which solar photoreduction is of crucial environmental relevance
because it is accompanied by organic pollutant oxidative
degradations. The compounds existing or formed in the environ-
ment are able to remove or at least abate a lot of natural and
anthropogenic pollutants, even those which are nonbiodegrad-
able and persistent.
II. Environmental Matter Under Sunlight ImpactThe composition of environmental compartments undergoes
continuous changes, the main driving force of which is solar radi-
ation. The most spectacular changes are brought about by photo-
synthesis, which is beyond the subject of this chapter. However,
beside photosynthesis a variety of photochemical processes are
observed in nature that results in the removal of many diverse
substances, in particular superfluous, degraded or even harmful
organic compounds.
The reactivity is influenced both by radiation and chemical
factors. In the former case, all parameters controlling sunlight
intensity and spectral range are important, such as intensity of
solar emission, altitude, latitude, season, day-time, etc. The chem-
ical factors originate from the characteristics of the substrate and
its surroundings. The relevant substrate features consist of spec-
tral fitting to the solar light, chemical reactivity at low energetic
excited states and high enough quantum yields. More complex is
the influence of the surroundings; for instance, dispersed matter
in the atmosphere favors photodissociation processes, whereas
the clay layers provide reaction fields which stabilize charge-
separated species generated in the photoredox processes ( 1 ).
In many cases, however, the effect is much more drastic: chro-
mate(VI), for example, in neat aqueous solution can be stored in
glass or quartz bottles without any decomposition induced by
sunlight; when, however, in aqueous milieu, where electron donors,
such as aliphatic alcohol, phenol, and its halogen derivates or
oxalate, appear ( 2 )photoreduction of chromate(VI) is observed,
leading to formation of the corresponding Cr(III) complex.
The photochemical reactions induced by sunlight are of essen-
tial relevance to pollution abatement only when they lead to oxi-
dative degradation of waste organic material. This can be done
either through direct or indirect photoredox reactions. The direct
photodecomposition of organic pollutants under sunlight is not
very common and applies mostly to aromatic compounds. Much
more common is indirect photoreactivity in which solar light is
METAL COMPLEXES AS SOLAR PHOTOCATALYSTS 293