X-Ray Spectroscopic Quantification of Struvite and Dittmarite Recovered from Wastewater

Phosphorus recovery from wastewater as struvite (MgNHPO⋅6HO) or dittmarite (MgNHPO⋅HO) can decrease water pollution risk, as well as produce a P-rich material suitable as fertilizer. However, most studies to date have focused on the removal of P from wastewater, rather than on characterization of the recovered P materials. The objective of this work was to apply microfocused X-ray fluorescence (XRF) spectroscopy, and both bulk and microfused X-ray absorption near edge structure (XANES) spectroscopy, to provide insight into the speciation of recovered P in various struvite-containing and struvite-like materials. Three materials were investigated: homogeneous crystalline struvite on apatite seed, homogeneous dittmarite, and heterogeneous struvite with sand contamination (referred to as the "sandy" material). The struvite materials were recovered from dairy wastewater, whereas the dittmarite was from a cheese processing plant. Phosphorus speciation in the crystalline struvite on apatite seed material was ∼17% apatite and 83% struvite; in the "sandy" material, P was ∼24% apatite and ∼76% struvite, with an uncertainty of approximately ±15%. The P -edge XANES spectra of recovered dittmarite appeared pure. These findings highlight the heterogeneity of recovered P materials and underscore the importance of P speciation to understand P release behavior and bioavailability from recovered phosphates.