Their oxidative degradation starts from PET analogous to that
shown for alcohols (Eq. 67). In the case of anionic electron
donors, electron transfer is assisted by external cations yielding
ion pairs ( 245 ) (Fig. 3).
Chromium(V) species, the main photoreduction products, are
stable only in exceptional cases (as in the polyvinylalcohol matrix
( 256 )). The reduction of chromium(V) to chromium(III) is a far
slower process when chromium is in the presence of strong che-
lating agents such as polycarboxylate derivatives ( 257 ), but in
general, Cr(V) undergoes a series of redox reactions and, in
absence of other redox and coordinating partners, is transformed
in part into a Cr(III) species and in part to Cr(VI), for example:
3Cr Vð Þ!Cr IIIð Þþ2Cr VIðÞ ð 68 ÞD. PHOTOCATALYTICCYCLES
Photochemical and photocatalytic processes are responsible for
self-cleaning of the atmosphere, soils, and water basins. Transi-
tion metal complexes contribute mainly to cleaning of the hydro-
sphere, although their cooperation with atmospheric gases is as
yet underestimated and the progress needs more detailed
investigations.
The photoassisted activity is especially useful for degradation
of strongly hazardous substances or recalcitrant pollutants,
which are resistant to removal in chemical and/or biochemical
processes. Transition metal photocatalysts are able to stimulate
the indirect photodegradation of molecules that resist direct
photolysis. Thus, they can activate species not absorbing sun-
light, or those whose excited states populated by sunlight
absorption are chemically inefficient.
Photocatalysis processes are environmental friendly, heading
for sustainable development because their driving force is sun-
light, their oxidizing agent is atmospheric molecular oxygen
and most photodegradations result in complete mineralization
of the DOM without any pollution by risky side-products.
Among the coordination compounds involved in environmental
photocatalysis, iron, copper, and chromium species belong to the
most effective. The photoassisted cycles driven by most transi-
tion metals need one photon excitation; the exception is the chro-
mium cycle, which needs twofold excitations of the chromium
species to close the cycle (Fig. 4).
330 ZOFIA STASICKA