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.
Highlights
In a world of an ever-growing global population and increasing use of raw materials, the recovery of critical elements from municipal, agricultural, and industrial wastewater streams is of paramount importance for the conservation of limited natural resources, and the prevention and remediation of environmental pollution
Powder X-ray diffraction (XRD) patterns of dried solid reaction products were collected on a PANalytical Empyrean X-ray diffractometer (Almelo, the Netherlands) using a Cu Kα radiation source operating at 40 kV and 40 mA
To evaluate the kinetics of crystal nucleation and growth, we considered two kinetic proxies crystallised directly from solution without the involvement of any structurally distinct precursor dependent phase(s).on the scattering features: (1) From SAXS, we investigated the evolution of I(qmin ), i.e., the scattering intensity the lowest q-value
Summary
In a world of an ever-growing global population and increasing use of raw materials, the recovery of critical elements from municipal, agricultural, and industrial wastewater streams is of paramount importance for the conservation of limited natural resources, and the prevention and remediation of environmental pollution. This becomes clear when looking at phosphorus (P) because modern agriculture increasingly relies on P-fertilizers, yet the natural phosphate rock reserves for fertilizer production are steadily declining and may be depleted in the mid-term future [1]. Research has primarily focused on optimizing the conditions under which struvite can be removed from the wastewater by bulk precipitation
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