Environmental Engineering Reference
In-Depth Information
reduction to solid As(0) was achieved, which is a very convenient method for As removal from water. However, careful attention
must be paid during these kinds of reactions as they can result in the formation of AsH 3 , one of the most toxic forms of As.
Proposed strategies for AsH 3 treatment include adsorption on suitable materials or oxidative treatment by a coupled gas-phase
photocatalytic system, which our research group is currently developing.
The photoconversion of As(III) and As(V) (arsenate) over mesoporous TiO 2 electrode was investigated in a photoelectro-
chemical cell (PeCC) [54] for a wide range of concentrations from micro- to nanomolar. It was shown that As(III) can be oxidized
and As(V) can be reduced in anoxic conditions under UV irradiation using the photoelectrochemical setup. The reduction or
increment of the photooxidation current depended on the As(III) concentration range. This abnormal concentration-dependent
behavior was ascribed to As(IV) species, which can be either oxidized or reduced depending on the experimental conditions.
Because As(V) is an electron acceptor, its addition consistently lowered the photocurrent in the entire concentration range.
Although, as mentioned earlier, inorganic As(III) and As(V) are the predominant contaminant species in water, the removal
or transformation of organoarsenic species cannot be neglected. MMA and dMA are used as herbicides in agriculture and
employed in industrial activities [41, 55] and are also introduced to the environment to some extent through As(V) methylation
carried out by a variety of organisms [56]. Also, complex organoarsenic species (e.g., with substituted phenyl rings in their
structure) are used to control intestinal parasites in poultry and swine [57, 58]. It has been shown that TiO 2 photocatalysis is
effective for the oxidation of some organic arsenic species.
xu et al. [41] studied the TiO 2 photocatalytic degradation of MMA and dMA. MMA seems to be the primary dMA degra-
dation product to be degraded later and the arsenic moiety oxidized to As(V). Results obtained in the presence of a radical
scavenger (terbutanol) indicated that the hydroxyl radical was the primary oxidant in the reaction. further work [59] undertaken
with other radical scavengers (superoxide dimutase (SOd), sodium bicarbonate, and sodium azide) also supported HO as the
primary oxidant and showed that the methyl groups in MMA and dMA are transformed into formic acid and possibly meth-
anol. Other authors [55] compared the photocatalytic oxidation on MMA and dMA by different AOPs, that is, UV/H 2 O 2 , UV/
TiO 2 , and UV/S 2 O 8 2− , finding the last one the most effective.
Zheng et al. [58] studied the TiO 2 photocatalytic degradation of phenylarsonic acid (used as feed additive in the poultry
industry), where HO was found to play a major role in oxidation, as was the case for dMA and MMA.
due to the extensive, good-quality work done on photocatalytic systems for As removal and the importance of the problem
of As in the environment, it is the vision of the authors that research on practical applications of these photocatalytic systems
should be strengthened in the coming years in order to provide cost-effective systems.
9.4
cHromium
Chromium is a metal with multiple industrial and technological applications including metallurgy, electroplating, the textile
industry, leather tanning, and wood preservation. Because of its extensive use, Cr is a frequent contaminant in wastewater,
mainly in the form of Cr(III) and Cr(VI). Cr(VI) presents the highest environmental threat due to its toxicity for biological
organisms together with its high solubility and mobility. The wHO [60] has established the maximum contaminant level of
Cr(VI) in drinking water to be 0.05 mg l −1 , while total Cr to be discharged should be below 0.1 mg l −1 according to the U.S.
environmental Protection Agency (ePA) [61]. Cr(III) is considered to be nontoxic or of very low toxicity [62, 63], and its
mobility is lower than that of Cr(VI). The conventional treatment for the removal of Cr(VI) dissolved in water involves its
reduction to Cr(III) with the use of sodium thiosulfate, ferrous sulfate [64, 65], sodium metabisulfite, sulfur dioxide, or other
chemicals, with subsequent economical costs and generation of residues [66].
figure 9.3 shows the redox potentials of chromium in acid conditions taken from reference [67].
0.95V
-0.74V
0.55V
1.34V
2.1V
-0.424V
-0.90V
Cr(VI)
Cr(V)
Cr(IV)
Cr(III)
Cr(II)
Cr(0)
1.72V
1.38V
figure 9.3
Latimer diagram connecting the different Cr species [67].
Search WWH ::




Custom Search