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Mesoporous phosphates are a group of nanostructured materials with promising applications, particularly in biomedicine and catalysis. However, their controlled synthesis via conventional template-based routes presents a number of challenges and limitations. Here, we show how to synthesize a mesoporous magnesium phosphate with a high surface area and a well-defined pore structure through thermal decomposition of a crystalline struvite (MgNH4PO4·6H2O) precursor. In a first step, struvite crystals with various morphologies and sizes, ranging from a few micrometers to several millimeters, had been synthesized from supersaturated aqueous solutions (saturation index (SI) between 0.5 and 4) at ambient pressure and temperature conditions. Afterwards, the crystals were thermally treated at 70-250 °C leading to the release of structurally bound water (H2O) and ammonia (NH3). By combining thermogravimetric analyses (TGA), scanning and transmission electron microscopy (SEM, TEM), N2 sorption analyses and small- and wide-angle X-ray scattering (SAXS/WAXS) we show that this decomposition process results in a pseudomorphic transformation of the original struvite into an amorphous Mg-phosphate. Of particular importance is the fact that the final material is characterized by a very uniform mesoporous structure with 2-5 nm wide pore channels, a large specific surface area of up to 300 m2 g-1 and a total pore volume of up to 0.28 cm3 g-1. Our struvite decomposition method is well controllable and reproducible and can be easily extended to the synthesis of other mesoporous phosphates. In addition, the so produced mesoporous material is a prime candidate for use in biomedical applications considering that magnesium phosphate is a widely used, non-toxic substance that has already shown excellent biocompatibility and biodegradability.

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.

Permeability is known as a key factor affecting the effectiveness of gas production from a natural gas hydrate reser-voir. In addition to the lithological effects of the hydrate-bearing sediment itself, the formation and dissociation processes of gas hydrate in the sediments may also have an influence on the permeability. In this study, samples from a gas hydrate reservoir in the Qilian Mountain permafrost (borehole DK-8 and SK-2 in northern Mulicoalfield) were taken for the permeability experiments. Permeabilities were measured before hydrate formation,with hydrate and also after hydrate dissociation. The presence of solid methane hydrate in the pores lowers the permeability depending on hydrate saturation. However an unexpected high permeability decrease was observed after the dissociation of methane hydrate. Six kinds of permeability tests were carried out to detect the reason for formation damage after hydrate dissociation. The results indicate that the fresh water released from the hydratedissociation may cause an activation and following the migration of fine particles which block the pore throats and finally result in a decrease of permeability. Scanning Electron Microscopy (SEM) analysis on the filter papers which attached on the inlet and outlet of the core sample provides visible evidence on fine migration. In our study we present the experimental results of the permeability tests under different conditions and discuss the potential reasons for our observations. One possible explanation for this phenomenon may be that the release of fresh water causes an increase in the double layer thickness at the water mineral surfaces and therefore increase the repulsion forces between rock particles. This process can release small particles which were attached to the surface of bigger sediment grains.