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Carbon nanotubes are efficient adsorbents for heavy metal removal because they possess chemical stability, large surface area, excellent mechanical and electrical properties, adsorption property and well-developed mesopores (Gupta et al. 2011; Mubarak et al. 2014a; Al-Khaldi et al. 2015). They can also be further modified by chemical treatment to increase adsorption capacity (Chen et al. 2009; Mubarak et al. 2013, 2015a, 2015b, 2015c, 2016a, 2016c; Ruthiraan et al. 2015b). Hu et al. (2009) removed chromium using oxidised multi-walled carbon nanotubes and 100% maximum removal was achieved at an optimum pH of 2.88. Gupta et al. (2011) combined the adsorptive property of multi-walled carbon nanotubes with the magnetic property of iron oxide. The advantages of this composite are high surface area, can be used for contaminant removal and can be controlled and removed from the medium using a simple magnetic process. A maximum removal of 88% at pH 6 was obtained. Luo et al. (2013) prepared manganese dioxide/iron oxide/acid oxidised multi-walled carbon nanotube nanocomposites for chromium removal. Manganese dioxide is a scavenger of aqueous trace metals because of its high adsorption capacity but the use of pure manganese dioxide is not favoured because of the high cost and its unfavourable physical and chemical properties. The maximum adsorption capacity of the above nanocomposite was 186.9 mg/g with a maximum removal of 85% at an optimum pH of 2. Mubarak et al. (2014b) functionalised carbon nanotubes for chromium removal using nitric acid and potassium permagnate in 3:1 volume ratio and compared the removal capacity with non-functionalised carbon nanotubes. They found that maximum adsorption capacity for functionalised carbon nanotubes was 2.517 mg/g while for non-functionalised carbon nanotubes it was 2.49 mg/g, and removal capacity for functionalised carbon nanotubes (87.6%) was higher compared to non-functionalised carbon nanotubes (83%). Mubarak et al. (2016b) produced carbon nanotubes using microwave heating for comparative study of the removal of chromium with another heavy metal (i.e., lead). Microwave heating provides a fast and uniform heating rate and it accelerates reaction and gives a higher yield. The maximum adsorption capacity obtained for chromium was 24.45 mg/g and removal efficiency obtained was 95% at an optimum pH 8. Table 3 summarises the reported use of carbon nanotubes for chromium removal from wastewater.

Table 3

Chromium removal using carbon nanotubes as an adsorbent

AdsorbentMetal concentration (mg/L)Optimum pHBest model fitContact time (min)Adsorbent dose (g/L)Adsorbent capacity (mg/g)Removal per cent (%)References
Nitric acid oxidised carbon nanotube – 150 0.5 18% Atieh et al. (2010)  
Composite of carbon nanotubes and activated alumina 100 Langmuir Freundlich 240 2.5 264.5 >95% Sankararamakrishnan et al. (2014)  
Nitrogen-doped magnetic CNTs 12.82 Langmuir 720 0.2 638.56 >97% Shin et al. (2011)  
CNT supported by activated carbon 0.5 Langmuir 60 0.04 72% Atieh (2011)  
Cigarette filter with MWCNT and graphene – – – – 63–79% Yu et al. (2015)  
Oxidised multi-walled carbon nanotubes 2.88 <2 Langmuir adsorption isotherm 9,900 75–1.25 4.2615 100% Hu et al. (2009)  
Composite of multi-walled carbon nanotubes and iron oxide 20 – 10–60 0.1–2 – 88% Gupta et al. (2011)  
Manganese dioxide/iron oxide/acid oxidised multi-walled carbon nanotube nanocomposites 50–300 Langmuir 150 186.9 85% Luo et al. (2013)  
Carbon nanotubes functionalised using nitric acid and potassium permagnate Langmuir and Freundlich 120 0.1 2.47, 2.48 87.6% Mubarak et al. (2014b)  
Carbon nanotube produced using microwave heating Langmuir and Freundlich 60 24.45 95% Mubarak et al. (2016b)  
AdsorbentMetal concentration (mg/L)Optimum pHBest model fitContact time (min)Adsorbent dose (g/L)Adsorbent capacity (mg/g)Removal per cent (%)References
Nitric acid oxidised carbon nanotube – 150 0.5 18% Atieh et al. (2010)  
Composite of carbon nanotubes and activated alumina 100 Langmuir Freundlich 240 2.5 264.5 >95% Sankararamakrishnan et al. (2014)  
Nitrogen-doped magnetic CNTs 12.82 Langmuir 720 0.2 638.56 >97% Shin et al. (2011)  
CNT supported by activated carbon 0.5 Langmuir 60 0.04 72% Atieh (2011)  
Cigarette filter with MWCNT and graphene – – – – 63–79% Yu et al. (2015)  
Oxidised multi-walled carbon nanotubes 2.88 <2 Langmuir adsorption isotherm 9,900 75–1.25 4.2615 100% Hu et al. (2009)  
Composite of multi-walled carbon nanotubes and iron oxide 20 – 10–60 0.1–2 – 88% Gupta et al. (2011)  
Manganese dioxide/iron oxide/acid oxidised multi-walled carbon nanotube nanocomposites 50–300 Langmuir 150 186.9 85% Luo et al. (2013)  
Carbon nanotubes functionalised using nitric acid and potassium permagnate Langmuir and Freundlich 120 0.1 2.47, 2.48 87.6% Mubarak et al. (2014b)  
Carbon nanotube produced using microwave heating Langmuir and Freundlich 60 24.45 95% Mubarak et al. (2016b)  

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