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 CaSO4 · 2 H2O 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 CaSO4 · 2 H2O crystals. Upon comparison, it is evident that the energy required for HPO42- substitution of SO42- is the lowest, suggesting that phosphates are most likely to enter the CaSO4 · 2 H2O lattice in the form of HPO42-. Furthermore, HPO42- substitution in CaSO4 · 2 H2O is likely localized, with no inclination towards intralayer substitution. In instances where phosphate replaces SO42-, 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. •Phosphorus is uniformly distributed throughout PG, with heightened density in regions where sulfur density decreases.•Increased Ca3(PO4)2 doping leads to an expansion of interplanar spacing and cell volume in CaSO4 · 2 H2O.•The formation energy of the HPO42- configuration is the lowest, indicating that phosphate is most likely to incorporate into the CaSO4 · 2 H2O lattice in the form of HPO42-.•Upon the substitution of SO42- by phosphate, the electron distribution around P atoms closely resembles that around S atoms.•We propose the most plausible model for PG as (CaSO4)1-x(CaHPO4)x·2 H2O.