of other substances such as TiO 2 or WO 3 , it seems to be more
useful rather for wastewater cleaning from hazardous species
than for environmental self-cleaning( 237 – 242 ).
C. CHROMIUMCOMPOUNDS
Chromium environmental abundance is much lower than that
of iron. Moreover, the chemistry and photochemistry of chro-
mium(III) complexes are significantly distinct from those of
iron(III). The main symptoms are lower stability of Cr(II) species
and lower solubility of Cr(III) complexes. Moreover, chromium
forms compounds in several higher oxidation states, among
which only Cr(VI) species are stable. As a consequence, in the
environment chromium occurs in two oxidation states, that is,
Cr(III) and Cr(VI), which are drastically different in charge,
physicochemical properties, as well as chemical and biochemical
reactivity. The immobile Cr(III) form is one of the trace elements
essential for the proper functioning of living organisms, whereas
mobile Cr(VI) exerts strong toxic effects on biological systems.
Their conversion into each other is possible but needs specific
reaction conditions, such as solution pH, oxygen concentration,
presence of appropriate electron donors, acceptors, and
mediators acting as reagents, ligands, or catalysts(243,244).
These prerequisites do not favor anticipation of any observable
chromium contribution to pollutant photodegradation in the
environment. But yet, cyclic fluctuations of the Cr(VI)/Cr(III)
concentration ratio in seawater, dependent on solar irradiance,
were detected, which is indicative for the detectable photo-
conversion that had to entail an oxidative photodegradation of
DOM ( 243 ). Moreover, the photochemical conversion between
Cr(VI) and Cr(III) was investigated with respect to the possible
existence in the nature of complete photocatalytic cycles func-
tioning in day-and-night intervals. Under conditions mimicking
the natural ones, the model system consisted of [Cr(C 2 O 4 ) 3 ]^3 ,
CrO 42 , and C 2 O 42 , the concentration ratio was oscillating
according to light and dark sequences, suggesting the cyclic
conversions of Cr(III) to Cr(VI) and Cr(VI) to Cr(III) (95,245).
C.1. Cr(III) complexes
Current knowledge of chromium(III) photochemistry relates
mostly to the decay of the metal-centered (MC) excited states( 246 ).
Photosubstitution and photoisomerization are, however, of minor
METAL COMPLEXES AS SOLAR PHOTOCATALYSTS 325