algae removal could be achieved for all initial pH values
with an electrolysis time of 60 minutes. It could be
concluded that increasing reaction time results in an
increasing amount of aluminum hydroxide flocs for removal
of algae, since hydroxide ions are produced along with H2
bubble and pH of treated water increases also (Fig. 7).
In acidic and neutral regions, hydrolysis reactions cause
aluminum ions to be hydrolized in the solution and produce
several aluminum species; for instance, monomeric species
(Al(OH)2
+
and Al(OH)2+) and polymeric species such as
Al13(OH)345+ and Al13O4(OH)247+. All of these would finally
transform into insoluble amorphous (Al(OH)3) through
complex polymerization/precipitation kinetics [18,24].
Time (min)
0 10 20 30 40 50 60
Algae removal efficiency (%)
0
20
40
60
80
100
Initial pH 6
Initial pH 7
Initial pH 8
Figure 6. Algae removal efficiency as a function of electrolysis time with
different initial pH (CD: 20 A/m2
).
There are 2 general mechanisms of algae destabilization.
Firstly, cationic hydrolysis products and positively charged
precipitates may strongly adsorb on negatively charged
algae leading to charge neutralization. Secondly, the
precipitation may occur either on the surface of algae or the
precipitates formed in the bulk may attach to the algae
(Sweep flocculation and enmeshment).
In the alkaline region, the aluminate anions (Al(OH)4-)
become the predominant species [25] which cannot be
adsorbed on the negatively charge algae, cause the declining
removal efficiency. To gain similar efficiency as acidic and
neutral cases, longer electrolysis time would be needed due
to increase of aluminum concentration required in order to
accomplish the sweep flocculation and enmeshment as the
main mechanism. From the experiment, it could be
considered that the result was similar to [15]