can be incorporated with most carbon-sulfur surface groups existing in the form of thiocarbonyls [170]. In addition to the benefit of introducing sulfur containing functional groups, sulfuration treatment may also change the porous structure of carbons with reported increase or decrease in specific surface area and pore volume, depending on the treatment conditions [171–173]. The decrease in surface area and pore volume is likely caused by sulfur-containing particles that may plug the pores.

Fig. 5.

Typical sulfuration synthesis routes to introduce sulfur containing functional groups into carbon adsorbents [175, 215–218].

Several publications have reported the enhanced heavy metal sorption capacity through sulfuration treatment of carbon adsorbents. For example, Tajar et al. [55] reported that surface modification of nut shells derived AC with SO₂ at ambient temperature can greatly enhance removal of Cd(II) from aqueous solutions. Macıas-Garcıa et al. [174] indicated that AC treated with SO₂ at 900 °C can improve the adsorption capacity of Cd(II) ions by about 70%. Macıas-Garcıa et al. [173] also examined adsorption of Pb(II) onto AC treated with SO₂ and/or H₂S at varying temperature and found that the amount of Pb(II) adsorbed increases with increasing sulfur content in the AC. Shen and Chen [175] treated graphene nanosheets with sulfanilic acid to obtain sulfonated graphene, demonstrating an excellent adsorption capacity (58 mg/g) to Cd(II) in aqueous solutions. Zhao et al. [176] found that sulfhydryl groups can be decorated onto GO surface through an easy reaction route and the sulfhydryl functionalized GO can effectively remove U(VI) from aqueous solutions.

4. Effects of functional groups

Research on sorption of heavy metals in aqueous solutions typically examines adsorption kinetics and isotherms to evaluate the performance of carbon adsorbents. The pseudo second order (PSO) model is perhaps the most widely used to fit the adsorption kinetics data, signifying chemisorption dominated mechanisms for heavy metal adsorption by carbons. While both Langmuir and Freundlich models are often used to simulate the isotherm data. The Langmuir isotherm, suggesting monolayer adsorption onto carbon surface, is more popular than the Freundlich isotherm, probably because it predicts the maximum adsorption capacity (Q_max). In this section, we summarize heavy metal adsorption performance of various carbon adsorbents with oxygen, nitrogen, sulfur, and other functional groups in to Table 2, Table 3, Table 4, and Table 5, respectively.

Table 2.

Selected carbon adsorbents with oxygen containing functional groups for heavy metal removal

Material	Heavy Metal	Qmax (mg/g)	Kinetic model	Isotherm	Functional Groups (FG)	Reference
Polyrhodanine modified multi-walled CNTs	Pb(II)	8118	Pseudo-second-order (PSO)	Langmuir, Freundlich	-OH	[193]
Lignosulfonate-modified graphene hydrogel	Pb(II)	1210	PSO	Langmuir	-OH, -COOH	[177]
					-OH, -COOH,
GO	Pb(II)	1119	PSO	Langmuir		[195]
					-C=O, -C-O
Graphene sand composite	Cr(VI)	2859.38	-	-	-OH, -COOH	[194]
Polyethyleneimine and GO	Cr(VI)	539.53	PSO	Langmuir	-COOH	[198]
Poly(m-phenylenediamine)-coated Fe3O4/o-MWCNTs	Cr(VI)	346	PSO	Langmuir	-OH, -COOH	[178]
CNT-COO−	Hg(II)	661.9	-	Langmuir	-COOH	[199]
Rice straw AC	Hg(II)	500	PSO	Freundlich	-OH, -C-O	[200]
CoFe2O4-chitosan-graphene	Hg(II)	361	PSO	Langmuir	-OH, -COOH	[201]
EDTA functionalized magnetic GO	Hg(II)	268.4	PSO	Freundlich	-OH, -COOH	[92]
					-OH, -COOH,
GO	Cd(II)	530	PSO	Langmuir		[195]
					-C=O, -C-O
Ear-like poly (acrylic acid)-AC	Cd(II)	473.2	PSO	Langmuir	-COOH	[202]
AC	Cd(II)	388.7	-	Langmuir	-OH, -COOH	[203]
GO	Ni(II)	180.893	PSO	Langmuir	C=O	[196]
					-OH, -COOH,
AC from Glycyrrhiza glabra residue	Ni(II)	166.7	PSO	Langmuir		[204]
					-C=O, -C-O
Polyvinylpyrrolidone-reduced GO	Cu(II)	1689	-	-	-OH	[89]
L-Tryptophan functionalized GO	Cu(II)	588	PSO	Langmuir	-C=O, -C-O	[181]
Xanthated Fe3O4-chitosangrafted onto GO	Cu(II)	426.8	PSO	Langmuir	-COOH	[166]
Zero valent iron (ZVI)/graphene composites	Co(II)	134.27	PSO	Freundlich	-OH	[197]
Amination GO	Co(II)	116.35	PSO	Langmuir	-COOH	[94]
AC from Glebionis coronaria L. biomass	Co(II)	45.75	-	Langmuir	-OH, -C-O	[191]
					-OH, -COOH,
GO	Zn(II)	345	PSO	Langmuir		[195]
					-C=O, -C-O
AC produced from Bambusa vulgaris striata chitosan/poly(ethylene-oxide)/AC electrospun nanofibrous membrane	Zn(II)	254.39	PSO	Langmuir	-COOH,-C=O	[191]
	Zn(II)	186.2
			-	Langmuir	-COOH	[72]
	Fe(III)	217.4
AC-AMP	Fe(II)	67.1	-	-	-OH	[205]
					-OH, -COOH,
Graphene Oxides	U(VI)	1330	-	Langmuir		[121]
					-C=O, -C-O-C
Magnetic cobalt ferrite/multiwalled CNTs	U(VI)	212.7	PSO	Langmuir	-OH	[206]
Three-dimensional graphene foam	As(V)	177.6	-	-		[98]
Fe(III) oxide coated ethylene-diamine modified MWCNTs	As(V)	23.47	PSO	Freundlich	-OH	[116]
Biochar-supported zerovalent iron	Ag(II)	655.4	-	-	-OH	[207]
					-OH, -COOH,
Graphene oxides	Th(IV)	58.59	PSO	Langmuir		[117]
					-C=O
					-OH, -COOH,
GO Nanosheets	Eu(III)	175.44	-	Langmuir		[182]
					-C=O, -C-O
GO-hydroxyapatite	Sr(II)	702.18	PSO	Langmuir	-OH	[208]
Chitosan/GO	Au(II)	1076.65	PSO	Langmuir	-OH, -COOH	[209]

Table 3.

Selected carbon adsorbents with nitrogen containing functional groups for heavy metal removal

Material	Heavy Metal	Qmax (mg/g)	Kinetic model	Isotherm	FG	Reference
				Langmuir,
Polyrhodanine modified multi-walled CNTs	Pb(II)	8118	PSO		-NH2	[193]
				Freundlich
Nitrogen-doped hierarchical microporous / mesoporous carbons	Pb(II)	212	PSO	Langmuir	-NH	[93]
Sulfuric acid doped poly diaminopyridine/graphene composite	Cr(VI)	609.76	PSO	Langmuir	-NH	[210]
polyethyleneimine and GO	Cr(VI)	539.53	PSO	Langmuir	-NH2	[198]
Poly(m-phenylenediamine)-coated Fe3O4/o-MWCNTs	Cr(VI)	346	PSO	Langmuir	-NH	[178]
CoFe2O4-chitosan-graphene	Hg(II)	361	PSO	Langmuir	-NH2	[201]
CNT-CONH2	Hg(II)	332.6	-	Langmuir	-NH2	[199]
EDTA functionalized magnetic GO	Hg(II)	268.4	PSO	Freundlich	-NH2	[92]
AC	Cd(II)	388.7	-	Langmuir	-NH	[203]
GO	Ni(II)	180.893	PSO	Langmuir	-C=N, -C-N	[196]
L-Tryptophan functionalized GO	Cu(II)	588	PSO	Langmuir	-NH	[181]
Xanthated Fe3O4-chitosangrafted onto GO	Cu(II)	426.8	PSO	Langmuir	-NH	[166]
NH2-rich polymer/GO	Cu(II)	349.04	PSO	Langmuir	-NH2	[211]
Chitosan/poly(ethylene oxide)/AC electrospun nanofibrous membrane	Cu(II)	195.3	-	Langmuir	-NH2	[72]
Amination GO	Co(II)	116.35	PSO	Langmuir	-NH2	[94]
Chitosan/poly(ethylene oxide)/AC electrospun nanofibrous membrane	Zn(II) Fe(III)	186.2 217.4	-	Langmuir	-NH2	[72]
Amidoxime-graftedmultiwalled CNTs	U(VI)	145	PSO	Langmuir	-NH2	[192]
Chitosan/GO	Au(II)	1076.65	PSO	Langmuir	-NH2	[209]

Table 4.

Selected carbon adsorbents with sulfur containing functional groups for heavy metal removal

Material	Heavy Metal	Qmax (mg/g)	Kinetic model	Isotherm	FG	Reference
	Pb(II)	29.98
Sulfur-Functionalized Ordered	Hg(II)	70.75			-C-S, -C=S,
			PSO	-		[189]
Mesoporous Carbon	Cd(II)	4.96			-COS, -SOx
	Ni(II)	1.2
	Pb(II)	226
Chitosan/Sulfydryl-functionalized GO composite	Cd(II)	117	PSO	Freundlich	-SH	[167]
	Cu(II)	235
	Cd(II)	202.429
Dithiocarbamate CNTs	Cu(II)	101.523	PSO	Langmuir	-C=S	[190]
	Zn(II)	16.625
Low-cost sulfurized AC derived from nut shells	Cd(II)	142.86	-	Frendlich	-C=S	[55]
Sulfonated Graphene Nanosheets	Cd(II)	58	PSO	Langmuir	-SO3H	[175]
Citrus Limettioides peel carbon	Ni(II)	38.46	PSO	Langmuir	-S=O	[212]
Xanthated Fe3O4-chitosan grafted onto GO	Cu(II)	426.8	PSO	Langmuir	-C-S	[166]

Table 5.

Selected carbon adsorbents with other functional groups for heavy metal removal.

Material	Heavy Metal	Qmax (mg/g)	Kinetic model	Isotherm	FG	Reference
Waste-art-paper biochars	Pb(II)	1555	PSO	Langmuir	-CO32-	[167]
Graphene sand composite	Cr(VI)	2859.38	-	-	-O-CH3	[194]
Sulfuric acid doped poly diaminopyridine / graphene	Cr(VI)	609.76	PSO	Langmuir	-C=C	[210]
Nitrogen-doped magnetic CNTs	Cr(III)	638.56	PSO	Langmuir	-C-C,	[165]
Polyvinylpyrrolidone-reduced GO	Cu(II)	1689	-	-	-C=C	[89]
Zero valent iron (ZVI)/graphene composites	Co(II)	134.27	PSO	Freundlich	-C=C	[197]
					-C=C,
AC from Glebionis coronaria L. biomass	Co(II)	45.75	-	Langmuir		[191]
					-P-O-C
AC-AMP	Fe(II)	67.1	-	-	-CH2	[205]
Amidoxime-grafted multiwalled CNTs	U(VI)	145	PSO	Langmuir	-C=N-OH	[192]
GO-hydroxyapatite	Sr(II)	702.18	PSO	Langmuir	-HPO42-	[208]
Chitosan/GO	Au(II)	1076.65	PSO	Langmuir	-C=C	[209]

4.1. Oxygen containing functional groups

Oxygen containing functional groups are so far the most important in influencing surface reactions, surface behavior, hydrophilicity, electrical and catalytic properties of carbons. Table 2 summarizes the effects of oxygen containing functional groups on the removal of heavy metals by carbon adsorbents in term of their maximum adsorption capacities, the best-fit kinetic and isotherm models, and the specific functional groups. The hydroxy and carboxyl groups, among the most common functional groups, have been found to contribute widely to the adsorption of heavy metals, such as Pb(II), Cr(VI), Hg(II), Cd(II) and Ni(II) ions onto various carbon adsorbents [177–180]. In addition, -C=O and -C-O functional groups have been reported more likely to make effects on the surface of GO based adsorbents [181, 182]. Yang et al. [183] revealed the great adsorption capacity of Cu(II) ions onto GO through an aggregation process caused by oxygen containing functional groups binding to Cu(II) in aqueous solutions. Bai et al. [34] and Bian et al. [50] have also pointed out the important role of oxygen-containing groups on surfaces of GO played in the adsorption of Pb(II) and Cd(II), respectively. Adsorption of Cr (VI) on H₃PO₄ treated biochar was through forming inner sphere complex with the surface oxygenic functional groups on the biochar [97]. Wang et al. [184] noted that the adsorption capacity of MWCNTs treated with concentrated HNO₃ is significantly increased due to creation of oxygen functional groups that can react with Pb²⁺ to form complex or salt precipitates on the CNTs surface. In addition, the adsorption equilibrium time for the acidified CNTs is much shorter than that for ACs. Different chemical and physical modification methods have been applied to oxidize biochars derived from various feedstocks and the modifications dramatically increased the contents of surface oxygen containing functional groups and thus biochars’ adsorption rates and capacities to various heavy metals including Pb(II), Cd(II), Cu(II), Ni(II), etc. [19, 37].

4.2. Nitrogen containing functional groups

Because incorporation of nitrogen groups onto carbon surface can increase its basic properties to enhance the adsorption of heavy metals, the nitrogen containing functional groups have received increasing attentions in the research of carbon adsorbents. On the other hand, the introduction of nitrogen functional groups also had negligible effects on porous properties of carbons [155]. Table 3 displays the related studies on heavy metal adsorption of carbon materials with nitrogen containing functional groups, summarizing the relevant adsorption capacity and the best-fit kinetic and isotherm models. The nitrogen containing functional groups, such as -NH₂ and -NH, contribute mostly to selective adsorption of heavy metals onto carbon nanomaterials, which are commonly synergistic with the oxygen containing functional groups. The nitrogen-containing ACs derived from ammonium humates show high adsorption ability to aqueous Cu(II) and Pb(II), even better than many commercial ACs [185].

Nitrogen-fictionalized carbon (biochar/AC) can be obtained directly from diammonium hydrogen phosphate treatment biomass through pyrolysis and have strong sorption ability to aqueous Cr(VI) [136]. Several other carbon materials including microporous carbon, mesoporous carbon, magnetic carbon nanoparticles, and carbon film have been doped with nitrogen to introduce nitrogen containing functional groups on carbon surfaces to increase their adsorption ability to various heavy metals in aqueous solutions [93, 135, 165, 186, 187].

4.3. Sulfur containing functional groups

In general, sulfur containing functional groups on carbon adsorbents are beneficial to their adsorption of heavy metals. Depending on the experimental conditions, however, introducing sulfur containing functional groups can either increase or decrease specific surface area and pore volume of the carbon adsorbents [171–173], which may also affect their adsorption performances. In addition, elemental sulfur has been loaded into carbon adsorbents through sulfuration treatment such as solution infiltration and chemical reaction-deposition [188], which can expand their adsorption ability to various aqueous heavy metals. Table 4 lists selected studies on the effects of sulfuric groups on carbon adsorption of heavy metals, summarizing the corresponding adsorption capacity as well as the best-fit kinetic and isotherm models. As can be seen, the sulfuric functional groups significantly enhance the adsorption performance of heavy metals onto carbons, especially with respect to the adsorption of Cu(II) and Cd(II) ions onto GO and CNT based adsorbents. Feng et al. [56] revealed that the elemental sulfur species deposited on carbonaceous surfaces after continuous exposure to hydrogen sulfide are the most effective mercury uptake sorbents comparing thiophene and sulfate. The preparation of sulfur-functionalized ordered mesoporous carbons with sulfur bearing salts activating can simultaneously increase its specific surface area (837–2,865 m²/g) and pore volume (0.71–2.3 cm³/g), the affinity of the sulfurized carbons towards heavy metals is in order of Hg(II)>Pb(II)>Cd(II)>Ni(II) at pH of 7 or greater [189]. Sulfonated graphene nanosheets not only hold a relatively complete sp²-hybridized plane with a high affinity to aromatic pollutants in water, but also have sulfonic acid groups and partial original oxygen-containing groups that exhibit fast adsorption kinetic rate and superior adsorption capacity towards aqueous Cd(II) [175]. It has also been reported that dithiocarbamate groups functionalized MWCNTs prepared by ethylenediamine and carbon disulfide have very high adsorption capacities for Cd(II), Cu(II), and Zn(II) in aqueous solutions [190].

4.4. Other functional groups

Selected studies about the effects of other functional groups on carbon adsorption for heavy metals are summarized in Table 5 along with the corresponding adsorption capacity and the best-fit kinetic and isotherm models. While oxygen, nitrogen and sulfur containing functional groups are predominant for the adsorption of heavy metals, other functional groups (e.g., -C=C, -CH₂, -O-CH₃ and -P-O-C) can also be introduced to carbon surface to promote heavy metal adsorption (Table 5). Xu et al. [39] confirmed precipitation of Pb(II) with the CO₃^2- group released from waste-art-paper biochar to form PbCO₃ and Pb₂(OH)₂CO₃ crystals on its surface and indicated that high temperature incineration of the spent biochar can turn it into value-added (high purity >96%) PbO nanoparticles. Tounsadi et al. [191] revealed that ACs prepared from Glebionis coronaria L. biomass by phosphoric acid activation are rich in phosphorus containing surface functional groups, which are beneficial to the removal of Cd(II) from aqueous solution. Amidoxime-grafted MWCNTs synthesized by plasma techniques can selectively adsorb U(VI) ions from nuclear industrial effluents, which was due to the binding contribution of different functional groups especially -C=N-OH [192].

5. Conclusions and perspectives

Clearly, surface functional groups of carbon materials play important roles in the removal of heavy metals from aqueous solutions. The interactions between the functional groups and the heavy metals can directly or indirectly affect the adsorption mechanisms such as electrostatic interaction, surface complexation, ion exchange, physical adsorption, and precipitation (Fig. 1). From a practical standpoint, surface modification of carbon adsorbents in optimizing their physicochemical and sorptive properties is essential in developing new technologies for environmental remediation. One key area of research deals with surface modification of carbon materials by incorporating heteroatoms on carbon surfaces (Fig. 2), thereby selectively adsorbing target heavy metals. Among the different chemical modification methods, oxidation, nitrogenation, and sulfuration are the most commonly used ones and can effectively introduce corresponding oxygen, nitrogen, and sulfur containing functional groups on the carbon surfaces to promote the adsorption of heavy metals (Fig. 3–5). Overall, carbon materials with designed surface functional groups are promising adsorbents for various environmental applications, especially with respect to the removal of heavy metal ions from aqueous solutions. To advance and further promote the research on functionalized carbon adsorbents, additional investigations are needed in several directions, such as: 1) Current surface modification techniques of carbon materials often require strong acid/base treatment, high heat/pressure treatment, or intensive oxidation/reduction reactions; therefore, it is necessary to develop novel, facile, cost-effective, and environmentally-friendly surface modification methods to functionalize carbon adsorbents. 2) Despite the efforts by researchers to investigate the mechanism of adsorption, it is commonly believed that more profound and comprehensive characterization methods on the surface properties of carbon adsorbents are required to further explain and elucidate the adsorption behaviors and mechanisms. 3) While the same functional group behaves similarly in the adsorption process, quantitative evaluation of the role of each type of function groups in heavy metal adsorption does not exist in the literature. 4) Because functionalized carbon materials have unique and tunable physicochemical properties, their applications in other fields such as energy and environmental applications should also be explored.