that control the Cu environmental speciation and thus its bio-
availability and toxicity.
The role of chromium compounds (Fig. 4b) is quite special.
Although two photons are needed to excite two different Cr forms
in one cycle, each photoreduction entails an organic pollutant
oxidation, which possibly means higher efficiency and larger
diversification of degraded pollutants. Moreover, the processes
result not only in the degradation of organic compounds but also
assist in decreasing the concentration of chromate(VI), one of the
most dangerous and toxic pollutants.
Beside monometallic cycles, mixed metal systems are also
active in nature. The cooperation between the photocatalytic
cycles of two transition metals can lead to diversification of the
degraded pollutants, but their efficiency can be either higher or
lower compared to that of the single metal cycle.
Photocatalysis of the mixed Cu(II)/Fe(III)/TiO 2 /edta system
was investigated and edta photooxidation was found
accompanied by simultaneous metal cation reduction, the rate
of which was directly correlated with the initial Cu(II) concentra-
tion ( 229 ).
In aspect of its toxicity, any pathway leading to abatement of
chromate(VI) pollution arouse a vivid interest. One of such pat-
hways seems to be created by cooperations between the iron
and chromium photocatalytic cycles, which were reported as
effectively converting chromate(VI) into Cr(III) species. Photo-
chemical coupling reactions between polycarboxylate Fe(III)
complexes and chromate(VI) were studied and strong collabora-
tion between both photocatalysts was demonstrated, which was
significantly affected by the oxygen concentration
(16,17,95,261). On the other hand, chromium(VI) reduction pho-
toinduced by iron(III) nitrilotriacetate accompanied by nta deg-
radation was found to be independent of the O 2 concentration,
whereas the oxidation state of the chromium product depended
on the pH ( 257 ).
In the Fe(III)/Cr(VI)/BPA ternary system (BPA¼bisphenol A,
(CH 3 ) 2 C(C 6 H 4 OH) 2 ), simultaneous photocatalytic reduction of
Cr(VI) and oxidation of bisphenol A was observed in the presence
of Fe(III)OH complexes, and the reaction rate was affected by
Fe(III) and Hþconcentrations ( 262 ).
The study of Cr(VI) photoreduction in the presence of ferric ion
and inorganic anions showed that direct irradiation of acidic
wastewaters containing Cl, NO 3 , and Fe(III) is a feasible strat-
egy for eliminating Cr(VI)( 263 ).
A synergistic photoreduction of Cr(VI) and Cu(II) mediated by
TiO 2 , or photocatalytic reduction of Cr(VI) and oxidation of
332 ZOFIA STASICKA