Wako Pure Chemical Industries,
Ltd. (Osaka, Japan). All reagents used in the research are of
analytical grade. All solutions were prepared by distilled water.
A stock HA solution (1 g/L) was prepared by dissolving a
measured amount of HA powder in 0.1 mol/L NaOH solution by
high shear mixing under the nitrogen atmosphere for several hours
until the HA powder dissolved completely. The solution pH was
adjusted to 7.0 by 0.1 mol/L HCl. The working HA solution was
prepared by diluting a calculated amount of stock solution with
distilled water and the concentration of HA was 50 mg/L. In
aggregation experiment, metal–HA complex solution was used.
The metal–HA complex solution was prepared by mixing 1 mL of
150 mg/L CaCl2, 1 mL of 150 mg/L MgCl2 and 5 mL of 1 g/L HA with
93 mL deionized water. The pH of this solution was adjusted to
7.0 with 0.1 mol/L HCl or 0.1 mol/L NaOH and shook for 2 h at room
temperature.