V. Photooxidation of Organic Pollutants by Transition Metal

Complexes in Hydrosphere and Soils

One of the most significant stimulus of photocatalytic processes
in water basins and soils are transition metal ions. Although abun-
dance of naturally occurring transition metals is in general less
than 100 ppm and concentrations of their water soluble forms are
even several orders lower, their presence has relevant effects on
the environmental processes and compositions. The important
prerequisites are readiness to decrease their oxidation numbers
in photoredox reactions and the possibility to generate higher oxi-
dation numbers in contact with molecular oxygen. As a conse-
quence, transition metal ions present in the aquatic
compartments are involved in photochemical cycles that play an
important role in the degradation of various pollutants.

A. IRONCOMPLEXES

Especially important for the cleaning of aquatic systems are
water-soluble iron(III) complexes, although in colloidal systems
Fe(III) species can also be light-activated ( 76 ). One of the
controlling parameters is the nature of the ligand ( 131 ). A special
role is played by hydroxo complexes, but ligands forming chelate
complexes are also of relevance, among them HS. The OH
ligand
undergoes photooxidation at the expense of the Fe(III) center:

FeIIIðÞOHL 5

hi

þH 2 O
!

hvðÞLMCT

FeIIðÞH 2 OL 5

hi

þOH ð 25 Þ

and OH radicals that belong to the strongest environmental
oxidants are responsible for most of the degradative oxidations
of organic pollutants. Alternatively, organic ligands coordinated
to Fe(III) are oxidized directly by the excited center:

FeIIIðÞLL 3

hi

!

hvðÞLMCT

FeIIðÞLL 2 ðÞH 2 O 2

hi

þLL ð 26 Þ

Typically sunlight absorption can excite Fe(III) complexes to
ligand-to-metal charge transfer (LMCT) excited states, which
decay via photoinduced electron transfer (PET) to the Fe-center
from the inner (ligand) or external electron donor. The photo-
chemically generated Fe(II) species is then reoxidized to the ini-
tial Fe(III) compound or its derivative (e.g., aqua complex)
closing the photocatalytic cycle. As result many environmental
pollutants are oxidized by molecular oxygen in reactions driven

316 ZOFIA STASICKA