A novel process approach of a circular economy practice in the production of hydroxyapatite from phosphogypsum

Abstract

Phosphogypsum (PG), a by-product of phosphate fertilizer industry, is chemically impure gypsum (CaSO₄ · 2 H₂O) containing phosphate residues. Provided by the calcium and phosphorus content, PG can be considered as a precursor for synthetic hydroxyapatite (s-HAp) production. This study proposes a two-step alkali route for s-HAp production from PG. Resulting samples were characterized by ICP-OES, SEM, XRD and FT-IR analyses in comparison with bone ash (BA) sample. Ca/P ratio (wt%) was determined as 2.46 and 2.78 for s-HAp and BA samples, respectively. SEM analysis showed the uniform distribution of spherical shaped particles in BA samples; however s-HAp particles showed irregular distribution of nearly spherical-like particles agglomerated as platelets on the surface. XRD analysis indicated that s-HAp particles possessed low crystallinity and ICDD references showed the appearance of apatite-CaOH phases. FT-IR spectrum showed the vibration bands of PO₄³⁻ bands in the range of 1018–601 cm⁻¹. According to characterization analyses, s-HAp samples show lower Ca/P ratio, irregular morphology, and low crystallinity due to possible impurities. Thus, further downstream operations regarding to impurity removal should be employed to develop a promising route to synthetic HAp production and employ a CE approach on industrial scale to evaluate s-HAp samples as a commercial substitute to BA.