Cu(I) produced by photoreduction of Cu(II) undergoes rela-
tively fast reoxidation mostly in the reaction with molecular
oxygen:
CuþþO 2 !Cu^2 þþO 2 ð 42 Þwhich makes the Cu(I) disappearance in oxic environments very
fast (in ms). In suboxic rivers, however, the Cu(I) concentration
varies with solar UV-irradiance and can reach even 50% of the
dissolved copper ( 2 ). Chloride ions retard the Cu(I) oxidation
through its stabilization in the CuCl form. Conversely, oxygen
dissolved in water reduces the Cu(I) concentration, and O 2
and H 2 O 2 are produced instead:
O 2
þHþ!HO 2 !H 2 O 2 þO 2 ð 43 ÞThese species play an important role in the copper cycle as they
can behave either as oxidants or as reducing agents. The Cu(I)
oxidation proceeds in two steps: slow (Eq. 44) and fast (Eq. 45).
CuþþH 2 O 2 !Cu^2 þþOHþOH ð 44 ÞCuþþOH!Cu^2 þþOH ð 45 ÞSimilarly, Cu(II) reduction goes through the slow (Eq. 46) and
the fast steps (Eq. 47)
Cu
2 þ
þHO 2 !Cu
þ
þO 2 þHþ ð 46 ÞCu^2 þþO 2 !CuþþO 2 ð 47 ÞDissolved Cu(II) compounds show exceptional susceptibility to
scavenge hydroperoxide (HO 2 ) and superoxide (O 2 ) radicals
( 230 ). Peroxide and superoxide species are thus not only formed
in the copper cycle but they have also an influence on the Cu(I)/
Cu(II) interconversion, which is of importance in natural waters.
The reaction of Cu(I) with H 2 O 2 generating OHradicals (Eq. 44)
resembles that of the iron(II) complexes (Eq. (27), Fenton reaction)
and although it is followed by a fast opposite reaction (Eq. 45) that
consumes the radicals, copper can contribute to the oxidative degra-
dation in the Fenton-type processes (205,232– 235 ). On the other
hand, the photophysical properties of copper(I) bis(phenanthroline)
complexesinducedbyaMLCTexcitationwereinterpretedasforma-
tion of the Cu(I)!Cu(II) excited state ( 236 ).
Photooxidation of pollutants with participation of Cu(II)
complexes may also occur in heterogeneous systems, viz. on
semiconductor surfaces. Since this system requires the presence
324 ZOFIA STASICKA