The effect of Ca2+ and CO3
2 on HA removal was examined by
the addition of CaCl2 and NaHCO3 solution and the results are
described in Fig. 4. The presence of Ca2+ and bentonite as a
coagulant aid has contributed towards the higher removal of HA
and these trends can also be seen in the presence of Ca2+, CO3
2 and
bentonite. On the other hand, the HA removal was lower than the
control experiments, when CO3
2 and bentonite were added to the
HA solution. Other researchers have also reported on the similar
results regarding the presence of turbidity-contributing materials
(kaolin) and the interaction or association of HA with Mg2+ and Ca2
+ ions which can enhance the removal of HA by PAC through
coagulation process [28,36,37]. This is possible because Ca2+ ions
are able to block the negatively charged groups of the surface
molecules and also act as bridges between functional groups of the
two adjacent molecules. It has also been reported that Ca2+
compresses the electrical double layer (EDL) of both the PAC
surface and the HA colloid particles [38]. This reaction proceeds to
compress the EDL of both HA and bentonite particles. Thus, the
impact of electrostatic repulsion between the negatively charged
bentonite particles and HA molecules may be reduced.