Environmental Engineering Reference
In-Depth Information
10.3 Carbon Nanotubes and Fullerenes for Contaminant Separation
10.3.1 Adsorptive Removal of Heavy Metal Cations
Heavy metal cations in aqueous environments are one of the most serious issues
nowadays. It is necessary to remove heavy metals from wastewater before discharging it
into the environment. Li et al. (2002) was the first group to using carbon nanotubes
(CNTs) as an adsorbent for the removal of heavy metal cations. Since then, the
adsorptive removal of a series of heavy metal ions including lead, copper, cadmium,
zinc, manganese, and nickel has been attempted using CNTs.
10.3.1.1 Effect of Adsorbents
As mentioned in the previous section, nanotubes themselves are not good
adsorbents. Therefore, a further treatment is necessary to adapt them for an individual
purpose. Usually, oxidation treatments, based on a reaction with concentrated acid or
oxidant agents, are utilized for the purpose of removing the end caps of the tubes and
producing functional groups on the surface of nanotubes (Kondratyuk and Yates, 2007).
In a study, Li et al. (2002) showed that acid-treated CNTs exhibited an adsorption
capacity of 15.6 mg/g at a Pb 2+ equilibrium concentration of 2.7 mg/L, which is much
higher than that of as-grown CNTs. This result is in agreement with the adsorption of
Cd 2+ (Li et al., 2003b), Ni 2+ (Kandah and Meunier, 2007), and Cu 2+ for CNTs (Wu,
2007b). This increased capacity can be explained due to the fact that oxidized CNTs
contain a higher amount of carboxyl, lactone, and phenolic groups (Li et al., 2003b).
This causes a rise in the negative charge on the carbon surface and/or induces the
oxygen atoms in the functional groups to donate a single pair of electrons to the metal
ions, leading an increase in their adsorption capability (Rao et al., 2007), although
sometimes the oxidation process reduces the surface area of CNT materials (Kondratyuk
and Yates, 2007). The period of refluxing CNTs in concentrated nitric acid or the
temperature of the treatment step can also cause variances in the results of the amount of
functional groups on the carbon surface, leading to a dissimilar adsorption capacity.
Stafiej and Pyrzynska (2007) found that soaking CNTs in 8 M HNO 3 at room
temperature produced more acidic groups on the surface, compared to as-grown CNTs;
however, refluxing that CNT with 8 M HNO 3 at 400 K somewhat reduced the amount of
acidic groups. By increasing the treatment time of CNTs in concentrated nitric acid, the
total amount of functional groups increases from 1.6 to 2.7 mmol/g and then 2.8 and 3.1
mmol/g at 1, 2, 6 and 10 h-acidified treatment, respectively (Wang et al., 2007a).
Although the amount of individual carboxylic, hydroxyl, and carbonyl groups changes
irregularly because of their different oxidization potentials, the adsorption capacity of
these acidified CNTs for Pb 2+ increases dramatically with an increase in acidification
treatment time from 1 to 6 h, from which point onwards it almost remains constant.
 
 
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