by solar energy, that is in photoassisted processes catalyzed by
Fe complexes (cf.Fig. 1).
Among Fe(III) chelate complexes those with HS are of special
relevance due to abundant occurrence of these ligands in the
nature. They are thermally very stable compounds, whereas
under sunlight undergo photoredox reactions yielding ferrous
species and free HSþradicals (seeSection III.A) (6,132).
A.1. Fenton and photo-Fenton reactions
The most common oxidation reaction induced by iron
complexes is generation of OHradicals from H 2 O 2 by Fe(II) salts
in the so-called Fenton reaction( 133 ):
Fe^2 þþH 2 O 2 !Fe^3 þþOHþOH ð 27 ÞThe reaction can be converted into a photocatalytic process by
photoreduction of Fe(III) complexes; when at least one of the lig-
ands is hydroxide, then the second OH radical is produced
(Eq. 25); such a reaction sequence is known as the photo-Fenton
reaction. It belongs to photoassisted processes because oxidation
of organic compounds requires the consumption of radiation
energy, whereas the iron photocatalyst is regenerated. However,
during this operation instead of molecular oxygen another sacri-
ficial acceptor (H 2 O 2 ) is required. Although hydrogen peroxide
does exist in the environment, its concentration is too low to play
any significant role in the natural photo-Fenton processes. More-
over, any continuous course of the reaction needs continuous
delivery of hydrogen peroxide, and thereby the photo-Fenton role
in natural processes is limited ( 134 ).
Instead, the photo-Fenton reaction is strongly recommended as
one of the most efficient and convenient among the advanced oxi-
dation methods resulting in effective oxidative degradation of
organic pollutants in waste and sewage treatments( 135 ). Kinet-
ics and mechanism of the photo-Fenton processes ( 136 – 141 ), pro-
cess parameters and control ( 142 ), pH influence ( 143 ), effect of
iron source (144,145), and anchored oxygen-donor coordination
to iron ( 146 ), were all studied extensively. The rate of Fe(II) reox-
idation necessary to close the photocatalytic cycle was
investigated as function of naturally occurring ligands, molecular
oxygen and radical scavenger availability, solution pH, and tem-
perature ( 147 ).
The conventional homogeneous photochemical system was pro-
posed to be enhanced by using electro-(photo)-Fenton(148,149)
METAL COMPLEXES AS SOLAR PHOTOCATALYSTS 317