Search

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 &lt, 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 &ge, 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 &lt, 0.3) or (ii) cobalt phosphate octahydrate (at Co/P &gt, 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.

This chapter focuses on time-resolved studies of nanostructure development in sol-gel liquids, that is, diluted sols, wet gels, and drying thin fffilms. The most commonly investigated classes of sol-gel materials are silica, organically modified silica, template-directed mesostructured silica, titania and titania-based materials, zirconia and zirconia-based materials, and few others. After a brief overview of small-angle X-ray scattering (SAXS) theory and some experimental details, the chapter discusses separately each of these classes of sol-gel materials. Nearly all time-resolved SAXS studies on silica have been carried out starting with alkoxysilanes. The chapter discusses studies of systems with a single precursor under acidic and basic conditions, respectively, and reviews combinations of alkoxysilanes with other metal precursors and mixed organic-inorganic systems.

Item does not contain fulltext

Although clays are crucial mineral phases in Earth's weathering engine, it is unclear how they form in surface environments under (near-)ambient pressures and temperature. Most synthesis routes, attempting to give insights into the plausible mechanisms, rely on hydrothermal conditions, yet many geological studies showed that clays may actually form at moderate temperatures (< 100 deg. C) in most terrestrial settings. Here, we present the mechanism of the low-temperature (25-95 deg. C) crystallization of a synthetic Mg-clay, saponite. We describe the pathway at the various sub-stages of the reaction, as we derived from high-energy X-ray diffraction, infrared spectroscopy and transmission electron microscopy data. Our results reveal that saponite crystallizes via a two stage process: 1) a rapid (several minutes) co-precipitation where ~20% of the available magnesium becomes incorporated into an aluminosilicate network followed by 2) a much slower crystallization mechanism (many hours to days) where the remaining magnesium becomes gradually incorporated into the growing saponite sheet structure., manuscript + supporting information