The synthesis of calcium hydroxyapatite (Ca-HA) starting from calcium carbonate and different orthophosphate sources, including orthophosphoric acid, potassium, sodium and ammonium dihydrogen orthophosphates, was investigated under ambient conditions. The reaction started with calcium carbonate dissolution in an acid medium, followed by rapid precipitation of calcium cations with orthophosphate species to form calcium phosphate based particles which were in the size range of 0.4-1 m. These particles then agglomerated into much larger ones, up to 350 m in diameter (aggregates). These aggregates possessed an unstable porous structure which was responsible for the porosity of the final products. The highest specific surface area and pore volume were obtained with potassium dihydrogen orthophosphate. On the other hand, orthophosphoric acid led to the highest dissolution of calcium carbonate and the complete precipitation of orthophosphate species. Under ambient conditions, calcium phosphate based solid products of low crystallinity were formed. Different intermediates were identified and a reaction pathway proposed.

The synthesis of calcium hydroxyapatite (Ca-HA, chemical formula Ca 10 (PO 4 ) 6 (OH) 2 ) from calcium carbonate and orthophosphoric acid was investigated under atmospheric conditions to determine their suitability as economical starting materials. The order in which reactants were added influenced only the particle size distribution of the solid products. Calcium carbonate particles were rapidly dissolved in acid medium of orthophosphoric acid. The divalent calcium formed was then precipitated with orthophosphate species to form small primary calcium phosphate particles (<1 lm) which partially agglomerated into larger particles with a particle size of up to 300 lm. The dissolution of calcium carbonate was not complete, even after a long reaction time of 72 h and large particles with core-shell structures were formed from residual calcium carbonate particles (core) and calcium phosphate based compounds (shell). Using different characterization techniques, the principal intermediates of the reaction could be identified including mono-calciumphosphate monohydrate (MCPM, Ca(H 2 PO 4 ) 2 ÁH 2 O), dicalcium phosphate dihydrate (DCPD, CaHPO 4 Á2H 2 O), dicalcium phosphate anhydrous (DCPAs) and octacalcium bis(hydrogenphosphate) tetrakis(phosphate) pentahydrate (OCP, Ca 8 (HPO 4 ) 2 (PO 4 ) 4 Á5H 2 O). Their evolution during the synthesis was followed. The final products obtained after 72 h of synthesis contained a mixture of Ca-HA, OCP, B-type calcium carbonate-apatite (CAP, Ca 10 (PO 4 ) 6 CO 3 ) with low crystallinity and some small amounts of intermediates. The removal of Pb 2+ ions, used as a model pollutant, was performed with the calcium phosphate sorbents in powder form or in suspension form. The highest sorption capacity was found at 625 mg g À1 for the sorbent in suspension form which resulted from the addition of calcium carbonate powder to the solution of orthophosphoric acid. From the identification of the composition of the sorbents, the reaction pathway for the removal of lead(II) is proposed.

et al.. Hydroxyapatite starting from calcium carbonate and orthophosphoric acid: synthesis, characterization, and ap-Abstract Hydroxyapatite [Ca 10 (PO 4 ) 6 (OH) 2 ], Ca-HA, is the emblematic mineral phase of bones, and is known for its complexity and difficult to reproduce chemical synthesis. Among the routes developed for obtaining this calcium phosphate, the so-called double-decomposition method is well described and often utilized. However, the Ca-HA synthesized by this way forms a larger mass of ammonium nitrate by-product than the desired product itself. Pure Ca-HA for orthopedic or dental applications usually uses thermal treatment to eliminate residual nitrogen compounds by releasing them in the atmosphere. Contemporary sol-gel methods currently in fashion produce even more degradation products including solvents and precursor organics. We now report on a green synthesis procedure which makes pure Ca-HA with minimum by-product. The synthesis calls for reacting phosphoric acid with calcium carbonate in water suspension to form a Ca-HA gel of fine particles. This gel can be filtered and the solids recovered, dried, and sintered, but can also be used as-is for environmental applications such as heavy metal ions or textile dye removal from polluted waste streams. This green Ca-HA has been used to trap heavy metals in flue gases and in municipal waste water treatment plants. This low-cost and low-environmental impact material can be developed for medical use because of its absence of impurities, and in catalytic productions for remediation of many environmental problems. Recent results show Ca-HA can also serve in reforming biogas compositions into useful products, after deposition of selected metal elements. Some of these results will be communicated in this paper.