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Struvite crystallisation and the effect of Co2+ ions

DOI: 10.3390/min9090503 DOI Help

Authors: Jorn Hovelmann (GFZ German Research Centre for Geosciences) , Tomasz M. Stawski (GFZ German Research Centre for Geosciences) , Helen Freeman (GFZ German Research Centre for Geosciences; University of Leeds) , Rogier Besselink (GFZ German Research Centre for Geosciences; Université Grenoble Alpes, CNRS) , Sathish Mayanna (GFZ German Research Centre for Geosciences) , Jeffrey Paulo H. Perez (GFZ German Research Centre for Geosciences; Freie Universität Berlin) , Nicole S. Hondow (University of Leeds) , Liane G. Benning (GFZ German Research Centre for Geosciences; Freie Universität Berlin)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Minerals , VOL 9

State: Published (Approved)
Published: August 2019
Diamond Proposal Number(s): 16256

Open Access Open Access

Abstract: The controlled crystallisation of struvite (MgNH4PO4∙6H2O) is a viable means for the recovery and recycling of phosphorus (P) from municipal and industrial wastewaters. However, an efficient implementation of this recovery method in water treatment systems requires a fundamental understanding of struvite crystallisation mechanisms, including the behavior and effect of metal contaminants during struvite precipitation. Here, we studied the crystallisation pathways of struvite from aqueous solutions using a combination of ex situ and in situ time-resolved synthesis and characterization techniques, including synchrotron-based small- and wide-angle X-ray scattering (SAXS/WAXS) and cryogenic transmission electron microscopy (cryo-TEM). Struvite syntheses were performed both in the pure Mg-NH4-PO4 system as well as in the presence of cobalt (Co), which, among other metals, is typically present in waste streams targeted for P-recovery. Our results show that in the pure system and at Co concentrations < 0.5 mM, struvite crystals nucleate and grow directly from solution, much in accordance with the classical notion of crystal formation. In contrast, at Co concentrations ≥ 1 mM, crystallisation was preceded by the transient formation of an amorphous nanoparticulate phosphate phase. Depending on the aqueous Co/P ratio, this amorphous precursor was found to transform into either (i) Co-bearing struvite (at Co/P < 0.3) or (ii) cobalt phosphate octahydrate (at Co/P > 0.3). These amorphous-to-crystalline transformations were accompanied by a marked colour change from blue to pink, indicating a change in Co2+ coordination in the formed solid from tetrahedral to octahedral. Our findings have implications for the recovery of nutrients and metals during struvite crystallisation and contribute to the ongoing general discussion about the mechanisms of crystal formation.

Journal Keywords: struvite; phosphate recovery; cobalt; crystal formation; nanoparticles

Subject Areas: Chemistry, Earth Science

Instruments: I22-Small angle scattering & Diffraction

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