Environmental Engineering Reference
In-Depth Information
Volatile Organic Compounds. MWNTs were used as an adsorbent for directly
trapping volatile organic compounds (VOC) in gaseous samples and indirectly from
water samples (Li et al., 2004). While it does not directly describe the separation of
VOC from water samples by MWNTs, this section partially expresses the potential of
MWNTs for capturing VOCs, even under high humidity conditions. A series of VOCs,
including benzene, toluene, p-xylene, o-xylene, ethyl benzene, n-pentane, n-hexane,
cyclohexane, n-heptane, dichloromethane, trichloromethane, tetrachloromethane, 1,2-
dichloroethane, acetone, ether, ethyl acetate, n-propanol, and n-butanol, were
investigated. The samples were purged by N 2 gas with a flow rate of 40 l/min at 35 o C.
In comparison with Carbopack B, a commercial adsorbent, all tested compounds on
MWNTs presented 2-3 orders of magnitude larger breakthrough volumes (BTVs),
showing much stronger retention. Consequently, the recoveries of all tested compounds
in an air sample using MWNTs were between 82 and 110% and were not affected by gas
humidity (up to 70%); whereas the Carbopack B's recoveries of acetone and n-propanol
decreased from 12 to 0% for acetone and 86 to 35% for n-propanol. Moreover, the
recoveries of dichloromethane on Carbopack B in both 0 and 70% humidity were 0. The
determination of VOCs in water samples by a purge-and-trap system using MWNTs
displayed satisfactory recoveries with all the tested compounds (88-110%). Thus, the
results show the potential of MWNTs for further practical use as a trap in collecting
analytes, both in air and water samples.
Non-Volatile Organic Compounds. Recently, there have been numerous
studies on the removal of toxic compounds such as herbicides, insecticides, pesticides as
well as chlorophenols, phthalate esters, endocrine disruptors, and dioxins by using
MWNTs as SPE adsorbents.
MWNTs have great potential for application in environmental analysis,
compared to conventional SPE adsorbents such as C 18 or C 8 silica, and polystyrene-
divinylbenzene polymers. MWNTs have much greater adsorption capability than that of
C 18 bonded silica for both dicamba and 2,4,5-T (Biesaga and Pyrzynska, 2006;
Pyrzynska et al., 2007). High recoveries (83-97%) and low detection limits (2.8-3.6
μg/L) of dicamba and 2,4,5-T were achieved using only 3 ml of the eluent
acetonitrile/NH 3 (80:20, v/v) at a flow rate of 2.5 ml/min and acidic solution of the
samples (pH = 1). MWNTs are an excellent adsorbent for the preconcentration of
sulfonylurea herbicides, including triasulfuron, bensulfuron-methyl (Zhou et al., 2007),
nicosulfuron, thifensulfuron-methyl, and metsulfuron-methyl (Zhou et al., 2006b), not
only in synthetic water samples but also in real water samples (e.g., tap water, seawater,
reservoir water, and well water). The recommended conditions for preconcentration
steps were to use a low sample solution pH (= 3), 12 ml of the eluent made by a mixture
of acetonitrile and 1% acetic acid, a flow rate of 8 ml/min. Using these conditions, the
recoveries of triasulfuron, bensulfuron-methyl, nicosulfuron, thifensulfuron-methyl, and
metsulfuron-methyl from environmental samples were in the range of 104.9-113.6%,
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