Environmental Engineering Reference
In-Depth Information
9
New AdvANces iN HeterogeNeous
PHotocAtAlysis for treAtmeNt
of toxic metAls ANd ArseNic
Marta I. Litter
1,2,3
and Natalia Quici
1,2
1
Gerencia Química, Comisión Nacional de Energía Atómica, Buenos Aires, Argentina
2
Consejo Nacional de Investigaciones Científicas y Técnicas, Buenos Aires, Argentina
3
Instituto de Investigación e Ingeniería Ambiental, Universidad de Gral. San Martín, Buenos Aires, Argentina
9.1
iNtroductioN
Conventional methods for water treatment such as biological methods, ozonation, and activated carbon are not completely
suitable to reduce the concentration of certain pollutants to regulation levels, especially in the case of resilient compounds;
furthermore, even when successful, these processes can be slow and expensive [1, 2]. A relatively new alternative to traditional
water treatments are the so-called advanced oxidation or reductive processes (AOPs/ARPs), which appear to be attractive
options for the removal of difficult contaminants. These processes are based on the formation of very active oxidative and
reductive species, mainly the hydroxyl radical, HO
∙
[3-9]. The most investigated technologies from the fundamental point of
view and their application in real systems have been AOPs. However, some pollutants can be transformed only via a reductive
pathway, for which ARPs can be good choices [3-11].
One of the most extensively investigated AOPs/AORs is heterogeneous photocatalysis (HP), where a broadband semicon-
ductor (SC) is irradiated by ultraviolet (UV) or visible light leading to oxidation-reduction reactions and to the transformation
of pollutants present in aqueous or gaseous systems. HP leads to the eventual mineralization of organic contaminants and to the
transformation of metallic ions and metalloids to less toxic species or species easier to be separated from the system in a
subsequent treatment step [4, 8, 12].
The main aspect that will be covered in this chapter is the utilization of reductive and oxidative HP for the treatment of heavy
metals (hexavalent chromium, uranyl, mercury (II) and lead (II)) and arsenic (As(III)/(V)), all of them known for their high
toxicity and environmental damage.
9.2
BAsic PriNciPles of HP
In HP, the SC is excited in a primary process by absorption of photons of energy equal to or higher than the SC band gap (
E
g
),
leading to the promotion of an electron from the valence band (VB) to the conduction band (CB), with the creation of a hole:
−
+
SC heh
CB
+→+
ν
(9.1)
VB
