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 CO3 2appeared on CO2 adsorption and were intensified by increasing the adsorbed amount. The physisorbed CO2 was converted into CO3 2ions 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 f CO3 2-+ H2O.

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