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 mu M (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 mu M sorbed as both octahedral and tetrahedral complexes (Zn-O 1.98-2.03 angstrom), with evidence for bidentate configuration (Zn-P 3.18 angstrom)., Bidentate complexes were incorporated into the near-surface structure, contributing to distortion of the struvite 113 PO43- band in the Fourier transform infrared spectra. At Zn concentrations >5 mu M, tetrahedral monodentate adsorbates (Zn-O 1.98 angstrom) dominated, transitioning to a Zn-phosphate precipitate at 100 mu M. When Zn is present during struvite precipitation, octahedral monodentate sorbates detected at 1 mu M (Zn-O 2.08-2.10 angstrom; Zn-P 3.60-3.64 angstrom) polymerized at 5-50 mu M, ultimately forming a Zn-hydroxide precipitate at 100 mu M. 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.