Treatment
2.3 Adsorption of Water Pollutants onto Graphene-Based Materials
2.3.2 Adsorption of Organic Contaminants
2.3.2.2 Other Organic and Emerging Contaminants
Mostly, GO and RGO have been tested as adsorbents, and the literature on the use of composite adsorbents based on graphene is scarce. Recently, Yan et al. [90] used GO with different degrees of oxidation, obtained by varying the oxidation time and KMnO4 amount, to adsorb aniline, nitrobenzene, and chlorobenzene (Table 2.3). Adsorption of the two latter compounds was not evidently influenced by pH. However, the pH affected aniline adsorption likely due to the presence of the amino group that may confer a better water solubility and hydrogen bonding ability. GO with lower oxidation degree led to higher removal efficiencies. The Freundlich model fitted the experimental adsorption isotherms slightly better than the Langmuir one, indicating a possible multilayer heterogeneous adsorption.
On the other hand, pesticides are also a matter of major concern due to their indiscriminate use and widespread presence in surface and ground waters.
For instance, the adsorption of endosulfan, chlorpyrifos, and malathion using GO and RGO was investigated [36]. The kinetics was very fast and more than 90% of the pesticides were removed in less than 10 min, with RGO being 10–20% more effective than GO. Adsorption of the pesticides was independent on pH, since none of the pesticides are ionizable. Their removal was related to the formation and precipitation of grapheme–water–pesticide complexes through electrostatic interactions. GO was also used for the adsorption of lysozyme, as a model protein of naturally occurring organic matter [91]. It was suggested that abundance of –OH and –COOH groups on GO surface facilitates the electrostatic bonding with lysozyme molecules, of zwitterionic nature. GO exhibited a sharp decrease in lysozyme adsorption capacity (200–300 mg g−1) when monoand divalent cations were dissolved in the protein solutions, possibly due to double-layer interactions on both, lysozyme and GO. Commercial graphene was also tested but a significantly lower adsorption capacity was obtained (52 mg g−1).
Phenol, recognized as a priority contaminant, and phenolic compounds are common targets of adsorption studies. In particular, the use of graphene- based adsorbents has been implemented by Wang et al. [93] who investigated the effect of the reduction degree of RGO and the amounts and distributions of remaining oxygen-containing functional groups on the adsorption of ten phenolic compounds (Table 2.3). Higher adsorption levels were found for
compounds with one or more electron-donating and withdrawing groups on the benzene ring. A linear correlation between their adsorption and the reduction degree of RGO was established. π–π interactions between aromatic molecules and RGO apparently increased with the more extended RGO reduction. Notably, firstly obtained GO sheets were less crumpled but RGO sheets were more folded and agglomerated, yet similar surface areas (~940 m2 g−1) were obtained in suspension. Previously, Li et al. [92] had achieved similar values of maximum adsorption capacity for phenol (Table 2.3) using RGO but for samples with considerable less surface area (SBET=306 m2 g
−1).
The removal of organic solvents and hydrocarbons is a less explored topic.
Compact porous RGO sponges with good structural stability were developed and used to adsorb chloroform and diesel oil with high removal capacity [94].
The specific surface area of the RGO sponge played a predominant role in the adsorption of these contaminants, whereas the surface charge had almost no effect. A remarkable feature, in terms of reuse of the adsorbent, is that the adsorbed oil and other organic solvents can be easily eliminated by their burning in air without destroying the sponge structure.
Emerging contaminants constitute another currently popular group that is receiving growing attention. They are new products or chemical compounds without regulatory status and whose effects on environment and human health are sometimes unknown. They include phthalates, bisphenol A (BPA), personal care products, and pharmaceuticals drugs and their metabolites, among others [95]. Table 2.4 summarizes some of the most recent studies reported using GO/RGO-based adsorbents.
Table 2.4 Summary of recent studies on the adsorption of emerging contaminants using graphene-based adsorbents.
BPA is a well-recognized endocrine disruptor, which interferes with metabolic processes of natural hormones. RGO and magnetic RGO have been applied to its removal. Xu et al. [99] achieved good adsorption capacity for BPA (Table 2.4) using RGO with moderate specific surface area (SBET = 327 m2 g−1). The increase in temperature led to a decrease in the adsorption capacity of 30% at 69 °C. On the other hand, the RGO-Fe3O4 [100] greatly improved dispersibility of the magnetic particles and reduced their agglomeration; however, some lost in adsorption capacity was observed as the amount of iron salts increased in the composite. Similarly to RGO, the adsorption capacity decreased with increasing temperature and the solution pH influenced the adsorption, mainly at alkaline values. As already mentioned in the section concerning graphene synthesis routes, interesting green alternatives to conventional chemical reduction using hydrazine have been proposed. Thakur and Karak [101] used banana peel ash base source and C. esculenta leaves extract as the reducing agent. Reduction of GO could be due to formation of complexes between polyphenols in the leaves with Fe(III) ions with a high release of protons and electrons. RGO-Fe3O4 was applied to the adsorption of tetrabromobisphenol A (TBBPA) removal, a BPA derivative used as flame retardant.
In the case of pharmaceutical products removed by grapheme-based adsorbents, recent works include the use of commercial GO with considerable specific surface area (SBET = 762 m2 g−1). A high adsorption capacity for clofibric acid (CA) (Table 2.4), an active metabolite of blood lipid regulators frequently detected in water, was obtained [97]. CA adsorption was pH dependent and affected by the presence of humic acids, which might act as a “bridge” between GO and CA at low pH values.
Adsorption of antibiotics has also been scarcely studied. Mainly GO and magnetic graphene-based adsorbents have been applied to adsorb fluoroquinolones, a group of frequently detected antibiotics [48]. The adsorption of ciprofloxacin (CIP) and norfloxacin (NOR) by RGO-Fe3O4 was reported as a spontaneous and exothermic process, and a synergic structural effect was suggested regarding the hindrance of the aggregation of magnetic microparticles and the improvement of the stability of RGO sheets [48]. Lab-prepared GO was used to test tetracycline (TC) adsorption [96] and
another commercial GO displayed a very high adsorption capacity for sulfamethoxazole (SMX) [98]. It was proposed that the adsorption mechanism was mainly based on hydrophobic interactions and that the amine aromatic ring of SMX was oriented parallel to the surface of GO, while the second ring was oriented away from the GO surface. Additionally, further sonication of this commercial GO improved adsorption more than twice, allegedly due to a reduction in the density of oxygen-containing functional groups in GO surface.
Regarding personal care products, the use of a magnetic β-CD-GO/Fe3O4 nanocomposite for the adsorption of p-phenylenediamine (PPD), a toxic component of hair coloration products, was studied [102]. CD was suggested to increase the adsorption capacity of the adsorbent through the formation of inclusion complexes in solution with organic molecules through host–guest interactions. Adsorption was found to increase with temperature (Table 2.4).