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In the context of circular economy and heavy metal (HM) recovery from municipal solid waste incineration (MSWI) fly ash (FA), detailed knowledge of HM binding forms is required for achieving higher extraction rates. The FA mineralogy is still poorly understood due to its low grain size and low metal concentration. To investigate the HM binding forms, a sophisticated thermodynamic reactive transport model was developed to simulate ash-forming processes. The stability of different binding forms was investigated at different flue gas conditions (varying ratios of HCl, SO2, O2) by simulating the gas cooling path in closed system and dynamic open system, where the gas composition is changing upon cooling due to precipitation of solids. The simulations predict that at flue gas conditions of molar ratio S/Cl 1, the HM precipitate as less soluble sulphates. The results indicate that the less soluble HM fraction in the electrostatic precipitator ash represent oxides and silicates that formed in the boiler section but were transported to the electrostatic precipitator. The model provides insight into the physical-chemical processes controlling the metal accumulation in the flue gas and FA during the cooling of the flue gas. The obtained data serve as valuable basis for improving metal recovery from MSWI FA.

Water boiling control evolution of natural geothermal systems is widely exploited in industrial processes due to the unique non-linear thermophysical behavior. Even though the properties of water both in the liquid and gas state have been extensively studied experimentally and by numerical simulations, there is still a fundamental knowledge gap in understanding the mechanism of the heterogeneous nucleate boiling controlling evaporation and condensation. In this study, the molecular mechanism of bubble nucleation at the hydrophilic and hydrophobic solid-water interface was determined by performing unbiased molecular dynamics simulations using the transition path sampling scheme. Analyzing the liquid to vapor transition path, the initiation of small void cavities (vapor bubbles nuclei) and their subsequent merging mechanism, leading to successively growing vacuum domains (vapor phase), has been elucidated. The molecular mechanism and the boiling nucleation sites' location are strongly dependent on the solid surface hydrophobicity and hydrophilicity. Then simulations reveal the impact of the surface functionality on the adsorbed thin water molecules film structuring and the location of high probability nucleation sites. Our findings provide molecular-scale insights into the computational aided design of new novel materials for more efficient heat removal and rationalizing the damage mechanisms.

For the first time, the structural investigation of a Pb-exchanged zeolite (Pb13.4(OH)10Al17.4Si54.6O144 ∙38H2O) with STI framework type, revealed a highly unusual and intriguing sudden volume increase under continuous heating. Understanding the fundamental mechanisms leading to such an unusual behaviour is essential for technological applications and interpretation of chemical bonding in zeolites. The dehydration was tracked in situ from 25 to 450 °C by single crystal X-ray diffraction, infrared and X-ray absorption spectroscopy. Further interpretation of the experimental observations was supported by ab initio molecular dynamics simulations. Initially, Pb-STI unit-cell volume contracts (ΔV = -3.5%) from 25 to 100°C. This agrees with the trend observed in STI zeolites. Surprisingly, at 125°C, the framework expanded (ΔV = +2%), adopting a configuration, which resembles that of the room temperature structure. Upon heating, the structure loses H2O but no de-hydroxylation occurred. The key mechanism leading to the sudden volume increase was found to be the formation of Pbx(OH)y clusters, which prevent the shrinking of the channels, rupture of the tetrahedral bonds and occlusion of the pores. This zeolite has therefore an increased thermal stability with respect to other STI metal-exchanged zeolites, with important consequences on its applications.

Small-pore zeolites are successfully employed as catalysts, sorbents and molecular sieves. Their physiochemical properties can be tuned by modifying their extraframework cation (EF) composition via ion exchange. In this study, we investigate the crystal structure of a Cd-exchanged levyne (LEV) intergrown with erionite (ERI) by combining Single Crystal X-ray Diffraction (SCXRD), Molecular Dynamic simulations (MD) and Extended X-ray Absorption Fine-Structure spectroscopy (EXAFS). Data obtained from the different techniques consistently indicate that Cd2+ in LEV is arranged in a nearly ordered fashion. In contrast, strong disorder of the EF species (Cd2+ and H2O) is observed in the ERI cavities. Here, Cd2+ forms aqueous complexes that are more mobile in comparison to LEV, where Cd2+ bonds to both H2O and framework-oxygen atoms. The formation of Cd-clusters is excluded based on EXAFS analysis. Finally, to discriminate between thermal and static disorder, we propose a new approach based on combined MD and geometry optimization.

The possibility to develop a process for lead removal from wastewater, based on calcium carbonate minerals, depends on the overall efficiency of the uptake process. Aqueous Pb tends to form cerussite (CER) via a dissolution–precipitation reaction when interacting with aragonite (ARG) and calcite (CAL). From a thermodynamic perspective, the two processes have a similar driving force, because the solubility of CAL and ARG is approximatively the same (Ks ≈ 1 × 10–8). Experimentally, the macroscopic yield of reaction was found to be very different. Using ex situ electron microscopy, diffraction, and in situ atomic force microscopy, we demonstrate that the Pb uptake mechanism by the two most abundant CaCO3 polymorphs is controlled by the kinetics of processes at the solid–solid and solid–liquid interfaces. Aragonite is isostructural with the product phase (CER) that easily precipitates taking advantage of the template effect offered by the surfaces of the substrate. The reaction proceeds through an interface-coupled dissolution–precipitation that leads to a mineral replacement. Because of a crystallographic mismatch, the reaction between CAL and CER mainly occurs as a simple solvent-mediated transformation. Our study unveiled the mechanistic reasons behind the different reaction yields shown by CAL and ARG toward Pb uptake. Similar conclusions can be extended to other contaminant(aq)-CaCO3(s) systems, thus increasing the predictability of limestone efficiency toward the uptake of heavy metals.

The effect of Ag+ incorporation into stellerite, a natural zeolite with STI framework type, was investigated by means of Single Crystal X-ray Diffraction (SC-XRD), Molecular Dynamics (MD) simulations and X-ray Absorption Fine Structure Spectroscopy (XAFS). At room temperature the complete exchange of the original extraframework ions with Ag+ provoked a distortion of the framework accompanied by symmetry reduction from orthorhombic Fmmm to monoclinic F2/m space group. Ag+ ions were strongly disordered, with occupancies ranging from 0.02 to 0.24, at partially-occupied sites within zeolitic cages. The combination of ab initio molecular dynamic simulations and XAFS spectroscopy suggested that Ag+ is coordinated by three H2O oxygen at 2.37 Å and by two framework oxygen atoms between 2.55 and 3.00 Å. The thermal stability was monitored in situ by SC-XRD (from 25 to 400°C) and by XAFS (from 25 to 650°C). Upon heating the structure transformed to three different topologies: B phase, D’ phase, observed here for the first time, and D phase. The unit-cell volume contracted from 4392.85(14) at room temperature to 3644.4(4) Å3, measured at 400°C. Possible Ag+ to Ag0 reduction could be excluded although the formation of Ag+-Ag+ clusters could not be unambiguously ruled out.

Ag-exchanged zeolites are known to have improved sorption and catalytic properties compared to the raw natural material. The application range of Ag-exchanged zeolites is linked to the stability of the exchanged form and its structural evolution at high temperature. In this study, we investigated the thermal stability of an Ag-exchanged zeolite with LEV framework type. The dehydration path was monitored in situ by Single Crystal X-ray Diffraction (SC-XRD) and X-ray Absorption Fine Structure Spectroscopy (XAFS). The experimental data were compared with those extrapolated from Molecular Dynamics (MD) trajectories. Our results showed that Ag-LEV follows a different dehydration path compared to that of the natural levyne (Ca-LEV). Between 25 and 350°C, the unit-cell volume contraction was -4% with respect to that measured at room temperature. Upon dehydration, Ag-LEV transformed to the LEV B topology: such transformation is accompanied by the change from R-3m to R3m space group and by the onset of the rupture of one T-O-T connection at 250°C. Differently from Ca-LEV, no additional change to LEV B’ configuration was detected. XAFS analysis indicated that each Ag is approximately surrounded by 4 oxygen atoms between 2.15 and 2.40 Å. This local environment was maintained up to 650°C. Weak Ag+1-Ag+1 interactions, detected in the whole investigated temperature range, are mainly ascribed to the presence of Ag-erionite (Ag-ERI) intergrown with Ag-LEV. No reduction to elemental Ag0 occurred upon heating.

The possibility of forming magnesite from brucite crystallites, in a CO2-rich environment, has attracted large interest as a possible industrial procedure to store CO2 in the form of carbonate minerals. The reaction mechanisms responsible for such processes are, however, not very well-known. In this work we first consider the compatibility of the magnesite and brucite structures, along specific crystallographic directions. In the second part we describe the sequence of events that leads to the formation of a magnesite layer, via the dehydroxylation of brucite and consequent adsorption and diffusion of carbon complexes. To observe these thermally activated events, we employ a method recently developed in our group1,2 for finding complex reaction pathways and reproducing the related free energy surface. We find that the (1100) brucite surface can be dehydroxylated quite easily. The formation of vacancies in the hydroxyl layers might favor the diffusion of OH and protons through the channels between two neighboring (0001) OH planes, which characterize the brucite structure. This mechanism creates the necessary conditions for further dehydroxylation and formation of magnesite layers