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Open AccessFeature PaperArticle

Struvite Crystallisation and the Effect of Co2+ Ions

1
GFZ German Research Centre for Geosciences, 14473 Potsdam, Germany
2
School of Chemical and Process Engineering, University of Leeds, LS2 9JT Leeds, UK
3
Université Grenoble Alpes, CNRS, ISTerre, 38000 Grenoble, France
4
Department of Earth Sciences, Freie Universität Berlin, 12249 Berlin, Germany
*
Author to whom correspondence should be addressed.
Minerals 2019, 9(9), 503; https://doi.org/10.3390/min9090503
Received: 12 July 2019 / Revised: 14 August 2019 / Accepted: 19 August 2019 / Published: 22 August 2019
(This article belongs to the Special Issue Crystal Nucleation and Growth)
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. View Full-Text
Keywords: struvite; phosphate recovery; cobalt; crystal formation; nanoparticles struvite; phosphate recovery; cobalt; crystal formation; nanoparticles
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MDPI and ACS Style

Hövelmann, J.; Stawski, T.M.; Freeman, H.M.; Besselink, R.; Mayanna, S.; Perez, J.P.H.; Hondow, N.S.; Benning, L.G. Struvite Crystallisation and the Effect of Co2+ Ions. Minerals 2019, 9, 503. https://doi.org/10.3390/min9090503

AMA Style

Hövelmann J, Stawski TM, Freeman HM, Besselink R, Mayanna S, Perez JPH, Hondow NS, Benning LG. Struvite Crystallisation and the Effect of Co2+ Ions. Minerals. 2019; 9(9):503. https://doi.org/10.3390/min9090503

Chicago/Turabian Style

Hövelmann, Jörn; Stawski, Tomasz M.; Freeman, Helen M.; Besselink, Rogier; Mayanna, Sathish; Perez, Jeffrey P.H.; Hondow, Nicole S.; Benning, Liane G. 2019. "Struvite Crystallisation and the Effect of Co2+ Ions" Minerals 9, no. 9: 503. https://doi.org/10.3390/min9090503

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