Adsorption of carbonate-derived molecules on the surface of carbonate-containing apatites

Abstract

Carbonated apatites synthesized at low temperatures were dried until constant weight at 25 °C. Two 99%¹³C-enriched samples were examined with EPR after X-irradiation. The observed spectra are interpreted in terms of five paramagnetic radicals, i.e. on O^–, two CO^–₃ and two CO^–₂ radicals. As a result of the ¹³C hyperfine coupling, practically no signals were found in the region around g= 2, where EPR signals originating from ¹²C-containing radicals (commonly called ¹²C signals) are to be expected. Afterwards, the apatite powders were spread out in order to ensure a good contact of the apatite surface with the surrounding atmosphere. The samples were again X-irradiated and re-examined with EPR. Strong ¹²C signals were detected and attributed to surface radicals. Two different paramagnetic species were tentatively identified as CO^–₂ and CO^– radicals adsorbed on the apatite surface. The g values for these radicals are g₁= 2.0030, g₂= 2.0015, g₃= 1.9973 for the CO^–₂ radical and g₁= 2.0058, g₂= 2.0041, g₃= 2.0023 for the assumed CO^– radical.