Environmental Engineering Reference
In-Depth Information
rate of organic pollutants is dependent on the diffusion rates of organic compounds in the active points of the anode, which
results in low degradation efficiency. in addition, direct electro-oxidation occurs via the production of hydroxyl radicals,
which are produced by the oxidation of water at the anode; water electrolysis often consumes much energy. The decompo-
sition of the water increases with increasing voltage; however, the degradation efficiency of pollutants may not increase
proportionally. Additionally, the pH of wastewater, which is an important parameter in the electrochemical treatment, could
be changed by water electrolysis. For these reasons, the development of efficient catalytic electrochemical methods for
water treatment based on the indirect electro-oxidation of pollutants involving electrogeneration of strong oxidants (such as
hydroxyl radicals) is now in progress. These catalytic electrodes can produce oxidants and the addition of other chemicals
is not required. in this process, the waste is oxidized in the bulk solution by these oxidants. in recent years, the electrode
surface modification process via advances in nanofabrication to improve electrocatalytic properties has been one of the most
active research areas. The improvement in performance appears therefore to be primarily caused by the increase in surface
area, which results in high contacting rates and pollutant adsorption. The addition of catalytically active nanoparticles is
shown to be an effective method to increase the transfer speed of electrons, reactivity, and diffusion in reaction [59]. in
recent years, several electrodes have been evaluated to rank anode materials in terms of high stability, high activity toward
organic oxidation, and cost. The types of electrodes tested included TiO 2 , ZnO, Nb 2 O 5 , WO 3 , SnO 2 , ZrO 2 , CdS, and ZnS,
among others.
The PeCh degradation of organic pollutants is an extension of the heterogeneous photocatalytic process, in which the cata-
lyst is placed on an electrode that is controlled potentiostatically. in a PeCh system, the photogenerated electron-hole pairs are
separated by means of an externally applied electric field [60]. A particulate TiO 2 film electrode is usually used as a photoanode
by coating TiO 2 nanoparticles on the supporting media such as a conducting glass or a metallic material. in these systems, uv
light must be used in order to excite TiO 2 [61]. To efficiently utilize the solar light and improve the photooxidation technique
for wastewater purification, TiO 2 film electrodes were modified by coupling with other semiconductors (such as CdS, ZnO, and
SnO 2 ), and doping with metal or nonmetal ions. dopants can shift the light absorption toward visible light (such as Fe-doped
TiO 2 or N-doped TiO 2 electrodes) or exhibit a significant blue shift in their uv absorption spectrums (such as Si-doped TiO 2
nanofilm electrodes) [62]. TiO 2 film thickness can affect the efficiency of both light energy conversion and electron transfer.
yan et al. demonstrated that there exists an optimal thickness of the TiO 2 film for the maximum degradation rate of phenol. in
their experiments, they observed an improvement in activity when the thickness was increased, but the activity decreased when
the thickness was increased further to an 11-layer film. They attributed this to its relatively high resistance to electron transfer.
in addition, the thick film can shield the catalysts from light [62, 63].
Highly oriented titania nanotube arrays with {101} crystal face were prepared by Hou et al. The PeCh catalytic activity
toward the oxidation of acid orange 7 of nanotube arrays was particularly higher for P-25 TiO 2 than for TiO 2 films [64]. The
degradation rate of organic pollutants was enhanced using Zn-doped TiO 2 nanotube [65] and TiO 2 nanotube/Sb-doped SnO 2
electrodes [66]. The PeCh degradation of dyes has also been performed using other nanostructured films like indium tin oxide
film [67] and Bi 2 WO 6 nanoflake film electrodes [61]. A higher PeCh degradation of naphthol blue-black has been observed for
WO 3 film electrodes than for TiO 2 nanoparticulate film electrodes [68].
it has been demonstrated that CNTs functionalized with ultrathin foreign species coatings are an ideal electrode material in
the field of electrocatalytic degradation. recently, SnO 2 /CNT composite electrodes, and eu 2 O 3 /CNT and CeO 2 /CNT core-shell
structures have been synthesized. The preparation method has a significant effect on the photoelectrocatalytic degradation of
the dye. For example, Zhang et al. synthesized SnO 2 /CNT composite electrodes by a liquid deposition method and a solvother-
mal method, which show a higher electrocatalytic performance than that of the liquid deposition method [69].
24.5
catalytic feNtoN process
24.5.1
introduction
Fenton chemistry involves the generation of a hydroxyl radical from hydrogen peroxide (H 2 O 2 ) in the presence of Fe 2+ or Fe 3+
ion (eq. 24.1). The generated hydroxyl radicals can decompose a wide range of organic compounds [70].
2
+
+ −
+ →++
3
Fe HO Fe
HO OH
(24.1)
22
it is well known that the homogeneous Fenton process has many disadvantages; it requires to be operated at pH < 3.0 for
preventing the precipitation of Fe 2+ and Fe 3+ ions [71], and therefore this system suffers from the draining of the ferrous catalyst
and the hydrolysis of iron ions (limited pH range) [72]. The resulting sludge of ferric hydroxide requires a further separation
and disposal, which makes it a costly process [73].
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