3. Results and discussion
3.1. Effect of UV irradiation on the degradation process of azo
dyes
Solutions of 119 mg L−1 AR88 in 0.05 M Na2SO4 and 127 mg L−1
AY9 in 0.1 M Na2SO4 of pH 3.0 were separately degraded by EF and
SPEF in the recirculation flow plant after adding 0.5 mM Fe2+ to test
the comparative oxidation power of both methods, as well as the
influence of UV irradiation supplied by sunlight on their degradation
processes. In these trials, a current density of 50 mAcm−2 was
imposed to the BDD/ADE cell for 360 min. It was found that the
solution pH remained practically constant, slowly decreasing up to
final values of 2.6–2.7, as expected if acidic by-products are formed
during the destruction of azo dyes.
The above experiments always yielded a fast decolorization of
the starting dye solutions. Fig. 2a exemplifies the decay of the percentage
of color removal calculated by Eq. (7) with electrolysis
time for the AR88 solution at max = 508 nm. As can be seen, overall
decolorization is easily attained in 40 min, at similar rate for
both EF and SPEF treatments. That means that AR88 and its colored
aromatic by-products are rapidly destroyed with BDD(•OH)
formed from reaction (1) and •OH generated from Fenton’s reaction
(2) with little participation of photolytic reaction (5). Similar
results were obtained for the AY9 solution, although in this case,
the decolorization rate at max = 386 nm for both EAOPs was much
lower and the solution became colorless after a longer electrolysis
time of 80 min.
Avery different behavior was observed for the TOC decay of both
dye solutions. As an example, Fig. 2b evidences that SPEF is much
more potent that EF for AR88 mineralization. In the former process,
TOC decreases rapidly to 3.5 mg L−1 (93% TOC removal) in 180 min,
whereupon it undergoes a quite slow decay up to a final value of
1.4 mg L−1 (97% TOC removal). In contrast, TOC is only reduced by
54% after 6 h of EF treatment, although its faster degradation takes
place over the first 60 min and at practically similar rate to that
found for SPEF, i.e., when hydroxyl radicals are the main oxidants
of aromatic pollutants. An analogous trend was obtained by com-