Substitution preferences of phosphate in gypsum: An experimental and DFT simulation study

Phosphogypsum (PG), a solid waste produced from the wet process of phosphoric acid production, primarily contains impurities such as phosphates. Understanding how phosphates exist is crucial for the purification and resource utilization of PG. This study employs Energy Dispersive X-ray Spectroscopy (EDS), Fourier Transform Infrared Spectrometer (FTIR), X-ray diffraction (XRD), and Density Functional Theory (DFT) simulations to investigate the forms of phosphates within the CaSO 4 · 2 H 2 O lattice. The research findings reveal that the density of phosphorus (P) increases in regions where the density of sulfur (S) decreases. Higher phosphorus doping enhances the interplanar spacing and unit cell volume of CaSO 4 · 2 H 2 O crystals. Upon comparison, it is evident that the energy required for HPO 4 2- substitution of SO 4 2- is the lowest, suggesting that phosphates are most likely to enter the CaSO 4 · 2 H 2 O lattice in the form of HPO 4 2- . Furthermore, HPO 4 2- substitution in CaSO 4 · 2 H 2 O is likely localized, with no inclination towards intralayer substitution. In instances where phosphate replaces SO 4 2- , the electronic distribution near phosphorus atoms mirrors that of sulfur atoms. This study provides a comprehensive, atomic-level elucidation of the presence of phosphates in gypsum, significantly contributing to the rational utilization of solid waste.