Environmental Engineering Reference
In-Depth Information
Metallic Hg was found as a solid deposit when PMA was used and Hg
2
Cl
2
in the case of PMA. The best condition for Hg(II)
removal and mineralization was at pH 11 in the absence of O
2
. Phenol was detected as an intermediate in the process in both
cases and with no formation of dangerous alkylmercury species. Calomel formation from PMC under nitrogen supported the
two successive one-electron transfer reactions hypothesis, as in the case of inorganic salts. A very interesting application for the
TiO
2
-Hg system is the use of the photocatalytic reduction of mercury in the field of analytical chemistry as a cost-effective, fast,
and sensitive method for mercury determination in environmental and biological samples. Chemical vapor generation (CVg)
is used as a sample introduction method for specific trace element determination using atomic spectrometry [213]. A photocata-
lytic TiO
2
system appears as an attractive alternative to achieve the transformation Hg(II) → Hg(0) without the need for the use
of concentrated chemical reductants. This application of the HP system for Hg(II) reduction has been studied by some authors
in the past few years [213-215]. yin et al. [213] studied Hg vapor generation by direct photocatalysis employing UV-TiO
2
nanoparticles with online atomic fluorescence spectrometry (AfS) determination for the first time. The authors employed
formic acid and sodium formate as the hole scavenger mixture. Han et al. [214] studied a similar system using low molecular
weight alcohols, aldehydes, or carboxylic acids such as glycol, 1,2-propanediol, glycerol, oxalic acid, and malonic acid. The
authors obtained good reduction of Hg(II) with all the tested compounds using only UV light irradiation initially (high-pressure
Hg vapor UV lamp); the addition of TiO
2
to the system led to improved efficiency, with very low detection limits (0.02-
0.04 µg l
−1
). Other authors [215] studied Hg concentration, pH, formic acid concentration, the effect of oxygen, and UV irradia-
tion time for this process. In their work, the authors investigated the determination of low Hg(II) concentrations (3 µg l
−1
) and
compared the photocatalytic activity of unmodified TiO
2
, Ag-TiO
2
, and ZnO for reduction of mercury. They obtained a recovery
in the range of 95-99% when it was catalyzed by nano Ag-TiO
2
in optimized conditions with a detection limit of 0.13 µg l
−1
.
despite the multiple investigations conducted to study the photocatalytic reduction of Hg(II), many of the processes involved
are still unknown due to the complexity of Hg(II) chemistry, which involves various species in solution and in the solid phase.
More fundamental research studies should be undertaken in order to clarify the mechanisms, especially for extremely toxic
organomercurial species. On the other hand, it is important to remember that many mercury species present high tendency toward
volatilization, and the coupling of liquid and gaseous HP setups can be the solution to the problems associated to these issues.
9.8
coNclusioNs
Heterogeneous photocatalytic treatment of metals and metalloids can be a valuable option for the removal of these species from
water, which does not require expensive reagents or equipment. UV lamps are economical and the possibility of using costless
solar light is open, with much more research in the past few years concerning the efficiency of photocatalysts with activity in
the visible range. However, an overview of the literature on the subject for the species described here—arsenic, chromium,
uranium, mercury and lead—indicates that important fundamental and applied research is still missing on several aspects.
The application of HP to arsenic species is very promising taking into account that both oxidative and reductive mechanisms
may lead to less toxic species (As(V) compared with As(III)) or the solid phases (As(0)). Although the oxidative system has
been studied in detail, the reductive pathway is a challenging alternative that should be taken into account and improved.
The mechanistic features of the photocatalytic reduction of chromium (VI) to the less toxic Cr(III) form have been thor-
oughly studied. The impressive synergistic effect of organic donors present simultaneously with Cr(VI) in wastewaters as a
result of different industrial processes reveals that this system is very appropriate to be scaled for real-world applications. In
order to achieve this, research on adequate photoreactor design and industrial innovation is a priority. Cr(VI) photocatalytic
reduction is unique because it is not inhibited by oxygen, at least at acidic pH, and this represents an additional advantage for
implementation.
Research on photocatalytic removal of uranium salts from water is still scarce. Photoreduction is possible only in the
presence of hole scavengers in the absence of oxygen with the formation of uranium oxides; however, rapid reoxidation and
redissolution takes place after exposure to air. These are aspects that should be considered and improved to fully demonstrate
the potential of the technology.
Mechanistic studies on Pb(II) HP are almost complete and ready for real-world applications. The best method is the reduc-
tive indirect pathway, driven by species formed in the presence of alcohols or carboxylates, which, as in the case of Cr(VI), can
be present together with Pb(II) in wastewaters, rendering the technology appropriate for Pb(II) removal.
Mercury (II) photocatalysis also has not been studied much, although it is known that reaction occurs easily through direct
reduction driven by CB electrons. Interesting and encouraging results have been found in the case of the extremely toxic organ-
omercurial species, and further investigation on possible applications deserves attention.
To sum up, fundamental research is still needed in order to improve TiO
2
photocatalysis of arsenic and heavy metal sys-
tems, such as photocatalyst performance, improvement in photon efficiencies, achievement of higher overall rates, and
