Chemistry Reference
In-Depth Information
a detailed analysis of the best pH conditions should include not only the initial substrate, but also the rest of
the compounds produced during the process. Moreover, pH also influences the concentration of the active
•OH species; their formation is favoured at higher pH values, being relevant for pH > 9 [5].
19.5.3 Catalyst concentration
Whether in static, slurry or dynamic flow photoreactors, the initial reaction rates were found to be directly
proportional to the catalyst mass. This indicates a truly heterogeneous catalytic regime. However, above a
certain value, the reaction rate levels off and becomes independent of catalyst mass. When catalyst
concentration is very high, after travelling a certain distance on an optical path, turbidity impedes further
penetration of light in the reactor; moreover, under these conditions the light scattering is very intense. An
optimum catalyst mass has thus to be found in order to avoid excess catalyst and ensure total absorption of
efficient photons: typically not more than around 2 g l
−1
. Moreover, consider that the minimum catalyst/
substrate ratio reported in most studies is around 10, thus a preliminary dilution of the wastes to be treated
could be necessary when the pollutant concentration is higher than around 200 mg l
−1
, otherwise the
degradation could be slow.
19.5.4 Degradation kinetics
It was found in most photocatalytic experiments that the primary degradation follows a pseudo-first order
kinetic law, according to the equation:
-dC
/dt
=
k
C
sub
obs
sub
where C
sub
is the substrate concentration and k
obs
is the observed rate constant.
The decrease of k
obs
by increasing the substrate concentration can be explained by assuming competition
between intermediates and substrate for the semiconductor active sites. The k
obs
values show an inverse
dependence on the initial substrate concentration and can fit in the Langmuir-Hinshelwood equation:
(
)
−
1
k
−
1
=
k Kk
+
−
1
C
obs
c
ads
c
0
where C
0
is the initial substrate concentration, K
ads
is the adsorption equilibrium constant of the substrate and
k
c
is the product of the second order rate constant (k') by
θ
rad
(
θ
rad
represents the coverage of the sites by the
reactive radicals) [15].
Previous detailed studies have demonstrated the influence of adsorption of substrates on the surface of TiO
2
particles, with a corresponding facilitated attack of such molecules from the active radical species present.
19.6
Additives reducing the e
−
/h
+
recombination
In order to increase the degradation rate it is possible to add reagents capable to trap the e
−
CB
, delaying the
electron-hole recombination and to favour the formation of active oxidizing species. Among them,
peroxydisulfates can offer beneficial effects since they provide both the electron scavenging and the
introduction of a new active radical (SO
4
•
−
) capable to originate
•
OH and to directly oxidize the organic
substrates [16]. The following reactions describe the formation of the radical active species:
S
2
O
8
=
+
e
−
CB
→
SO
4
=
+
SO
4
•
−
SO
4
•
−
+
H
2
O
→
SO
4
=
+
•
OH
+
H
+
