4. Conclusions
It has been demonstrated that the SPEF process in a recirculation
flow plant with a BDD/ADE cell coupled with a solar photoreactor
yields almost overall mineralization of AR88 and AY9 solutions
at pH 3.0. This EAOP is much more effective than the comparative
EF treatment, where poor TOC removal is reached. Both
azo dye solutions are rapidly decolorized at similar rate by EF
and SPEF, indicating that initial compounds and its colored aromatic
products are oxidized by hydroxyl radicals. A mixture of
carboxylic acids is detected as ultimate by-products. Oxalic acid
is accumulated in large extent in EF, being the major component of
final electrolyzed solutions. The quick photolysis of Fe(III)-oxalate
complexes by UV light of solar irradiation explains the higher
oxidation power of SPEF. The incident UV light also photolyzes several
N-derivatives favoring the predominance of NH4
+ over NO3
−
ions, with a large release of volatile N-compounds. In SPEF, the
use of lower current densities is preferable because higher efficiency
and lower specific energy cost are obtained, although longer
time is required for achieving 95% TOC removal. The degradation
rate and MCE values increase up to a catalytic content of
0.5 mM Fe2+, which is optimal for SPEF. When the azo dye concentration
increases, the SPEF process becomes more viable because
of the gradual rise in efficiency and decrease in specific energy
cost. For the higher AY9 concentration tested of 200 mg L−1 TOC
operating with 0.5 mM Fe2+ at 50 mA cm−2, 93% TOC removal is
reached after 360 min of SPEF treatment with 73% efficiency and
a specific energy cost of about 1 kWh g−1 TOC (18 kWh m−3). Our
results indicate that the application of this technology to azo dyes
mineralization is feasible at industrial level for wastewaters containing
high contents of these pollutants and small amounts of
catalytic Fe2+ by supplying low current densities to the BDD/ADE
cell.