ReviewA review on heavy metal ions adsorption from water by graphene oxide and its composites
Graphical abstract
Introduction
Since the discovery of graphene in 2004, the 2-dimensional (2D) hexagonal network of carbon atoms graphitic material has received worldwide attention and research [1]. Graphene is extensively used in electronics, biological engineering, filtration, lightweight/strong composite materials, photovoltaic and energy storage, due to its excellent electrical conductivity, high mechanical strength and thermal conductivity, high impermeability to gases and optical transparency [2], [3], [4], [5]. Various strategies such as mechanical exfoliation or ball milling of graphite [6], epitaxial growth by thermal graphitization of silicon carbide [7], bottom-up organic synthesis [8], epitaxial growth of graphene by chemical vapor deposition (CVD) on substrates [9], liquid-phase exfoliation [10], unzipping carbon nanotubes [11], [12], and reduction of graphene oxide [13], [14] have been proposed to synthesize graphene. GO is considered as the oxidized form of graphene [15], functionalized by a range of reactive oxygenous functional groups [16], [17]. It is generally prepared by chemical oxidation of graphite resulting in extended graphene sheets decorated with epoxy and hydroxyl functional groups in the basal planes and carboxylic acid groups at the edges [18], and the oxygenous functional groups on GO make a significant contribution to its hydrophilicity and high negative charge density [19].
Heavy metal contaminants in water can result in various undesirable consequences [20]. Various methods such as adsorption, coagulation, precipitation, ion exchange, filtration and oxidation processes [21], [22] have been proposed to remove the contaminants from water. Among these, adsorption has been considered as an attractive method and widely employed in industries due to its low cost, simplicity of design, ease of operation, insensitivity to toxic pollutants and smaller amounts of harmful substances [23]. Compared to other adsorbents including rice husk [24], orange peel [25], neem leaf [26], red mud [27], bagasse fly ash, sawdust [28], conducting polymers [29], activated carbon [30] and carbon nanotubes [31], GO is regarded as the most promising absorbent to adsorb various heavy metal ions [32], [33], [34], [35] due to its large theoretical specific surface area, surface hydrophobic π-π interaction, hydrophilicity, high negative charge density and easily synthesized from the abundant natural graphite in large-scale using chemical oxidation and exfoliation method [36], [37], [38].
The current state of the application of GO and its composites in the adsorption of heavy metal ions from water is therefore reviewed in this paper. Basically, the review focuses on the adsorption affinity and mechanisms, affecting factors and regeneration of the adsorbents, and intends to provide ideas and inspirations to spark rapid development of the adsorbents for the adsorption of various heavy metal pollutants, eventually resulting in the commercial application of the GO and its composites.
Section snippets
Preparation of GO
GO is conventionally prepared by chemical oxidation and subsequent exfoliation of pristine graphite with either the Brodie, Staudenmaier, or Hummers methods, or some variations of these methods. In 1859, Brodie first found that only graphitizable carbons containing regions of graphitic structure could be oxidized to generate graphite oxide (GrO) by potassium perchlorate and concentration nitric acid mixture [39]. Then, Staudenmaier heated the mixture of graphite, sulfuric acid, nitric acid and
Heavy metal ions adsorption by GO
The various heavy metal ions present in water are harmful to aquatic-life, human beings and environment [64], which has drawn global concern for many years. GO is considered as a promising adsorbent for the removal of heavy metal ions such as Au(III) and Pt(IV) [16], Pb(II) [65], Cu(II) [66], Zn(II) [67], Cd(II) [68], and Co(II) [69], with the corresponding adsorption capacities of which were much higher than those of other adsorbents under similar conditions. The maximum adsorption capacities
Adsorption of heavy metal ions on GO composites
In order to increase the adsorption capacity, improve the adsorption selectivity of heavy metal ions or facilitate the separation of the spent GO from water, numerous materials have been used to functionalize GO. Ethylene diamine tetraacetic acid (EDTA) [85], [96], chitosan (Ch) [99], [100], aromatic diazonium salt [101], ethylenediamine [102], sulfanilic acid [103], triethylenetetramine (TET) [98], manganese oxide [104], MgAl-layered double hydroxides [105], FeOOH [106], ion nanoparticles [107]
Desorption and regeneration
An ideal adsorbent should not only hold higher adsorption capability, but also show better desorption performance, which would significantly enhance the efficiency and reduce the overall cost. Therefore, desorption and regeneration are very important for the commercial application of GO and its composites. The desorption percentages of Zn(II) eluted from the spent GO were 91.6%, 73.4% and 53.2% with using 0.1 M HCl, 0.1 M HNO3 and H2O, respectively, and HCl demonstrated its superior ability of
Summary and outlook
Currently, GO and its composites are extensively explored as advanced adsorbent materials for the removal of heavy metal ions from water due to their high efficiency, fast kinetics, and strong affinity to various metal ions. Electrostatic attraction, ion exchange and surface complexation are the possible mechanisms that mainly responsible to the adsorption. It has been confirmed that the oxygenous functional groups and thickness of GO, species of heavy metal ions in solution and experimental
Acknowledgements
The financial supports for this work from the National Natural Science Foundation of China under the project No. 51504176 and No. 51474167 are gratefully acknowledged. Also, the work is supported by the Fundamental Research Funds for the Central Universities No. 2016-YB-026.
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