Full Length ArticleGraphene and graphene oxide as adsorbents for cadmium and lead heavy metals: A theoretical investigation
Graphical abstract
Introduction
Since the discovery of graphene in 2004 [1], it has become an essential material in a wide range of research and technologies due to its special properties. One of its important features is that, it has a unique electronic band structure with zero band gap [2]. This leads to its excellent electrical transport and its fast charge mobility [3], [4]. Moreover, graphene has a high thermal conductivity, a high strength, a large specific surface area (2630 m2/g), it is biocompatible and it can be easily functionalized to improve its properties and extend its applications [5], [6], [7]. Because of these exceptional properties, graphene and modified graphene have been studied in various application fields such as sensors [8], [9], [10], catalysis [11], [12], [13], energy storage [14], [15], electronics [16], [17], [18], biomedical [19], [20] and water remediation [21], [22], [23].
Nowadays, heavy metal pollution is a growing public health concern. There are many efforts to get rid of these metals such as cadmium (Cd) and lead (Pb) due to their carcinogenicity and toxicity. Among the various methods used to remove heavy metals, adsorption has shown to be the most powerful process, because it is environmentally friendly and relatively efficient [24]. Owing to the large surface area of graphene, it can be a good adsorbent for different pollutants [25]. Unlike graphene, graphene oxide (GO) has different oxygen-containing functional groups which can act as binding sites for heavy metals. Also, GO is easily dispersed in water, it is hydrophilic, it has fast kinetics and a higher surface area than pristine graphene. Therefore, GO has high adsorption capacity for heavy metals removal [26], [27] and in general is a good candidate for water treatment and desalination [28], [29], [30].
Utilizing graphene and graphene derivatives in heavy metals removal requires a good understanding of the nature of bonding. Therefore, several theoretical studies on the adsorption of metals on graphene have been carried out since its discovery. For example, Nakada and Ishii studied the adsorption for most elements using the local density approximation (LDA) of the density functional theory (DFT) [31]. They reported that the adsorption energy of elements with filled d- and s-orbitals, as in Cd, is very small and that adsorption is accompanied with a minimum migration energy. However, in their research spin polarization was not considered. Moreover, Manadé et al. [32] have considered the van der Waals (vdW) dispersive forces using Grimme’s D2 dispersion correction along with the PBE functional for the interaction of 3d, 4d and 5d transition metals with the graphene surface for only one coverage of 0.031 monolayer (ML).
For Pb adsorption on graphene, the bond character and preferred site for metal adsorption have been previously investigated using spin-polarized DFT [33]. The Pb atom is found to prefer positioning on top of the graphene carbon atoms, with a mixed ionic and covalent character. At low temperature, Pb can easily migrate on the graphene surface, then forms small clusters [34]. Lately, the adsorption of Cd, Hg and Pb was investigated by Shtepliuk et al. [35] in a search for graphene-based heavy metals sensors. The B3LYP and PBE-D3 functionals have been applied by these authors to analyze the interaction of the aforementioned heavy metals with graphene quantum dots and extended graphene, respectively. It has been found that Pb binds more strongly than Cd and Hg and acts as an electron donor. Unlike Pb, Cd and Hg do not tend to form clusters on the surface of graphene.
To date, few theoretical studies have been done on the adsorption of Cd and Pb on graphene oxide (GO). It is obvious that the presence of carboxyl or hydroxyl functional groups on the graphene sheet increases the adsorption stability with respect to pristine graphene. For instance, previous DFT calculations were utilized for the study of the interaction of Pb and Zn atoms with carboxylated and hydroxylated graphene sheets, and the hydroxyl group was found to be more effective in the stabilization of the adsorption process of both metals [36].
In the current study, we use periodic DFT and Gaussian type function basis sets to study the adsorption of two heavy metals that cause water pollution, namely: Cd and Pb onto graphene monolayers. Here, different sizes of graphene supercells are considered to examine the effect of coverage on both adsorption energies and geometries. The effect of these metals on the electronic properties of graphene has been investigated by analyzing density of states (DOS) diagrams and Mulliken (confirmed by Hirshfeld) populations. In order to fill the gap of the past reports, as the adsorption of Cd and Pb on GO has not been fully elucidated, three models of GO with the most favourable coverages and configurations found by Boukhvalov et al. [37] have been considered to examine the enhancement in the adsorption process. The changes in the structure of GO, the DOS diagram, and the charge population have also been studied. Finally, the role of DFT-D3 correction of dispersion interactions on the adsorption process of both Cd and Pb on graphene and GO is considered. Herein, the solvent effect is not considered due to software limitation, while according to recent previous studies on graphene-like-structure (coronene and graphene quantum dots) [38], it is expected that the relative stabilities if not improved will be at least preserved.
Section snippets
Computational methods
Periodic restricted (closed shell) and unrestricted (spin-polarized) DFT calculations have been performed using the CRYSTAL17 [39], [40] software to obtain the optimal geometries and electronic properties of Cd-graphene and Pb-graphene, respectively. All DFT calculations have been carried out using PBE0 [41], a hybrid and parameter-free functional. Indeed, PBE0 has proved to be effective in describing extended systems [42]. All-electron basis sets are used for all atoms except Cd and Pb, for
Geometrical and energetical properties
Here, we consider different graphene supercells that are built as 2 × 2, 3 × 3, 4 × 4, 5 × 5, 6 × 6 and 7 × 7 expansions of the graphene primitive cell. The supercells contain 8, 18, 32, 50, 72 and 98 carbon atoms respectively, see Fig. 1(a). As one heavy metal is placed on each graphene supercell, the following adatom surface coverages (x) are accordingly realized: 0.125 (1/8), 0.056 (1/18), 0.031 (1/32), 0.020 (1/50), 0.014 (1/72) and 0.010 (1/98) ML. The adsorption of Cd and Pb has been
Conclusions
In this paper, we have investigated the adsorption of two toxic heavy metals (Cd and Pb) on graphene and graphene oxide using periodic density functional theory (PBE0 functional). In order to study the effect of coverages, different sizes of graphene monolayers are considered. In general, Pb is found to adsorb more strongly on graphene than Cd and this result is supported by the calculated adatom height (h), Mulliken and Hirshfeld population analysis, DOS spectra and CDD maps. Moreover, because
CRediT authorship contribution statement
Sara M. Elgengehi: Visualization, Investigation, Writing - original draft. Sabry El-Taher: Supervision, Validation. Mahmoud A.A. Ibrahim: Software, Resources. Jacques K. Desmarais: Formal analysis, Resources. Khaled E. El-Kelany: Conceptualization, Data curation, Writing - review & editing, Supervision, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge Science and Technology Development Fund (STDF), Egypt for their financial support to establish an HPC-unit at CompChem Lab, Minia University. As well, we acknowledge Compute Ontario (Graham) in partner with Compute Canada (www.computecanada.ca) for providing the necessary computational facilities.
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