Environmental Engineering Reference
In-Depth Information
24.5.2
heterogeneous fenton-like systems
Heterogeneous Fenton-like systems using iron-supported catalysts, for example, zero-valent iron ((Zvi) Fe
0
) [74], goethite
(α-FeOOH) [75], Fe
3
O
4
[76], and Fe
0
/Fe
3
O
4
[77], have recently been developed [78]. However, the reaction rate of heteroge-
neous Fenton catalysts is generally slower than that of homogeneous catalysts due to the agglomeration of solid particles,
decreasing reactive sites and catalytic activity [71, 79]. in order to improve the rate of the catalytic degradation reaction, most
heterogeneous Fenton systems require an external energy input, such as irradiation with uv light or sunlight [80]. Chu et al.
developed a hybrid system of Fenton and photo-Fenton processes to increase the degradation rate as well as power consump-
tion [81]. The deposition of iron species supported over different materials (zeolites, clays, silicas, CNTs, etc.) has been widely
reported as photo-Fenton-like heterogeneous catalysts to overcome the recovery of the homogeneous iron ions [82]. However,
these matrices introduce mass transfer resistance between catalytic sites and organic pollutants and reduce efficiency; they also
reduce light penetration and inhibit photodegradation [72, 79]. This disadvantage can be overcome with the use of nanomag-
netic Fenton's catalysts. These catalysts increase the rate of mass transfer near the catalyst surface and enhance light absorption.
They also combine the advantage of the high surface area of nanoparticles with the settling properties of much larger particles
by the application of an external magnetic field [82a]. Several studies on the degradation of organic pollutants investigate the
potential use of magnetic heterogeneous Fenton catalysts. For example, Zhu and Tang [83] synthesized a magnetic nano-
BiFeO
3
catalyst for the photo-Fenton decomposition of organic pollutants in the presence of H
2
O
2
. in another study, Zhang
et al. investigated a heterogeneous Fenton system based on Fe-Fe
2
O
3
core-shell nanowires [84].
The application of nanoparticles as catalysts of the Fenton-like and photo-Fenton reactions has been described by several
investigators (Table 24.2 [72, 79, 83, 85]). in comparison with their microsized counterparts, nanoparticles show a higher
catalytic activity. The advantage of using nanoparticles as catalysts for Fenton-like reactions would more than offset the disad-
vantage of requiring uv radiation to accelerate the reaction [86]. For example, valdés-Solís et al. [87] have developed manganese
ferrite nanoparticles with a high surface area that can accelerate the Fenton-like reaction without requiring uv radiation.
Zelmanov and Semiat [88] investigated the catalytic degradation of ethylene glycol and phenol by iron(iii) oxide nanoparticles
in the absence of uv radiation.
recently, Fenton processes without needing the addition of toxic chemical reagents based on H
2
O
2
electrogeneration have
gained a lot of interest. H
2
O
2
is formed through an electrochemical reaction of oxygen (cathodic half-reaction (eq. 24.2) [89]:
OHe HO
2
++→
2
+
2
−
(24.2)
2
2
in the electro-Fenton process, hydroxyl radicals are produced from a reaction between electrogenerated H
2
O
2
and added Fe
2+
through Fenton's reaction (eq. 24.1) in the acidic solution [90]. Fe
2+
can also be regenerated by the reduction of Fe
3+
at the
cathode via reaction 24.3 [91]:
Fe
3
+
+→
e e
−
2
+
(24.3)
Since the optimal pH for the photo-Fenton process (pH < 3) is not agreeable with the optimal pH values for the electrochem-
ical generation of H
2
O
2
(pH > 7), the degradation rate in the electro-Fenton oxidation is not fast. Fe@Fe
2
O
3
core-shell nanow-
ires as an iron reagent offer one quantitative solution to this problem. A neutral electro-Fenton system is easily obtainable with
their combination with CNTs and AC fibers as cathodes [84, 92].
An alternative technology to overcome this problem was developed by ding et al., who designed a PeCh/electro-Fenton
system by coupling visible light-driven PeCh oxidation and electro-Fenton oxidation in an undivided cell. Bi
2
WO
6
nanoplates
deposited on fluorine-doped tin oxide (FTO) glass and Fe@Fe
2
O
3
core-shell nanowires supported on AC fiber were used as the
anode and the cathode in this system, respectively [93].
Photoelectro-Fenton (PeF) is another technology that has been proposed in recent years in which the solution treated under
electro-Fenton conditions is simultaneously irradiated with uv light. The irradiation causes the photolysis of Fe(OH)
2+
, the
predominant species of Fe
3+
in the pH range 2.5-4.0, yielding more Fe
2+
and
·
OH and accelerating the photolysis of complexes
of Fe
3+
with oxidation by-products such as carboxylic acids [70a, 90b]. One of the new approaches in developing an efficient
advanced oxidation processes is to use PeF combined with heterogeneous photocatalysis [85h, 94]. recent work has reported
an enhancement of PeF treatment when it is combined with TiO
2
[85a] or ZnO photocatalysis (Table 24.2). As mentioned ear-
lier, the modification of TiO
2
with a short bandgap semiconductor such as Fe
2
O
3
can enhance activity and shift the wavelength
of irradiation into the visible region [95]. The negative effect of Fe
2
O
3
modification on TiO
2
was also reported. Jeon et al. mod-
ified TiO
2
with Fe
2
O
3
. in comparison to the pure TiO
2
, the modified TiO
2
(Fe
2
O
3
/TiO
2
) nanoparticles displayed a lower PeCh
activity that was attributed to the hematite-induced charge recombination due to an energy level mismatch between TiO
2
and
