The role of nanomaterials as effective adsorbents and their applications in wastewater treatment

AC was used initially as sorbents; however, due to the difficulties associated with heavy metals and dye removal at ppb levels, CNTs, fullerenes, and graphene were used as nanosorbents to overcome this difficulty. AC typically has high porosity, high surface area, and can be prepared from readily available carbonaceous precursors such as coal, wood, coconut shells and agricultural wastes [72, 73, 74, 75]. AC is extensively used for the removal of inorganic and organic pollutants from effluent streams and in water treatment [22]. In addition, it possesses a significantly weak acidic ion exchange character, enabling it to remove metal contaminants and to adsorb pollutants from wastewater [22]. The sorption of pentavalent arsenic on granular activated carbon (GAC) was experimentally studied [75]. AC prepared from coconut tree sawdust was used as an adsorbent for the removal of Cr(VI) from aqueous solution [76]. Sorption and stability of mercury on AC for emission control were also reported [72]. Powdered activated carbon (PAC) prepared from Eucalyptus camaldulensis Dehn bark was studied and showed a sorption capacities (q_m) at 60 °C, of 0.85 and 0.89 mmol g⁻¹ for Cu(II) and Pb(II), respectively [73]. A novel sodium polyacrylate grafted AC was produced using gamma radiation to increase the number of functional groups on the surface which increased the efficiency of metal ions sorption by AC [74]. Their high sorption ability and low price make AC promising materials for heavy metals and dye removal.

CNTs (Fig. 2), which were first developed by Iijima [77], have a unique structural, electronic, optoelectronic, and semiconductor, as well as mechanical, chemical and physical properties [22, 23]. CNTs have been applied widely to remove heavy metals and dyes in wastewater treatment [6, 7, 9, 11, 12, 13, 14, 15, 16, 41, 71, 78, 79, 80, 81, 82].

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CNTs are considered to be one of the most promising adsorbents for wastewater treatment because of their large adsorption capacity for synthetic dyes [15]. Multi-walled carbon nanotubes (MWCNTs) have been shown to surpass cadmium hydroxide nanowire-loaded AC (Cd(OH)₂–NW–AC) with respect to their efficient removal of safranin O (SO) from wastewater [83]. However, only few studies were reported on the application of CNTs for dye removal from aqueous solution [41, 69, 76, 84, 85, 86]. Moreover, CNTs were typically used directly without further treatment [41, 69, 85]. Therefore, CNT functionalization has been initiated to introduce various functional groups that provide new adsorption sites [15]. Among such modifications, oxidation is an easy method for introducing hydroxyl and carbonyl groups to the sidewalls of CNTs. Oxidized MWCNTs were found to be effective in the removal of methylene red (MR) and methylene blue (MB) from aqueous solutions [87, 88]. Yao et al. [89] reported an adsorption capacity of 41.63 mg g⁻¹ at 333 K for the removal of MB onto CNTs. Shahryari et al. [90] performed the same batch of experiments on MWCNTs having a higher surface area of 280 m² g⁻¹ as compared to that of CNTs (160 m² g⁻¹) used by Yao et al. and reported a higher MB adsorption of 132.6 mg g⁻¹ at 310 K. In addition, cellulose grafted with soy protein isolate/hydroxyapatite rod-like nanocrystals showed a high MB adsorption capacity of 454 mg g⁻¹ [91].

The adsorption capacity also depends on the experimental conditions, nature and type of adsorbent. The comparative adsorption of anionic orange II (OII) from aqueous solution using MWNTs and carbon nanofibers (CNF) as adsorbents was studied in batch experiments by Rodríguez et al. [75]. They found that the adsorption of II (OII) onto MWCNTs was slightly higher than CNF (the adsorption capacity in case of MWCNTs was 77.83 mg g⁻¹, while it was 66.12 mg g⁻¹ in case of CNF [75]). In addition, MWCNTs showed higher adsorption than PAC for removal of reactive red M-2BE (RRM). The maximum amounts of RRM uptake were 335.7 and 260.7 mg g⁻¹ for MWCNTs and PAC, respectively [76]. The higher adsorption capacity can be explained on the basis of higher average pore diameter of MWCNTs, which was 7.62 nm as compared to 3.52 nm of PAC. It seems that dye molecules can easily be diffused from the surface to pores of MWCNTs due to larger pore size.

CNT-impregnated chitosan hydrogel beads (CSBs) have been developed for the removal of congo red (CR). CSBs demonstrated a higher maximum adsorption capacity (450.4 mg g⁻¹) than chitosan without impregnation (200 mg g⁻¹) based on Langmuir adsorption modeling [72]. A new generation of CSBs has been prepared by sodium dodecyl sulfate and MWCNTs to improve the mechanical properties [73]. The new CSBs have demonstrated a high maximum adsorption capacity for CR of 375.94 mg g⁻¹ [73]. Compared to MWCNTs and hybrid CNTs (HCNTs), single wall carbon nanotubes (SWCNTs) can demonstrate better adsorption properties for organic contaminants because of their higher specific surface area. SWCNTs are more efficient for removing benzene and toluene and have shown maximum adsorption capacities of 9.98 and 9.96 mg g⁻¹, respectively [74]. A maximum adsorption capacity of 496 mg g⁻¹ was achieved when a reactive blue 29 (RB29) has been removed from aqueous solution by using SWCNTs [92].

On the other hand, CNTs showed high sorption efficiency of divalent metal ions. The advantages and drawbacks of Co(II) and Cu(II) removal using AC, CNTs, and carbon-encapsulated magnetic nanoparticles were reported by Pyrzyńska and Bystrzejewski [80]. The results showed that the carbon nanomaterials have significantly higher sorption efficiency compared to commercial AC. Meanwhile, Stafiej and Pyrzynska [79] found out that the solution conditions such as pH and metal ion concentrations could affect the adsorption characteristics of CNTs. Oxidized CNTs have also shown exceptionally high sorption capacity and efficiency for Pb(II), Cd(II) and Cr(VI) from water [78, 93, 94]. CNTs were also reported as good adsorbents for multi-component sorption of metal ions [71]. The sorption mechanisms were reported to be governed by the surface features, ion exchange process and electrochemical potential [95]. The latter plays a significant role in multi-component sorption where redox reactions, not only on the adsorbent surface but also among the different adsorbates, are likely to occur. MWCNTs were found to adsorb ²⁴³Am with extraordinarily high efficiency by forming very stable complexes [96]. The sorption characteristic of Pb(II) from aqueous solution was studied using oxidized MWCNTs [81]. The reported results showed a slope of V/m and intercept of C_o V/m for the same initial concentration of Pb(II) and the same content of oxidized MWCNTs for each experimental data [81]. Oxidized MWCNTs were also applied to adsorb Ni(II) from aqueous solution [41]. The predominant mechanism of Ni(II) sorption onto MWCNTs was reported to be ion exchanged at low pH values and by strong surface complexation at high pH values [41]. It was also reported that oxidized MWCNTs can be potentially promising materials for the pre-concentration and solidification of heavy metal ions [70].

The sorption and kinetic desorption of ^152+154Eu(III) on MWCNTs have also been studied [97]. It was found that MWCNTs were a suitable material for pre-concentration of lanthanides from large volumes of aqueous solutions in radioactive nuclear waste management with the strong surface complexation and/or chemisorption mechanism [97]. With the aid of FITEQL 3.2, Chen et al. [82] characterized the surface properties of MWCNTs at different ionic strengths and pH values, and modeled Sr(II) and Eu(III) sorption onto oxidized MWCNTs by applying surface complexation model, and found that the diffuse layer model (DLM) fit the experimental data very well. The removal of divalent metal ions [Cd(II), Cu(II), Ni(II), Pb(II), Zn(II)] from aqueous solution using various kinds of CNTs have been studied [98]. It has been proved that the CNTs are very promising adsorbents for environmental protection applications because of their superior sorption capacity and in the same time ability to effective desorption of divalent metal ions [98].

It is worth noting that the CNTs can be potentially produced on a large scale using different methods such as chemical vapor deposition to reduce the cost of production and increase their future use in environmental protection applications.

Graphene (Fig. 3), which can be used as nanosorbents, typically consists of one or more atomic-layered carbon atoms, and possesses a unique two-dimensional structure and excellent mechanical, thermal and electrical properties [99, 100]. Using Van der Waals’ forces and π–π stacking interactions, the adsorption of dyes on few-layered graphene nanosheets can be realized (see Fig. 4). For modify the chemical and physical properties and improve the manufacturing of reduced graphene oxide (rGONSs) graphene oxide nanosheets (GONSs), it can be first incorporated into composite materials. Both rGONSs layers and single GONSs layers have high aspect ratios and large π-electronic surfaces that provide strong intermolecular forces among adsorbates [101]. Due to the opened-up layer structure, rGONSs exhibit markedly faster adsorption kinetics than CNTs [63, 102]. Among different carbon-based materials (coal base AC (HD4000), SWCNTs and MWCNTs), rGONSs exhibited better adsorption capacities for two synthetic organic compounds (SOCs; phenanthrene and biphenyl) in aqueous solutions [52]. More importantly, rGONSs are much cheaper than SWCNTs [103]. rGONSs have already been used as adsorbents for the removal of cationic red X-GRL [104], MB [66, 105], methyl orange (MO) [106], CR [107] and other organic materials from aqueous solutions. The maximum adsorption capacities of p-toluenesulfonic acid (p-TA), 1-naphthalenesulfonic acid (1-NA) and MB on GNS reach up to 1430, 1460 and 1520 mg g⁻¹ at 303 K, respectively, which are the highest among all nanomaterials studied in this field to date [108]. The observed adsorption capacities for the adsorption of three types of pesticides [chlorpyrifos (CP), endosulfan (ES), and malathion (ML)] onto GONSs and rGONSs from water are as high as 1200, 1100 and 800 mg g⁻¹, respectively, and GONSs and rGONSs were unprecedented substrates for these adsorption technologies [109].

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The few-layered GONSs through the modified Hummers’ method have been synthesized [111]. These rGONSs can be used as sorbents for the removal of Cd(II) and Co(II) ions from aqueous solution [111]. It was reported that heavy metal ions sorption on nanosheets is dependent on pH and ionic strength [111]. The abundant oxygen-containing functional groups on the surfaces of graphene oxide nanosheets were reported to play an important role on sorption [111].

Magnetite–graphene composite adsorbent with a particle size of ~10 nm was reported to give a high binding capacity for As(III) and As(V) [112]. The high binding capacity was due to the increased adsorption sites in the graphene composite [112]. The strong functional groups on the graphene oxide (GO) surface make it a potential adsorbent for metal ion complexation through both electrostatic and coordinate approaches. Generally, GO showed high adsorption capacity for cationic metals. rGONSs can be used for adsorption of both cationic and anionic metals. After modification of GO with organics or metal oxides, its composites can also be used for anionic metal removal due to functionalization. Cu(II)–GO interaction in aqueous solution showed that Cu(II) causes GO sheets to be folded and form large aggregates [113]. The coordination between Cu(II) and oxygen atoms on GO was the primary driving force. GO has a Cu(II) adsorption capacity of 46.6 mg g⁻¹, which is higher than that of CNTs (28.5 mg g⁻¹) and AC (4–5 mg g⁻¹) [113].

The removal of Cd(II) [71, 111], Co(II) [111], Pb(II) [71, 114], and U(VI) [115] ions from aqueous solutions have been studied using few-layered GONSs. It was found that the abundant oxygen-containing functional groups on GONSs surfaces play an important role in metal sorption, which was in agreement with the results of Zhao et al. [111]. It was also reported that Cd(II) and Co(II) sorption on GONSs is strongly dependent on pH and weakly dependent on ionic strength. The presence of humic acid reduced Cd(II) and Co(II) sorption on GONSs at pH < 8. The maximum sorption capacities of Cd(II) and Co(II) on GONSs at pH 6.0 and 303 K were about 106.3 and 68.2 mg g⁻¹, respectively. For Pb(II), the maximum adsorption capacities were about 842, 1150, and 1850 mg g⁻¹ at 293, 313, and 333 K, respectively [114], but sorption capacity of U(VI) at pH 5.0, 293 K was 97.5 mg g⁻¹ [115].

Modification of GO with organic materials can change the surface functional groups for better adsorption of various metal ion species. A modified GO with thiol (SH) groups by diazonium chemistry was reported to adsorb sixfold higher concentration of Hg(II)ions than GO and AC [116]. When N-(trimethoxysilylpropyl) ethylenediaminetriacetic acid (EDTA-silane) was used to obtain a chelating GO for Pb(II) removal, the adsorption was fast and completed within 20 min, with an adsorption capacity of 479 mg g⁻¹ at pH 6.8 [117]. Compared to AC and CNTs, GO and GNs present stronger adsorption for many water pollutants.