Struvite (MgNH4PO4●6H2O, MAP) precipitation can be induced from nutrient-rich wastewaters to mitigate phosphorus content. Struvite also precipitates naturally from biological wastes, occurring as urinary calculi in humans, canines, felines, small ruminants, and in equine enteroliths. Zinc is ubiquitous in wastewaters and biological wastes, acting as a micronutrient or a toxic species contingent upon concentration. The extent and mechanisms of Zn interaction with struvite from controlled model solutions (0.065-6.54 mg/L Zn), greenhouse wastewater (0.69 mg/L Zn), and equine colonic fluids is compared. The impact of Zn, and other associated contaminants, on the mineral structure and properties was assessed. Struvite mineralogy was confirmed by X-ray diffraction (XRD), and structural distortion due to impurities determined by Fourier transform infrared (FTIR) spectroscopy. The struvite Zn loadings were measured at 54-6577 mg/kg from model solutions, 64-247 mg/kg from wastewater and 197-238 mg/kg for the enterolith. Additional elements and organic carbon were also detected in wastewater struvite and the enterolith. X-ray absorption fine structure spectroscopy (XAFS) revealed differences in the Zn speciation and bonding environment contingent upon concentration and crystallization environment. Struvite from model solutions had tetrahedrally or octahedrally coordinated Zn adsorbates and polymers. For struvite recovered from wastewater, Zn sorbed as tetrahedrally coordinated polymers forming a Zn-P phase with structural linkages. The horse enterolith Zn configuration was unique, with distorted ZnO4 tetrahedra located between crystallographic planes, forming bidentate linkages to struvite phosphate groups. Simultaneous thermal analysis (STA) coupled with evolved gas analysis (EGA) revealed that struvite from all solutions is more susceptible to decomposition compared to pure struvite, releasing NH3(g) and H2O(g), along with CO(g) and CO2(g) contingent upon the organic content. The Zn concentration and configuration, and the susceptibility to decomposition has implications for reuse of struvite derived from wastes as fertilizer, and for potential treatment of struvite biological concretions.

Mechanisms of zinc association with struvite in model, wastewater and biological systems

256th National Meeting (2018)

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