Environmental Engineering Reference
In-Depth Information
Recently, the adsorption of natural organic matter (NOM) on CNTs has been
considered (Lu and Su, 2007). In this study, CNTs exhibited high efficiency in the
removal of NOM. In fact, the CNTs possessed 1.4-1.8 times more adsorption capacity in
terms of dissolved organic carbon (DOC) and assimilable organic carbon (AOC) than
GAC (an adsorbent commonly used for NOM removal). The CNTs obtained after
thermal treatment at 400
o
C for 1 hour, showed a higher efficiency than raw CNTs. This
difference is because, as compared with the heated CNTs, the surface of raw CNTs is
more positive at a pH less than 5.0, but more negative at a pH greater than or equal to
5.0, resulting from a larger amount of functional groups on raw CNTs. The adsorption of
NOMs onto CNTs was also found to be strongly dependent on the solution pH. The
adsorption capacity slowly decreased as pH increased from 4 to 5, which could be
attributed in part to competition between NOM and OH
-
for the same CNT site. As the
pH increased from 4 to 10, adsorption capacity decreased gradually as a result of
electrostatic repulsion between the NOM and the CNT surface. Adsorption isotherms of
NOM onto CNTs can be described well by the Langmuir isotherm, and an increase in
DOC and AOC adsorption onto CNTs with increasing ionic strength was observed. This
increase could be addressed as an increase in the activity coefficient of DOC and AOC,
which induced these organic molecules to become more coiled and less soluble. The
times needed for reaching equilibrium in terms of DOC and AOC for the adsorption of
NOM onto CNTs were quite short, 80 and 120 minutes for DOC and AOC, respectively.
10.3.4.4 Derivatives of Benzene
Dichlorobenzene.
1,2-dichlorobenzene (DCB) was adsorptively removed on
both as-grown and graphitized CNTs, which were obtained by treating the as-grown
CNTs in a nitrogen atmosphere at 2200
o
C for 2 hours. It took only 40 minutes for CNTs
to attain equilibrium, and the adsorption capacity of as-grown and graphitized CNTs was
found to be 30.8 and 28.7 mg/g, respectively, from a 20 mg/L solution of DCB (Peng et
al., 2003). Generally, it takes a longer time for activated carbon to attain equilibrium
(e.g., it takes 20 h for the adsorption of phenol). Removal of DCB by CNTs fluctuated
very little in the pH range between 3 and 10, although when the pH exceeds 10, the
removal drops suddenly, which is similar to the case of THM adsorption (Lu et al.,
2006b). At higher pH values, the oxygen-containing groups on the surface of CNTs can
easily be ionized and adsorb more water, thereafter the formation of water clusters
hinders access of adsorbates to the surface. The adsorption capacity of graphitized CNTs
decreased much less when the pH exceeded 10; after being treated at a high temperature,
the number of oxygen groups on the surface of graphitized CNTs was reduced
(Kondratyuk and Yates, 2007). The sorption isotherms of DCB on CNTs were fit well
with the Freundlich isotherm, and the calculated thermodynamic parameters showed that
the adsorption of DCB is spontaneous and endothermic. Adsorption of DCB to CNTs
increased with an increase in temperature, and as-grown CNTs showed a higher
adsorption affinity for DCB than graphitized CNTs. This is because as-grown CNTs
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