Thermodynamic model of MSWI flue gas cooling path: Effect of flue gas composition on heavy metal binding forms

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, Cu and Zn precipitate as oxides (and Zn silicates) at approximately 650°C. At temperatures <300°C, Zn, Cu, Pb and Cd are predicted to precipitate as easily soluble chlorides. In flue gas with 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.

Cyano(fluoro)borate and cyano(hydrido)borate ionic liquids: low-viscosity ionic media for electrochemical applications

The synthesis and detailed characterization of low-viscosity room-temperature ionic liquids (RTILs) and [BnPh₃P]⁺ salts with the cyano(fluoro)borate anions [BF(CN)₃]^- (MFB), [BF₂(CN)₂]^- (DFB), and [BF₃(CN)]^- as well as the new mixed-substituted anion [BFH(CN)₂]^- (FHB) is described. The RTILs with [EMIm]⁺ or [BMPL]⁺ as countercations were obtained in yields of up to 98% from readily available alkali metal salts and in high purities that allow application in electrochemical devices. Trends in thermal stability, melting and freezing behavior, density, electrochemical stability, dynamic viscosity, specific conductivity and ion diffusivity have been assessed and compared to those of the related tetracyanoborate- and cyano(hydrido)borate-RTILs. The crystal structure analysis of the [BnPh₃P]⁺ salts of [BF_n(CN)_4-n]^- (n = 0-4), [BH_n(CN)_4-n]^- (n = 1-3) and [BFH(CN)₂]^- provided experimental access to anion volumina that together with ion molecular mass, electrostatic potential, shape and chemical stability have been correlated to physicochemical properties. In addition, the cytotoxicity of the [EMIm]⁺-ILs and potassium or sodium salts was studied.

Unconventional Pathways to Carbide Phase Synthesis via Thermal Decomposition of UI4(1,4-dioxane)2

UI₄(1,4-dioxane)₂ was subjected to laser-based heating─a method that enables localized, fast heating (T > 2000 °C) and rapid cooling under controlled conditions (scan rate, power, atmosphere, etc.)─to understand its thermal decomposition. A predictive computational thermodynamic technique estimated the decomposition temperature of UI₄(1,4-dioxane)₂ to uranium (U) metal to be 2236 °C, a temperature achievable under laser irradiation. Dictated by the presence of reactive, gaseous byproducts, the thermal decomposition of UI₄(1,4-dioxane)₂ under furnace conditions up to 600 °C revealed the formation of UO₂, UI_x, and U(C_1-xO_x)_y, while under laser irradiation, UI₄(1,4-dioxane)₂ decomposed to UO₂, U(C_1-xO_x)_y, UC_2-zO_z, and UC. Despite the fast dynamics associated with laser irradiation, the central uranium atom reacted with the thermal decomposition products of the ligand (1,4-dioxane = C₄H₈O₂) instead of producing pure U metal. The results highlight the potential to co-develop uranium precursors with specific irradiation procedures to advance nuclear materials research by finding new pathways to produce uranium carbide.

Ammonium Formate as a Safe, Energy-Dense Electrochemical Fuel Ionic Liquid

While solid and liquid energy carriers are advantageous due to their high energy density, many do not meet the efficiency requirements to outperform hydrogen. In this work, we investigate ammonium formate as an energy carrier. It can be produced economically via a simple reaction of ammonia and formic acid, and it is safe to transport and store because it is solid under ambient conditions. We demonstrate an electrochemical cell that decomposes ammonium formate at 105 °C, where it is an ionic liquid. Here, hydrogen evolves at the cathode and formate oxidizes at the anode, both with ca. 100% Faradaic efficiency. Under the operating conditions, ammonia evaporates before it can oxidize; a second, modular device such as an ammonia fuel cell or combustion engine is necessary for complete oxidation. Overall, this system represents an alternative class of electrochemical fuel ionic liquids where the electrolyte is majority fuel, and it results in a modular release of hydrogen with potentially zero net-carbon emissions.

Porewater compositions of Portland cement with and without silica fume calculated using the fine-tuned CASH+NK solid solution model

The CASH+ sublattice solid solution model of C-S-H aims to predict the composition of C-S-H and its ability to take up alkalis. It was originally developed for dilute systems with high water-solid ratios, and thus in this paper further optimized and benchmarked against measured pore solution compositions of hydrated Portland cement (PC) and PC blended with silica fume (SF) at realistic water-binder ratios. To get an improved agreement with the pore solution data, the stability of two CASH+ model endmembers, TCKh and TCNh, has been fine-tuned with standard Gibbs energy corrections of + 7.0 and + 5.0 kJ·mol^-1, respectively (at 1 bar, 25 °C). The agreement was maintained with the experiments used to originally parameterize the CASH+ model for the uptake of K and Na in dilute systems. The K and Na concentrations predicted using the fine-tuned CASH+NK model are in a good agreement with the measured values for PC and PC + SF system at different water to binder ratios, silica fume additions, and at temperatures up to 80 °C.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1617/s11527-022-02045-0.

A comparison of methods for the estimation of the enthalpy of formation of rare earth compounds

Rare earth elements are helping drive the global transition towards a greener economy. However, the way in which they are produced is far from being considered green. One of the major obstacles to developing greener production methods and the design of novel processes and materials involving rare earth elements is the limited thermodynamic data available. In the present work, we apply a suite of methods to estimate the enthalpy of formation of several rare earth compounds, including a new method based on a linear relationship, established by the authors. Experimental values of the enthalpy of formation of LnCl₃, LnOCl, LnPO₄, Ln₂O₂S, Ln₂O₂CO₃ and NaLnO₂ were collated and used to assess the accuracy of the different methods, which were then used to predict values for compounds for which no data exists. It is shown that Mostafa et al.'s group contribution method and the linear relationship proposed by the authors give the lowest mean absolute error (<9%). The volume based thermodynamics (VBT) method yields estimates with absolute mean errors below 16.0% for LnPO₄ and Ln₂O₂S, but above 26.0% for other compounds. Correction of the VBT method using an improved estimate of the Madelung energy for the calculation of the lattice enthalpy decreases the absolute mean error below 12.0% for all compounds except LnPO₄. These complementary methods provide options for calculating the enthalpy of formation of rare earth compounds, depending on the experimental data available and desired accuracy.

The effective volumes of waters of crystallization: general organic solids

Using a list of compatible hydrate/anhydrate pairs prepared by van de Streek and Motherwell [CrystEngComm (2007), 9, 55-64], we have examined the effective volume per water of crystallization for 179 pairs of organic solids using current data from the Cambridge Crystallographic Structural Database (CSD). The effective volume is the difference per water molecule between the asymmetric unit volumes of the hydrate and parent anhydrate, and has the mean value 24 ų. The conformational changes in the reference molecule between the hydrate and its anhydrate are shown in two figures: one for a relatively rigid standard organic molecule and (in the supplementary file) one for a more flexible linear molecule. Using data from Nyman and Day [Phys. Chem. Chem. Phys. (2016), 18, 31132-31143], we have also established a generic volumetric coefficient of thermal expansion of organic solids with a value of 147 ± 56 × 10^-6 K^-1. There is a significant number of outliers to the data, negative, near zero, and large and positive. Some explanation for the existence of these outliers is attempted.

The effective volumes of waters of crystallization: non-ionic pharmaceutical systems

The physical properties of organic solids are altered when hydrated (and, more generally, when solvated) and this is of particular significance for pharmaceuticals in application; for instance, the solubility of a hydrate is less than that of its parent. The effective volumes of waters of crystallization for non-ionic pharmaceuticals (where the `effective' volume is the difference per water molecule between the hydrate volume and the volume of the anhydrous parent) are here examined. This investigation contrasts with our earlier study of effective volumes of waters of crystallization for ionic materials where the coulombic forces are paramount. Volumetric properties are significant since they correlate strongly with many thermodynamic properties. Twenty-nine hydrate/parent systems have been identified, and their volumetric properties are reported and analysed (apart from aspartame and ephedrine for which the structural data are inconsistent). Among these systems, the data for paracetamol are extensive and it is possible to differentiate among the volumetric properties of its three polymorphs and to quantify the effect of temperature on their volumes. The effective volumes in both ionic and non-ionic systems are similar, with a median effective volume of 22.8 Å3 for the non-ionic systems compared with 24.2 Å3 for the ionic systems, and both are smaller than the molecular volume of 30 Å3 of ambient liquid water – which appears to be an upper limit to the effective volumes of waters of crystallization under ambient conditions. These results will be supportive in checking and confirmation of hydrated crystal structures and in assessing their thermodynamic properties.

Understanding the Stability of Salt-Inclusion Phases for Nuclear Waste-forms through Volume-based Thermodynamics

Formation enthalpies and Gibbs energies of actinide and rare-earth containing SIMs with silicate and germanate frameworks are reported. Volume-based thermodynamics (VBT) techniques complemented by density functional theory (DFT) were adapted and applied to these complex structures. VBT and DFT results were in closest agreement for the smaller framework silicate structure, whereas DFT in general predicts less negative enthalpies across all SIMs, regardless of framework type. Both methods predict the rare-earth silicates to be the most stable of the comparable structures calculated, with VBT results being in good agreement with the limited experimental values available from drop solution calorimetry.

Physicochemical characterizations of novel dicyanamide-based ionic liquids applied as electrolytes for supercapacitors

Novel ionic liquids (ILs), containing a dicyanamide anion (DCA-), are synthesized and applied as suitable electrolytes for electrochemical double layer capacitors (EDLCs). The prepared ILs are either composed of triethyl-propargylammonium (N222pr +) or triethyl-butylammonium (N2224 +) cations paired with the DCA- anion. The structure of the cation influences its electrostatic interaction with the DCA- anion and highly impacts the physical and electrochemical properties of the as-prepared ILs. The geometry and the length of the alkyl chain of the propargyl group in N222pr + enhance the ionic conductivity of N222pr-DCA (11.68 mS cm-1) when compared to N2224-DCA (5.26 mS cm-1) at 298 K. It is demonstrated that the Vogel-Tammann-Fulcher model governs the variations of the transport properties investigated over the temperature range of 298-353 K. A maximum potential window of 3.29 V is obtained when N222pr-DCA is used as electrolyte in a graphene based symmetric EDLC system. Cyclic voltammetry and galvanostatic measurements confirm that both electrolytes exhibit an ideal capacitive behavior. The highest specific energy of 55 W h kg-1 is exhibited in the presence of N2224-DCA at a current density of 2.5 A g-1.

A density functional theory based approach for predicting melting points of ionic liquids

Accurate prediction of melting points of ILs is important both from the fundamental point of view and from the practical perspective for screening ILs with low melting points and broadening their utilization in a wider temperature range. In this work, we present an ab initio approach to calculate melting points of ILs with known crystal structures and illustrate its application for a series of 11 ILs containing imidazolium/pyrrolidinium cations and halide/polyatomic fluoro-containing anions. The melting point is determined as a temperature at which the Gibbs free energy of fusion is zero. The Gibbs free energy of fusion can be expressed through the use of the Born-Fajans-Haber cycle via the lattice free energy of forming a solid IL from gaseous phase ions and the sum of the solvation free energies of ions comprising IL. Dispersion-corrected density functional theory (DFT) involving (semi)local (PBE-D3) and hybrid exchange-correlation (HSE06-D3) functionals is applied to estimate the lattice enthalpy, entropy, and free energy. The ions solvation free energies are calculated with the SMD-generic-IL solvation model at the M06-2X/6-31+G(d) level of theory under standard conditions. The melting points of ILs computed with the HSE06-D3 functional are in good agreement with the experimental data, with a mean absolute error of 30.5 K and a mean relative error of 8.5%. The model is capable of accurately reproducing the trends in melting points upon variation of alkyl substituents in organic cations and replacement one anion by another. The results verify that the lattice energies of ILs containing polyatomic fluoro-containing anions can be approximated reasonably well using the volume-based thermodynamic approach. However, there is no correlation of the computed lattice energies with molecular volume for ILs containing halide anions. Moreover, entropies of solid ILs follow two different linear relationships with molecular volume for halides and polyatomic fluoro-containing anions. Continuous progress in predicting crystal structures of organic salts with halide anions will be a key factor for successful prediction of melting points with no prior knowledge of the crystal structure.

Absolute ion hydration enthalpies and the role of volume within hydration thermodynamics

This paper reports that various thermodynamic properties in aqueous media for certain individual ions and for compounds are linear functions of the inverse cube root of the solid respective ionic and compound solid state volumes, V_m^−1/3.