The effect of the current density (ratio current intensity/anodic
area) was analysed between 120 and 500 A/m2 and the studies
were performed at pH 3.2, with a distance between electrodes of
1.0 cm and NaCl concentration of 1.5 g/L. Fig. 3a displays the
removal of phenolic content over time. As can be observed, there
is a significant decrease of TPh (84.0–87.3% of removal) during
the first 30 min of reaction with 250 and 500 A/m2
. In the case of
the experiment with 120 A/m2 more time (120 min) was needed
to achieve a similar removal value. Regarding COD decrease
(Fig. 3b), the test with 500 A/m2 has a sharply depuration of
38.5% during the first 15 min, followed by the experiment with
250 A/m2 (15–30 min), whereas the assay using 120 A/m2 continues
to be the one that takes longer time (60 min) to get removals
similar to the other experiments; this because at the beginning
of the process the COD depletion is practically null. The current
density applied to the system determines the amount of ions
released as Zn2+ and therefore the amount of the resulting coagulant.
Thus, the higher the amount of dissolved Zn2+ ions in solution
the greater is the rate of Zn(OH)2 formation and consequently a
higher COD and TPh removal efficiency is achieved [21,32].
Additionally, the increase of current density promotes the generation
of hydrogen bubbles and decreases its size, resulting in an
intensification of the upward flow and a superior removal of pollutants
by flotation. Consequently, it seems to be advantageous to
use high current densities which enable high electrocoagulation
efficiencies in short running times. However, at the end of
180 min of treatment time, the experiment with 500 A/m2 led to
the worst COD removal value. This might be due to the fact that
higher current values lead to a high turbulence in the system and
consequently the particles responsible for coagulation do not have
enough time to agglomerate themselves and remove the pollutants.
Important factors such as energy cost and durability of the
electrodes must be taken into account in the process optimisation.
Therefore, the current density of 250 A/m2 was chosen to continue
the experiments, because after 30 min the highest COD removal
(51%) was attained and if the higher current density would be used,
it would cause higher consumption of the anode material.
It was still observed that using different density current conditions
no significant effect over pH evolution was detected and the
final value for all cases was neutral (Fig. 3c). However, for the initial
times the quickest experiment was for 500 A/m2 (5 min), followed
by 250 (10 min) and 120 A/m2 (30 min).