Nanomaterials have wide applications as sorbents for water purification as they have high selectivity as well as capacity for organic/inorganic pollutants and contaminants in aqueous solutions. Non-essential toxic heavy metals present in water have become a major cause of many diseases, ageing, and genetic problems. Carbon-based nanomaterials have generated great interest in their use as sorbent materials for removal of heavy metals from water/wastewater as they are stable, have limited reactivity, wide surface area, and are strong antioxidants. This study explored the possibility of ranking different carbon-based nanomaterials for efficient removal of lead ions from water which are routinely detected in water samples. Lead may be introduced in water because of its intensive use in many products. The adsorbents based on graphenes and carbon nanotubes have been analyzed on the basis of adsorption capacity, reusability, toxicity/biocompatibility, and cost. The effects of surface area of sorbent, pH of water, thermodynamics/kinetic studies have also been analyzed to evaluate the efficiency of sorbents. The information can be utilized to select the appropriate, most efficient and safe sorbents for removal of lead ions from water.

INTRODUCTION

Water pollution due to heavy metal ions (non-biodegradable pollutants) continues to be a challenge in all parts of the world as it poses serious health risks. Non-essential heavy metals that are toxic to living organisms and detected in water samples are cadmium, chromium, mercury, lead, arsenic, and antimony. On 6 February 2014, Waterkeeper Alliance and Yadkin Riverkeeper issued the results of water sampling from the Dan River in the wake of the third largest coal ash spill in US history (EcoWatch 2014). Results revealed that the water was contaminated with extremely high levels of arsenic, chromium, iron, lead, and other toxic metals typically found in coal ash. A lead concentration of 0.129 mg/L was detected in the water which was much higher than that recommended by the Environmental Protection Agency (EPA) and World Health Organization (WHO) for drinking water and for protection of other aquatic species. The EPA and WHO have recommended the maximum acceptable concentration of Pb(II) in drinking water as 0.015 mg/L and 0.01 mg/L, respectively (Waid & Hossam 2007).

Heavy metals may accumulate in different organs of aquatic species. Fish are the primary source of protein and omega-3 fatty acids for many human beings. The fish contaminated with toxic metals may raise health concerns for consumers (Ashraf et al. 2005; Kojadinovic et al. 2006; Yilmaz et al. 2009). Removal of heavy metals from water is important in terms of health and environment due to the risk of accumulation in living tissues throughout the food chain. Many methods have been proposed to remove heavy metals from water such as precipitation, coagulation-sedimentation, reverse osmosis, ion exchange, cementation, and adsorption onto activated carbon (Charerntanyarak, 1999; Qdais & Moussa 2004; Dabrowski et al. 2004). Among these, adsorption is one of the best methods for removal of heavy metals from water because it is quite simple, time saving and economical, involving no complicated instrumentation and space. Natural and traditional sorbents could remove heavy metal ions from water with low sorption efficiency and capacity limits (Norotry et al. 2000; Omer et al. 2005; Bang et al. 2005; Pratik & Choksi, 2008). To overcome this difficulty nanomaterials of size 1–100 nm have been used to remove heavy metal ions from water. Many nanoscale materials such as dendrimers, metal oxides, zeolites, carbon-based nanoparticles, nanoclay, magnetic nanoparticles, and chitosan nanospheres (Ngomsik et al. 2005; Wang et al. 2010; Zhao et al. 2011; Hua et al. 2012; Gupta et al. 2012; Liang et al. 2013; Kumar & Chawla, 2014; Kumar et al. 2015; Onnby et al. 2014; Ayati et al. 2014) have been used for adsorption of heavy metals from aqueous solution. It has been reported that carbon nanotubes (CNTs) and carbon-encapsulated magnetic nanoparticles have significantly higher sorption efficiency toward metal ions in comparison with activated carbons (Pyrzyńska & Bystrzejewski 2010).

This work focuses on the removal of lead from aqueous solution because of its intensive use in many applications and industries. Elemental lead is a bluish grey metal. Lead ores comprise 0.002% of the earth's crust. They include lead sulfides, sulfates, carbonates, chloroarsenates, and chlorophosphates. Apart from naturally occurring ores, lead may be used in the form of organic and inorganic lead. Lead can be used as a pure metal, as an alloy, or in the form of a chemical compound. It is very likely that lead enters the water cycle through different sources. The main sources of exposure to lead in drinking water are lead piping, lead solders, electronic waste, effluents, lead-based paints and use of lead-containing ceramics for cooking or drinking (Rojas et al. 1994; Mathee et al. 2007; Zheng et al. 2008). It is very costly to replace old piping and lead continues to dissolve even from old pipes. The amount of lead dissolved in water depends on pH, temperature, and contact time in water. Lead may enter the blood stream and be distributed to the liver, kidney, spleen, brain, bones, and central nervous system through ingestion of water contaminated with lead. Lead exposure can cause severe neurological and physiological disorders. Children under the age of six are at risk of developing cognitive health effects due to lead exposure (Sciarillo et al. 1992; Schneider & DeCamp, 2007; WHO 2008). So, it is very important to design/tailor the adsorbents for efficient removal of lead ions from water. This study identifies and prioritizes the different types of CNTs and graphenes used as sorbents for removal of lead ions from water on the basis of efficiency, reusability, toxicity, and cost. Toxicity has been taken into consideration because of health implications of nanoparticles (Wang et al. 2009; Kolosnjaj et al. 2010; Bhatt et al. 2011). The information is further compiled to understand the existing data gaps and challenges in the area.

METHODS

The study presents the following steps for analyzing the performance of carbon-based nanomaterials for the removal of lead ions from water:

  • Overview of carbon-based nanomaterials used as adsorbents.

  • CNTs and graphene-based adsorbents for removal of Pb(II) ions from water.

  • Variation of adsorption capacity of carbon-based nanosorbents with pH.

  • Effect of surface area of sorbent on adsorption capacity.

  • Isothermal adsorption models for adsorption of lead.

  • Thermodynamic/kinetic studies and mechanism for adsorption of lead.

  • Performance of sorbents on the basis of efficiency, reusability, toxicity, and cost.

  • Identification of knowledge gaps and conclusions.

RESULTS AND DISCUSION

Overview of carbon-based nanomaterials used as adsorbents

Carbon-based nanomaterials such as graphenes, CNTs, and fullerenes have attracted great attention among researchers due to their unique combination of chemical and physical properties. Owing to properties such as high mechanical strength, thermal and electrical conductivity, and optical properties, modified/functionalized carbon-based nanomaterials are also being utilized for various industrial applications, such as high-strength materials, electronics, biomedical engineering and water purification (Cha et al. 2013).

Graphene is a two-dimensional (2D) nanomaterial consisting of sp2 hybridized carbon with high specific surface area. Graphene possesses a unique structure and is a type of one or a number of atomic layered graphite. It possesses good mechanical and thermal properties. Graphene oxide (GO) is a sheet of carbon functionalized with epoxy and hydroxyl functional groups on the basal plane, with carboxyls and lactols at the edges (Dreyer et al. 2010). CNTs are considered as a sheet of graphite that has been rolled into a tube with single, double or multiple walls. CNTs usually have a diameter in the range 0.1–10 nanometres and a length of up to centimeters. CNTs surfaces have a strong interaction with other molecules. They possess excellent adsorption ability.

A fullerene, also commonly known as the buckyball, is a spherical closed-cage structure made of sp2 carbon. Its consistent size and shape as well as availability for chemical modification led many scientists to develop C60 derivatives for many applications (Kroto et al. 1985). Graphene and CNTs have similar electrical, thermal, and optical properties, but the 2D structure of graphene enables more diverse electronic characteristics. Graphene is structurally tough yet highly flexible, which makes it attractive for engineering thin, flexible materials (Kim et al. 2009; Eda et al. 2008). In a number of reported studies modified graphenes and CNTs exhibit good adsorption capacity for environmental pollutants. In reported adsorption studies, the adsorption space was found to be on the external surface and not on the inner cavity or the inter-wall space of multi-walled CNTs (MWCNTs) (Zhao et al. 2011; Madadrang et al. 2012; Li et al. 2002; Li et al. 2003a, 2003b; Peng et al. 2003; Yang et al. 2006a, 2006b; Mubarak et al. 2013, 2014a, 2014b). Fullerenes have also been used for the adsorption of organic compounds and adsorption depends on the dispersion of fullerene (Cheng et al. 2004; Cheng et al. 2005). Ballesteros et al. (2000) studied the analytical potential of fullerene as a sorbent for organic and organometallic compounds. It has been concluded that fullerene adsorbs many types of organic substances with maximum efficiency of 60% and adsorption efficiency decreased with increase in polarity of the organic compound. Fullerenes and CNTs have different geometries and have different spaces for adsorption. Theoretically, the full surface of a fullerene should be available for adsorption, but experiments confirmed that much less surface was available on fullerenes for adsorption (Yang et al. 2006a, 2006b) because of aggregation of fullerene particles.

CNTs and graphene-based adsorbents for removal of Pb(II) ions from water

Activated carbon was used initially as sorbent to remove heavy metals up to a certain concentration level. Then CNTs, fullerene, and graphene were synthesized to be used as nanosorbents. These carbon-based materials were also modified to further improve the sorption capacities. Modified graphene such as GO, EDTA-graphene oxide (EDTAGO), and chitosan-GO (Zhao et al. 2011; Madadrang et al. 2012; Sitko et al. 2013) have been studied as sorbents for removal of lead ions from water. CNTs can also be used as supporting material for adsorption. Adsorption capacities of CNTs can be significantly enhanced by modification. CNTs/modified CNTs have been used as adsorbents for a number of heavy metal ions such as copper, nickel, cobalt, vanadium, silver, cadmium, and rare earth elements (Stafiej et al. 2007). A detailed review of carbon-based nanomaterials, graphenes, and CNTs used as sorbents for removal of lead ions from water is provided in the following sections.

Graphene-based adsorbents for removal of Pb(II) ions from water

The data reported to date for graphene-based sorbents used for removal of lead ions from water are compiled in Table 1. Zhao et al. (2011) used few-layered graphene oxide (FGO) to adsorb Pb(II) ions from aqueous solutions. The abundant oxygen-containing groups on the surfaces of FGO played an important role in Pb(II) ion adsorption on FGO. The maximum adsorption capacities of Pb(II) ions on FGO calculated from the Langmuir model were about 842 mg/g, 1,150 mg/g, and 1,850 mg/g at 293 K, 313 K, and 333 K, respectively. Sitko et al. (2013) prepared GO for removal of Cu(II), Zn(II), Cd(II), and Pb(II). The results indicate that maximum adsorption can be achieved in broad pH ranges of 3–7 for Pb(II). The maximum adsorption capacity of Pb(II) on GO at pH 5 was 1,119 mg/g. The competitive adsorption experiments showed the maximum affinity for Pb(II).

Table 1

Graphene-based adsorbents for removal of lead ions from water

Adsorbent Preparation method Qmax (mg/g) Surface area (m2/g) Temp (K) pH Isotherms Other metal ions? Reference 
FGO nano sheets FGO were synthesized from graphite using the modified Hummers method 842 Graphite 3.0 & FGO 120 293 K NA Langmuir No Zhao et al. (2011
1,150 313 K 
1,850 333 K 
GO GO was prepared through the oxidation of graphite using potassium dichromate 1,119 *NA RT Langmuir Cu (II), Zn (II), Cd (II) Sitko et al. (2013
EDTAGO Chelating groups are successfully linked to GO surfaces through a silanization reaction between N-(trimethoxysilylpropyl) ethylenediamine triacetic acid (EDTA-silane) and hydroxyl groups on GO surface 479 ± 46 NA RT 6.8 Langmuir No Madadrang et al. (2012
MCGO MCGO was fabricated through an easy and fast process and its application as an excellent adsorbent for metal ions was also demonstrated 76.94 NA RT Langmuir No Fan et al. (2012)  
GNSs GNSs were obtained by vacuum-promoted low-temperature exfoliation 35–40 NA NA NA NA No Huang et al. (2011
Graphene sheets before and after modification with 1- PBA Graphene sheets before and after modification with 1- PBA via the stacking interaction for heavy metal ions such as Cu(II), Cd(II) 99.3 mg/g for graphene sheet (GS), 124.2 mg/g for graphene/PBA(GSPBA) NA NA Langmuir No Kong et al. (2013
Adsorbent Preparation method Qmax (mg/g) Surface area (m2/g) Temp (K) pH Isotherms Other metal ions? Reference 
FGO nano sheets FGO were synthesized from graphite using the modified Hummers method 842 Graphite 3.0 & FGO 120 293 K NA Langmuir No Zhao et al. (2011
1,150 313 K 
1,850 333 K 
GO GO was prepared through the oxidation of graphite using potassium dichromate 1,119 *NA RT Langmuir Cu (II), Zn (II), Cd (II) Sitko et al. (2013
EDTAGO Chelating groups are successfully linked to GO surfaces through a silanization reaction between N-(trimethoxysilylpropyl) ethylenediamine triacetic acid (EDTA-silane) and hydroxyl groups on GO surface 479 ± 46 NA RT 6.8 Langmuir No Madadrang et al. (2012
MCGO MCGO was fabricated through an easy and fast process and its application as an excellent adsorbent for metal ions was also demonstrated 76.94 NA RT Langmuir No Fan et al. (2012)  
GNSs GNSs were obtained by vacuum-promoted low-temperature exfoliation 35–40 NA NA NA NA No Huang et al. (2011
Graphene sheets before and after modification with 1- PBA Graphene sheets before and after modification with 1- PBA via the stacking interaction for heavy metal ions such as Cu(II), Cd(II) 99.3 mg/g for graphene sheet (GS), 124.2 mg/g for graphene/PBA(GSPBA) NA NA Langmuir No Kong et al. (2013

*NA = not available

EDTAGO was found to be a good adsorbent for Pb(II) removal with a higher adsorption capacity (Madadrang et al. 2012). The adsorption capacity for Pb(II) removal was found to be 479 ± 46 mg/g at pH 6.8, and the adsorption process was completed within 20 minutes. The Langmuir adsorption model agreed well with the experimental data. It has been reported that EDTAGO can be reused after washing with HCl. Fan et al. (2012) used magnetic chitosan/graphene oxide (MCGO) as sorbents for the removal of Pb(II) ions from aqueous solutions. The results indicated that adsorption of Pb(II) ions on MCGO surface was strongly dependent on pH value of the solution. The abundant functional groups on the surfaces of MCGO played an important role in Pb(II) ion sorption. Equilibrium studies showed that the data of Pb(II) ion adsorption followed the Langmuir model. The maximum adsorption capacity for Pb(II) was estimated to be 76.94 mg/g. The MCGO was stable and easily recovered. Graphene nanosheets (GNSs) (Huang et al. 2011) that were obtained by vacuum-promoted low-temperature exfoliation were used to adsorb lead ions from an aqueous system with an adsorption capacity in the range of 35–40 mg/g.

Kong et al. (2013) studied the adsorption capacity of graphene sheets before and after modification with 1-pyrenebutyric acid (PBA) via the stacking interaction for heavy metal ions such as Cu(II), Cd(II), and Pb(II) at different pH and contact time. PBA modified graphene (GS-PBA) was found to be a better adsorbent for heavy metal ions than graphene. The adsorption capacity of graphene was 99.3 mg/g for Pb(II), while that of graphene/PBA was 124.2 mg/g for Pb(II) at pH of 5.

CNTs as adsorbent for removal of Pb(II) ions from water

CNTs have attracted enormous attention due to their unique tubular structures and large length/diameter ratio. CNTs can be either metallic or semi-conducting. Li et al. (2003a, 2003b) found that the sorption of Pb(II) on to MWCNTs was 3–4 times larger than for conventional sorbents in water purification. Table 2 gives the compiled data for CNTs used as sorbents for removal of lead ions from water. It is clear from Table 2 that CNTs are good adsorbents for multi-component sorption of metal ions. CNTs are also modified by oxidation, used with metal ions or metal oxides, and coupling with organic compounds to increase the sorption capacity. The adsorption properties of CNTs were found to be strongly dependent on the pH value of the solution.

Table 2

CNT-based adsorbents for removal of lead ions from water

Adsorbent Preparation method Qmax (mg/g) Surface area (m2/g) Temp (K) pH Isotherms Other metal ions? Reference 
Pristine MWCNTs and acidified MWCNTs Acidified MWCNTs were obtained with different durations of soaking in nitric acid solution 91(Acidified) NA R.T Langmuir No Wang et al. (2007
7.2 (Pristine) 
Raw MWCNTs, Oxidized MWCNTs, MWCNTs with tris (2-aminoethyl) amine (MWCNTs-TAA) Oxidized MWCNTs were prepared by oxidation with 3M HNO3 followed by synthesis of MWCNTs-TAA 6.7 (raw), 27.8 (oxidized), 71 (MWCNTs-TAA)  NA R.T 5.7–6 Langmuir No Tehrani et al. (2014
MWCNTs were functionalized with 6-arm amino PEG, and synthesized PEG-MWCNTs Oxidation followed by treatment with SOCl2 and amination with PEG- -NH2 47.5 22.5 298 Koble-Corrigan Cd, As Velikovic et al. (2013
Oxidized CNTs (O-CNTs) Chemical vapor deposition followed by dispersion in concentrated nitric acid and refluxed at 140 °C 63.29 153 298 4.2 Langmuir Cd, Cu Li et al. (2004
Oxidized carbon nanotubes (O-CNTs) CNTs-2 (soaked in HNO3) CNTs-3 (refluxed with HNO32.96 NA R.T Freundlich Cu, Co, Zn, Mn Stafiej et al. 2007  
Oxidized MWCNTs (O) and (OMWCNTs) polymerized with cyclodextrin (POMWCNTs) Synthesis of MWCNTs by spray pyrolysis followed by polymerization with cyclodextrin polymer 54.38 (O) 78.61 (O) R.T 5–6 Langmuir Co Mamba et al. (2010
28.86 (PO) 5.46 (PO) 
Pristine MWCNTs (P) and modified with 8-HQ 8-HQ-MWCNTs (M) Pb(II) adsorption increased from 80% to 95.1% upon the modification of MWCNTs with 8-HQ 0.064 (P) 69.1(P) 298 Langmuir Cu, Pb, Zn, Cd Kosa et al. (2012
0.076 (M) 76.2 (M) 
Pristine carbon nanotubes (P) and CNTs grown on A12O3 particles (MWCNTs-A12O3Carbon nanotubes (MWCNTs) were grown on the surface of microsized A12O3 particles in CH4, atmosphere at 700 °C under the catalysis of Fe-Ni nanoparticle 11.23 (P) 95.68 (P) R.T Langmuir Pb, Cu, Cd Hsieh & Horng (2007)  
67.11 (CNTs- A12O318.61 (CNTs- A12O3
CNTs/iron oxide magnetic composites CNTs/iron oxide magnetic composites, CNTs were first oxidized and then treated FeCl3. 6H2O and FeSO4 6H2O at 70 oC under N2 atmosphere. NaOH solution was added drop wise, stirred, dried and ground 0.5 m mol/g 134.0 R.T Freundlich Cu Peng et al. (2005
CNT/MnO2 CNTs were prepared by catalytic pyrolysis, MnO2 coating through redox process 78.74 275 323 Langmuir Cu Wang et al. (2007
Diethylenetriamine modified MWCNTs (d-MWCNT) NA d-MWCNT 58.26 NA 318 NA Langmuir No Vukovic et al. (2011
MWCNTs/iron oxide magnetic composites (MWCNTs- MCs) Co-precipitation method 10.02– 31.25 NA 293–353 5.5 Langmuir No Hu et al. (2011
Carbon nanotubes on granular activated carbon The thermal CVD process using 1% nickel as catalyst 0.853 NA NA Langmuir No Onundi et al. (2011
Sugarcane bagasse/MWCNT composite NA 56.6 mg g−1 (composite) 23.8 mg g−1 (bagasse) NA 301 4.5 Langmuir No Hamza et al. (2013
Optimized MWCNTs (ethylenediamine-grafted MWCNTs (MWCNTs-EDA-I and MWCNTs-EDA-II) Two oxidization routes of MWCNTs were carried out to provide carboxylic acid (CA)-functionalized MWCNTs (MWCNTs-CA-I and II). After separation samples are treated with SOCl2 and ethylenediamine 157.19 mg/g (MWCNTs-EDA-I) NA R.T Langmuir No Hu et al. (2012
89.16 mg/g (MWCNTs-EDA-II) 
Adsorbent Preparation method Qmax (mg/g) Surface area (m2/g) Temp (K) pH Isotherms Other metal ions? Reference 
Pristine MWCNTs and acidified MWCNTs Acidified MWCNTs were obtained with different durations of soaking in nitric acid solution 91(Acidified) NA R.T Langmuir No Wang et al. (2007
7.2 (Pristine) 
Raw MWCNTs, Oxidized MWCNTs, MWCNTs with tris (2-aminoethyl) amine (MWCNTs-TAA) Oxidized MWCNTs were prepared by oxidation with 3M HNO3 followed by synthesis of MWCNTs-TAA 6.7 (raw), 27.8 (oxidized), 71 (MWCNTs-TAA)  NA R.T 5.7–6 Langmuir No Tehrani et al. (2014
MWCNTs were functionalized with 6-arm amino PEG, and synthesized PEG-MWCNTs Oxidation followed by treatment with SOCl2 and amination with PEG- -NH2 47.5 22.5 298 Koble-Corrigan Cd, As Velikovic et al. (2013
Oxidized CNTs (O-CNTs) Chemical vapor deposition followed by dispersion in concentrated nitric acid and refluxed at 140 °C 63.29 153 298 4.2 Langmuir Cd, Cu Li et al. (2004
Oxidized carbon nanotubes (O-CNTs) CNTs-2 (soaked in HNO3) CNTs-3 (refluxed with HNO32.96 NA R.T Freundlich Cu, Co, Zn, Mn Stafiej et al. 2007  
Oxidized MWCNTs (O) and (OMWCNTs) polymerized with cyclodextrin (POMWCNTs) Synthesis of MWCNTs by spray pyrolysis followed by polymerization with cyclodextrin polymer 54.38 (O) 78.61 (O) R.T 5–6 Langmuir Co Mamba et al. (2010
28.86 (PO) 5.46 (PO) 
Pristine MWCNTs (P) and modified with 8-HQ 8-HQ-MWCNTs (M) Pb(II) adsorption increased from 80% to 95.1% upon the modification of MWCNTs with 8-HQ 0.064 (P) 69.1(P) 298 Langmuir Cu, Pb, Zn, Cd Kosa et al. (2012
0.076 (M) 76.2 (M) 
Pristine carbon nanotubes (P) and CNTs grown on A12O3 particles (MWCNTs-A12O3Carbon nanotubes (MWCNTs) were grown on the surface of microsized A12O3 particles in CH4, atmosphere at 700 °C under the catalysis of Fe-Ni nanoparticle 11.23 (P) 95.68 (P) R.T Langmuir Pb, Cu, Cd Hsieh & Horng (2007)  
67.11 (CNTs- A12O318.61 (CNTs- A12O3
CNTs/iron oxide magnetic composites CNTs/iron oxide magnetic composites, CNTs were first oxidized and then treated FeCl3. 6H2O and FeSO4 6H2O at 70 oC under N2 atmosphere. NaOH solution was added drop wise, stirred, dried and ground 0.5 m mol/g 134.0 R.T Freundlich Cu Peng et al. (2005
CNT/MnO2 CNTs were prepared by catalytic pyrolysis, MnO2 coating through redox process 78.74 275 323 Langmuir Cu Wang et al. (2007
Diethylenetriamine modified MWCNTs (d-MWCNT) NA d-MWCNT 58.26 NA 318 NA Langmuir No Vukovic et al. (2011
MWCNTs/iron oxide magnetic composites (MWCNTs- MCs) Co-precipitation method 10.02– 31.25 NA 293–353 5.5 Langmuir No Hu et al. (2011
Carbon nanotubes on granular activated carbon The thermal CVD process using 1% nickel as catalyst 0.853 NA NA Langmuir No Onundi et al. (2011
Sugarcane bagasse/MWCNT composite NA 56.6 mg g−1 (composite) 23.8 mg g−1 (bagasse) NA 301 4.5 Langmuir No Hamza et al. (2013
Optimized MWCNTs (ethylenediamine-grafted MWCNTs (MWCNTs-EDA-I and MWCNTs-EDA-II) Two oxidization routes of MWCNTs were carried out to provide carboxylic acid (CA)-functionalized MWCNTs (MWCNTs-CA-I and II). After separation samples are treated with SOCl2 and ethylenediamine 157.19 mg/g (MWCNTs-EDA-I) NA R.T Langmuir No Hu et al. (2012
89.16 mg/g (MWCNTs-EDA-II) 

Functionalization of CNTs can be performed using oxidizing agents such as nitric acid, sulfuric acid, potassium permanganate, hydrogen peroxide, and ozone (Cuentas-Gallegos et al. 2006; Otvos et al. 2006; Lu & Chiu 2008; Peng et al. 2011). Functionalized CNTs have unique properties, such as high aspect ratio, high mechanical strength, light weight, high electrical conductivity, high thermal conductivity, and high surface area, which make them suitable for different applications (Ajayan 1999). Functional groups such as carboxyl, lactones, and phenols induce negative charge on the CNTs surface and oxygen atoms of these functional groups donate electrons to the metal ions, increasing the cation exchange capacity of the CNTs. Protons in the functional groups of the CNTs are exchanged with the metal ions. Chemical interaction (bond) between the metal ions and the surface acidic functional groups of the CNTs is mainly responsible for adsorption. To increase the absorption capacity of CNTs, they can be oxidized with nitric acid. Some studies showed that un-derivatized CNTs tend to be water insoluble and toxic.

Kosa et al. (2012) modified MWCNTs with 8-hydroxyquinoline (8-HQ), which are used to remove Pb(II), Cu(II), Cd(II), and Zn(II). The results showed that the modification of CNTs with 8-HQ enhanced the removal process. It is interesting to note that reported adsorption capacities were lower than those of other studies using different CNTs as given in Table 2. Authors have explained that this is due to the lower specific area of the pristine MWCNTs and 8-HQ-MWCNTs, which were 69.1 m2/g and 76.2 m2/g, respectively. Li et al. (2003a, 2003b) reported that oxidized CNTs showed exceptionally high sorption capacity and efficiency for Pb(II) ions from water. They have investigated the sorption of Pb(II), Cu(II), and Cd(II) onto MWCNTs and reported maximum sorption capacities of 97.08 mg/g for Pb(II), 24.49 mg/g for Cu(II), and 10.86 mg/g for Cd(II) at room temperature, pH 5 and metal ion equilibrium concentration of 10 mg/L. They also found that the sorption capacities of the MWCNTs were 3–4 times larger than those of powder activated carbon and granular activated carbon. Xu et al. (2008) used oxidized MWCNTs as adsorbent to study the sorption characteristic of Pb(II) ions from aqueous solution, and the results indicated that sorption of Pb(II) on oxidized MWCNTs is strongly dependent on pH values, and independent of ionic strength and the type of foreign ions. Li et al. (2002) also found that CNTs could have good Pb(II) adsorption capacity at pH 5.

Carbon nanotubes/metal oxide (CNT/MO) composites combine the properties of CNTs and metal oxide nanoparticles, and show unique properties as a result of interaction between them (Chu et al. 2010). Metal oxide nanoparticles supported on CNTs have been found to be effective adsorbents for the removal of heavy metal ions and organic chemicals from water. CNT/MO composites may be synthesized using various techniques such as pressure-less sintering, in situ chemical vapor deposition, ultra-sonication, or sol-gel method with spark plasma sintering. MWCNT/alumina nanocomposite has been found to be an effective sorbent for the removal of Pb(II) ions from aqueous solution at pH range of 3–7. The coated nanotubes exhibit better removal ability over uncoated nanotubes (Gupta et al. 2011). The adsorption percentage of Pb(II) ions increased as the composite dose was increased over the range 1–50 mg. Approximately 100% removal was achieved with 50 mg of the coated MWCNTs compared with 85% removal with 50 mg of uncoated MWCNTs. Increase in adsorption was attributed to the electrostatic attraction between the pairs of electrons on the oxygen atoms of alumina and the cationic lead. CNT-iron oxides magnetic composite has also been used as adsorbent for removal of Pb(II) and Cu(II) from water (Peng et al. 2005). The experimental data were well described by the Freundlich adsorption isotherm with correlation coefficient of 0.9794 for Pb(II). Scanning electron microscope characterization showed an entangled network of CNTs with clusters of iron oxides attached to it and suggests the formation of CNTs/iron oxides composites. Furthermore, Wang et al. (2007) prepared manganese oxide-coated CNTs for the removal of Pb(II) from aqueous solution. It was found that the adsorption capacity increased with increasing MnO2 load because of the availability of more binding sites. The optimum reported load for best performance is 30 wt% MnO2. It was reported that adsorption capacity of MnO2/CNT was much higher than that of CNT. At zero loading level, the adsorption capacity of MnO2/CNT was almost three times that of CNT.

Hamza et al. (2013) investigated the adsorption of Pb(II) from aqueous solution onto a sugarcane bagasse/MWCNT composite and compared it with sugarcane bagasse. The adsorption capacity for the composite was found to be higher at 28 °C and pH 4.5: 56.6 mg/g for the composite and for bagasse the reported value was 23.8 mg/g. The Langmuir adsorption isotherm provided the best fit to the data and Elovich kinetics model was found to be suitable for kinetics studies. The thermodynamic parameters of adsorption suggested the spontaneous adsorption process for both but for the bagasse adsorbent it was enthalpy-driven, whereas for the sugarcane bagasse/MWCNT composite it was entropy-driven. Desorption of the lead-loaded sorbents was quite proficient with 0.1 mol/dm3 HCl.

Hu et al. (2012) reported that Pb(II) can be adsorbed on the ethylenediamine-grafted multi-walled carbon nanotubes (MWCNTs-EDA-I and MWCNTs-EDA-II) in the pH range of 4–7 and MWCNTs-EDA-I has a higher maximum Pb(II) adsorption capacity (157.19 mg/g) than MWCNTs-EDA-II (89.16 mg/g). The adsorbed Pb(II) can be eluted completely using 5 mL of 1 mol/l HNO3.

In another study by Perez et al. (2010), nitrogen-doped MWCNTs were oxidized and used to adsorb Pb(II) from aqueous solution. Carboxylic and nitro groups on the surface of oxidized MWCNTs increased the adsorption capacity. Pristine, oxidized, ethylenediamine, diethylenetriamine, and triethylenetetramine modified multi-walled carbon nanotubes (raw-MWCNT, o-MWCNT, e-MWCNT, d-MWCNT, and t-MWCNT, respectively) were employed as sorbents to study the adsorption characteristics of Pb(II) and Cd(II) ions (Vukovic et al. 2011). Adsorption data were described by a pseudo-second-order kinetic model and by the Langmuir isotherm. The maximum adsorption capacity of Pb(II) was 58.26 mg/g. Pb(II) ion selectivity with respect to d-MWCNT and e-MWCNT was greater than Cd(II). Atieh et al. (2010) studied the adsorption of Pb(II) from water by using COOH functionalized CNTs. Results of the study showed that 100% of Pb(II) was removed at pH 7.

Velickovic et al. (2013) used MWCNTs functionalized with 6-arm amino polyethylene glycol (PEG), and synthesized PEG-MWCNTs as adsorbent for Cd(II), Pb(II), and As(V) ions. Time-dependent adsorption was described by the Weber-Morris kinetic model and adsorption process by the Koble-Corrigan isotherm. The maximum adsorption capacities of Cd(II), Pb(II), and As(V) on PEG-MWCNTs, at pH 4 and temperature 298 K, were 77.6 mg/g, 47.5 mg/g and 13 mg/g, respectively. The competitive adsorption studies showed that the adsorption affinity of ions toward PEG-MWCNTs showed highest adsorption of Cd(II) at pH 8, followed by Pb(II) at pH 6, and As(V) at pH 4.

Variation of adsorption capacity of carbon-based nanosorbents with pH

pH of the medium plays an important role in the adsorption of particular metal ions on CNTs/graphenes as they exist as different species depending on the pH. Zhao et al. (2011) reported that the adsorption of Pb(II) ions on FGO increases with increasing pH from 1 to 8 and then, at pH>8, decreases with increasing pH values. It is important to point out that final pH values were lower than the initial pH values if the initial pH value was lower than 9.5. The adsorption of Pb(II) was partly due to ion exchange of Pb(II) with H+ (oxygen-containing functional groups on the surface of FGO are deprotonated) on FGO surfaces, and the exchanged H+ ions are released to solution thereby resulting in the decrease in pH values. At low pH values, the main species of Pb(II) ions are Pb(II) and Pb(OH)+, while at high pH values, the predominant Pb(II) species is Pb(OH)3 which is difficult to adsorb on to FGO because of the negative surface charge of FGO. At high pH values, the functional groups are progressively deprotonated to form a negative surface charge and thereby the negatively charged Pb(II) species is difficult to adsorb on the negatively charged surfaces of FGO. In a study by Huang et al. (2011), it was found that after heat treatment adsorption against lead ions increased in case of graphene nanosheets, although the oxygen complexes of GNSs showed a considerable decrease. Also, lead ion uptake resulted in an increase in the pH value of the solution. It was supposed that the Lewis basicity of GNSs improved by heat treatment in favor of simultaneous adsorption of lead ions and protons onto sheets. So, better performance of graphene-based adsorbents was observed at low pH values.

Tehrani et al. (2014) studied the effect of pH on adsorption of Pb(II) on oxidized MWCNTs with tris (2-aminoethyl) amine (MWCNTs-TAA) from aqueous solution at pH in the range of 3–11 using NaOH (0.01–0.1 mol/L) or HNO3 (0.01–0.1 mol/L) for pH adjustment. It was reported that sorption initiated from pH 3. This trend has been attributed to competition between H+ and Pb(II) ions for capturing the active sites. The mechanism suggested that the reaction between the lone pair on the nitrogen of the tris (2-aminoethyl) amine groups and the metal ions results in the formation of complexes. At pH less than 3, the active sites of TAA (electron lone pair) are protonated, which inhibited the reaction. Below pH 6, Pb(II) exists as free ions in solution, making their interaction with the surface unhindered. At pH values above 6, Pb(OH)+, [Pb(OH)2], and [Pb(OH)3] species exist and can precipitate from solution. These species would be adsorbed to a greater extent on a less polar carbon surface of the adsorbents compared with Pb(II) ions. As the pH increases, more negatively charged surface becomes available thus facilitating greater metals removal.

Li et al. (2002) also concluded that the removal of Pb(II) from water by acid-refluxed CNTs depends on pH value, which affects the surface charge of the adsorbent, degree of ionization and speciation of the adsorbates. It was found that adsorption capacity of the CNT increases with the pH value from 3 to 7. Precipitation of metal ions occurs as the pH exceeds 7. At pH 3, the adsorption effect was very weak due to the competition of protons with Pb(II) on the adsorption sites. However at pH 5, the adsorption capability increases due to the role of functional groups on the CNTs surfaces. In another study by Kosa et al. (2012), it was reported that the removal of Pb(II) by 8-HQ-MWCNTs changed significantly for Pb(II), from 1.2 to 14.3% when the pH was increased from 2 to 6. The minimum adsorption observed at low pH values was attributed to the fact that the higher concentration and mobility of hydrogen ions at lower pH favored the preferential adsorption of hydrogen ions compared with metal ions. Also, at low pH values the surfaces of the MWCNTs are covered by hydrogen ions, which prevents the metal ions from approaching the sorbent binding site. An increase in the pH favors the adsorption of Pb(II) ions as positive surface charge leads to less Coulombic repulsion of the metal ions.

Gupta et al. (2011) also reported that alumina coated MWCNTs were found to be an effective adsorbent for the removal of lead ions from aqueous solution and with increasing pH from 3 to 7, the percentage of lead removed was increased. Hsieh & Horng (2007) also reported that Pb(OH)2 begins to precipitate at pH 9. Li et al. (2003a, b) reported that the sorption capacities of Pb(II) by CNTs decreases with an increase in solution ionic strength. Xu et al. (2008) reported that sorption of Pb(II) on oxidized MWCNTs is strongly dependent on pH values, and independent of ionic strength and the type of foreign ions. The efficient removal of Pb(II) from aqueous solution is limited at pH 7–10. The effective pH range for efficient removal of lead according to the studies discussed above is 3–7.

Effect of surface area of sorbent on adsorption

Adsorption is a surface-based phenomenon. Nanomaterials have high surface area to mass ratio, but they also tend to aggregate in solution. The stability of nanomaterial suspensions could be increased by oxidation or functionalization (Xu et al. 2012). It is clear from Figure 1 that oxidation can significantly enhance the adsorption capacity of CNTs. In the case of MWCNT, pristine MWCNT with surface area of 69.1 m2/g had an adsorption capacity of 0.064 mg/g (Kosa et al. 2012), whereas oxidized MWCNT with surface area 78.61 m2/g was found to have an adsorption capacity much higher than that of pristine at 54.38 mg/g (Mamba et al. 2010). Mamba et al. (2010) also polymerized the oxidized MWCNT and reported decreased adsorption capacity of 28.86 mg/g with decreased surface area of 5.46 m2/g. If we compare the adsorption capacity of pristine MWCNT with modified MWCNT it is clear that it increased from 0.064 to 0.076 mg/g (Kosa et al. 2012). This was attributed to the increase in surface area of modified MWCNT. Li et al. (2002) reported that CNT with surface area 145 m2/g has 49.95 mg/g adsorption capacity which is lower than the adsorption capacity (Li et al. 2004) of CNT (63.29 mg/g) which has a larger surface area (153 m2/g). In another study carried by S. G. Wang et al. (2007), CNT/MnO2 was reported to have maximum surface area of 275 m2/g with maximum adsorption capacity of 78.74 mg/g. Hsieh & Horng (2007) reported that CNT with a larger surface area (95.68 m2/g) has a lower adsorption capacity (11.23 mg/g) compared with CNTs/Al2O3 which has 67.11 mg/g adsorption capacity and 18.61 m2/g surface area. This was explained by the tendency of pristine CNT to agglomerate and settle down so that they have less opportunity to adsorb metal ions present in water. In summary, we can tailor the adsorption properties of nanomaterials by oxidation/functionalization. Pristine CNTs have a lower sorption capacity compared with oxidized ones. Further, the nature of modification also decides the sorption behavior of adsorbents. The detailed comparison for graphene sorbent cannot be done because of data gaps (Figure 2). Future studies of these sorbents should also incorporate the surface area studies of sorbent used. It will be very helpful while tailoring the properties of sorbents in a systematic way.

Figure 1

Schematic representation of different types of carbon nanotubes used as sorbents along with adsorption capacity in mg/g followed by surface area in brackets if available.

Figure 1

Schematic representation of different types of carbon nanotubes used as sorbents along with adsorption capacity in mg/g followed by surface area in brackets if available.

Figure 2

Schematic representation of different types of graphene used as sorbents along with adsorption capacity in mg/g followed by surface area in brackets if available.

Figure 2

Schematic representation of different types of graphene used as sorbents along with adsorption capacity in mg/g followed by surface area in brackets if available.

Isothermal adsorption models for adsorption of lead

The isothermal adsorption model of Pb(II) onto adsorbents was analyzed by the following equations:

Langmuir isotherm 
formula
1
Freundlich isotherm: 
formula
2
where Ke and Kf are the Langmuir and Freundlich constants, respectively.

By comparing the regression coefficient (R2), the Langmuir isotherms were the best fit models for maximum adsorbents on adsorbing metal ions and the maximum capacities (mg/g) of adsorbents were determined. Langmuir adsorption indicates that one atomic layer of the metal ion formed on the surface of adsorbents by the process of adsorption. The Langmuir isotherm adsorption model with higher R2 (R2 ∼ 0.999) value indicates the adsorption process is better than the Freundlich model (R2 ∼ 0.979) for Pb(II). This implies the monolayer coverage of lead ions on the surface of the carbon-based nanosorbents.

Thermodynamic/kinetic studies and mechanism for adsorption of lead

Adsorption of metal ions on the adsorbent surfaces can be easily explained by different kinetic models, e.g. pseudo-first-order, pseudo-second-order, and diffusion or intraparticle diffusion kinetic models (Tsai et al. 2001; Demirbas et al. 2004). Kinetic models are also helpful to describe the mechanism of adsorption of metal ions on the adsorbent surface. The pseudo-first-order kinetic model for a solid–liquid system can be expressed by 
formula
3
A linear form of the equation is 
formula
4
where k1 is the adsorption rate constant of first-order kinetic models in L/mg/min, qe is equilibrium amount adsorbed at time t (mg/g) and qt is equilibrium amount adsorbed at time t = ∞. The values of k1 were calculated from the slope of the linear plot of ln(qeqt) versus t.
The pseudo-second-order kinetic models can be expressed by Demirbas et al. (2004) 
formula
5
A linear form of the equation is 
formula
6
where k2 is the adsorption rate constants of second-order kinetic models in L/mg/min, qe is the equilibrium amount adsorbed at time t (mg/g), and qt is the equilibrium amount adsorbed at time t = ∞. The values of qe and k2 can be calculated by the slope and intercept of plot of t/qt versus t.
Thermodynamic parameters can be calculated from the variation of the equilibrium constant K0 with the change in temperature using the following equation: 
formula
7
where as is the activity of adsorbed ion and ae is the activity of the ion in solution at equilibrium. The adsorption standard free energy changes (ΔG0) can be calculated by equation: 
formula
8
where R is the gas constant and T is the temperature in K.

Zhao et al. (2011) reported that adsorption of Pb(II) ions on FGO was mainly dominated by strong surface complexation. The thermodynamic parameters calculated from the temperature dependent adsorption isotherms indicated that the adsorption of Pb(II) ions on FGO was a spontaneous and endothermic process. Endothermic means that sorption increases with increase in temperature. Adsorption isotherms and kinetic studies (Sitko et al. 2013) for sorption of lead ions on GO nanosheets predicted that monolayer coverage and adsorption was controlled by chemisorption involving the strong surface complexation of metal ions with the oxygen-containing groups on the surface of GO. The adsorption experiments showed the tendency to agglomerate and precipitate post adsorption.

Yu et al. (2013) used O2-plasma oxidized MWCNTs for adsorption of lead ions and reported the enhanced adsorption capacity after plasma oxidation. The adsorption kinetics was described by a pseudo-second-order model and thermodynamic parameters suggested endothermic and spontaneous adsorption. The adsorption of lead on MnO2/CNTs was also described by the pseudo-second-order rate equation (Wang et al. 2007). Thermodynamics of Pb(II) was studied at various temperatures and adsorption capacity was found to increase with the rise of temperatures, which indicated an endothermic reaction. Hsieh & Horng (2007) studied the adsorption of lead ions on CNTs grown on microsized alumina particles and results matched well with the second-order kinetic model. Hu et al. (2011) used MWCNTs/iron oxide magnetic composites for adsorption of lead and predicted it as an endothermic process. Xu et al. (2008) reported that sorption of Pb(II) on oxidized MWCNTs can be described by a pseudo-second-order model very well. The maximum studies reported to date described the kinetics using a pseudo-second-order model and thermodynamic studies suggested the endothermic and spontaneous adsorption of lead ions on the reviewed carbon-based sorbents (i.e. graphenes and CNTs).

Proposed mechanism of adsorption of Pb(II) ions

The adsorption mechanism in the case of carbon-based nanomaterials depends on the pH of the solution and the functionalization of the adsorbent. The ion exchange mechanism mainly contributes to Pb(II) uptake on oxidized MWCNTs at low pH and the surface complexation mechanism to Pb(II) uptake at high pH values. Figure 3 explains the ion exchange of Pb(II) by protons of functional groups in the case of functionalized CNTs at low pH. The adsorption mechanism became more and more complicated for modified systems and showed different order. In a study by Tehrani et al. (2014), it was concluded that adsorption on the surface of MWCNT modified with TAA was due to sharing of a lone ion pair of amine groups to lead ions. It was discussed that adsorption cannot be explained by one mechanism. Kinetic studies are also helpful to describe the mechanism of adsorption of lead ions onto the adsorbent surface. The value of the correlation coefficient (R2) was close to one and a relatively high correlation coefficient is a more applicable model to represent the adsorption mechanism of lead onto adsorbents. The value of the correlation coefficients (R2) of the pseudo-first-order model was slightly lower than the pseudo-second-order model for most of the reviewed nanosorbents which indicates that the kinetic model is best explained by the pseudo-second-order model rather than the pseudo-first-order model. This also indicates that adsorption of lead ions onto the reviewed nanosorbents may be explained by a chemisorption mechanism which involves the valence forces through exchange or sharing of electrons between nanosorbents and lead ions.

Figure 3

Adsorption of lead by graphene-based adsorbents from water samples at pH 3–7 and temperature 298–353 K.

Figure 3

Adsorption of lead by graphene-based adsorbents from water samples at pH 3–7 and temperature 298–353 K.

Major findings for removal of Pb(II) ions using carbon-based nanomaterials as sorbents

Carbon-based nanosorbents have been analyzed in the pH range 5–7 and temperature range 298–353 K on the basis of adsorption capacity, reusability, and cost, and are discussed in detail in the following section.

Adsorption capacity of graphene-based sorbents under a given set of conditions (pH 5–7) follows the following order: GNS < MCGO < GS < GSPBA < EDTAGO < FGO < GO (Figure 4). This order indicates that GO prepared through the oxidation of graphite using potassium dichromate had a higher adsorption capacity than the other graphene-based sorbents for removal of lead ions from water at pH 5.

Figure 4

Adsorption of lead by carbon nanotube-based adsorbents from water samples at pH 3–7 and temperature 298–353 K.

Figure 4

Adsorption of lead by carbon nanotube-based adsorbents from water samples at pH 3–7 and temperature 298–353 K.

Adsorption capacity of CNT-based sorbents under a given set of conditions (pH 5–7) follows the following order: pristine MWCNTs < oxidized MWCNTs < PEG-MWCNT < MWCNTs-A12O3 < MWCNTs-TAA < CNT/MnO2 < MWCNTs-EDA-II < acidified MWCNTs < MWCNTs-EDA-I (Figure 5). This order indicates that oxidation and modification on the surface of CNTs make them more efficient absorbents for adsorption of lead ions from water at pH range 5–7. However, adsorption capacity of lead ions by CNT mainly depends on the types of nanotubes and their method of preparation. Adsorption capacity of commercial MWCNTs (Conyuan MWCNTs 2040) is higher than the MWCNTs with 10–20 nm outer diameter, 30 nm length, and >95% purity at approximately the same pH. The adsorption capacity of oxidized CNTs prepared by chemical vapor deposition followed by dispersion in concentrated nitric acid and refluxed at 140 °C is much higher than the catalytic pyrolysis using Ni particles followed by dispersion in concentrated nitric acid and refluxed at 140 °C for 1 hour. Method of oxidation of CNTs also plays a very important role in adsorption. Adsorption capacity of MWCNTs oxidized at 55 °C for 24 hours using a 1:3 H2SO4:HNO3 mixture is much higher than the MWCNTs oxidized by HNO3 dispersed for 45 minutes in an ultrasonic bath (35 KHz, 70 W) followed by stirring at 110 °C for 48 hours using sulfuric acid and hydrogen peroxide mixture.

Figure 5

Schematic diagram of ion exchange mechanism with sidewall groups of carbon nanotubes by lead ions in solution.

Figure 5

Schematic diagram of ion exchange mechanism with sidewall groups of carbon nanotubes by lead ions in solution.

Reusability of adsorbent has an important role in deciding its performance in removal of heavy metal ions from water. Adsorbents that offer excellent reusability results without compromising the efficiency are preferred over those that cannot be regenerated. Reusability results of MWCNT-TAA revealed that the sorbent ability after regeneration did not change considerably (Tehrani et al. 2014). The efficiency of lead removal was around 96% for up to three cycles and decreased after the fourth adsorption/desorption cycle. The recycling, desorption and regeneration of the MWCNTs were evaluated by Kosa et al. (2012) and the results predicted that Pb(II) ions desorbed at pH values lower than 2, and the MWCNTs could then be used in up to three cycles of adsorption/desorption without losing efficiency. Regeneration of CNTs/A12O3 can be promoted by increasing the concentration of the acid (Hsieh & Horng 2007). The recovery capacity was maintained above 90% even after six successive adsorption/desorption cycles. Some of the Pb(II) ions can be desorbed from FGO at low pH values, and the desorption percentage of Pb(II) from FGO decreases with increasing pH values, which is attributed to the physical sorption of Pb(II) at low pH and strong surface complexation at high pH values. The experimental results of Madadrang et al. (2012) suggest that EDTAGO can be reused after washing with HCl, suggesting potential applications in environmental clean up. Fan et al. (2012) also reported that prepared MCGO was stable and easily recovered. Yu et al. (2013) reported that O2-plasma oxidized MWCNTs may be regenerated very easily by altering the pH of solution. Peng et al. (2005) separated CNTs/iron oxide magnetic composites by a simple magnetic process and a recovery rate of above 98%. It can be concluded that reconditioning of all reported sorbents reported satisfactory results.

Cost is also an important factor in large-scale applications of carbon-based nanosorbents. CNTs and graphenes are relatively expensive in comparison with conventional sorbents. Cost comparison of different carbon-based sorbents follows the following trend (Cheap Tubes Inc., 2014; www.cheaptubes.com): functionalized CNTs > unfunctionalized SWCNT ≈ mono layer GO ≈ few layer GO > unfunctionalized MWCNT. Cost also depends on the quality of material synthesized and the type of modification. Cost wise analysis of reported sorbents indicates that GOs are better candidates that can be incorporated in water treatment technologies.

Environmental implications

It is also necessary to check the impact of nanosorbents on environment and human health before commercialization. However, to date, data related to toxicity have not been abundant and are not being evaluated on the basis of all the factors that control toxicity (Chawla & Kumar 2013). Only a few peer-reviewed studies of the toxicity comparison studies of CNTs and graphenes have been published by various researchers. Toxicity studies have shown that non-derivatized/pristine CNTs tend to be water insoluble and toxic. CNTs after functionalization with various functional groups (e.g. hydroxyl, carboxyl, amines, etc.) become more biocompatible.

The results of rodent studies showed that CNTs produce inflammation, epithelioid granulomas, fibrosis, and biochemical/toxicological changes in the lung (Lam et al. 2006). Single-walled CNTs were found to be more toxic than quartz and carbon black. Zhang et al. (2010) compared the cytotoxicity level of graphene to that of CNTs in neuronal PC12 cells. Toxicity was found to depend on composition of the nanomaterial. Toxicity of CNTs depends on many factors (Chawla & Kumar 2013) such as size, type of modification, wall structure, method of synthesis, disagglomeration, and dispersion. Toxicity studies of graphene are very limited compared with CNTs. Yan et al. (2011) reported that PEG-coated GNSs do not cause appreciable toxicity to mice at the tested dose (20 mg/kg) in a period of 3 months. In a study by Akhvan & Ghaderi (2010), it was reported that sharp GNS edges cause considerable damage to the cell membrane of bacteria. It has been pointed out that this antibacterial property has the potential to be useful.

Sasidharan et al. (2011) reported that pristine graphene was found to accumulate on the cell membrane causing high oxidative stress leading to apoptosis, whereas carboxyl functionalized hydrophilic graphene was internalized by the cells without causing any toxicity. Wang et al. (2011) predicted dose-dependent toxicity of GOs to cells and animals. Bussy et al. (2013) suggested strategies to enhance the overall safety of graphenes. Hydrophilic or modified forms of graphene sheets have been found to be degradable and safe. GO nanosheets were found to be biocompatible with yeast cells (Yang et al. 2012). Wojtoniszak et al. (2012) reported that GO functionalized with PEG in the concentration range 25–3,125 μg/mL was biocompatible with mice fibroblast cells.

Liao et al. (2011) reported more hemolytic activity with the smallest size graphene in comparison with aggregated graphene sheets. Coating GO with chitosan nearly eliminated hemolytic activity. It was shown that particle size, state, and oxygen content/surface charge of graphene have a strong impact on biological/toxicological responses to red blood cells. In addition, graphene sheets were found to be more damaging to mammalian fibroblasts than the less densely packed GO. It was emphasized that toxicity of graphene/modified graphene depends on the exposure environment and type of interaction with cells. With the wide range of morphologies, coatings, and hybrid structures available for graphenes, more detailed and longer-term studies are required before serious in vivo biomedical graphene applications are implemented. The results of toxicity studies are also different at cellular, tissue and whole body level. However, for all reported sorbents, prioritization on the basis of toxicity cannot be done because there have been insufficient studies.

CONCLUSIONS

It is important to highlight that functionalized/modified carbon-based materials have better removal capacity compared with pristine nanomaterials. Modification can considerably improve the adsorption capacity of carbon-based nanomaterials by providing more active binding sites. Compared with CNTs, GOs are more efficient sorbents for removal of Pb(II) ions from water in terms of adsorption capacity, selectivity, equilibrium time, and regeneration. Adsorption capacity of GO prepared through the oxidation of graphite using potassium dichromate was found to be higher than the other graphene-based/carbon-based sorbents for removal of Pb(II) ions from water at pH 5. The adsorption kinetics was described by a pseudo-second-order model and thermodynamic parameters suggested endothermic and spontaneous adsorption. Removal of lead from water takes place mainly by monolayer coverage on the adsorbent surface. Cost is also an important factor in large-scale applications of carbon-based nanosorbents. Graphene-based sorbents were found be better than CNTs with similar cost and reusability. However, adsorption capacity also depends on the method of synthesis of carbon-based nanomaterials and the method of modification. The identified issues are limited studies that followed similar synthesis methodologies, purity, size, and surface area of materials proposed as sorbents. More studies are required for real water samples in order to check their practical application. It is also necessary to check the potential health risk and environmental impacts of carbon-based nanomaterials before proposing them for different applications. The toxicity studies of nanosorbents must be considered before using them as sorbents.

ACKNOWLEDGEMENT

The authors are grateful to the administration and management of Manav Rachna International University Faridabad, India, for providing infrastructure and other support for preparation of this review and other ongoing research.

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