Development of industrial waste management approaches for adaptation to circular economy strategy: the case of phosphogypsum-derived hydroxyapatite

Abstract

Phosphogypsum (PG, CaSO₄·2H₂O), impure gypsum with phosphate residues, is a by-product of fertilizer industry. Due to calcium and phosphorus in its structure, PG can be evaluated for synthetic hydroxyapatite (HAp) production. This study provides a two-step reaction sequence for PG-derived synthetic HAp production. Synthesized HAp (s-HAp) particles were calcined at 900 °C for 4 h, followed by characterization studies with respect to commercial bone ash (CBA) sample to be evaluated as its substitute in bone china industry. Ca/P ratio (wt%) of the samples was investigated by microwave-assisted temperature-controlled acid digestion, followed by ICP-OES analysis. Crystallinity of the samples was analyzed by XRD, and morphological structure was investigated by SEM–EDS analyses. ICP–OES analyses revealed that s-HAp and CBA samples have similar Ca/P ratio, 2.0979 for s-HAp and 2.0917 for CBA samples, respectively. Low-intensity β-TCP phases in the XRD patterns of s-HAp samples revealed low crystalline structure. SEM images indicated a uniform distribution with spherical-like morphology in CBA samples, however, a layer-by-layer morphology and amorphous structure was observed in s-HAP samples. Structural and morphological differences indicate that PG-derived s-HAp samples require purification processes and reaction parameters should be further developed to increase yield. However, this study provides a high potential for a novel approach of an industrial scale valorization strategy for PG. Because the supply–demand cycle is price dependent, the availability of phosphorus in agriculture as a raw material has been a major concern for the usage of phosphate rock. Novel methods for substitute supplies of phosphorus in agricultural applications have been suggested. While HAp is a common ceramic material, its application as a substitute for phosphorus fertilizer is still in its infancy. PG has great potential as a raw material for the synthesis of hydroxyapatite and ammonium sulfate. Large-scale PG production in the phosphoric acid industry puts pressure on research to develop alternate methods for using PG aggregate resources in an economical and environmentally responsible way. The use of PG as a calcium and phosphorus precursor in the synthesis of HAp will result in an economical raw material and an effective waste management plan.