The role of ammonium citrate washing on the characteristics of mechanochemical-hydrothermal derived magnesium-containing apatites

The role of citrate washing on the physical and chemical characteristics of magnesium-substituted apatites (HAMgs) was performed. HAMgs were synthesized by a mechanochemical-hydrothermal route at room temperature in as little as 1 h, which is five times faster than our previous work. Magnesium-substituted apatites had concentrations as high as 17.6 wt% Mg with a corresponding specific surface area (SSA) of 216 m²/g. A systematic study was performed to examine the influence of increasing magnesium content on the physical and chemical characteristics of the reaction products. As the magnesium content increased from 0 to 17.6 wt%, magnesium-doped apatite crystallite size decreased from 12 to 8.8 nm. The Mg/(Mg + Ca) ratio in the product was enriched relative to that used for the reacting precursor solution. During mechanochemical- hydrothermal reaction, magnesium doped apatites co-crystallize with magnesium hydroxide. Citrate washing serves to remove the magnesium hydroxide phase. The concomitant increase in surface area results because of the removal of this phase. Possible mechanisms for magnesium hydroxide leaching are discussed to explain the measured trends.