The amount of adsorption could be strongly influenced by the pH
of solution and the electrostatic interactions play a role in
controlling the amount of HA adsorption [39]. At neutral pH,
the CaCO3 particles are neutral or slightly positively charged. Thus,
a high adsorption affinity of the negatively charged HA was
observed. With increasing the pH, the CaCO3 particles become
negatively charged and electrostatic interactions between the
CaCO3 particles and HA were repulsive, and subsequently,
adsorption was low. A study conducted by Petrovic et al. [40]
also observed the decrease of HA adsorption onto CaCO3 with
increasing pH.