Civil Engineering Reference
In-Depth Information
13.5.1 Air depollution
Investigations on photocatalytic activity of cement pastes, mortars and con-
crete containing TiO
2
nanopowders have been performed in most cases by
fl owthrough methods. Nitrogen monoxide, or directly a NO
x
(NO
NO
2
)
mixture, are used as polluting source with typical concentration of approxi-
mately 1 ppmv (1 part per million in volume), and NO and NO
2
concentra-
tions are compared in the inlet and outlet gas fl ow; a chemiluminescent NO
x
analyser is used as measuring unit. Tests involve a fi rst phase of gas fl ow in
the absence of any irradiation, to reach an equilibrium of air composition
in the reactor chamber and of the NO absorption in the cementitious mate-
rial itself, which clearly must not be considered in the calculation of NO
degradation. Afterwards, a UV light source (or solar spectrum lamp) is
switched on and the outlet gas composition sampled at predefi ned time
intervals: NO concentration usually drops immediately by some percent,
and fi nally goes back to its initial value when the lamp is switched off and
the test interrupted.
In NO
x
degradation tests, it is important to keep in mind that the fi rst
chemical reaction taking place at the TiO
2
surface is the reduction of NO
to NO
2.
Therefore, after a rapid decrease of NO
x
concentration, a slight
increase in its value can be observed in the steady state of reaction, since
the drop of NO concentration is accompanied by the formation of NO
2
.
Finally, NO
2
is reduced as well, by the following reactions:
NO
+
+
+
OH
i
→+
NO
H
[13.1]
2
NO
+
OH
→ +
NO
−
H
+
[13.2]
i
2
3
Degradation effi ciencies of laboratory specimens range from approxi-
mately 50-60% in fl ow conditions to the total degradation of pollutants in
batch conditions, depending on gas concentration, fl ux and irradiation time.
Humidity was observed in many cases to slightly reduce photocatalytic
activity when above a certain threshold, as the adsorption of water mole-
cules on the surface of TiO
2
nanoparticles is competitive with respect to
pollutant adsorption and consequent degradation (Hüsken
et al.
, 2009).
The result of the reaction chain of NO
x
degradation has drawn much
attention and some concern, since the fi nal reaction step involves the dis-
solution of nitrate ions in rain to form nitric acid, and the consequent acidi-
fi cation of rainwater that reaches the sewers. Nonetheless, the concentration
of NO
x
in air is in the order of magnitude of tens of ppb (parts per billion),
and consequently the possible concentration of HNO
3
that could reach
sewers is extremely limited and is not expected to produce any detrimental
effect.
Similar tests are performed with VOCs as polluting source, among which
propanol, butanol, toluene, formaldehyde and acetone are the most diffuse
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