Page 196 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Reactive Oxygen Species Generation on Nanoparticulate Material 181

.
produce surface-bound SO3 . The relatively high quantum yields are
.
attributed in part to the desorption of SO3 from the -Fe 2 O 3 surface and
subsequent initiation of a homogeneous aqueous-phase free radical chain
oxidation of S(IV) to S(VI). The following photochemical rate expression
describes the observed kinetics over a broad range of conditions:

2
d[SsIVd] K [HSO 3 ]
s
2 5 fI 0 s1 2 10 2e[a-Fe 2 O 3 ]/ da 2 b (45)
dt 1 1 K [HSO 3 ]
s
where the quantum yield is defined as follows [40, 41]:
[ of molecules reacting via pathway i
f i sld; (46)
total number of photons absorbed by reacting molecule

or

moles of compound transformed
21
f sldsmol einstein d 5 (47)
r
moles of photons absorbed
where
f 5 1 (48)
i
i
A similar kinetic expression [38] was observed for the photocatalytic
oxidation of S(IV) on TiO . In this case, for 385 nm, quantum yields
2
in excess of unity (e.g., 0.5 300) were observed and attributed also
.
to desorption of the SO 3 radical anion from the TiO surface leading
2
to the initiation of homogeneous free radical chain reactions. These chain
reactions have an amplified effect on the measured quantum efficiency.
Depending on the free radical chain length, the measured values can
be greater than one. In addition, the observed quantum yields depend
on the concentration and nature of free radical inhibitors present in the
heterogeneous suspension.
For SO in water, the free radical chain reactions involve the forma-
2
.
.
tion of sulfur radical species such as SO , SO , and SO . that are
5
3
4
alternative forms of ROS with similar reactivity to superoxide and
hydroxyl radicals.
Iron oxides and iron oxide polymorphs initiate the chain reaction as
follows:
hv
2 2
O 1 2 HSO 3 h 2 HSO 4 (49)
2
-Fe 2 O 3
2
2
. FeOH 1 HSO 3 m . FeSO 3 1 H O (50)
2