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, 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.

Humic Acids Limit the Precipitation of Cadmium and Arsenate at the Brushite-Fluid Interface.

Bioavailability and mobility of cadmium (Cd2+) and arsenate (As5+) in soils can be effectively lowered through the dissolution of brushite (dicalcium phosphate dihydrate, CaHPO4·2H2O) coupled with the precipitation of a more stable mineral phase containing both Cd and As. Due to the ubiquitous presence of humic acid (HA) in soil environments, it is more complex to predict the fate of dissolved Cd and As during such sequestration. Here, we used in situ atomic force microscopy (AFM) to image the kinetics of simultaneous precipitation of Cd and As at the brushite-fluid interface in the presence of HA. Results show that HA inhibits the formation of both amorphous and crystalline Cd(5- x)Ca x(PO4)(3- y)(AsO4) y(OH) on the (010) face of brushite. A combination of X-ray photoelectron spectroscopy (XPS) and real-time surface-enhanced Raman spectroscopy (SERS) reveals that part of As5+ reduction into As3+ with HA and [HA-Cd] complexation occurs, modulating the concentrations of free Cd2+ and As5+ ions to inhibit subsequent precipitation of a Cd(5- x)Ca x(PO4)(3- y)(AsO4) y(OH) phase on the dissolving brushite surface. A combination of AFM imaging, SERS analyses, and PhreeqC simulations suggests that environmentally relevant humic substances can limit the precipitation of Cd and As at mineral surfaces through a mechanism of oxidation/reduction and aqueous/surface complexation. This may exacerbate the transportation of these contaminants into waters by subsurface fluid flow, and research attempts to weaken the negative effect of HA are needed.

Imaging Organophosphate and Pyrophosphate Sequestration on Brucite by in Situ Atomic Force Microscopy.

In order to evaluate the organic phosphorus (OP) and pyrophosphate (PyroP) cycle and their fate in the environment, it is critical to understand the effects of mineral interfaces on the reactivity of adsorption and precipitation of OP and PyroP. Here, in situ atomic force microscopy (AFM) is used to directly observe the kinetics of coupled dissolution-precipitation on cleaved (001) surfaces of brucite [Mg(OH)2] in the presence of phytate, glucose-6-phosphate (G6P) and pyrophosphate, respectively. AFM results show that the relative order of contribution to mineral surface adsorption and precipitation is phytate > pyrophosphate > G6P under the same solution conditions and can be quantified by the induction time of OP/PyroP-Mg nucleation in a boundary layer at the brucite-water interface. Calculations of solution speciation during brucite dissolution in the presence of phytate or pyrophosphate at acidic pH conditions show that the solutions may reach supersaturation with respect to Mg5H2Phytate.6H2O as a Mg-phytate phase or Mg2P2O7 as a Mg-pyrophosphate phase that becomes thermodynamically stable before equilibrium with brucite is reached. This is consistent with AFM dynamic observations for the new phase formations on brucite. Direct nanoscale observations of the transformation of adsorption/complexation-surface precipitation, combined with spectroscopic characterizations and species simulations may improve the mechanistic understanding of organophosphate and pyrophosphate sequestration by mineral replacement reactions through a mechanism of coupled dissolution-precipitation occurring at mineral-solution interfaces in the environment.

In Situ Nanoscale Imaging of Struvite Formation during the Dissolution of Natural Brucite: Implications for Phosphorus Recovery from Wastewaters.

As phosphorus (P) resources are diminishing, the recovery of this essential nutrient from wastewaters becomes an increasingly interesting option. P-recovery through the controlled crystallization of struvite (MgNH4PO4·6H2O), a potential slow-release fertilizer, is highly attractive, but costly if large amounts of Mg have to be added. In this context, natural Mg-minerals like brucite (Mg(OH)2) could provide more cost-effective Mg-sources compared to high-grade Mg-compounds such as MgCl2. Here we used in situ atomic force microscopy (AFM) to study the interactions of ammonium phosphate solutions with brucite (001) cleavage surfaces. Brucite dissolution was strongly enhanced in the presence of H2PO4- ions, most likely due to the formation of negatively charged surface complexes. Simultaneously with brucite dissolution, we directly observed the formation of a new phase that was identified as struvite by Raman spectroscopy. Our results suggest that brucite dissolution and struvite precipitation were coupled at the mineral-fluid interface within a thin fluid boundary layer. An interpretation is proposed where the heterogeneous nucleation and growth of struvite occurs via a particle-mediated process involving the formation of primary nanoparticles, followed by their continuous aggregation, fusion and possible transformation to crystalline struvite. These observations have implications for the feasibility of using brucite in phosphorus recovery processes.

In situ imaging of interfacial precipitation of phosphate on Goethite.

Adsorption and subsequent immobilization of orthophosphate on iron oxides is of considerable importance in soil fertility and eutrophication studies. Here, in situ atomic force microscopy (AFM) has been used to probe the interaction of phosphate-bearing solutions with goethite, α-FeOOH, (010) cleavage surfaces. During the dissolution of goethite we observed simultaneous nucleation of nanoparticles (1.0-3.0 nm in height) of iron phosphate (Fe-P) phases at the earliest nucleation stages, subsequent aggregation to form secondary particles (about 6.0 nm in height) and layered precipitates under various pH values and ionic strengths relevant to acid soil solution conditions. The heterogeneous nucleation rates of Fe-P precipitates at phosphate concentrations ranging from 5.0 to 50.0 mM were quantitatively defined. Enhanced goethite dissolution in the presence of high concentration NaCl or AlCl3 leads to a rapid increase in Fe-P nucleation rates, whereas low concentration MgCl2 inhibits goethite dissolution, this in turn influences Fe-P nucleation. Moreover, kinetic data analyses show that low concentrations of citrate caused an increase in the nucleation rate of Fe-P phases. However, at higher concentrations of citrate, nucleation acceleration was reversed with much longer induction times to form Fe-P nuclei. These in situ observations may improve the mechanistic understanding of processes resulting in phosphate immobilization by goethite-rich acid soils in the presence of various inorganic and organic additive molecules.

A template-free and low temperature method for the synthesis of mesoporous magnesium phosphate with uniform pore structure and high surface area.

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.