In water treatment, coagulation is an essential process for the
removal of fine particulate matters [24]. Metal salt coagulant of
polyaluminum chloride (PAC), aluminum sulfate (alum) and ferric
chloride (FC) are commonly used in this process.They are widely
applied for water treatment due to their large coagulation activity.
Coagulation could further reduce DBPs precursors in the raw water
supply [25]. Many studies have reported that coagulation–
precipitation process can be applied in removing HA. Unfortunately,
they tend to overlook the effects of the metal ions
associated with HA. At natural pH of water, HA is positively
charged by the presence of metal ions such as Ca2+ and Mg2+ via ion
exchange reaction between these cations and the hydrogen ions
associated with the carboxyl and phenolic hydroxyl group in HA
[26]. It is most likely that positively charged HA makes coagulation
harder because the metal–salt coagulants also are positively
charged whereby the removal of HA becomes suppressed