Production of nano hydroxyapatite and Mg-Whitlockite from biowaste-derived products via continuous flow hydrothermal synthesis : a step towards circular economy

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Nigar, Farah and Johnston, Amy-Louise and Smith, Jacob and Oakley, William and Islam, Md Towhidul and Felfel, Reda and Grant, David and Lester, Edward and Ahmed, Ifty (2023) Production of nano hydroxyapatite and Mg-Whitlockite from biowaste-derived products via continuous flow hydrothermal synthesis : a step towards circular economy. Materials, 16 (6). 2138. ISSN 1996-1944 (https://doi.org/10.3390/ma16062138)

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Abstract

Biowastes from agriculture, sewage, household wastes, and industries comprise promising resources to produce biomaterials while reducing adverse environmental effects. This study focused on utilising waste-derived materials (i.e., eggshells as a calcium source, struvite as a phosphate source, and CH3COOH as dissolution media) to produce value-added products (i.e., calcium phosphates (CaPs) derived from biomaterials) using a continuous flow hydrothermal synthesis route. The prepared materials were characterised via XRD, FEG-SEM, EDX, FTIR, and TEM analysis. Magnesium whitlockite (Mg-WH) and hydroxyapatite (HA) were produced by single-phase or biphasic CaPs by reacting struvite with either calcium nitrate tetrahydrate or an eggshell solution at 200 °C and 350 °C. Rhombohedral-shaped Mg-WH (23–720 nm) along with tube (50–290 nm diameter, 20–71 nm thickness) and/or ellipsoidal morphologies of HA (273–522 nm width) were observed at 350 °C using HNO3 or CH3COOH to prepare the eggshell and struvite solutions, and NH4OH was used as the pH buffer. The Ca/P (atomic%) ratios obtained ranged between 1.3 and 1.7, indicating the formation of Mg-WH and HA. This study showed that eggshells and struvite usage, along with CH3COOH, are promising resources as potential sustainable precursors and dissolution media, respectively, to produce CaPs with varying morphologies.

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