crucial roles. Mono- or mixed-metal photocatalytic cycles are
stimulated, in which the complex photoreduction is accompanied
by oxidation of ligands and/or external molecules (sacrificial elec-
tron donors), whereas the sacrificial acceptor, closing of the
photocatalytic cycle, is predominantly molecular oxygen. The self-
cleaning mechanisms by photocatalysis and photosensitization
apply also to organic matter included in microorganisms, thus they
can lead as well to the environmental self-disinfection. The review
shows fast progress in our understanding of the natural processes
driven by the solar energy and catalyzed by transition metal
complexes; treating the cycles as anti-photosynthesis is also
discussed.
Keywords:Environmental homogeneous photocatalysis; Photore-
duction of transition metal complexes; NO-carriers;
Photodegradation of organic pollutants; Photocatalytic cycles
stimulated by iron, copper and chromium complexes.
I. IntroductionThe metal reservoirs are lithosphere and deeper Earth shells.
The presence of metal species in the atmosphere and hydro-
sphere, and their migration between the environmental com-
partments, depend predominantly on the nearest surroundings
and especially on ligands present in their direct neighborhood.
Metal oxidation state and ligand nature belong thus to the most
important factors controlling metal transport in the environ-
ment. The best illustration of this relation can be seen in mer-
cury transport from the solid phase (e.g., in the form of HgS or
Hg 2 Cl 2 ) to the hydrosphere (e.g., as HgCl 2 ) and atmosphere
(e.g., as Hg(CH 3 ) 2 or Hg). Other examples can be chromium com-
pounds, which are composed of mobile and water-soluble CrO 42
anions, or sparingly soluble hydroxo and macrocyclic complexes
of Cr(III). A similar difference in solubility is observed between
Fe(III) and Fe(II) compounds.
The role of photochemistry in nature is difficult to overesti-
mate especially when we consider conversion of solar energy into
the chemical energy in natural photosynthesis or ozone produc-
tion, which protects the Earth from hazardous UV radiation.
Much less is known about the role of sunlight in environmental
self-cleaning processes.
The latter processes are stimulated by transition metal com-
pounds and mostly by iron, copper, and chromium complexes,
292 ZOFIA STASICKA