Nanoscale Carbon Materials for Contaminant Separation - Nanotechnologies for Water Environment Applications - page 288

effect” as observed in the case of the adsorption of THMs (Lu et al., 2005); thereby
implying the existence of other influential mechanisms. Adsorption isotherms of o-
xylene and p-xylene on as-grown and purified-CNTs can be described by the Langmuir
isotherm. Compared to activated carbon, both as-grown and purified CNTs have lower
adsorption capacities, due to the large specific surface area of activated carbon; though
the adsorption capacity per unit of the surface area of as-grown CNTs is higher than that
of activated carbon (Chin et al., 2007).
Polycyclic Aromatic Hydrocarbon. The adsorption affinity and desorption
behavior were studied of pyrene, phenanthrene, and naphthalene on CNTs and fullerene
as well as the competitive sorption of these PAHs on CNTs (Yang et al., 2006a; Yang et
al., 2006b; Yang and Xing, 2007). They determined that the single-solute sorption
isotherms of naphthalene, phenanthrene and pyrene were well fitted with the Polanyi-
Manes models (PMM) (Yang et al., 2006b). Good fits of the Freundlich model were
obtained for naphthalene on MWNT15 (MWNTs with an outer diameter < 15 nm) and
phenanthrene by SWNTs and fullerene, since the Freundlich model is a special form of
PMM. Adsorption affinities of adsorbates on MWNT15 followed the order: naphthalene
< phenanthrene < pyrene, consistent with their hydrophobicity (K ow ). For different
adsorbents, adsorption capacity seems to be relative to surface area, micropore volume,
and the volume ratio of mesopore to micropore. The adsorption capacity of
phenanthrene on carbon nanomaterials is in the order: SWNTs > MWNTs >> fullerene.
The phenanthrene adsorbed by CNTs was 2-4 orders of magnitude higher than fullerene.
Competitive sorption among three PAHs on MWNT15 revealed that significant
competition occurs at a relatively low concentration of a primary solute and a high
concentration of competitors, whereas competition was much weaker in the case of
relatively high concentrations of the primary solute and low competitor concentrations
(Yang et al., 2006a). In bi-solute systems, the more similar the molecular structure and
physicochemical properties between primary and the competing solutes are, the greater
the occurrences of competition. Phenanthrene has a molar volume and the other
physicochemical properties such as water solubility between those of pyrene and
naphthalene, resulting in a greater competition with pyrene than naphthalene or with
naphthalene than pyrene (Yang et al., 2006a).
Based on the desorption experiments of pyrene, phenanthrene, and naphthalene,
it was found that the sorption isotherms of carbon nanotubes had no significant
adsorption-desorption hysteresis, whereas adsorption-desorption hysteresis was present
and irreversible for fullerene (Yang and Xing, 2007). The existence of adsorption-
desorption hysteresis for the adsorption of naphthalene by fullerene was also observed
and investigated (Cheng et al., 2004; Cheng et al., 2005), and will be mentioned in
further detail later. Yang and Xing (2007) estimated that the available adsorption space
of carbon nanotubes was found to be the cylindrical external surface, not the inner