The mineral struvite (MgNH4PO4•6H2O) can precipitate both intentionally and spontaneously from phosphorus-bearing wastes, and may be used as a source of phosphate fertilizer. Zinc is a heavy metal that is ubiquitous in many wastes, and can act as a micronutrient or a toxic species contingent upon concentration. Despite the potential for co-occurrence of Zn with struvite in waste systems, the mechanism(s) of Zn interaction with the mineral, and any impacts on the mineral structure remain unknown. In this study Zn interaction 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 initial Zn concentrations were within the range detected in several phosphorus-rich wastes. 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 (XAFS) confirmed that Zn added to struvite-bearing solutions at concentrations ≤5 µM sorbed as both tetrahedral and octahedral 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 PO43- band in the Fourier transform infrared (FTIR) spectra. At Zn concentrations &gt;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 initial Zn concentrations and resultant solid loadings from these experiments are consistent with those reported from wastewaters, suggesting that similar Zn sorption processes may occur in more complex systems. That struvite can sorb heavy metals from solution, and with notable impacts on the mineral structure, needs to be considered for its use as a fertilizer.

Zinc interaction with struvite during and after mineralization from phosphorus-rich sources

249th National Meeting (2015)

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