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.