Bone-derived hydroxyapatite: ultrastructure and tuning for controlled dissolution characteristics for a model nanofertilizer

Abstract

Calcium phosphate nanoparticles have been increasingly propounded as an efficient phosphorus (P) fertilizer in crop production. In this study, we used sustainable sourcing of calcium phosphates by recycling bovine bones and modifying the extracted hydroxyapatite using three approaches: calcination and mechano-activation, alkaline hydrolysis, and subcritical water extraction. Hydroxyapatites derived from these treatments were analyzed for ultrastructure, particle size distribution and crystal chemistry to interpret the dissolution characteristics under a flow-through nanofiltration system. While calcination was the most effective treatment for removing organic molecules—with only 0.22 wt% of total organic carbon and 0.03 wt% nitrogen remaining on hydroxyapatite—it led to the largest growth in crystallite size (113–139 nm) compared to hydroxyapatites treated through alkaline hydrolysis (45–93 nm) or subcritical water (33–78 nm). Surprisingly, the mechano-activated hydroxyapatite following the calcination exhibited a sustained and high P release profile, driven by the presence of ultrafine (10–35 nm) amorphous particles mixed with well-ordered and structurally defective apatitic structures. Hydroxyapatites recovered by alkaline hydrolysis and subcritical water treatments exhibited slow and steady P release profiles reflecting fewer structural and surface imperfections compared to mechano-activated hydroxyapatite. These findings demonstrate that processing-induced ultrastructural and compositional changes in bone-derived hydroxyapatite exert a major role in P-release characteristics. Consequently, these products could be tuned for both sufficiency and efficiency of P fertilization, thus generating sustainably sourced nano P fertilizer for crop production.