Environmental Engineering Reference
In-Depth Information
[171]. Fe
3
O
4
-activated carbon MNPs have been proposed for the removal of aniline, with
an adsorption capacity of 90.9 mg/g at pH 6 [172]. Activated carbon/CoFe
2
O
4
magnetic
composite was developed to remove malachite green, and the adsorption capacity reached
89.29 mg/g [173]. Humic substances, dissolved organic matters in water resistant to biodeg-
radation, known as precursors of carcinogenic trihalomethanes, can be effectively removed
using magnetic mesoporous carbon, with a removal ratio up to 60% [174]. CNTs are another
class of important functionalized medium for modifying magnetic components as a result
of their excellent adsorption properties [175-180]. In the work of Qu et al., methylene blue
and neutral red were effectively removed by multiwalled CNTs illed with Fe
2
O
3
particles;
the adsorption capacities were as high as 42.3 and 77.5 mg/g, respectively [181]. In addition,
graphene, chitin, and even polymers such as polyaniline have also been proposed as func-
tional media [142,182-185]. In those adsorption processes using chemically functionalized
MNMs, the solution pH is usually critical for the removal of organic contaminants because
it controls the chemical species of surface-active groups and the contaminants [186].
14.3.1.3 Photocatalytic Technology
Fe
2
O
3
, with band gap of 2.2 eV, is a promising
n
-type semiconductive material and a can-
didate catalyst for photodegradation [187]. Their inexpensive separation and low quantum
yield often limit their application for photocatalysis of toxic compounds. Now, consider-
able efforts, including decreasing photocatalyst size to increase surface area, combining
photocatalyst with some novel metal nanoparticles, and increasing hole concentration
through doping, have been developed to enhance photocatalytic activity [131]. In addi-
tion, improved charge separation and inhibition of charge carrier recombination have
also been taken into account for improving the overall quantum eficiency for interfacial
charge transfer [188-190].
It has been reported that some species of Fe(III) oxides (i.e., α-Fe
2
O
3
, γ-Fe
2
O
3
, α-FeOOH,
β-FeOOH, and γ-FeOOH) present potential for degrading organic pollutants and reducing
their toxicity via the enhanced photocatalysis effect [191]. For example, electron-hole pairs
can be generated through the narrow band gap of Fe
2
O
3
under illumination (Equation 14.1)
[192].
(
)
−
+
Fe O v eO eh
cb
+→ +
(14.1)
23
23
vb
Those MNMs are illustrative to manipulate the catalytic properties of iron oxide for
photocatalysis. Khedr et al. have reported that iron oxide nanoparticles synthesized by
thermal evaporation and coprecipitation method can be used to photodegrade Congo red
dye (C
32
H
24
N
6
O
6
S
2
) [193]. During this process, irradiation has no pronounced effect on the
catalytic decomposition capacity. Danielsen and Hayes reported the reduction of carbon
tetrachloride (CCl
4
) using synthetic magnetite. In this system, the primary reaction prod-
uct is carbon monoxide (CO), followed by chloroform (CHCl
3
) [194].
Although iron oxide NMs themselves have been widely used as photocatalysts, their
activities frequently decline as a result of the electron-hole charge recombination on the
surface at levels of nanoseconds [195]. Deposition or coating a noble metal on a magnetite
(Fe
3
O
4
) or maghemite (γ-Fe
2
O
3
) surface is one of the effective approaches for solving this
problem. For example, gold/iron oxide aerogels have been prepared as photocatalysts for
degrading disperse Blue 79 azo dye in aqueous solution under ultraviolet light illumination