Spectroscopic and DFT study on the species and local structure of arsenate incorporated in gypsum lattice

The incorporation of arsenate (As(V)) in the gypsum structure is an important process for arsenic (As) fixation during industrial effluent treatment and may influence the mobility and bioavailability of As in surface environment. However, spectroscopic evidence is still lacking for its species and local structure. The species and local structure of As(V) in a gypsum lattice were investigated using Fourier transform infrared (FTIR) spectroscopy, density functional theory (DFT) modelling, and full-potential multiple scattering (FPMS) simulations. Ascorbic acid-treated As(V)-gypsum co-precipitates were used to avoid the influence of amorphous calcium arsenate on the characterization. The lack of the FTIR band in the range of 750–860 cm^{− 1} was an indicative that no AsO₄^{3 −} species was incorporated into the gypsum structure. DFT calculations proved that the incorporation of AsO₄^{3 −} was energetically much harder than HAsO₄^{2 −} species. The FPMS structural refinement yielded the optimal As–O interatomic distances of 1.77, 1.67, 1.65, and 1.66 Å, with an average of 1.69 ± 0.057 Å, in agreement with the DFT and EXAFS results. Our work conclusively showed that HAsO₄^{2 −} dominated as the species of As(V) incorporated into the gypsum lattice, with the H atom in the HAsO₄^{2 −} group adjacent to water layer, regardless of pH.