Nanoscale Carbon Materials for Contaminant Separation - Nanotechnologies for Water Environment Applications - page 286

10.3.4 Adsorptive Removal of Organic Compounds
To date, there have been numerous reports related to the adsorptive removal of
organic compounds such as polycyclic aromatic hydrocarbon (PAH), natural organic
matter (NOM), and dyes from aqueous solutions, using CNTs as the adsorbent. It has
also been noted that CNTs are effective solid-phase extraction (SPE) adsorbents for the
preconcentration of organic matter to be used for further analysis of a HPLC system. For
convenience, the use of CNTs for the preconcentration of organic matter is mentioned
separately in Section 10.3.5.
10.3.4.1 Trihalomethanes
Adsorption of trihalomethanes (THMs) onto concentrated acid-purified MWNTs
from water was investigated (Lu et al., 2005, 2006b). After being purified into a mixture
of concentrated nitric acid and sulfuric acid, MWNTs became more hydrophilic and
possessed a large surface area, suitable for the adsorption of low molecular weight and
relatively polar THM molecules. Adsorption of trihalomethanes, including CHCl 3 ,
CHBrCl 2 , CHBr 2 Cl, and CHBr 3 , reached equilibrium in 150 minutes for an initial
concentration of the trihalomethanes of 3.2 mg/L, which was significantly shorter than
the contact time required to reach equilibrium for PAC (360 minutes). A longer contact
time to reach equilibrium at lower initial THM concentrations was observed, which may
be explained by the fact that diffusion mechanisms control the adsorption of THMs onto
CNTs (Lu et al., 2005). Among the four trihalomethanes, CHCl 3 (the smallest molecule)
had the highest adsorption rates onto CNTs, followed by CHBrCl 2 , CHBr 2 Cl, and CHBr 3 .
This is supported by the known benefits of smaller molecules during diffusion in the
pores. The adsorption rates also increased with an increase in temperature (Lu et al.,
2006b) because increasing the temperature results in an increase in the diffusion rate of
THM molecules across the external boundary layer and within the pores of CNTs due to
a decrease in solution viscosity. Adsorption kinetics of THMs followed Lagergren's first
order rate (Lagergren, 1898), with a high correlation coefficient (R 2 > 0.96).
Adsorption capacities of THMs are in the order: CHCl 3 > CHBrCl 2 > CHBr 2 Cl >
CHBr 3 (Lu et al., 2005). There are two possible reasons for the higher adsorption
capacity of CHCl 3 onto CNTs. First, the surface tension of CHCl 3 (27.14 dyn/cm) is
much lower than that of CHBr 3 (46.2 dyn/cm), making it easier for CHCl 3 to wet the
CNT surface and fill open CNTs by capillarity, thereby enhancing the adsorption
capacity of CHCl 3 . Second, the dipole moment of the C-Cl bond (1.56 Debye) is higher
than that of the C-Br bond (1.48 Debye), causing the polarity of THMs to follow the
order: CHCl 3 > CHBrCl 2 > CHBr 2 Cl > CHBr 3 . As such, CHCl 3 is more preferentially
adsorbed onto the hydrophilic surface of CNTs owing to the presence of plenty of
oxygen-containing groups. It was found that the adsorption of THMs onto CNTs was
dependent on the solution pH and temperature; the adsorption capacities of THMs