These results are in agreement with the Pourbaix diagram for
zinc material [31]. When the pH medium of the effluent is acidic,
the dissolution of the electrode may occur to the solution in the
form of Zn2+, that subsequently reacts with the hydroxyl anion
formed in the cathode and precipitates in the form of hydroxides,
Zn(OH)2. The possible reactions that may occur when zinc is used
as anode are presented in Eqs. 1–4.
At the anode Zn ! Zn2þ þ 2e ð1Þ
At the cathode 2H2O þ 2e ! H2 þ 2OH ð2Þ
In the solution Zn2þ þ 2OH ! ZnðOHÞ2 ð3Þ
Global reaction Zn þ 2H2O ! H2 þ ZnðOHÞ2 ð4Þ
This process promotes the coagulation with the aggregation of
the pollutants, reducing the organic load and the concentration
of pollutants in the liquid phase [21].
For pH 7 and 9, the initial phase corresponds to the lower dissolution
of the electrode with Zn2+ formation. For this reason, the
process of formation of hydroxide flocs in solution to aggregate
pollutants is reduced, leading to a low efficacy of the electrocoagulation
process in what regards COD.
Fig. 1c depicts the pH medium for all experiments followed over
time. As can be observed, the pH increased for all tests. This
increase may be explained by the reactions occurring at the cathode:
during the process, at this electrode, water molecules receive
electrons and dissociate into hydrogen bubbles and hydroxyl ions,
causing the pH medium increase. During the process with pH 3.2,
the solution rose to neutral pH, which is in the ideal range, not
needing pH adjustment, to be directly discharged or even to be
used in biological processes.
In order to verify the mechanisms that exist in this ECG process,
the ratio TPh/COD is illustrated in Fig. 2. As can be seen, this ratio is
not constant over time, suggesting that the precipitation and
adsorption of pollutants are not the only phenomena present once,
if that would be the case, TPh and COD removal should follow the
same trend. In fact, one can infer that it may also occur the oxidation
of organic compounds into intermediates which are accounted
for as COD but no longer have phenolic character.
3.