Concentration-Dependent
Photoredox Conversion of As(III)/As(V)
on Illuminated Titanium Dioxide Electrodes
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Abstract
The photoconversion of As(III) (arsenite) and As(V) (arsenate)
over a mesoporous TiO<sub>2</sub> electrode was investigated in a
photoelectrochemical (PEC) cell for a wide range of concentrations
(μM–mM), under nonbiased (open-circuit potential measurements)
and biased (short-circuit current measurements) conditions. Not only
As(III) can be oxidized, but also As(V) can be reduced in the anoxic
condition under UV irradiation. However, the reversible nature of
As(III)/As(V) photoconversion was not observed in the normal air-equilibrated
condition because the dissolved O<sub>2</sub> is far more efficient
as an electron acceptor than As(V). Although As(III) should be oxidized
by holes, its presence did not increase the photooxidation current
in a monotonous way: the photocurrent was reduced by the presence
of As(III) in the micromolar range but enhanced in the millimolar
range. This abnormal concentration-dependent behavior is related with
the fate of the intermediate As(IV) species which can be either oxidized
or reduced depending on the experimental conditions, combined with
surface deactivation for the water photooxidation process. The lowering
of the photooxidation current in the presence of micromolar As(III)
is ascribed to the role of As(IV) as a charge recombination center.
Being an electron acceptor, the addition of As(V) consistently lowers
the photocurrent in the entire concentration range. A global concentration-dependent
mechanism is proposed accounting for all the PEC results and its relation
with the photocatalytic oxidation mechanism is discussed