Environmental Engineering Reference
In-Depth Information
0.16V
0.38V
-0.52V
-1.66V
UO
4
2-
UO
2
+
U
4+
U
3+
U
0.27V
-1.38V
figure 9.5
Latimer diagram connecting the different U species [152].
Reports concerning HP for uranium treatment in water are still scarce. In our previous reviews, a few cases were reported
[154-156] but, due to their importance and limited diffusion, they are included again in this work. Most of the investigations
focus on uranyl ion reduction, with final formation of a deposit of uranium oxide (IV) (UO
2
).
If standard reduction potentials are taken into account (fig. 9.5), U(VI) can be photocatalytically reduced by TiO
2
e
CB
to
−
U(V) and then to U(IV).
In addition, U(V) rapidly disproportionates to U(VI) and U(IV) in the presence of water or hydrolytic compounds [157] with
a very complex chemistry; U(V) is very sensitive to atmospheric oxygen with rapid reoxidation to U(VI) [158].
In a pioneering work, Amadelli [155] reported photoreduction of uranyl solutions (initial concentration between 1.2
and 12.4 mM) on UV-illuminated TiO
2
suspensions and electrodes in the presence of excess of hole scavengers (2-propa-
nol, acetate, or formate); working with TiO
2
suspensions, the final product was uranium oxide of stoichiometry close to
U
3
O
8
found as a dark gray solid on the SC surface. Later, Chen et al. [154] presented results of U(VI) HP photocatalytic
removal from aqueous solutions in the presence of edTA (initial U(VI) concentration of 50 mg l
−1
and variable U(VI):
edTA ratio) and absence of oxygen; the authors remarked on the importance of the presence of adequate donors to sus-
tain appreciable reaction rates. further exposure to air reoxidized and redissolved the uranium species. This method was
proposed for recovering uranyl from wastewaters of nuclear power plants, where chelating agents (normally edTA,
hydroxyethylenediaminetetraacetic acid (HedTA) and oxalic and citric acid) and their uranium (VI) complexes are
usually present.
In another work [159], using methanol as hole scavenger, U(VI) from phosphate-containing waste was photocatalytically
reduced in a 98% from an initial concentration of 40 mg l
−1
. Complexation of uranyl with the hole scavenger was found to play
an essential role in the photoredox process, and, in addition, this indicates the predominance of the indirect reductive pathway
(see Section 9.1) [155]. eliet and Bidoglio [160] studied the U(VI) photocatalytic reduction in TiO
2
aqueous suspensions by
time-resolved laser-induced fluorescence (TRLIf), proposing a reaction mechanism based mainly on the role of adsorbed
uranium species. Later, Selli et al. [156] observed that the U(VI) photoreduction in TiO
2
aqueous suspensions was enhanced
when complexed with HA. The authors proposed a kinetic model considering that the photoreduction takes place in both the
aqueous phase and the photocatalyst solution interface. Boxall et al. [161] explored the use of HP in actinide valence state
control as a possible application of waste minimization in nuclear fuel processing and with the separation of Np, Pu, and U.
Other authors [162] studied the U(VI) photocatalytic reduction on the surface of TiO
2
anatase nanotubes with and without UV
light irradiation. xPS measurements of the catalyst after the experiments showed adsorbed U(VI) and U(IV) species, with a
predominant amount of the reduced form both in the dark and in illuminated experiments; a higher amount of U(IV) was
obtained when light was used.
while mechanistic studies on the photocatalytic removal of uranium-related species are scarce and merit further research,
additional investigation is mandatory to clarify major reaction engineering issues following this approach.
9.6
leAd
Lead has many industrial uses such as in the production of lead acid batteries, solder, alloys, cable sheathing, pigments, rust
inhibitors, ammunition, glazes, and plastic stabilizers. Consequently, lead pollution is mainly anthropogenic, coming from
industrial effluents. Lead has been extensively used in plumbing fittings and as solder in water distribution systems, and lead
pipes still subsist in old structures. Polyvinyl chloride (PVC) pipes now in use also contain lead compounds that can be
leached resulting in high lead concentrations in drinking water. Lead effects on the central nervous system can be serious,
with pregnant women, fetuses, infants, and children up to 6 years of age the most likely to suffer the adverse health effects of
its cumulative poison [163]. A guideline value of 0.01 mg l
−1
is maintained but designated as provisional by the wHO [163].
