Environmental Engineering Reference
In-Depth Information
13.3.2.4 Sequential processes
In other recent work (Bandala et al., 2012), the same research group reported the use
of sequential coupled disinfection processes, in this case SDPAF processes followed
by chlorine. In their work they suggested that the high efficiency of ozone-chlorine
sequential disinfection previously reported could be generalized to different reactive
oxygen species (i.e. hydroxyl radicals) which could synergistically enhance the oxida-
tive properties of chlorine, thus improving the inactivation process. If true, it means
that other methods to produce hydroxyl radicals might produce a similar synergis-
tic effect in sequential processes. They assessed the photo-assisted Fenton process
alone under different H
2
O
2
and Fe
2
+
concentrations to test its ability to inactivate
Ascaris suum
eggs. The effect of free chlorine alone was also tested. Using the best
reaction conditions, free chlorine only treatment achieved 83% egg inactivation after
120 min of reaction time, while the sequential photo-assisted Fenton process plus
chlorine treatment achieved over 99% of egg inactivation after 120 kJ L
−
1
(about one
hour of solar radiation). No effect on helminth eggs inactivation was observed with
free chlorine alone after 550 mg min L
−
1
, whereas egg inactivation in the range of 25-
30% was obtained for sequential processes (ENPHOSODIS then chlorine) using only
150 mg min L
−
1
.
13.4 SOLAR HETEROGENOUS PHOTOCATALYSIS
Heterogeneous photocatalytic degradation is usually related with the use of a sta-
ble, solid semiconductor capable of stimulating, under the effect of irradiation, a
redox reaction at the solid/solution interface, while remaining unchanged after many
turnovers of the redox system. When the semiconductor is in contact with a liquid elec-
trolyte solution containing a redox couple, charge transfer occurs across the interface
to balance the potentials of the two phases. An electric field is formed at the surface
of the semiconductor, bending its energetic bands from the bulk of the semiconduc-
tor toward the interface. During the absorption of a photon with appropriate energy
(photo excitation) by the semiconductor, band bending provides the conditions for
carrier separation. The two generated charge carriers should react at the semiconduc-
tor/electrolyte interface with the species in solution and, under steady state conditions;
the amount of charge transferred to the electrolyte must be equal and opposite for
the two types of carriers. When the charge carriers, usually denominated electron/hole
pairs, are generated in the semiconductor the electron moves away from the surface to
the bulk of the semiconductor as the hole migrates towards the surface.
Metal oxides are common semiconductor materials suitable for photocatalytic
purposes. Table 13.4.1 lists some selected semiconductor materials, which have been
used for photocatalytic reactions in the past, along with their band gap energy and
the wavelength range required to activate the catalysts. Among all these possibilities,
TiO
2
is a widely analyzed, low-cost, nontoxic, stable, highly photoreactive, and chem-
ically and biologically inert photocatalyst. Solar driven heterogeneous photocatalysis
(SDHP) using titanium dioxide is the AOP most widely used as an alternative to conven-
tional water decontamination and disinfection as well as air remediation technologies
(Castillo et al., 2011).
Search WWH ::
Custom Search