Environmental Engineering Reference
In-Depth Information
electron acceptor and HO
−
and H
2
O are available as electron donors to yield hydroxyl radicals. It is well documented that
these trapping reactions occur in a lapse time shorter than 30 ps [22]
.
Considering the importance of mass transference in the process, initial practical approaches for a quantitative description of
HP kinetics have been commonly carried out using a Langmuir-Hinshelwood (L-H) kinetics model [23]
.
This mathematical
model assumes that the reaction occurs on the catalyst surface. According to the L-H model, the reaction rate (
r
) is proportional
to the fraction of particle surface covered by the pollutant (
θ
x
). Mathematically,
dC
dt
kKC
KC KC
r
=− ==
++
k
θ
r
(2.2)
rx
1
ss
where
k
r
is the reaction rate constant,
K
is the pollutant adsorption constant,
C
is the pollutant concentration at any time,
K
s
is
the solvent adsorption constant, and
C
s
is the concentration. During the 1980s, many authors presented their data using the L-H
kinetic approach. Nevertheless, despite fitting well with the experimental data, the L-H approach does not consider the role of
the radiation field on the mechanism [24, 25]
.
Other kinetic studies on HP suggest that the reaction rate increases with catalyst concentration to get a maximum value
concentration depending on the compound and the reactor used. for these concentrations, the reaction rate remains
unchanged or decreases with further increments of catalyst concentration [25]
.
An interesting problem is the relation
between catalyst concentration, reaction rate, radiation absorption, and process improvement. Considering this, many
different models have been proposed. studies have suggested relationships aiming to estimate the radiation absorbed by the
catalyst [24, 25]
.
from these results, several models, most of them based on complex mathematical or statistical computational
approaches, have been developed. These models are able to predict radiation absorption and scattering as a function of
catalyst concentration, optical path and catalyst type, and its relation to pseudokinetic constants obtained experimentally
[24-26]
.
Based on the radiation absorbed by the catalyst, Alfano's group as well as other authors have focused on the a
priori design of photochemical reactors, the improvement of HP reactions, and the generation of intrinsic reaction kinetic
that may lead to process scaling-up [27-29]
.
Besides reactor design, heterogeneous PC degradation reaction can be enhanced by the use of higher active catalyst or inor-
ganic oxidizing species. In the first case, activation of TiO
2
under visible light is a desirable technological approach. In order to
utilize visible light for TiO
2
excitation, dye-sensitized and ion-doped TiO
2
has been developed in recent years and has yielded
promising results for the photocatalyzed degradation of different substrates [30, 31]
.
2.4
visible light absorbiNg seMicoNductors
According to reaction sequence (2.1), the production of charge carriers is a fundamental step in the degradation processes using
HP. Once generated, these species may lead to hydroxyl radical generation (and the subsequent organic matter degradation)
or can recombine to generate the initial state and energy emission. This latter reaction, known as recombination, is a practical
problem when using a TiO
2
catalyst, and it is extremely efficient (reaction rate = 10
-9
s) when no proper electron acceptor is
present in the reaction media [32]
.
This side process is energy-wasting and limits the achievement of high quantum yields (i.e.,
number of primary chemical reactions per photon absorbed). In most of the cases, dissolved oxygen is used as the electron
scavenger, and several works have dealt with its efficiency as an oxidant agent to complete organic matter mineralization [33]
.
Nevertheless, it has been demonstrated that only low mineralization is reached when dissolved oxygen is used as an oxidant
agent [34]
.
The recombination of charge carriers seriously affects the actual photonic yield of the photocatalyst; however, this is not the
only variable related with the effectiveness of the process. One of the main disadvantages in its use is related to the wavelength
required for activating TiO
2
. The HP process using titanium dioxide occurs only when it is irradiated with ultraviolet
(UV,
λ
< 400 nm) radiation and photon energy is absorbed by the crystal structure of the semiconductor, transferring electrons
from the valence to the conducting band. This specific characteristic limits the photocatalyst sensitivity when solar radiation is
intended to be used since only a small part of the solar spectrum (about 5%) falls within the UV radiation [2, 35]. In order to
avoid this limitation, several modifications have been attempted to the TiO
2
PC structure in order to make it active under visible
spectral irradiation, improve its photosensitivity and quantum yield, as well as reduce its band-gap energy requirements for PC
activation. These modifications follow different approaches: (i) dye sensitization, (ii) TiO
2
coupling with other semiconducting
materials with appropriate band-gap energy, (iii) surface deposition of metal clusters, and (iv) doping the crystal lattice with
metallic and nonmetallic foreign atoms; see figure 2.1 [36, 37]
.
