Sorption of Chromium with Struvite During Phosphorus Recovery

Struvite (MgNH4PO4·6H2O; MAP) precipitation is a viable means of phosphorus (P) recovery from animal and human wastes. The behavior of metal contaminants such as chromium (Cr) during struvite precipitation, however, requires consideration. Here the influence of both Cr concentration and oxidation state on sorption is assessed. The Cr content of struvite precipitated in the presence of 1–100 μM Cr as Cr(III) (22.3–3030.1 mg/kg) was higher than that of solids from Cr(VI) (4.5–5.1 mg/kg) solutions. For 1–20 μM Cr(III) solids, scanning electron microscopy (SEM) revealed etch pit formation on struvite crystal surfaces, indicative of a surface interaction. The formation of an adsorbate was confirmed by X-ray absorption fine structure spectroscopy (XAFS). At initial concentrations ≥20 μM Cr(III), XAFS confirmed the formation of a Cr(OH)3·nH2O(am) precipitate. Fourier transform infrared (FT-IR) spectroscopy revealed that both Cr(III) and Cr(VI) sorption resulted in distortion of the PO4 3– tetrahedra in the mineral structure. This, combined with SEM results revealed that even at low sorbed concentrations, the Cr impurity can affect the mineral surface and structure. Thus, the initial Cr concentration and oxidation state in wastes targeted for P recovery will dictate the final Cr content, the mechanism of sorption, and impact on the struvite structure.