n a larger range of current using Fe electrodes, maintaining a nearly
constant decolorization yield about 90% (Fig. 9a). The comparison of
Edye and μ confirms these results (Fig. 9b): contrary to Al, no minimum
was observed in the range of j investigated, Edye always followed a
parabolic trend and μ was proportional to t with Fe electrodes, as Y
was nearly constant. In Fig. 9b, Edye became similar for Al and Fe when
Y curves achieved a similar plateau value, above 15 mA/cm2
.
Conversely, μ values differed widely: first, because the electrodissolution
of 1 mol Fe requires only 2 mol electrons, while 1 mol Al needs 3
at constant current; then, because the molar mass of Fe is about twice
that of Al. As these differences were only slightly reduced by the
higher Φ values of Al electrodes, Δmexp was far higher at constant j
with iron. The same was expected for the mass of sludge, but it
appears that mS≈2.5Δmexp for iron, whereas mS≈4Δmexp for Al,
which reduces the difference in terms of sludge production.
As a conclusion, Fe electrodes are able to achieve a similar
decolorization yield, but at lower current and also conductivity than
Al electrodes, as complementary runs showed that values close to 90%
yield could be maintained under the same conditions when κ was
about 6 mS/cm. The main disadvantage of iron results from Faraday's
law which induces higher μ and mS values at constant current, but this
can probably be circumvented by working at lower current: the
optimum j value was, indeed, the smallest value investigated in this