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Nanomaterials are efficient adsorbents for the removal of heavy metals from wastewater because of their high surface area, enhanced active sites and the functional groups that are present on their surface (Gopalakrishnan et al. 2015). Graphene is a carbon-based nanomaterial with a two-dimensional structure, high specific surface area and good chemical stability. It is available in various forms such as pristine graphene, graphene oxide and reduced graphene oxide. Graphene may be oxidised to add hydrophilic groups for heavy metal removal (Thangavel & Venugopal 2014). Yang et al. (2014a) adsorbed chromium onto the surface of graphene oxide and the maximum adsorption capacity found was around 92.65 mg/g at an optimum pH of 5. This adsorption of chromium on graphene oxide was found to be endothermic and spontaneous. Gopalakrishnan et al. (2015) have also oxidised graphene for the addition of −COOH, −C=O and −OH functional groups onto the surface using a modified Hummer's method (Hummers & Offeman 1958). The novelty of their work is that only 70 mg of graphene oxide has been utilised for 100% removal of chromium from wastewater effectively at an optimum pH of 8. Graphene composite materials have been developed by a number of authors for the removal of heavy metals. Li et al. (2013) functionalised graphene oxide with magnetic cyclodextrin chitosan for the removal of chromium since magnetic cyclodextrin chitosan has favourable properties such as high adsorption capacity and magnetic property which assists in the separation process. Guo et al. (2014) functionalised graphene with a ferro/ferric oxide composite for chromium removal with a maximum adsorption capacity of 17.29 mg/g which is higher as compared to the adsorption capacity of other magnetic adsorbents, such as Fe@Fe2O3 core-shell nanowires (Ai et al. 2008), chitosan-coated MnFe2O4 nanoparticles (Xiao et al. 2013) and Fe3O4-polyethyleneimine (PEI)-montmorillonite (Larraza et al. 2012), i.e., 7.78 mg/g, 15.4 mg/g, 8.8 mg/g, respectively. Table 1 summarises the graphene-related work that has been reported in this area.

Table 1

Chromium removal using graphene, graphine oxide and modified graphine as an adsorbent

AdsorbentMetal concentration (ppm-mg/L)Optimum pHBest model fitContact time (min)Adsorbent dose (g/L)Adsorbent capacity (mg/g)Removal per cent (%)References
Graphene oxide based inverse spinel nickel ferrite composite 1,000 Langmuir 120 0.125–2.5 45 – Lingamdinne et al. (2015)  
Zero-valent iron assembled on magnetic Fe3O4/graphene nanocomposites 40–100 Langmuir 120 – 101 83.8% Lv et al. (2014)  
Zero-valent iron decorated on graphene nanosheets 15–35 Langmuir 90 1.0 – 70% Li et al. (2016)  
Copolymer of dimethylaminoethyl methacrylate with graphene oxide – 1.1 – 45 – 82.4 93% Ma et al. (2015)  
Graphene sand composite (GSC) 8–20 1.5 Langmuir 90 10 2859.38 93% Dubey et al. (2015)  
Graphene oxide 52 Langmuir 12 – 43.72 92.65% Yang et al. (2014a)  
Modified graphene (GN) with cetyltrimethylammonium bromide 50, 100 Langmuir 60 400 21.57 98.2% Wu et al. (2013)  
AdsorbentMetal concentration (ppm-mg/L)Optimum pHBest model fitContact time (min)Adsorbent dose (g/L)Adsorbent capacity (mg/g)Removal per cent (%)References
Graphene oxide based inverse spinel nickel ferrite composite 1,000 Langmuir 120 0.125–2.5 45 – Lingamdinne et al. (2015)  
Zero-valent iron assembled on magnetic Fe3O4/graphene nanocomposites 40–100 Langmuir 120 – 101 83.8% Lv et al. (2014)  
Zero-valent iron decorated on graphene nanosheets 15–35 Langmuir 90 1.0 – 70% Li et al. (2016)  
Copolymer of dimethylaminoethyl methacrylate with graphene oxide – 1.1 – 45 – 82.4 93% Ma et al. (2015)  
Graphene sand composite (GSC) 8–20 1.5 Langmuir 90 10 2859.38 93% Dubey et al. (2015)  
Graphene oxide 52 Langmuir 12 – 43.72 92.65% Yang et al. (2014a)  
Modified graphene (GN) with cetyltrimethylammonium bromide 50, 100 Langmuir 60 400 21.57 98.2% Wu et al. (2013)  

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