Potassium–carbonate co-substituted hydroxyapatite compositions: maximising the level of carbonate uptake for potential CO2 utilisation options

Abstract

CO₂ utilisation is a rapidly growing area of interest aimed at reducing the magnitude of anthropogenic greenhouse gas emissions. We report the synthesis of potassium–carbonate (K–CO₃) co-substituted hydroxyapatites with potassium and carbonate contents ranging from approximately 0.4–0.9 wt% and 3.4–13.0 wt% respectively via an aqueous precipitation reaction between calcium hydroxide, phosphoric acid and either potassium carbonate or potassium hydrogen–carbonate. The incorporated carbonate is situated on both hydroxyl and phosphate sites. A subsequent heat treatment in dry CO₂ at 600 °C allowed for a K–CO₃ co-substituted apatite containing approximately 16.9 wt% CO₃²⁻ to be prepared, amongst the largest carbonate contents that have been reported for such a material to date. Although this work shows that K–CO₃ co-substituted apatites with high levels of carbonate incorporation can be prepared using simple, room temperature, aqueous precipitation reactions with starting reagents unlikely to pose significant environmental risks, testing of these materials in prospective applications (such as solid fertilisers) is required before they can be considered a viable CO₂ utilisation option. A preliminary assessment of the effect of potassium/carbonate substitution on the solubility of the as-prepared compositions showed that increasing carbonate substitution increased the solubility.