Environmental Engineering Reference
In-Depth Information
HP appears as a suitable alternative for Hg reduction, and many authors have studied this technology with TiO
2
, ZnO, and
wO
3
, exploring the use of UV, visible, and solar irradiation [10, 177, 190-207]. The main conclusions drawn from published
works can be condensed in the following points:
1. Mercury photocatalytic reduction strongly depends on pH conditions. Higher conversion rates can be obtained at basic
pH. The reason for this behavior seems to be a larger driving force for Hg(II) reduction produced by the shift in the poten-
tial of the CB electrons with the increase in the pH: eCB decreases 0.059 V per unit of pH (at 25 °C) [206]. In acidic
conditions, Hg(0) photooxidation can occur inhibiting Hg(II) reduction, as found by Serpone et al. [200] in experiments
at pH 0. In a very recent work, López-Muñoz et al. [208] studied the influence of pH and the addition of methanol, formic
acid, and oxalic acid as sacrificial agents for the photocatalytic removal of Hg(II). At pH 10, an efficient removal of
Hg(II) was achieved even in the absence of organic additives, attaining final mercury concentrations in the solution at
trace levels (µg l
−1
). In acidic conditions, the addition of sacrificial organic molecules significantly increased the rate and
extent of aqueous Hg(II) removal.
2. The presence of oxygen inhibits mercury reduction in acidic or neutral pH conditions, due to the competition between
equations 9.5, 9.37, and 9.38 for a direct reduction mechanism:
2+
−
+ →()
(9.37)
Hg
e gI
CB
−
(0
(9.38)
Hg(I)
C
+ →
e g
A mechanism involving monoelectronic steps has been proposed by our group [212] for direct Hg(II) HP reduction, par-
ticularly in the case of inorganic salts. evidence from experiments performed with HgCl
2
yielding calomel Hg
2
Cl
2
(together with Hg(0)) support this hypothesis.
3. The addition of electron donors to the system favors reductive Hg(II) HP indicating the parallel occurrence of an indirect
mechanism. These donors can be added to the system with or without the formation of complexes with Hg(II), or as part
of an organic mercury species, with the organic moiety acting as electron donor. electron donors added to the system can
avoid reoxidation of mercury species by reacting with h O
VB
+
•
and preventing the following reaction from taking place
/
[209, 210]:
+
•
(9.39)
Hg
()/( )
0
I hHO
+
/
→
Hg IHgII
()/
(
)
VB
The nature and distribution of mercury products deposited on the catalyst depend on the reaction conditions. Starting from
inorganic mercury salts, Hg(NO
3
)
2
, HgCl
2
and Hg(ClO
4
)
2
, Hg(0), HgO, or Hg
2
Cl
2
are formed on the photocatalyst surface [209].
In the absence of additives, Hg
2
Cl
2
and Hg(0) were respectively identified in acidic and neutral/alkaline media as main reduced
species on the titania surface [208]. The addition of organic additives enhances the photocatalytic reduction to Hg(0). On the
other hand, a direct correlation between Hg(II) dark adsorption on the TiO
2
[208] surface and the efficiency of Hg(II) photore-
duction could not be established [206, 208].
Recently, Lenzi et al. [211] studied Hg(II) photoreduction using HgCl
2
in the presence of formic acid at pH 4 on TiO
2
and
Ag/TiO
2
prepared by sol-gel and impregnation methods. The Ag/TiO
2
sample prepared by impregnation proved to be better than
the sample prepared by the sol-gel method, because, due to the high dispersion of Ag in the last sample, Ag may act as an
electron-hole recombination center, leading to decreased Hg(II) photoreduction. In contrast, in the sample prepared by impreg-
nation, Ag addition seems to prevent electron-hole recombination, with better photoactivity.
As was pointed out before, organic mercury species are much more toxic than inorganic ones. Metallic mercury has been
reported as the product of HP treatment of methylmercury in the presence of methanol and in the absence of oxygen [200, 212].
Later, Miranda et al. [198] found optimal conditions at alkaline pH for this reaction. Although reduction of Hg(II) was found
both in the presence and in the absence of oxygen, different products were identified from the organic moiety of the organome-
tallic compounds: methane was found in oxic conditions while methanol formed under a nitrogen atmosphere.
Two monoelectronic successive steps are proposed where the organic moiety acts as an electron donor. Our group studied
the UV/TiO
2
photocatalysis of phenylmercury salts in aqueous solutions starting from acetate (C
6
H
5
HgCH
3
CO
2
, phenylmercury
acetate (PMA)) and chloride (C
6
H
5
HgCl, phenylmercury chloride (PMC)) salts [195]. The results were very promising as it was
possible to remove mercury to a large extent with the simultaneous mineralization of the organic portion of the compound.
