Environmental Engineering Reference
In-Depth Information
It was reported that Pb(0) produced via indirect reduction forms colloidal zero-valent Pb that eventually deposits on the lamp
used as an irradiation source when it is immersed in the suspension.
Recently, Li et al. [185] explored the photocatalytic Pb(II) reduction over TiO
2
and Ag/TiO
2
catalysts in the presence of
formic acid. They showed that the initial rate of Pb(II) reduction over Ag/TiO
2
is dependent on the concentrations of both Pb(II)
and formic acid. Solution pH impacts on Pb(II) reduction with 98.6% removal at pH 3.5 and just 11.8 at pH 0.8; the result can
be explained by a stronger adsorption of formate at the higher pH on the photocatalyst surface favoring the formation of CO
•−
.
Previously, other authors [186] employed a quartz crystal microbalance (QCM) technique to study the kinetics of photocatalytic
reduction of Pb(II) on nanocrystalline TiO
2
coatings. The photocatalyst was used as an ultrathin coating deposited on the
surface of quartz crystal, and Pb(II) removal could be monitored by measuring the change in frequency of the quartz crystal
resonator. Again, it was found that the indirect reduction of Pb(II) depends greatly on the organic additives, with the following
order of efficiency: HCOOH > H
2
C
2
O
4
> CH
3
OH > C
2
H
5
OH.
Titania-silica (TiO
2
-SiO
2
) photocatalyst [187] (silica essentially being used as support to increase the specific surface area)
was synthesized and employed in a study that focused on the simultaneous oxidation of cyanide and the removal of heavy metal
ions, Pb(II) among them, under mild conditions in aqueous solutions. The photocatalytic performance was markedly dependent
on the catalyst, target concentrations, and reaction time.
In conclusion, the photocatalytic treatment of Pb(II) has many potential advantages, including the following:
1. Mixtures of lead (II) and organic scavengers may be present in industrial wastes, which could give rise to economical
methods for its removal.
2. The photocatalytic treatments of Pb(II) generate its transformation and immobilization as lead oxides, metallic deposits,
or colloidal zero-valent lead, making the ulterior separation and treatment of solid residues easier.
Although the Pb photocatalytic system has been extensively studied, significant effort must be undertaken to transfer the
knowledge, in the search for technological solutions.
9.7
mercury
Mercury is a metallic element that can be found naturally in the environment. The solubility of mercury in water and therefore
its presence in natural or residual waters depends on its chemical state: while Hg(0) and HgS are insoluble, Hg(II) is easily sol-
uble and Hg(I) presents slow solubility [188]. Organic mercury species such as methyl or phenylmercury salts have a higher
toxicity than inorganic species, with organic mercury being chemically stable due to the carbon-mercury bond. The toxicity of
mercury (II) is a matter of concern; the toxic effects of inorganic mercury compounds are mainly renal, whereas methyl- and
ethylmercury salts are responsible for neurological disturbances. The guideline value for inorganic mercury is 0.006 mg l
−1
in
drinking water [188].
The use of mercury in industrial processes is significant, and it is used in the electrolytic production of chloride and caustic
soda, in electrical appliances, and as raw material for various mercury compounds. Mercury compounds are used as agricultural
pesticides, fungicides, antiseptics, preservatives, pharmaceuticals, electrodes, and reagents [188].
figure 9.7 shows the Latimer diagram for Hg.
0.854V
0.911V
0.796V
Hg
2+
Hg
2
2+
Hg
0.268V
Hg
2
Cl
2
figure 9.7
Latimer diagram connecting the different Hg species [192].
