Removal of cadmium ion from wastewater by carbon-based nanosorbents: a review

Natural waters are mostly polluted by heavy metal ions caused by industrial wastage. Due to their toxic effects metal ions pose a serious threat to living organisms (Khan et al. 2008; Singh et al. 2010; Zhao et al. 2011a, b) and the environment in terms of growth, yield and quality of crops and plants (Singh & Agrawal 2010; Ok et al. 2011). Mercury, cadmium, arsenic, chromium and lead have the most serious effects on human beings as well as on the environment. Cadmium is of toxicological concern because of its bioaccumulation and non-biodegradability property even at very low concentration (Waalkes 2000; Li et al. 2012; Amin et al. 2013). Cadmium is widely used in making nickel-cadmium batteries, pigments, phosphate fertilizers, alloys and metal plating (John & Santhi 1994; Mhiaoui et al. 2003; Lambert et al. 2007; Martinis et al. 2009). Runoff from waste batteries and paints, corrosion of galvanized pipes and discharge from metal refineries are identified sources of cadmium exposure in water (Hutton & Symon 1986; Akesson 2011). Cadmium may be introduced into water naturally by volcanic eruption. The typical permissible cadmium concentration as per the WHO (2008) and EPA (2008) guidelines in drinking water is 0.003 mg/L and 0.005 mg/L, respectively. High concentration of cadmium above the recommended value in water causes serious poisoning in human beings.

Many research groups have focused their interest on removal of heavy metal ions from water/wastewater. A number of methods have been developed to remove heavy metal ions from water, such as ion exchange, reverse osmosis, precipitation, cementation and adsorption (Charerntanyarak 1999; Dabrowski et al. 2004; Qdais & Moussa 2004; Hua et al. 2012). Among them, adsorption is considered as an effective, efficient and economic method (Sud et al. 2008; Lo et al. 2012). Among the various traditional adsorbents for water purification, nanosorbents have emerged with great potential and accuracy because of their high surface area and high efficiency. Nanosized metal oxide, nanocomposites, magnetic nanoparticles and carbon nanotubes have been most effectively used and extensively reviewed for removal of heavy metals from water (Ngomsik et al. 2005; Wang et al. 2010; Zhao et al. 2011a, b; Hua et al. 2012; Kumar & Chawla 2014). However, removal of cadmium metal ions by different nanomaterials needs to be studied in greater detail to explore their toxicity, reusability, cost, and maximum efficiency.

This review provides an overview of carbon-based nanosorbents for removal of cadmium from water with the challenge of the environmental impacts, regeneration and reusability characteristics of these nanosorbents. The review is divided into the following sections:

• Cadmium poisoning and health effects

• Nanosorbents for removal of heavy metals from water

• Review of carbon-based nanosorbents for removal of cadmium

• Result analyses

• Conclusions.

Cadmium poisoning is associated with various clinical complications in human beings. Cadmium is a well-known nephrotoxic agent (Wu et al. 2008). Cadmium has a very long biological half-time of 10–30 years in humans (Swaddiwudhipong et al. 2012) and accumulates primarily in kidney cells with development of chronic disease. The accumulation of cadmium in the kidney leads to dysfunction of the kidney. The ingestion of cadmium in human beings poses a great health risk even at very low concentrations (Waalkes 2003). Kidney damage and lung emphysema are the primary effects of high exposure of cadmium in the body. However, the other target organs that may be affected by the accumulation of cadmium in the body are liver, intestines, placenta, pancreas, heart, skeletal system, immune systems, reproductive system and nervous system (Fowler 2009; Ciarrocca et al. 2013). Cadmium poisoning sometimes causes hypertension, diabetes, nerve or brain damage, liver disease and urinary stones (Horiguchi et al. 2004; Fowler 2009). Cadmium poisoning may also cause bone damage (Jarup & Akesson 2009) and affects teeth and the nervous system permanently by replacing calcium. Long-term exposure to cadmium and excessive intake of cadmium causes serious illnesses such as itai-itai disease in human beings (Bolan et al. 2013). In addition, recently cadmium has been identified as a human carcinogen; increasing concentrations of cadmium in water increases cancer risk in humans (Mahalik et al. 1995; Fowler 2009).

Toxic heavy metal ions removal from water/wastewater by using synthetic compounds, polymers and other materials as sorbents is of great concern in environmental research applications (Moradi et al. 2009; Fu & Wang 2011). A number of traditional sorbents have been used for removal of cadmium from water/wastewater. Agricultural waste materials such as stems (Tan & Xiao 2009), husks (Panda et al. 2006), bark (Ghodbane et al. 2007), shell (Cimino et al. 2000), peels (Li et al. 2007), leaves (Sharma & Bhattacharyya 2005) and saw dust (Memon et al. 2007) have been extensively used as traditional sorbents for removal of cadmium. However, the use of traditional sorbents has been restricted by the inconvenience of separation in multi-metal ions solution, poor performance and efficiency. Over the last decade nanomaterials have been extensively studied for their potential applications in various applied fields, such as energy application, quantum dots for biomedical application, and photocatalytic applications (Krishna et al. 2013; Han & Ho 2014; Lee & Jang 2014). The sorption potential of nanomaterials has not been studied extensively. Nanomaterials are a promising material in terms of separation, performance and efficiency for removal of heavy metals from water because of their unique properties, such as small size, high specific surface area and morphological features (Hua et al. 2012). Nanomaterials have been found to be superior sorbents for various organic and inorganic pollutants, such as benzene (Gauden et al. 2006), fluoride and ethidium bromide (Li et al. 2003c; Najafi et al. 2013). The synthetic method of nanomaterials has been intensively developed and designed not only for their fundamental scientific interest but also their important application in sorption fields in order to get maximum efficiency (Shamsudin et al. 2013). The sorption capacity of nanomaterials was found to be improved by oxidation, modification or functionalization on the surface of nanomaterials (Khani & Moradi 2013). The European Commission on 18 October 2011 adopted the definition of a nanomaterial as a natural, incidental or manufactured material containing particles, in an unbound state or as an aggregate or as an agglomerate and where, for 50% or more of the particles in the number size distribution, one or more external dimensions are in the size range 1–100 nm. In specific cases and where warranted by concerns for the environment, health, safety or competitiveness the number size distribution threshold of 50% may be replaced by a threshold between 1 and 50%. A number of carbon-based nanoparticles, nanometal oxides, magnetic nanoparticles, nanocomposites, dendrimers, zeolites, nanoclays and modifications of functionalized nanomaterials have been widely used as adsorbent to remove heavy metal ions from water or wastewater (Hu et al. 2006; Ghorbani & Eisazadeh 2013).

Carbon-based nanomaterials include carbon nanotubes, oxidized carbon nanotubes, carbon-based fibres, graphene, carbon nanocomposites and fullerene. Raw carbon-based nanomaterials and modified carbon-based nanomaterials have been extensively used for removal of heavy metal ions from water. The following section describes the detailed study of carbon-based nanomaterials used as nanosorbent for removal of cadmium ions from water/wastewater.

The study of adsorption properties of carbon nanotubes (CNTs) plays an important role in the advance of research and technology for water purification. The applications of CNTs for the removal of heavy metal ions from aqueous systems have been extensively studied because of their large specific surface area (3,000 m²/g), mechanical stability and highly porous structure (Yin et al. 1999; Sheikh 2008; Moradi et al. 2012). Heavy metal ion adsorption capacity of CNTs is directly related to their total active surface acidity. Generally, CNTs can be synthesized by electric arc discharge, pulse laser vapour and catalytic decomposition of gaseous hydrocarbons (Ando et al. 2004; Balasubramanian & Burghard 2005). CNTs synthesized by two-stage catalytic chemical vapour deposition (CVD) method gives the highest aspect ratio, renders the lowest integrated intensity ratio and diameter size distribution, and provides excellent purity of CNTs (Shamsudin et al. 2013). Defects on the walls of CNTs decrease their adsorbent capacity. The sorption capacity of Cd²⁺ ion onto 10 mg of CNTs dosage of the average diameters in the range of 20–30 nm and 0.789 mg/L initial concentration observed was 19.14% at pH 7.0 for 120 minutes of contact time (Kabbashi et al. 2011). On the other hand, CNTs were grown on the surface of micro-sized Al₂O₃ particles for removal of Cd²⁺ and other heavy metal ions from aqueous solutions (Hsieh & Horng 2007). The adsorption capacity of CNTs grown on Al₂O₃ was superior to that of active carbon powders, commercial carbon nanotubes and Al₂O₃ particles. Although the absorption capacity of Cd²⁺ ion on CNTs grown on Al₂O₃ was lower than the Pb²⁺ and Cu²⁺ ions, it could be increased by further treatment or oxidation. The calculated saturation amount for Cd²⁺ was 2.0, 6.19 and 8.89 mg/g, adsorbed by 1 g of Al₂O_3, commercial CNTs and CNTs grown on Al₂O₃ respectively.

Different oxidizing reagents such as HNO₃, H₂O₂ and KMnO₄ were used for oxidation of CNTs (Khani & Moradi 2013). Oxidized CNTs show exceptionally high adsorption capacity and efficiency for removal of heavy metal ions. Enhancement in the adsorption capacity and efficiency of oxidized CNTs is explained due to incorporation of the oxygen containing functional groups on the surface of CNTs by chemical oxidation. Carboxylic and phenolic groups present on the carbon surface exhibit high affinity for various metal ions in water (Kadirvelu & Namasivayam 2003). Langmuir maximum adsorption capacity of Cd²⁺ ion onto oxidized carbon nanotube was higher than other metal ions such as Co²⁺, Zn²⁺, and Cu²⁺ (Gao et al. 2009; Tofighy & Mohammadi 2011). The adsorption efficiency of oxidized CNTs for cadmium and other metal ions has also been investigated in single, binary, ternary and quaternary systems (Gao et al. 2009). The amount of Cd²⁺ ions adsorbed on the oxidized CNTs was higher than the Zn²⁺ for single and binary systems. However, the adsorption process became more and more complicated for ternary and quaternary systems and showed a different order. Ion exchange or outer-sphere surface mechanism was suggested to be predominant over the complex mechanism of the removal process. However, ion exchange or outer surface mechanism mainly contributes to Cd²⁺ uptake on oxidized multi-walled carbon nanotubes (MWCNTs) at low pH and inner-sphere surface mechanism is the main uptake mechanism at high pH values (Yang et al. 2012). The effect of different oxidizing reagent on carbon nanotubes was also investigated to find out the maximum adsorption capacity of Cd²⁺ ion from aqueous solution. Three oxidizing reagents, H₂O₂, KMnO₄ and HNO₃, have been used to compare the adsorption efficiency of oxidized CNTs (Li et al. 2003b). As compared to CNTs, KMnO₄ oxidized CNTs showed high absorption capacity. The adsorption capacity of Cd²⁺ on CNTs was only 1.1 mg/g at a 4 mg/L Cd²⁺ ion equilibrium concentration while it reached 2.6, 5.1 and 11.0 mg/g for the H₂O₂, HNO₃ and KMnO₄ oxidized CNTs, respectively, at the same concentration. Adsorption capacities for three types of oxidized CNTs were greater because oxidation of CNTs introduces functional groups on the surface and increases the surface area. The specific surface area of H₂O₂, KMnO₄ and HNO₃ oxidized carbon nanotubes was found to increase from 122 to 130, 128 and 154 m²/g, respectively, by the oxidation process. The adsorption capacity of Cd²⁺ for KMnO₄ oxidized CNTs was sharply increased with increasing the dosage of CNT because host manganese residuals on oxidized CNTs surface by KMnO₄ contribute to cadmium sorption. Furthermore, the adsorption capacities and efficiency of cadmium ion also depend on the type of CNTs, i.e., single-walled carbon nanotubes (SWCNTs) and MWCNTs.

Single-walled carbon nanotubes are composed of a single rolled-up graphene sheet and MWCNTs are composed of a concentric arrangement of many cylinders. In a graphite arc process that forms fullerenes from atomized carbon, multi-walled CNTs are deposited at the graphite anode (Iijima 1991). Single-walled carbon nanotubes were also synthesized in the presence of metal catalyst (Iijima & Ichihashi 1993). Limited work has been done on the removal of cadmium ion by SWCNTs and modified SWCNTs. The removal efficiency of Cd²⁺ ions by SWCNTs has been investigated in detail only by Moradi (Moradi et al. 2010, 2011; Moradi 2011; Moradi & Zare 2011). Langmuir maximum adsorption capacity of Cd²⁺ ion observed was 24.07 mg/g on SWCNT at pH 5.0–6.0. The degree of sorption with respect to temperature on the sorbent surface has been explained on the basis of thermodynamic parameters, such as change in free energy (ΔG), change in enthalpy (ΔH) and change in entropy (ΔS). The negative value of free energy of adsorption indicates the feasibility of the process. However, a negative value of enthalpy of adsorption shows an exothermic process and a positive value of enthalpy of adsorption shows an endothermic process. The positive value of entropy of adsorption indicated spontaneous sorption process with high affinity (Tadjarodi et al. 2013). Thermodynamic parameters values showed that the adsorption process of ions was taking place by physi-sorption on the surface of SWCNTs and was spontaneous and endothermic in nature at 283–313 °K (Moradi et al. 2010, 2011).

Multi-walled carbon nanotubes are formed by two or more rolled-up graphite sheets. The available adsorption space is either external surface or inter-wall spaces, i.e., the spaces between the coaxial tubes of MWCNTs (Yang & Xing 2007). Three carbon-based materials including MWCNTs (5–10 nm), activated carbon (AC) and carbon encapsulated magnetic nanoparticles (CEMNPs) (40–90 nm) were used for the comparative study of cadmium adsorption under similar conditions (Pyrzynska 2010). The results showed that the theoretical saturation capacity for cadmium was 9.91, 20.37 and 91.0 mg/g for AC, CNTs and CEMNPs, respectively, at pH 8.0. High adsorption capacity of CEMNPs towards cadmium ions was explained by the synergistic effect between CNTs and iron oxides which improves the affinity of the adsorbent. The results indicate that specific active sites on the sorbents play an important role in metal ions’ retention. There was competition among the different heavy metal ions for binding of the active sites present on the MWCNTs’ surface. However, it was found that Cd²⁺ ion has slightly low affinity for binding of the active sites present on the MWCNTs’ surface compared to other metal ions such as Cu²⁺, Zn²⁺ and Pb²⁺ (Li et al. 2003a; Salam et al. 2012). The spontaneity of the adsorption process was confirmed by the calculated thermodynamics parameters (Salam 2013). Calculated thermodynamics parameters showed that the value of the free energy was negative while the change in entropy value was positive for cadmium ions. Positive value of entropy is due to the physical adsorption of cadmium ions from the aqueous solution to the surface of carbon nanotubes which increases randomness on the surface.

Modification on the surface plays an important role in increasing the sorption capacity of CNTs. It was found that amines with oxygen containing groups on the surface might be coordinating sites for sorption of heavy metal ions (Vukovic et al. 2011; Wang et al. 2013). The surface modification of CNTs can be done either by chemical bond formation or by physical adsorption (Wildgoose et al. 2006). Nanotubes are slightly less stable in polar solvents and tend to aggregate in solution. Surface modification also helps in stabilizing the dispersion in solvents and imparts high stability in solution. Both SWCNTs and MWCNTs were modified with different groups to increase the sorption capacity of CNTs for removal of heavy metals. The adsorption properties of carbon nanotubes were significantly improved by modification with different groups for removal of cadmium metal ions. Different parameters such as effects of temperature, pH, ionic strength, metal ion concentration and the amount of modified CNTs were studied for adsorption of heavy metal ions from water. The interaction of cadmium metal ions with carboxylate group functionalized single-walled carbon nanotube (SWCNT-COOH) surface was found to be higher than the single-walled carbon nanotube (SWCNT) surface due to the carboxyl group which increases negative charge on carbon nanotube surface (Moradi et al. 2011). Also, surface functionalization improved their stability in water and adsorption efficiency towards heavy metal ions (Li et al. 2003a; Feng et al. 2008). Langmuir maximum adsorption capacity of Cd²⁺ ion was increased from 24.07 to 55.89 mg/g on SWCNT-COOH at pH 5.0–6.0. Thermodynamic parameter values showed that adsorption processes of ions was taking place by chemi-sorption on the surface of SWCNTs-COOH and physi-sorption on the surface of SWCNTs and were spontaneous and endothermic in nature at 283–313 °K (Moradi et al. 2010, 2011; Moradi 2011; Moradi & Zare 2011).

The adsorption properties of MWCNTs were significantly improved by modified multi-walled carbon nanotubes with different groups such as six-arm amino polyethylene glycol (PEG), 8-hydroxyquinoline, ethylenediamine (En) (Vukovic et al. 2010; Kosa et al. 2012; Salam 2013; Velickovic et al. 2013). The maximum adsorption capacities of Cd²⁺ ions on the surface of modified MWCNTs by six-arm amino PEG (MWCNTs-PEG) was 77.6 mg/g for initial cadmium ion concentration of 10 mg/L at pH 8.0 (Velickovic et al. 2013). Minimum adsorption was observed at lower pH because at lower pH high concentration of hydrogen ions are present and, according to surface complex formation theory, hydrogen ions are preferentially adsorbed rather than the metal ions (Kosa et al. 2012). The percentage adsorption of Cd²⁺ ions increased from 14.3 to 61.7% by MWCNTs and from 40.67 to 100% by modified multi-walled carbon nanotubes when pH increases from 6.0 to 9.0 because cadmium usually precipitates at higher pH values (Lv et al. 2005). Intra-particular Weber–Morris kinetic model was used to describe the time-dependent adsorption. The initial adsorption reached equilibrium gradually at 97% of cadmium adsorption by physic-sorption process.

The MWCNTs with very few oxygen containing functional groups showed relatively low sorption capacities for Cd²⁺ (Tian et al. 2012). The sorption capacity of Cd²⁺ ions by MWCNTs was increased by surface coating with manganese dioxide (MnO₂), (MWCNTs-MnO₂) or with humic acids (Yang et al. 2011; Tian et al. 2012; Luo et al. 2013). Surface coating with four sequentially extracted humic acids on three multi-walled CNTs (MWCNTs) with different surface areas affected the adsorption process of heavy metal ions significantly. However, surface coating with the humic acids lowered the surface areas of the MWCNTs but greatly increased their sorption capacities because of complexation and electrostatic attraction (Tian et al. 2012). Surface coating with the humic acids introduced oxygen containing functional groups and increased negative charge on the MWCNTs’ surface. The adsorption efficiency of multi-walled carbon nanotubes for the removal of heavy metal ions could also be increased by modification with chitosan (Salam et al. 2011). Two real water samples were collected from the Red Sea and a wastewater treatment plant to evaluate the applicability of the nanosorbent (Kosa et al. 2012). Most of the metal ions were removed from the solution by using modified MWCNTs with 8-hydroxyquinoline. When MWCNTs were used to remove metal ions, only 0.3% Cd²⁺ ions were removed while modified MWCNTs had a high affinity for the removal of Cd²⁺ ions (5.77%) from real water samples (Kosa et al. 2012). If we look at the mechanism of ion removal, cadmium ions are removed from the water by ion-exchange as well as adsorption mechanisms. In the ion-exchange mechanism, cadmium ions are exchanged with the protons present on the oxygen atoms of the sidewall functional groups of carbon nanotubes (Figure 1). However, adsorption of cadmium ions on the surface of CNTs is due to strong chemical interaction between the cadmium ions and the surface sites of carbon nanotubes and is known as inner-sphere adsorption, while weaker interaction between cadmium ions and the surface sites of CNTs is known as outer-sphere adsorption.

Graphene possesses a two-dimensional structure of one- or multi-atomic-layered graphites. Graphene is used as another carbon-based nanosorbent, which has good thermal and mechanical properties. A few-layered graphene oxide nanosheets (GOx) were prepared by modified Hummers method which introduces –COOH, −C = O and –OH groups on the surface to increase the sorption capacity of heavy metal ions. Cadmium sorption on graphene oxide nanosheets was carried out in the presence of NaClO₄ in polyethylene test tubes (Zhao et al. 2011b). The sorption of Cd²⁺ increased with increasing pH from 6 to 9 because of deprotonation at high pH which made the surface more negative to combine with metal ions. The maximum sorption capacity of Cd²⁺ was 106.3 mg/g at pH 6.0 (303 °K). However, the sorption of Cd²⁺ on graphene oxide nanosheets decreased with increasing pH in the presence of humic acid because of formation of soluble metal–humic acid complex.

Modified or functionalized graphene was also used for removal of Cd²⁺ ions from water samples. Magnetic graphene oxide nanocomposite was synthesized for simultaneous removal of Cd²⁺ ions and ionic dyes such as methylene blue and orange G (Deng et al. 2013). The maximum sorption capacities in ultrapure water for Cd²⁺ ion was 91.29 mg/g. The sorption capacity of nanocomposite was suppressed for Cd²⁺ in the presence of methylene blue dye. The sorption capacity for Cd²⁺ ion in tap water samples was 65.39% of that in ultrapure water. Functionalized graphene was also fabricated by electrolysis method with potassium hexa-fluorophosphate solution as electrolyte under the static potential of 15 V (Deng et al. 2010). Langmuir maximum adsorption capacity observed was 73.42 mg/g at pH 6.2 and the adsorption equilibrium was achieved in just 40 minutes for cadmium ions. Iron-iron oxide dispersed graphene was used for removal of Cd²⁺ ions from water samples (Bhunia et al. 2012). Iron oxide nanoparticles in the presence of H₂/Ar atmosphere were transformed to a matrix of iron-iron oxide on the graphene surface at an elevated temperature. Iron-iron oxide dispersed graphene was highly porous and robust. Iron-iron oxide matrix dispersed on reduced graphene oxide (rGO), rGO–Fe(0)–Fe₃O₄, with high surface area and high efficiency was basically used for removal of arsenic metal from water samples but it was also used to evaluate adsorption efficiency of other toxic heavy metals like Cd²⁺ and Hg²⁺ from water. The maximum adsorption capacity of this sorbent was slightly poor towards Cd²⁺ ions because the standard reduction potential of Cd²⁺ (−0.35 V) was more negative than the other metal ions.

It is possible to assemble the nanoscale units in nanocomposites in order to explore their properties. In carbon nanocomposites active materials can be dispersed into a carbon matrix or nanocarbon can be dispersed into a polymer. Various magnetic nanocomposites with carbon-coated nickel structure have been used for removal of heavy metals from water (Xiao et al. 2012). Sankararamakrishnan et al. (2014) synthesized nanofloral clusters (NFCs) of CNTs over iron and nickel doped activated alumina by CVD for removal of cadmium ion from water. Langmuir maximum adsorption of Cd²⁺ ion on nanofloral clusters of CNT (NFC-CNT) was 229.9 mg/g at pH 7.5. A solid-phase extraction method for separation and pre-concentration determination of Cd²⁺ ion in environmental samples has been developed by using multi-walled carbon nanotubes/poly(2-amino-thiophenol) nanocomposites (MWCNT-NC) (Nabid et al. 2012). The sorbent showed high adsorption affinity for heavy metal ions on macromolecular chains due to the presence of numerous good extractive sites (S and N) of conducting polymer. The maximum adsorption capacity of sorbents at optimum conditions for cadmium ions was found to be 178.7 mg/g at pH 6.0 with the detection limit of 0.3 ng/mL. Recently, nanocarbon conjugated polymer nanocomposites (NCPCs) were prepared for effective removal of heavy metal ions from water (Khaydarov et al. 2010). NCPNs were synthesized from nanocarbon colloids and polyethylenimine. The bonding capacity of NCPC was 5.7 mmol/g at pH 6.0 for most of the divalent metal ions. The sorption percentage of Cd²⁺ and other divalent metal ions was higher than 99%. A novel composite of silver with multi-walled carbon nanotubes (Ag-MWCNTs) has been synthesized from functionalized multi-walled carbon nanotubes reduced with N,N-dimethylformamide and cross-linked with silver nitrate for effective removal of Cd²⁺ ion (Ramana et al. 2013). The Langmuir isotherm model was consistent with the experimental data at different temperatures. Maximum adsorption of Cd²⁺ ion was observed at pH 7.0 (more than 90% removal, 10 mg/L initial concentration of cadmium). The thermodynamic parameters indicated that the adsorption process of Cd²⁺ ion onto Ag-MWCNTs was endothermic, spontaneous and feasible in the temperature range 293–313 °K.

The reported concentration of cadmium in different environmental water samples is more than the recommended permissible concentration as per WHO guidelines (0.003 mg/L). The reported concentration of cadmium in different environmental water samples are in the range of 0.002–1.46 mg/L (Igwilo et al. 2006; Akoteyon 2012; Ambedkar & Muniyan 2012). High concentration of cadmium in water causes different poisonous effects on human beings. Therefore, traditional and nanomaterials are used for removal of cadmium ions from water. Carbon-based nanosorbents are preferred over conventional carbonaceous sorbents because of their high surface area to volume ratio, controlled pore size distribution and they are easy to fabricate on the surface to enhance surface chemistry. Adsorption on the surface of carbon-based nanosorbents is driven by hydrophobic dispersion and weak dipolar forces. This section describes the ranking of carbon-based nanosorbents for removal of cadmium ions on the basis of different parameters and detailed study of adsorption isotherms and kinetics of adsorption of the cadmium ions on carbon-based nanosorbents.

The removal efficiency of different synthetic, naturally occurring and waste materials used as adsorbents was 10–80% with respect to the initial cadmium ion concentration. However, adsorption capacity of these materials can be significantly increased by chemical modification on the surface or by pretreatment methods (Rao et al. 2010). Various adsorption parameters are required to evaluate their effects on removal efficiency and ranking of carbon-based nanosorbents for removal of cadmium ions from water samples. The adsorption capacity of different carbon-based nanosorbents has been compared at pH 5.0–7.5 and temperature range of 25–30 °C for their selection for treating real water samples. Available important data up to the present is listed in Table 1. Solution pH plays a key role for removal of cadmium from water and wastewater. As can be seen from the reviewed analysis, the removal efficiency of Cd²⁺ for all reviewed carbon-based materials tended to increase with increasing pH value. Increasing pH value could precipitate metal as metal hydroxide and the functional groups on the sorbent surface exist in the deprotonated form. However, at low pH, higher concentration and mobility of hydrogen ions favoured hydrogen ions’ adsorption rather than metal ions on adsorbent surfaces. The percentage adsorption of Cd²⁺ ions increased up to 100% by modified multi-walled carbon nanotubes with 8-hydroxyquinoline when pH increases from 6.0 to 9.0. However, the adsorption capacity of carbon-encapsulated magnetic nanoparticles (CEMNPs) was also better as compared to other reviewed carbon-based nanosorbents at higher pH value. Also, the removal efficiency of Cd²⁺ increased up to a limit range with increasing the temperature and time, respectively.

The adsorption capacity of the reviewed carbon-based nanosorbents at pH 5.0–7.5 follows roughly the following order: modified or dispersed MWCNTs > graphene oxide > SWCNTs-COOH > SWCNTs > oxidized CNT > MWCNTs > CNTs (Figure 2). However, the adsorption capacity of carbon-based nanosorbents precisely depends on the type of nanosorbents, e.g., non-functionalized and functionalized nanosorbents. The adsorption capacity of non-functionalized carbon nanotubes (CNTs) follows the following order: oxidized-CNT > CNTs-Al₂O₃ > CNTs. Oxidized raw CNTs gave better results in comparison to raw CNTs because of more active oxygen containing groups on the surface of CNTs. The adsorption capacity of functionalized or dispersed multi-walled CNTs follows the following order: NFC-CNT > MWCNTs-NC > MWCNTs-PEG > MWCNTs-MnO₂ > MWCNTs-En > MWCNTs. Modification or dispersion of CNTs can considerably increase the surface area as well as more active sites on the surface. The results analysis also showed that SWCNT-COOH surfaces were more effective toward Cd²⁺ ion adsorption than SWCNTs surface because of the carboxyl group which increases negative charge on carbon nanotubes surface. However, SWCNTs offered better performance regarding Cd²⁺ ion adsorption than the MWCNTs because of the presence of more active sites on the surface of SWCNTs. It is very difficult to correlate all the reviewed results in terms of all parameters because of their different preparation conditions, purification methods, various size and surface areas of reported sorbents.

The choice of nanosorbents for removal of cadmium also depends on toxicity and reusability of the carbon-based nanosorbents. Although a number of in vitro and in vivo toxicological studies have been performed, toxicity of carbon nanotubes has not been significantly evaluated because of the number of factors involved. Toxicity of carbon nanotubes depends on many factors, such as their physical form, diameter, length and the nature of attached molecules on the sidewall or surfaces (Wang et al. 2009; Firme & Bandaru 2010; Kolosnjaj et al. 2010). Also, the results of toxicity are often conflicted at the cellular and whole body levels. The size and length control of CNTs is very important to control the toxicity. Chronic toxicity of CNTs can be reduced by using short CNTs and keeping their length to less than 5 μm (Chawla & Kumar 2013). The functionalization on the surface of carbon nanotubes improves their biocompatibility and reduces the toxic effects (Chawla & Kumar 2014). It was found that at a particular concentration, surface modification or functionalization reduce the toxicity of carbon nanotubes (Foldvari & Bagonluri 2008). Thus, it can be concluded that modified or functionalized carbon nanotubes with smaller size and smaller length may be the best sorbent for removal of cadmium metal ions in terms of toxicity. In addition, further studies are required in this area to make these materials safer and compatible for different applications. Regeneration is another very important aspect for ranking of these nanomaterials. Carbon nanotubes can be regenerated and reused for removal of heavy metal ions. Regeneration significantly reduces the overall costs of the procedure. Regeneration and recycling of carbon nanotubes were evaluated in detail for a number of heavy metal ions (Wang et al. 2007; Tehrani et al. 2013). There are very few studies available to date for regeneration of carbon nanotubes for cadmium metal ions (Hsieh & Horng 2007; Kosa et al. 2012). Most of the metal ions desorbed on the surface of carbon nanotubes in acidic medium (pH lower than 2.0) and could be used without losing efficiency for up to three cycles. Research adopting standard and similar procedures while considering all parameters in all aspects is further required for finding the most efficient, cheapest and safest alternatives for removal of cadmium ions from water.

The results showed that the experimental q_e values for cadmium ions observed by using carbon nanoparticles (Pyrzynska 2010), carbon nanotubes (Hsieh & Horng 2007; Moradi et al. 2010; Moradi & Zare 2011) and modified multi-walled carbon nanotubes (Vukovic et al. 2010; Luo et al. 2013) have good agreement with the calculated value of saturation amount with the pseudo-second-order kinetic models. Conformity between the predicted value calculated by kinetic model and experimental value was expressed by correlation coefficient (R²). The value of correlation coefficient (R²) was close to 1 and the relatively high value of correlation coefficient is the more applicable model to represent the adsorption mechanism. The value of correlation coefficients (R²) of the pseudo-first-order model were slightly less than the pseudo-second-order model for most of the carbon-based nanosorbents which indicates that the kinetic model is best explained by the pseudo-second-order model rather than the pseudo-first-order model. This indicates that adsorption of cadmium onto carbon-based nanosorbents may be explained by chemi-sorption mechanism which involves the valence forces through exchange or sharing of electrons between nanosorbents and cadmium ions.

For all reviewed carbon-based nanosorbents, the kinetic data were fitted to pseudo-first-order and intra-particle diffusion models for an initial period of the first reaction step only. However, there was better correlation to the pseudo-second-order model over a long period of time for most of the systems studied (Hsieh & Horng 2007; Moradi et al. 2010; Pyrzynska 2010; Vukovic et al. 2010; Moradi & Zare 2011; Luo et al. 2013).

The remarkable physical and chemical properties of carbon-based nanomaterials make these materials excellent candidates for waste water treatment. The aim of this review is to compare the maximum sorption capacity for Cd²⁺ ions by carbon-based nanomaterials from aqueous solutions. However, the comparison of maximum sorption capacity for Cd²⁺ is restricted by several parameters, such as characteristics of sorbents prepared in a similar way with similar properties, i.e., surface area, surface total active sites, surface acidity, point of zero charge and also the solution properties such as pH of solution, time of contact of solution, initial metal ions concentration, etc. In conclusion, adsorption capacity of the reviewed carbon-based nanosorbents at pH 5.0–7.5 follows the following order: modified or dispersed MWCNTs > graphene oxide > SWCNT-COOH > SWCNTs > oxidised-CNT > MWCNTs > CNTs. The removal capacity of functionalized carbon nanotubes was found to be higher than the non-functionalized carbon nanotubes. Nanofloral clusters of carbon nanotubes over alumina (NFC-CNT) were found to be the best carbon-based nanomaterial for removal of cadmium ions from water at pH 7.5. Modification or dispersion of CNTs can considerably improve the adsorption capacity by providing more active binding sites. The amount of the contaminant cadmium that binds to the modified or dispersed MWCNTs is much higher than the AC due to π-π electron-donor acceptor properties of the modified or dispersed MWCNTs. Removal of cadmium from water takes place mainly by monolayer coverage on the adsorbent surface adopted Langmuir isotherm with great value of regression coefficient. The identified knowledge gaps are unavailability of systematic studies including similar preparation method, purity, size and surface areas of carbon-based nanomaterials. Most of the research is still based on laboratory scale only, therefore how to utilize all these nanomaterials for industrial application is still a challenging interest. It is also necessary to study the potential health risks and environmental impacts of carbon-based nanomaterials before their widespread use in different applied fields.

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.