Carbon Nanomaterials for Chromium (VI) Removal from Aqueous Solution
4.2 Carbon Nanomaterials for Heavy Metal Removal
Researchers have reported different types of nanomaterials to remove heavy metals from water or wastewater such as nanosorbents including CNTs, graphene, GO (Figure 4.1), zeolites and dendrimers. These nanomaterials show exceptional adsorption properties [14]. Among these nanomaterials,
CNTs are very significant in adsorption of different metal pollutants such as chromium [15], cadmium [16], lead [17], copper [18], nickel [19] and zinc [20] and metalloids such as arsenic (As) compounds [21]. Some researchers showed that the composites of CNTs with Fe and cerium oxide (CeO2) can also remove heavy metal ions [12, 22, 23]. Di et al. [15] reported that cerium oxide nanoparticles supported on CNTs can be used to remove As. CNTs show fast adsorption kinetics, which is mainly due to the highly accessible adsorption sites and the short intraparticle diffusion distance [24].
Figure 4.1 The carbon-based materials for Cr(VI) remediation—single-wall nanotubes (SWNT), multi-wall nanotubes (MWNT), graphene, GO and activated carbon.
CNTs can be considered as very good candidates for adsorption kinetics study because of their large specific surface area and the high thermal and chemical stabilities, which is related to their easy large-scale synthesis [25]. It has been found that the adsorption capacity of CNTs is much higher than that of the activated carbon due to the high surface area, which helps in the strong interaction between CNTs and dioxins [26].
The metal ions are sorbed onto CNTs using a very complicated mechanism possibly involving electrostatic attraction, sorption–precipitation and chemical interaction between the metal ions and the surface functional groups of CNTs [24]. Raw CNTs show very low sorption capacities for the metal ions, whereas oxidized CNTs (by HNO3, NaClO and KMnO4 solutions) can show significantly higher sorption. The sorption capacities of metal ions by raw CNTs are very low but significantly increase after oxidation. Wang et al.
[27] showed that MWNTs can adsorb Pb(II) after acidification, and they found the oxygenous functional groups on MWNTs play an important role in Pb(II) adsorption to form chemical complex adsorption.
In a separate study, Wang et al. [28] showed that the role of functional groups in the adsorption of Pb2+ to create a chemical complex was critical for efficient adsorption. MWNTs can adsorb lead in the form of PbO, Pb(OH)2 and PbCO3. Result showed that the MWNTs can adsorb Pb2+
species on their ends and at defective sites. Mn oxide–coated CNTs (MnO2/CNTs) can also be used to remove Pb2+ from aqueous solution.
Wang et al. [29] showed that the removal capacity of MnO2/CNTs was decreased with decreasing pH. The adsorption of lead ions started during the first 15 min of contact with MnO2/CNTs, and full equilibrium was reached in 2 h.
Zinc can be considered as a common heavy metal contaminant in wastewater. Zn ion can interact with neutral or ionic compounds of water to form different compounds including inorganic salts, stable organic complexes or inorganic or organic colloids. The adsorption of Zn by CNTs depends on pH. Lu and Chiu [20] showed that this adsorption was increasing with increasing pH and maximum was between pH of 8 and 11, whereas at pH 12 the adsorption was found to be decreased. The time taken for the adsorption
to reach the equilibrium for the CNTs (1 h) was shorter than that of powdered activated carbon (2 h). The adsorption of Zn can also depend on the rising temperature [30]. Through thermodynamic analysis, it has been found that the sorption of Zn2+ onto the CNTs is endothermic and spontaneous. Results showed that 0.1 mol/L nitric acid solution can be used to remove Zn2+ ions easily from the surface site of the CNTs, and the sorption capacity was maintained after 10 cycles of the sorption/desorption process. All these data suggest that both the SWNTs and MWNTs can be reused through many cycles of water treatment and regeneration.
Yang et al. [31] reported that the adsorption capacity of MWNTs increases with increasing pH and the capacity can be 0–99% in the pH range of 2–9.
The experiment showed that oxidized MWNTs were the most suitable material for the solidification and pre-concentration of Ni2+ from aqueous solutions. Based on different experiment, it can be concluded that the CNTs are the most effective nickel ion absorbent based on the high adsorption capacity as well as the short adsorption time [19].
The graphene honeycomb lattice is composed of two equivalent sublattices of carbon atoms bonded together with σ bonds. Each carbon atom in the lattice has a π orbital that contributes to a delocalized network of electrons.
Graphene possess 1D structure, and monolayer or few layer graphene is known. Graphene preparation by chemical vapor deposition (CVD) growth on epitaxially matched metal surfaces was first reported by May [66] and appeared term monolayer of graphite. Large-area monolayer or multilayer graphene was prepared on copper by deposition of carbon [32–34]. GO is also planar, but there are also oxygen atoms involved in the structure. GO is mostly prepared by the Hummers method [35]. It involves oxidation of graphite with potassium permanganate and sulphuric acid. Graphite salts made by intercalating graphite with strong acids such as sulphuric acid, nitric acid or perchlorate acid have also been used as precursors for the subsequent oxidation to GO [36]. Both, graphene and GO have very large surface area that can be used for a wide range of applications including adsorption of metal ions. Oxygen atoms in the structure of GO can be used for modifications of the surface and thus improve binding capacity of modified or reduced GO.