Novel insight into adsorption and co-adsorption of heavy metal ions and an organic pollutant by magnetic graphene nanomaterials in water
Graphical abstract
Introduction
Water quality has been deteriorated continuously due to the rapid growth of population, urbanization, industrialization, and other environmental issues [1], [2], [3]. Multiple pollutants often coexist in aquatic environments and change chemical, physical or biological properties of water [4], [5], [6]. If inadequately treated, aquatic ecosystems could be adversely affected by the combined pollutions [7], [8]. The treatment of soil and water co-contaminated with heavy metals and organic pollutants poses a significant challenge because these two types of pollutants have different fates and transport mechanisms [9]. Sorption is widely used to eliminate both inorganic and organic pollutants from aqueous solutions owing to the simplicity of design, low cost, high efficiency, and wide adaptability [1], [10].
As two-dimensional carbon-based materials with honeycomb structures that are sp2-hybridized with an atom thickness, graphene and its derivatives were reported to show excellent performance in adsorption of various environmental contaminants, such as aromatic compounds [11], [12], [13], dyes [14], [15], [16], antibiotics [17], [18], estrogens [19], [20], [21], heavy metal ions [22], [23], anions [24], [25] and combined pollutions [6], [26]. Especially, graphene oxide (GO), the main precursor of graphene, has been widely applied in waste-water treatment or water purification due to its large specific surface area and a variety of O-containing functional groups such as hydroxide, epoxide, carbonyl and carboxyl groups [4], [26], [27]. However, the separation procedures for this type of absorbents are often complicated due to their high dispersibility in water, which could lead to new environmental risks [4], [28]. Magnetization by introducing Fe3O4 nanoparticles (NPs) onto GO and reduced graphene oxide (RGO) can bring about separation convenience, even increased adsorption capacity, which favor the practical applications [29], [30], [31].
A number of studies have been conducted on the adsorption of magnetic graphene nanomaterials (MGNs) for heavy metals and organic contaminants in aqueous systems [32], [33]. However, the existing literature is inadequate in concerning the interactions between multi-pollutants in systems, whereas coexistence of various pollutants is much more common in real environments [6], [12]. Antibiotics (e.g., TC) and heavy metals (e.g., Cd(II)) are extensively employed as growth promoters and usually coexist in wastewater from livestock farm, causing considerable toxicological concerns [34]. Furthermore, multiple contaminations in natural environments influence the removal of individual contaminants via electrostatic interaction, cation–π interaction, precipitation or/and complexation [5], [35], [36].
Chemically-synthesized graphene nanosheets are generally defective and contain surface heterogeneity with various structures such as flat surface, wrinkles, defects, and O-containing functional groups, which are valuable for the high capacity of pollutant sorption [37], [38], [39]. The surface properties could be greatly altered by these structures, and affect the intrinsic properties and environmental applications of graphene-based materials. For instance, metal ions are preferentially adsorbed onto GO rather than RGO, due to the abundant surface functional groups on GO [4], [5]. The reduction of exfoliated GO removed the O-containing functional groups and repaired a sp2-hybridized structure [40], leading to weakened surface complexations of heavy metal ions with oxidized sites [41], enhanced π–π interactions between graphene nanosheets and organic pollutants, and reduced competition of water molecules with organic pollutants at the oxidized sites [5], [11]. Several preparative strategies have been explored for graphene morphology regulation [37], [38], while designing MGNs with special physicochemical properties and structural features is needed in practical applications.
In this study, we prepared and characterized magnetic graphene oxide (MGO), magnetic chemically-reduced graphene (MCRG) and magnetic annealing-reduced graphene (MARG), which contained different microstructures, iron and oxygen contents, and displayed various sorption capacities. Cadmium [Cd(II)] and arsenate [As(V)] were selected as model cation and oxyanion contaminant, respectively [6], [42], and tetracycline (TC) was selected as a model organic contaminant [43]. The main objective was to examine the properties and unique molecular adsorptive and co-adsorptive mechanisms of these magnetic graphene nanomaterials. The adsorption isotherms were established and effects of pH were examined to evaluate the adsorption behaviors of each of these pollutants on MGO, MCRG, and MARG in single systems. Besides, MGO was selected to further study the adsorption behaviors, interactions between the pollutants and associated mechanisms in three diverse types of binary solutions.
Section snippets
Synthesis of magnetic graphene nanocomposites
Graphene oxide (GO) was synthesized from natural graphite flakes via a modified Hummers’ method [44]. The adhesive GO was exfoliated by sonication and dialyzed to remove acids and other impurities. The Fe3O4 nanoparticles (NPs) were prepared by chemical co-precipitation of FeCl3·6H2O and FeSO4·7H2O. The sol of Fe3O4 NPs was obtained after sonication for 2 h [45]. The GO solution was then added dropwise into the sol of Fe3O4 NPs under appropriate concentrations and pH conditions under mechanical
Characterization of magnetic graphene nanocomposites
The TEM image (Fig. 1a) of MGO showed that the Fe3O4 NPs (10–20 nm size) were well dispersed in 2–5 layers of GO matrix, and the folding nature of graphene sheets was clearly visible. The basal plane of the GO was almost unoccupied by the magnetic NPs, consistent with the previous finding [47]. Dense aggregates of Fe3O4 NPs were observed in MCRG and MARG (Fig. 1b-c), indicating that the reduction of MGO induced serious wrinkles and overlaps on graphene nanosheets. Thermal reduction of MGO to
Conclusions
This study presents a new strategy to prepare diverse types of magnetic graphene nanomaterials, MGO, MCRG, and MARG, with different microstructures and properties. The three magnetic graphene nanomaterials had a good magnetic separability and showed excellent adsorption capacities for TC, Cd(II) and As(V), while MGO was shown to be the best adsorbent. High dispersibility and thin nanosheets contributed to the superior adsorption capabilities of MGO. Moreover, 59% O-containing functional groups
Acknowledgement
This work was financially supported by the National Natural Science Foundation of China (41721001), the Science and Technology Program of Zhejiang Province (2018C03028), and Agriculture Research System of China (CARS-01-30).
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