Zinc interaction with struvite during and after mineral formation

Sorption of Zn with struvite was assessed both during and after mineral formation at pH 9.0 for 1-100 μM (0.065-6.54 mg L(-1)) aqueous Zn. The Zn loadings of recovered solids were lower when Zn was present during struvite precipitation compared to when Zn was added to struvite-bearing solutions. X-ray absorption fine structure spectroscopy confirmed that Zn added to struvite-bearing solutions at concentrations≤5 μM sorbed as both octahedral and tetrahedral complexes (Zn-O 1.98-2.03 Å), with evidence for bidentate configuration (Zn-P 3.18 Å). Bidentate complexes were incorporated into the near-surface structure, contributing to distortion of the struvite ν3 PO4(3-) band in the Fourier transform infrared spectra. At Zn concentrations>5 μM, tetrahedral monodentate adsorbates (Zn-O 1.98 Å) dominated, transitioning to a Zn-phosphate precipitate at 100 μM. When Zn is present during struvite precipitation, octahedral monodentate sorbates detected at 1 μM (Zn-O 2.08-2.10 Å; Zn-P 3.60-3.64 Å) polymerized at 5-50 μM, ultimately forming a Zn-hydroxide precipitate at 100 μM. The lowest initial Zn concentrations (0.065 mg L(-1)) and resultant solid loadings from precipitation experiments (13 mg kg(-1)) are consistent with those reported for struvite recovered from wastewater, suggesting that similar Zn sorption processes may occur in more complex systems.