Ni and Co Struvites Revealing Crystallization Mechanisms and Crystal Engineering toward Applicational Use of Transition Metal Phosphates

Industrial and agricultural waste streams waste water, sludges, tailings, etc. which contain high concentrations of NH4 , PO43 , and transition metals are environmentally harmful and toxic pollutants. At the same time, phosphorous and transition metals constitute highly valuable resources. Typically, separate pathways have been considered to extract hazardous transition metals or phosphate independently from each other. Investigations on the simultaneous removal of multiple components have been carried out only to a limited extent. Here, we report the synthesis routes for Ni and Co struvites NH4MPO4 6H2O, M Ni2 and Co2 , which allow for P, ammonia, and metal co precipitation. By evaluating different reaction parameters, the phase and stability of transition metal struvites as well as their crystal morphologies and sizes could be optimized. Ni struvite is stable in a wide reactant concentration range and at different metal phosphorus M P ratios, whereas Co struvite only forms at low M P ratios. Detailed investigations of the precipitation process using ex situ and in situ techniques provided insights into the crystallization mechanisms crystal engineering of these materials. M struvites crystallize via intermediate colloidal amorphous nanophases, which subsequently aggregate and condense to final crystals after extended reaction times. However, the exact reaction kinetics of the formation of a final crystalline product varies significantly depending on the involved metal cation in the precipitation process several seconds Mg to minutes Ni to hours Co . The achieved level of control over the morphology and size makes precipitation of transition metal struvites a promising method for direct metal recovery and binding them in the form of valuable phosphate raw materials. Under this paradigm, the crystals can be potentially up cycled as precursor powders for electrochemical or electro catalytic applications, which require transition metal phosphate

Phase stability studies on transition metal phosphates aided by an automated synthesis

Transition metal phosphates TMPs have attracted interest as materials for electro catalysis, and electrochemistry due to their low cost, stability, and tunability. In this work, an automated synthesis platform was used for the preparation of transition metal phosphate crystals to efficiently explore the multidimensional parameter space, determining the phase selection, crystal sizes, shapes. By using X ray diffraction and spectroscopy based methods and electron microscopy imaging, a complete characterization of the phase stability fields, phase transitions, and crystal morphology sizes was achieved. In an automated three reactant synthesis, the individual effect of each reactant species NH4 , M2 , and PO43 amp; 8722; on the formation of transition metal phosphate phases M struvite NH4MPO4 6H2O, M phosphate octahydrate M3 PO4 2 8H2O with M Ni, Co and an amorphous phase, was investigated. The NH4 concentration dictates the phase composition, morphology, and particle size in the Ni system crystalline Ni struvite versus amorphous Ni PO4 phase , whereas in the Co system all reactant species NH4 , Co2 , and PO43 amp; 8722; influence the reaction outcome equivalently Co struvite vs. Co phosphate octahydrate . The coordination environment for all crystalline compounds and of the amorphous Ni PO4 phase was resolved by X ray absorption spectroscopy, revealing matching characteristics to its crystalline analogue, Ni3 PO4 2 8H2O. The automated synthesis turned out to be significantly advantageous for the exploration of phase diagrams due to its simple modularity, facile traceability, and enhanced reproducibility compared to a typical manual synthesi

Evidence for liquid liquid phase separation during the early stages of Mg struvite formation

In this work, we propose hydroxyapatite HA as a hard template to unlock the porosity of Fe N C catalyst materials. Using HA, a naturally occurring mineral that can be removed with nitric acid, in the synthesis generates a catalyst material with a unique porous network comprising abundant pores and interparticle cavities ranging from 10 to 3000 nm. Hard templating with HA alongside ZnCl2 as a micropore former results in a Fe N C catalyst based on naturally abundant peat with excellent oxygen reduction activity in alkaline conditions. A half wave potential of 0.87 V vs RHE and a peak power density of 1.06 W cm amp; 8722;2 were achieved in rotating ring disk electrode and anion exchange membrane fuel cell experiments, respectively, rivaling the performance of other state of the art platinum free catalysts presented in the literature. A combined approach of using renewable peat as a carbon source and HA as a hard template offers an environmentally friendly approach to high performance Fe N C catalysts with abundant porosit

Evidence for liquid-liquid phase separation during the early stages of Mg-struvite formation

The precipitation of struvite, a magnesium ammonium phosphate hexahydrate (MgNHPO · 6HO) mineral, from wastewater is a promising method for recovering phosphorous. While this process is commonly used in engineered environments, our understanding of the underlying mechanisms responsible for the formation of struvite crystals remains limited. Specifically, indirect evidence suggests the involvement of an amorphous precursor and the occurrence of multi-step processes in struvite formation, which would indicate non-classical paths of nucleation and crystallization. In this study, we use synchrotron-based in situ x-ray scattering complemented by cryogenic transmission electron microscopy to obtain new insights from the earliest stages of struvite formation. The holistic scattering data captured the structure of an entire assembly in a time-resolved manner. The structural features comprise the aqueous medium, the growing struvite crystals, and any potential heterogeneities or complex entities. By analysing the scattering data, we found that the onset of crystallization causes a perturbation in the structure of the surrounding aqueous medium. This perturbation is characterized by the occurrence and evolution of Ornstein-Zernike fluctuations on a scale of about 1 nm, suggesting a non-classical nature of the system. We interpret this phenomenon as a liquid-liquid phase separation, which gives rise to the formation of the amorphous precursor phase preceding actual crystal growth of struvite. Our microscopy results confirm that the formation of Mg-struvite includes a short-lived amorphous phase, lasting >10 s.We thank BAM and Helmholtz-Zentrum Berlin (HZB) for providing us with the beamtime at mySpot of BESSY II

Evidence for liquid-liquid phase separation during the early stages of Mg-struvite formation

The precipitation of struvite, a magnesium ammonium phosphate hexahydrate (MgNH4PO4⋅6H2O) mineral, from wastewater is a promising method for recovering phosphorous. While this process is commonly used in engineered environments, our understanding of the underlying mechanisms responsible for the formation of struvite crystals remains limited. Specifically, indirect evidence suggests the involvement of an amorphous precursor and the occurrence of multi-step processes in struvite formation, which would indicate non-classical paths of nucleation and crystallization. In this study, we use synchrotron-based in situ X-ray scattering complemented by cryogenic transmission electron microscopy to obtain new insights from the earliest stages of struvite formation. The holistic scattering data captured the structure of an entire assembly in a time-resolved manner. The structural features comprise the aqueous medium, the growing struvite crystals, and any potential heterogeneities or complex entities. By analysing the scattering data, we found that the onset of crystallization causes a perturbation in the structure of the surrounding aqueous medium. This perturbation is characterized by the occurrence and evolution of Ornstein-Zernike fluctuations on a scale of about 1 nm, suggesting a non-classical nature of the system. We interpret this phenomenon as a liquid-liquid phase separation (LLPS), which gives rise to the formation of the amorphous precursor phase preceding actual crystal growth of struvite. Our microscopy results confirm that the formation of Mg-struvite includes a short-lived amorphous phase, lasting >10 seconds

Solution-driven processing of calcium sulfate: The mechanism of the reversible transformation of gypsum to bassanite in brines

Here, we show that calcium sulfate dihydrate (gypsum) can be directly, rapidly and reversibly converted to calcium sulfate hemihydrate (bassanite) in high salinity solutions (brines). The optimum conditions for the efficient production of bassanite in a short time ( 4 M and maintaining a temperature, T > 80 °C. When the solution containing bassanite crystals is cooled down to around room temperature, eventually gypsum is formed. When the temperature is raised again to T > 80 °C, bassanite is rapidly re-precipitated. This contrasts with the better-known behaviour of the bassanite phase in low-salt environments. In low-salinity aqueous solutions, bassanite is considered to be metastable with respect to gypsum and anhydrite, and therefore gypsum-to-bassanite conversion does not occur in pure water. Interestingly, the high-salinity transformation of gypsum-to-bassanite has been reported by many authors and used in practice for several decades, although its very occurrence actually contradicts numerical thermodynamic predictions regarding solubility of calcium sulfate phases. By following the evolution of crystalline phases with in situ and time-resolved X-ray diffraction/scattering and Raman spectroscopy, we demonstrated that the phase stability in brines at elevated temperatures was inaccurately represented in the thermodynamic databases. Most notably for c(NaCl) > 4 M, and T > 80 °C gypsum becomes readily more soluble than bassanite, which induces the direct precipitation of the latter from gypsum. The fact that these transformations are controlled by the solution provides extensive opportunities for precise manipulation of crystal formation. Our experiments confirmed that bassanite remained the sole crystalline phase for many hours before reverting into gypsum. This property is extremely advantageous for practical processing and efficient crystal extraction in industrial scenarios.We thank BAM (project MIT1-20-09) and Helmholtz-Zentrum Berlin (HZB) for providing us with the beamtime at mySpot of BESSY II (proposal 211-10162-ST-1.1-P) and access to electron microscopes. AESVD acknowledges funding from the Junta de Andalucia (Spain) through project PROYEXCEL_00771. CP has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 101021894 [CARS-CO2]