reactions involving DOM and both ferrous and ferric iron, along
with various reactive oxygen species (superoxides, peroxy
radicals, hydrogen peroxide, hydroxo radicals); a net photooxida-
tion process may indicate the importance of the reactive oxygen
species in the overall observations( 208 – 211 ). Some inorganic
ingredients, such as phosphate or nitrate ions, are also of signifi-
cance in the photodegradation processes (212,213).
The solution pH is of special significance. Most of the processes
are favored by alkaline medium that is connected to the forma-
tion of hydroxo complexes and generation of the hydroxo radicals
(Eq. 25), which are the main oxidizers in photoinduced degrada-
tion of pollutants such as: phenol (C 6 H 5 OH), 2-aminophenol
(NH 2 C 6 H 4 OH), 3-chlorophenol (ClC 6 H 4 OH), cyclohexanol
(C 6 H 11 OH), and nitrobenzene (C 6 H 5 NO 2 )(132,214– 219 ). A simi-
lar mechanism was also suggested in the case of benzoic acid
(C 6 H 5 COOH) removal by homogeneous Fe(III) photocatalysis
( 220 ), although a maximum rate was noted for pH3.5, and at
pH values higher than 5.5 the system showed no reactivity at
all. The results are thus inconsistent with the effect of pH on
the concentration of the [FeIII(OH)(H 2 O) 5 ]^2 þform (pKa¼3.05).
Quite another mechanism was suggested in case of ligands
which form strong complexes with Fe(III), such as mono- and poly-
carboxylates: acetate (CH 3 COO
), citrate (OH)C 3 H 4 (COO) 33
,
malonate (CHCOO) 22
, oxalate (C 2 O 42
), and ethylenediamine-
tetraacetate (OOCCH 2 ) 2 NCH 2 CH 2 N(CH 2 COO) 24
, being common
constituents of environmental compartments. Moreover, these
polycarboxylate complexes undergo rapid photochemical reactions
under sunlight irradiation leading to the formation of oxidative
species. It was reported that light irradiation of Fe(III)-poly-
carboxylate complexes initiates the series of redox reactions by
LMCT excitation followed by producing both Fe(II) and ligand free
radicals.
Thus for Fe(III) acetate the pathway of homogeneous
photocatalytic mineralization of the acetate ligand was proposed
to start from R¼CH 3 radical generation and CO 2 release ( 221 ):

FeIIIðÞRCO 2

hi 2 þ

!

hvðÞLMCT

FeIIþRþCO 2 ð 28 Þ

A somewhat other mechanism was concluded from laser flash
photolysis experiments carried out for chelate Fe(III) complexes,
such as [Fe(edta)(H 2 O)]
and [Fe(edta)(OH)]^2
(16,17,87,95,222).
The edta complexes were suggested to undergo inner-sphere pho-
toreduction induced by LMCT excitation:

METAL COMPLEXES AS SOLAR PHOTOCATALYSTS 319