SINTERED B-TYPE CARBONATED HYDROXYAPATITES: IMPLICATIONS FOR TEMPERATURE-DRIVEN PROTEIN SELECTIVITY

Abstract: B-type carbonated hydroxyapatite (B-CHA) is widely used as an adsorbent because its labile lattice carbonate and calcium vacancies create surface chemistries that interact strongly—but not uniformly—with different protein classes. Tuning those chemistries by heat treatment offers a route to tailor selectivity without chemical post-modification. To establish how sintering-induced changes in crystallinity, phase composition, surface area and porosity of B-CHA correlate with structural features known to govern selective protein adsorption. Nanometric B-CHA was precipitated via an aqueous route and sintered for 5 h at 0 °C (HAE0), 500 °C (HAS5), 700 °C (HAS7) and 1000 °C (HAS10). Samples were examined by XRD/Rietveld, FTIR, Raman spectroscopy, BET/BJH N₂ sorption, SEM and EDXRF for Ca/P determination. Sintering progressively increased crystallinity from 0 % in HAE0 to 85.6 % in HAS10 and enlarged crystallite size from 12.1 nm to 25.9 nm. Concurrently, octacalcium-phosphate (OCP) residuals fell from 39.7 % (HAS5) to 5 % (HAS10). Specific surface area dropped steeply from 65 m² g⁻¹ (HAE0) to 16, 9 and 3 m² g⁻¹ for HAS5, HAS7 and HAS10, respectively, accompanied by densification and loss of mesopores >25 nm detected in the nitrogen-sorption profiles. Lattice parameter an expanded from 9.391 Å to 9.560 Å while c contracted from 6.878 Å to 6.811 Å, reflecting carbonate depletion and crystal growth along the a-axis. Despite these structural changes, Ca/P ratios remained within 1.51–1.60, confirming overall hydroxyapatite stoichiometry. The data reveal two temperature windows of interest for adsorption design: (i) 500–700 °C—carbonate bands are largely removed, OCP falls >75 %, and SSA remains 9–16 m² g⁻¹. The higher density of exposed Ca²⁺ sites and reduced competing phosphate acidity are expected to favour electrostatic binding of basic proteins while diminishing affinity for acidic proteins, thereby amplifying selectivity. (ii) ≥ 1000 °C—maximal crystallinity and minimal porosity create diffusion-limited surfaces suitable when non-specific adsorption must be suppressed. Simple geometrical considerations (surface Ca density versus area loss) indicate that HAS5 should exhibit the highest theoretical selectivity coefficient; ongoing adsorption isotherms will test this prediction. Sintering enables precise, reagent-free modulation of B-CHA structure. Heating to 500–700 °C balances crystallinity with accessible surface, generating matrices whose physicochemical signatures forecast enhanced protein selectivity. These insights supply quantitative benchmarks for engineering B-CHA adsorbents and motivate future coupling of structural metrics with adsorption kinetics to verify performance in real separation tasks.