An experimental and theoretical investigation of the formation of hydroxyapatite particles prepared by an electrochemical method

Electrochemical synthesis of hydroxyapatite particles was performed galvanostatically from a homogeneous solution of Na₂H₂EDTA·2H₂O, KH₂PO₄ and CaCl₂. The electrogeneration of OH^- ions by water reduction at the cathode plays an important role in the formation of hydroxyapatite by an electrochemical method. The OH^- ions induce the liberation of Ca²⁺ ions and the dissociation of phosphoric acid, which serve as the reactants for the formation of hydroxyapatite. We found that the final product obtained, i.e. brushite or hydroxyapatite (HA), corresponds to the pH of the solution. Brushite is formed when the pH is <7, and HA is formed when the pH >7. Aging the suspension for 72 h at 40°C transforms brushite to HA even at pH <7. The presence of Ca²⁺ during aging accelerates brushite conversion. EDTA serves as a chemical agent to mediate particle nucleation and growth. The reaction mechanism and its kinetic model proposed for the HA formation could predict the experimental results well.