The phosphate mineral struvite (MgNH4PO4.6H2O) can accumulate inorganic contaminants when crystallized from wastes. Heavy metals such as As, Cr, Zn, Cu, Ni, Fe and Mn are associated with struvite recovered from livestock, plant-based and synthetic wastewaters. Metal accumulation also occurs during in vivo crystallization of struvite biological concretions such as kidney and colon stones in humans and animals. In this study, synchrotron based techniques are coupled with geochemical tools to evaluate metal content, speciation and spatial distribution in struvite equine enteroliths (horse colon stones). Comparisons are made to environmental struvite samples precipitated from wastewaters for nutrient recovery purposes. Struvite metal loadings, as analyzed by inductively-coupled plasma optical emission spectroscopy (ICP-OES), ranged from 101 to 103 mg/kg. Dominant metals in enterolith samples were Fe (447-1850 mg/kg), Mn (160-280 mg/kg) and Zn (30-238 mg/kg). Sectioned enteroliths were analyzed by micro-X-ray fluorescence (μ-XRF) and micro-X-ray absorption fine structure spectroscopy (μ-XAFS) to determine spatial distribution and speciation of detected metals. Results were correlated with distinct mineral crystallization patterns, as observed by light microscopy and scanning electron microscopy (SEM). Powdered samples were analyzed by bulk XAFS to determine metal speciation, dominant binding sites, and location in the struvite structure. XAFS results and metal loadings were linked to Fourier transform infrared spectroscopy (FTIR) data to assess the impact of sorbed metals on the struvite structure. During slow struvite crystallization from horse colonic fluids, metals are predominantly incorporated into the mineral structure, with minimal structural distortion. This contrasts rapid struvite precipitation during wastewater nutrient recovery processes, where metals sorb at the surface and/or in the near surface structure, often with structural distortion. Results provide insight into trace element accumulation in struvite biological concretions during crystallization, and facilitates comparison with environmental samples undergoing analogous processes.

Heavy metal speciation and spatial distribution in phosphate minerals crystallized from wastes

256th National Meeting (2018)

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