Environmental Engineering Reference
In-Depth Information
5.2.1
cNts for BteX adsorption
Since their discovery in 1991 by Sumio iijima, CNTs, a new member of the carbon family, have gained widespread attention
because of their outstanding physicochemical properties [9, 10]. Nanotubes are categorized as (i) multiwalled carbon nanotubes
(MWCNTs) that consist of up to several tens of graphitic shells with adjacent shell separation of ~0.34 nm, diameters of ~1 nm,
and a large length to diameter ratio and (ii) single-walled carbon nanotubes (SWCNTs) that usually have a diameter close to
1 nm, with a tube length that can be many millions of times longer [9, 11].
Because CNTs provide chemically inert surfaces, large specific surface areas, hollow structure, and light mass density, they have
been tested as adsorbents for various types of hazardous pollutants such as heavy metals, radionuclides, and organic compounds
[12-14]. Furthermore, CNTs demonstrated superior adsorption capabilities than conventional adsorbents (e.g., activated carbon)
because their structure at the atomic scale is far more well defined and uniform [9, 15]. The adsorption properties of CNTs depend
on the adsorption sites, surface area, surface functional groups, purity, and porosity [16].
CNTs have four possible adsorption sites: internal sites, interstitial channels, external groove sites, and external surfaces.
On the external sites (grooves and outer surfaces), the adsorption reaches equilibrium much faster than on the internal sites
(interstitial and inside the tube) under same pressure and temperature conditions [9]. This is attributed to the direct exposure
of the external sites to the adsorbing materials as compared to the adsorption process on the internal sites, which has to be
initiated at the end of the pores, followed by diffusion to the internal sites [17, 18].
Su et al. [2] employed MWCNTs fabricated by the catalytic chemical vapor deposition method and oxidized by sodium hypo-
chlorite (NaOCl) solution as adsorbent for BTEX in aqueous solution. The CNTs (NaOCl) exhibited significantly higher BTEX
adsorption capacity than raw CNTs. The oxidation process of the CNTs using NaOCl resulted in a decrease in the surface area
of CNTs (NaOCl) in all pore size ranges (1.7, 1.7-5, and 5-100 nm). The authors attributed this decrease in surface area to the
formation of functional groups on the external and internal surface of CNTs (NaOCl) that are a direct product of oxidation. The
introduction of surface oxygen functional groups (e.g., carboxylic groups) into CNTs provides various chemical sites for BTEX
adsorption. Additionally, X-ray diffraction (XRD) data showed that the intensity of the CNT peak become considerably stronger
after CNTs are oxidized. This indicates an increase in the purity of CNTs after oxidation.
The affinity of BTEX to CNTs (NaOCl) was in the order of X > E > T > B. The increase in molecular weight (B < T < E, X), the
decrease in solubility (B > T > E > X), and the increase in boiling point (B < T < E, X) may explain the observed order of BTEX
adsorption on CNTs (NaOCl). Changes in solution pH [3-11] did not significantly impact the BTEX adsorption on CNTs
(NaOCl), which imply that BTEX are in molecular form during the adsorption process and that ion exchange does not play a part
in the BTEX solution. Similar to pH, changes in solution ionic strength (0-0.2 M NaCl) had no significant impact on BTEX
adsorption, reflecting high stability of CNTs (NaOCl) as BTEX adsorbents in a wide range of ionic strengths for solutions.
in another study by Su et al. [19] NaOCl-oxidized CNTs were compared with CNTs oxidized by other chemical agents
including HCl, H
2
SO
4
, HNO
3
, or NaOCl solutions for enhancing BTEX adsorption in aqueous solution. The results showed that
NaOCl-oxidized CNTs showed the greatest enhancement followed by HNO
3
-oxidized CNTs, and then H
2
SO
4
-oxidized CNTs.
in the same study, Su et al. [19] tested BTEX adsorption on granular activated carbon (gAC) and gAC (NaOCl) for
comparison with CNTs (NaOCl). The gAC (NaOCl) achieved lower BTEX adsorption than gAC as a result of the collapse of
the pore structure after oxidation. The NaOCl-oxidized CNTs showed the best adsorption performance of BTEX followed by
gAC and then CNTs. This adsorption affinity trend may be explained with two possible reasons: (1) the surface carboxylic
groups concentration followed the order of CNT (NaOCl) (1.039 mmol/g) > gAC (0.252 mmol/g) > CNTs (0.161 mmol/g) and
(2) the surface total acidity that can enhance electrostatic interactions between BTEX molecules and adsorbent surface followed
the order of CNTs (NaOCl) (1.17 mmol/g) > gAC (0.7 mmol/g) > CNTs (0.22 mmol/g). Furthermore, the order of BTEX adsorp-
tion was inconsistent with the order of surface area, pore volume, and average pore diameter. This clearly indicates that BTEX
adsorption is dependent on the surface chemistry of the adsorbents rather than on its characteristics.
The mechanism responsible for the adsorption of BTEX on CNT (NaOCl) was identified as π-π electron-donor-acceptor
mechanism involving the carboxylic oxygen atom of the CNTs (NaOCl) acting as the electron donor and the aromatic ring of
BTEX acting as the electron acceptor, as presented in Figure 5.1 [19]. This mechanism is similar to the adsorption mechanism
of BTEX on powdered activated carbon.
Carrillo-Carrion et al. [20] exploited the strong adsorption affinity of CNTs toward organic compounds for the
development of analytical methodologies. in their study, BTEX were liquid-liquid extracted from olive oil samples
using aqueous dispersion of sodium dodecyl sulfate (SDS)-coated MWCNTs as the extracting medium. Then, head-
space/gas chromatography/mass spectrometry (HS/gC/MS) was used for the analysis of the aqueous phase. liquid-
liquid extraction was the key step in the entire analytical process for measuring BTEX concentrations in the samples as
it is the source for sensitivity and selectivity enhancement. Selectivity and sensitivity determination is enhanced by the
presence of SDS-coated MWCNTs in the aqueous phase resulting in detection limits at least 10 times lower than the
direct HS method. Coating the MWCNTs with surfactants such as SDS enhances their adsorption properties because it
