Environmental Engineering Reference
In-Depth Information
Natural organic matter (NOM) removal by cNTs has also been studied by researchers, and the results were found to be promising.
lu and Su [18] investigated the role of thermally treated MWcNTs (at 400 ° c for 60 min) in the removal of NOM in terms of dis-
solved organic carbon (DOc) and assimilable organic carbon (AOc). A comparative study of cNT and granular activated carbon
(GAc) adsorption of NOM was also conducted. The NOM had anionic character across the pH range 3.0-9.0. The thermally
treated cNTs had greater surface area than their counterparts. Boehm titration results confirmed that the acidic and basic site con-
centrations were 0.4 and 0.25 mmol/g, respectively for raw (untreated) cNTs and 0.3 and 0.2 mmol/g, respectively for heated
cNTs. The kinetics of DOc removal was faster than that of AOc onto cNTs due to higher initial DOc concentration that creates
a greater diffusion driving force on the cNT surface. The experimental isotherm model fitted well with the langmuir model, and
the equilibrium adsorption of DOc and AOc obtained by the treated adsorbent was found to be more than that obtained by raw
cNTs. The increase in solution pH indicated that the hydroxyl ions were released from the cNT surface when NOM adsorption
occurred. But on increasing the initial pH from 4.0 to 5.0, DOc and AOc removal decreased due to the competition between NOM
and OH - on the same cNT site. GAc exhibited DOc and AOc adsorption capacities as 14.711 and 0.341 mg/g, respectively. The
raw and modified cNTs exhibited 22.003 and 26.138 mg/g adsorption capacity for DOc and 0.485 and 0.548 mg/g for AOc.
Joseph et al. [19] investigated NOM removal from a variety of potential drinking water sources through a combination of coagu-
lation and adsorption with cNTs. Both SWcNTs and MWcNTs were examined for their removal efficiencies of NOM parallel to
the coagulation process in the presence of alum and ferric chloride metal coagulants. Synthetic seawater (SW) and brackish water
(BW) (contained humic acid as its representative NOM), synthetic (old and young) landfill leachates (Ol, Yl) (contained glucose
as its representative NOM), and broad river water (BRW) in columbia, South carolina were chosen to obtain a detailed knowledge
of NOM and its behavior with regard to adsorption and coagulation. Humic acid used in the study contained approximately 40%
DOc. The isotherm data of BW, SW, and BRW fitted well with the freundlich model, and SWcNTs consistently had a higher
adsorption capacity than MWcNTs for NOM. The higher specific surface area of SWcNTs (407 m 2 /g) as compared to that of
MWcNTs (60 m 2 /g) was assumed to be responsible for the greater NOM uptake capacity of the former.
Adsorption of trihalomethanes (THMs) from water using cNTs has also been investigated [11]. MWcNTs were fabricated
by catalytic decomposition of the cH 4 /H 2 mixture at 700°c using Ni particles as catalyst. The as-prepared cNTs were treated
with concentrated acids for 24 h to remove metal catalysts and then washed by deionized water. After cleaning, the cNTs were
again dispersed into concentrated acid solutions and refluxed at 80°c for 2 h to remove amorphous carbon. finally, the cNTs-
containing solution was filtered to obtain purified cNTs. The THM solution consisted of equivalent concentrations of cHcl 3 ,
cHcl 2 Br, cHclBr 2 , and cHBr 3 . The SEM images confirmed the cylindrical shape of the cNTs, and the transmission electron
microscopy (TEM) image exhibited that cNTs possessed multiple walls with hollow inner diameter. The purity of cNTs
increased from 95.05 to 98.87% after acid treatment as revealed by TGA. The kinetic studies showed that the diffusion mech-
anism controlled the THMs adsorption onto cNTs and the smallest molecule, cHcl 3 , was preferred over other THMs (cHcl 2 Br,
cHclBr 2 , and cHBr 3 ) because cHcl 3 could easily enter the inner cavities through the pores. The adsorption capacity of cHcl 3
was observed to be the highest (2.41 mg/g), followed by cHBrcl 2 (1.23 mg/g), cHBr 2 cl (1.08 mg/g), and then cHBr 3
(0.92 mg/g). As the surface tension of cHcl 3 (27.14 dyn/cm) is much lower than that of cHBr 3 (46.2 dyn/cm), adsorption of the
former onto cNTs was much easier. Also, the polarity of the different THM compounds influenced the adsorption process.
Since the polarity of cHcl 3 is the highest, maximum adsorption was noticed in the case of cHcl 3 . THMs adsorption fluctuated
only a little in the pH range of 3.0-7.0. This was attributed to the purification of cNTs by an acid solution that made the cNTs
resistant to the acidic environment. But at higher pH (>7.0), THMs adsorption decreased due to the ionization of oxygen-con-
taining groups on cNTs surface resulting in the blocking of access of THM molecules to adsorption sites. The comparative
adsorption study of THMs by powdered activated carbon (PAc) and cNTs showed that the adsorption capacity of cHcl 3 on
cNTs was twice that on PAc due to the presence of functional groups on cNTs surface. Several other reports are also available
where carbon nanomaterials have been used for the removal of different pollutants [20-32].
15.2.2
metal oxide-based nps
Several adsorption studies have also been conducted to evaluate the performance of iron oxide-based nanomaterials. Many
studies have successfully shown the use of iron oxide as composite materials with host materials to fabricate magnetic sorbents.
Agrawal and Bajpai [33] synthesized iron oxide-based gelatin NPs and evaluated their use for the removal of cr(VI) from
aqueous solutions. An emulsion cross-linking method was followed to synthesize the gelatin-based NPs. The cross-linking
occurred between gelatin and glutaraldehyde. The gelatin NPs were allowed to swell in fe 2+ /fe 3+ solution and further treated
with NH 4 OH (an alkali) to precipitate the fe ions into iron oxide within the NP matrix. The SEM characterization revealed the
smooth, homogeneous, and relatively spherical particle surface of the gelatin NPs. The TEM images confirmed that after cr(VI)
adsorption onto the nanomaterial surface, the average size of the adsorbent increased. The XRD patterns of the feOGel nano-
composites showed the impregnation of iron oxide into gelatin NPs. The percent crystallinity of the feOGel was also higher
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