Conversion of waste animal bones into porous hydroxyapatite by alkaline treatment: effect of the impregnation ratio and investigation of the activation mechanism

Microporous biochars, mainly composed by hydroxyapatite (HAp), were prepared from animal waste bones. In this study, a three-step procedure including pre-pyrolysis, chemical treatment with NaOH, KOH and K2CO3, and pyrolysis was investigated. The effects of the activation agent and its concentration were analysed by N-2 adsorption-desorption, SEM, EDX, FTIR, and XRD techniques. The activation mechanism was investigated by TG-MS. FTIR and XRD data confirm that the obtained biochars were mainly composed of HAp. K2CO3 was the most effective with porosity being increased by 30 % (up to 234 m(2)/g) compared to non-alkali-treated sample. New pores were generated mainly in the microporous range. Analysis of the pyrolysis gases by MS revealed that the main effect of the alkali treatment is the incorporation of OH- ions, which then react with bone matrix to generate porosity. Other gas-phase reactions, such as reverse WGS and methanation reactions, promoted by K2CO3, may be involved in the activation process. In contrast, KOH caused little modification of the HAp structure. Statistical analysis supported the relationship among the release of certain compounds during pyrolysis and the textural properties of the final material.