FTIR Study of Adsorption of CO₂ on Nonstoichiometric Calcium Hydroxyapatite

The surfaces of calcium hydroxyapatite (CaHAP) particles with different Ca/P atomic ratios, synthesized by the precipitation method, were characterized and the adsorption mechanism of CO2 on CaHAP at 25 °C was elucidated mainly by FTIR. The number of surface P−OH groups was maximum at Ca/P = 1.6 while the quantity of irreversibly adsorbed CO2 was minimum at the same Ca/P. This indicates that the surface P−OH groups not only are the reversible adsorption sites for CO2 but also interfere with the irreversible adsorption of CO2. IR bands due to H2O and CO32- appeared on CO2 adsorption and were intensified by increasing the adsorbed amount. The physisorbed CO2 was converted into CO32- ions with the course of time. These results demonstrated that the irreversible adsorption of CO2 progresses via the reaction of CO2 with surface OH-: 2OH- + CO2 → CO32- + H2O.