Treatment
2.3 Adsorption of Water Pollutants onto Graphene-Based Materials
2.3.1 Adsorption of Heavy Metals
Heavy metals represent an important kind of pollutant species that are discharged from a large variety of industries such as metal smelting and finishing, electroplating, papermaking, and mining [49]. Table 2.1 summarizes some selected studies reported over the past 5 years regarding adsorption of heavy metals onto GO, RGO, and related materials. A concise description of the preparation method used in each case is included.
Likewise, main experimental conditions applied in adsorption assays, as well as adsorption capacities are listed.
Table 2.1 Selected studies on adsorption of heavy metal ions using graphene- based adsorbents
As seen in Table 2.1, copper, iron, lead, cadmium, and chromium are among the most commonly heavy metals tested as target contaminants.
Likewise, the modified Hummers method is almost exclusively employed to synthesize the GO used as such or to further develop GO/RGO-based adsorbents. As for conventional carbon-based adsorbents, the adsorption of heavy metal ions depends on the solutions’ pH as well as on the temperature [68–70]. The pH affects the surface charge of the adsorbents as well as the speciation of the metal ions [60]. In most cases, pH below 6 was carefully chosen to avoid precipitation of the metal hydroxides formed at higher pH values. However, some works [55] report adsorption capacities at pH 10. The influence of temperature is also frequently determined and thermodynamic parameters are reported. Most authors agree in that heavy metals adsorption on GO/RGO and on their based adsorbents is a spontaneous, endothermic process [43, 49, 53, 62, 64, 66].
For the majority of the heavy metals investigated, the Langmuir model usually fitted better than the Freundlich one, thus indicating a uniform binding energy and a monolayer coverage of the metal ions onto the adsorbents. For the sake of comparison, maximum adsorption capacities as evaluated from the Langmuir model are reported in Table 2.1, when informed by the authors; otherwise, removal efficiencies are presented. High adsorption capacities for heavy metals were obtained using dispersed GO sheets [50, 53, 54]. It is worth noting that differences in the adsorption capacities for a given metal ion may be related not only to the adsorption experimental conditions but also to the variability usually observed in GO structure when obtained as a product of graphite oxidation, since it depends on the oxidants and their amounts, on the graphite source, and on the reaction conditions employed [71].
As mentioned previously, the disadvantages of using disperse GO and RGO have led to the development of a variety of functionalized and composite adsorbents. Functionalization of GO with polymers have been largely investigated. Recently, Tan et al. [42] prepared different GO membranes loaded with polyvinyl alcohol (PVA), by varying the volume of the PVA solution while keeping the amount of GO constant (best ratio 1 GO:
3 PVA). The amount of PVA anchored on the interlamination of the GO membranes was a crucial factor in determining the adsorption capacity of
Cu(II), Cd(II), and Ni(II). PVA not only seems to act as a cross-linking agent and supporting pillar, preventing aggregation, but also the enlargement of the layer-to-layer distance of GO sheets (d-spacing of 1.13 nm) favors the availability of more adsorption sites and the diffusion of the metal ions. PVA caused negligible impact on the structure of GO and their interactions were attributed to hydrogen bonding between the oxygen-containing groups on GO and the hydroxyl groups on PVA chains.
Mejias Carpio et al. [50] functionalized GO with EDTA and evaluated the adsorption capacity for Cu(II) and Pb(II) ions. The time to achieve equilibrium of adsorption was as fast as 5 min, which was explained by the 2D sheets structure and the accessibility of metal ions to EDTA on the GO sheets. The higher maximum adsorption capacity for Pb(II) than for Cu(II) ions (Table 2.1) was attributed to a higher affinity for the former. It was also higher than that obtained by using only GO. The behavior was related to an increased number of carboxyl groups supplied by EDTA which enhanced adsorption by chelation. In another study [57], the presence of EDTA in the metal ions solutions was also found to increase more than three times the adsorption of Cu(II) and Pb(II).
Xu et al. [52] prepared polyacrylamide grafted RGO (PAM-γ-RGO) by γ- rays-induced reduction of the synthesized GO. C/O molar ratio increased from 1.9 in GO to 10.3 after irradiation, indicating the elimination of most functional groups and a partial reduction of the sheets. The degree of grafting with PAM was about 24% and the resulting BET surface area of PAM-γ- RGO was 128 m2 g−1. This low surface area was related to the aggregation that occurs when the adsorbent is dried. Despite this value, a very high maximum adsorption capacity for Pb(II) in solution was achieved, due to the good dispersibility of the adsorbent in water and the functional groups on PAM. To overcome the low compatibility of some polymers with GO sheets, which may result in low grafting rates by direct functionalization, Dong et al.
[45] added polydopamine (PD) as an interlayer to bind GO and polyethylenimine (PEI) to produce composite nanosheets of nano-thick PEI layer coated PD-modified GO (PEI–PD/GO). PD not only enhanced the hydrophilicity of GO sheets but also provided abundant active sites for further grafting PEI with a high density, and improved the poor dispersity of simple PEI/GO. After grafting PEI onto PD/GO surface, the PEI–PD/GO
composite still maintained the nanosheet structure and the thickness increased from 1 nm (for GO sheets) to 7 nm. Adsorption of Pb(II) increased in the order GO<PD/GO<PEI/GO<PEI–PD/GO. For the four metals tested, improved adsorption was obtained with PEI–PD/GO.
Aerogels and sponges represent other promising designs. Hydrothermal reduction of the nanosheets detailed above [45] led to RGO, PD/RGO, PEI/RGO, and PEI–PD/GO aerogels. Although RGO displayed much larger open pores than PEI–PD/RGO, which showed smaller but more uniform pores, both materials had similar surface areas (~370 m2 g−1). Interestingly, the latter was strong enough to support a weight 1300 times heavier than itself. The adsorption performance was also enhanced by the use of PD. PEI–
PD/RGO showed a greater adsorption of the four metal ions investigated.
Hybrid GO–biopolymer gels were obtained using bovine serum albumin (BSA), chitosan (CS), and double-stranded DNA as polymeric cross-linking agents [6]. Uniform distributions of the biopolymers chains with GO sheets were achieved and these gels were tested for the removal of two cationic dyes, Cu(II) and Pb(II) ions. Similar adsorption capacities with the three biopolymeric gels were obtained (Table 2.1). Adsorption equilibrium was attained sooner for the metal ions than for the dyes, since the metal ions have smaller diameters and greater diffusion coefficients than the cationic molecules. A higher adsorption was obtained for Cu(II). It was attributed to a higher stability constant for the complexation of Cu with GO–biopolymer gels compared to that for Pb(II). An S-doped graphene sponge was prepared by Zhao et al. [37] by a one-step hydrothermal method, during which the reduction of GO and the doping of sulfur were simultaneously achieved. A mass ratio of 1:1 of GO and l-cystine was found as the best one to achieve good structure and adsorption capacity for Cu(II) (228 mg g−1), as can be seen in Table 2.1. The mass ratio had to be carefully selected since for other ratios (1:2 or 2:1), the sponges collapsed and adsorption capacities decreased sharply (71 mg g−1 for 1:2 and 58 mg g−1 for 2:1). The adsorption capacity of Cu(II) on S-doped RGO sponge was determined at four temperatures between 0 °C and 55 °C, but it was not sensitive to this variation. It was also found ionic strength-independent, attributable to the strong affinity between S atoms and Cu(II). Sulfur, as sulfhydryl groups, was also introduced into GO
sheets by Li et al. [49], leading to improve the thermostability of the material.
GO-SH was further self-assembled with CS to fabricate an hybrid nanocomposite (CS/GO-SH) which combined the abundant –NH2 and –OH groups of CS with the –COOH, –OH, and –SH groups of GO. This nanocomposite proved to have a strong ability to catch metal ions through chelate reactions, even from multi-ions systems. The convenience of mixing CS goes beyond the introduction of functional groups. For the aerogel developed by Yu et al. [43], the GO was aggregated to CS in order to facilitate the separation by filtration or by low speed centrifugation, and over stacking of GO sheets was avoided in the aerogel.
Several authors have also investigated GO as reinforcing filler for polymeric adsorbents. GO–CS monoliths were developed by unidirectional freeze-drying [56]. This technique is attractive because water can be used as the porogen. Adding GO increased the compressive strength of the monoliths in wet state and the unidirectional porous structures obtained also facilitated the diffusion of Cu(II) and Pb(II) ions. Micro-ribbons, formed during freeze- drying, enlarged the surface area of the adsorbent; therefore, more functional groups (for example, carboxyl and amino groups) were exposed. Addition of GO was also found to change the balance of chelating groups (amino groups in CS, and carboxyl and hydroxyl groups in GO), influencing the maximum adsorption capacities for the metal ions. This effect was stronger on Pb(II) adsorption, possibly because the binding force between chelating groups and Pb(II), with higher electronegativity and a larger ionic radius than Cu(II), was greater than the interactions among the chelating groups. This effect was barely perceived for Cu(II). The same research group developed another GO–
CS monolith with the addition of gelatin [57]. The monoliths had similar structural characteristics to those described earlier. Hydroxyl and carboxyl groups on the GO nanosheets appeared to increase the hydrophilicity and water permeability of the monoliths, which should further facilitate the diffusion of metal ions. Interestingly, the monoliths without GO almost completely collapsed after 5 days. Besides, a great influence of the pH of the Cu(II) solution was observed. Removal increased from 16% to 89% as pH changed from 2 to 6. The trend was mainly related to electrostatic interactions. More recently, foamy monoliths of ordered porous architecture were prepared with GO and carboxymethyl cellulose (CMC) [47]. The size of
the pores developed was found significantly larger as the GO concentration increased and many micro-ribbons were also observed, protruding from the pore walls. They seemed to improve the compressive strength of the monolith as well as to increase its ability to adsorb metal ions.
Another significant group of GO/RGO-based adsorbents are magnetic composites, which are often developed under the premise of ease of separation from the water treated. A thiol-functionalized magnetic graphene composite was synthesized via simultaneous hydrothermal reduction of GO and coating with Fe3O4 nanoparticles, where thiol groups could provide of extra ion exchange sites for Cd(II) [62]. Adsorption equilibrium was achieved within 30 min. Fast kinetics is usually reported for the adsorption of heavy metal ions onto GO/RGO-based adsorbents [46, 60, 64] and even equilibrium attained immediately after contact has been reported [37]. Fan et al. [58] and Li et al. [59] investigated the adsorption of Pb(II) and Cr(VI), respectively, using hybrid GO–CS composites with magnetic properties. GO was added to strengthen magnetic Fe3O4 particles covered with CS [58]. The mesoporous adsorbent had a moderate surface area (SBET = 382 m2 g−1) and a reduced saturation magnetization due to the CS coating. The latter was still sufficient to achieve separation of the adsorbent by means of a magnet. Adsorption was attributed to chelation and interactions with functional groups of GO and CS.
Li et al. [59] added β-cyclodextrin (CD) into the composite, resulting in a mutual structural benefit: GO sheets appear to hinder agglomeration of CD- CS spheres and, in turn, the latter separated GO sheets and prevented their aggregation. The saturation magnetization was also sufficient for a proper separation from water, and a higher SBET was obtained (~446 m2 g–1).
Removal of Cr(VI) was best at low pH (Table 2.1) and would occur by a combination of adsorption by electrostatic attraction, reduction to Cr(III) and joint bonding with Cr(VI) into stable inclusion complexes. A similar adsorption capacity for Cr(VI) was obtained with magnetic β CD- ethylenediamine/GO particles [61]. Jabeen et al. [64] reported the reduction of Pb(II) ions by nano-zerovalent-iron (ZVI) enhanced by graphene sheets in RGO/ZVI nanoparticles. They not only scavenged free Pb(II) ions from water by adsorption, but immobilized the reduced Pb(0) very efficiently, as this reduction facilitates magnetic separation of the adsorbent from water. In the
absence of RGO, ZVI nanoparticles not only aggregated into chainlike structures but also their adsorption capacity was lower because they mainly adsorbed Pb(II). RGO sheets, decorated with non-aggregated iron nanoparticles, could adsorb both Pb(II) and free Pb(0) through basic functional groups in RGO.
Multi-solute adsorption of heavy metals and possible interferences are other relevant issues for a closer approach to real wastewater systems. Competitive adsorption of Cd(II), Pb(II), and Cu(II) ions from a ternary mixture was found to decrease the adsorption capacity of CS/SH-GO composite with respect to those for the individual metal ions [49]. Kinetics of adsorption was also affected by multi-ions presence; most of them were removed within 10 min from single-solute solutions but removal from ternary solutions lasted 40 min. The impact of the presence of the other ions was less significant for Cd (II), which presented the lowest adsorption from single metal ion systems. On the contrary, in another study [65], adsorption of Cd(II) onto magnetic GO supported SAc composite was strongly affected by background electrolyte cations (Na+, K+, Ca2+, Mg2+, Mn2+, Zn2+, and Ni2+) and anions (Cl−, NO3−, ClO4−, and PO43–). Even more, when facing real effluents, results might change drastically. Luo et al. [51] performed selectivity adsorption tests with different ions in solution, finding that the efficiency of removal was in the order: Cu > Pb > Cd > Mn > Zn > Ni. However, when a real smelter industry effluent was used as feed solution, only Pb and Fe (out of eight metal cations) were removed, probably due to the stronger coordination ability of these ions. The presence of other metal ions was not the only reason for a diminished adsorption capacity. Li et al. [44] found that the adsorption of Cu(II) using GO was also affected by the presence of humic and fulvic acids. Likewise, the presence of aniline also affected the adsorption of Cr(VI), whether enhancing or reducing it at low and high pH values, respectively [61].