First-principle based modeling of urea decomposition kinetics in aqueous solutions

Copper and zinc removal from anaerobic digestates via Sporosarcina pasteurii induced precipitation: Effect of volatile fatty acids on process performance

Microbial induced carbonate precipitation (MICP) shows great potential for metals recovery from secondary sources, which is vital for circular economy. This study explores the feasibility of using Sporosarcina pasteurii for MICP to recover copper (Cu) and zinc (Zn) from acidogenic anaerobic digestates at laboratory scale. Pre-cultured S. pasteurii was inoculated into solutions containing 20 g L-1 of urea and varying concentrations of Cu and Zn (0-25 mg L-1). The system was maintained at 30 °C with continuous agitation for seven days to assess Cu and Zn removal at initial pH values of 5, 6 and 7. The influence of volatile fatty acids (VFAs) on urea hydrolysis and Cu and Zn removal via S. pasteurii-induced MICP was evaluated by adding 3 g COD L-1 of acetic and propionic acids to metal solutions. Results showed that S. pasteurii enhanced Cu and Zn removal, with yields varying from 22% to 100% depending on the initial pH. In the presence of VFAs, Cu and Zn removal was significantly reduced (p < 0.05), however, only S. pasteurii-incubated samples exhibited Cu and Zn removal, indicating exclusive biological-driven removal. The primary mechanisms of action inferred for Cu and Zn removal in VFAs-spiked samples involved urea hydrolysis, which increased local pH and facilitated metals precipitation, as well as the adsorption of metal ions onto the negatively charged S. pasteurii cell wall. This study demonstrates the potential of S. pasteurii to enhance Cu and Zn removal from VFAs-containing media paving the way for a sustainable metals recovery alternative from waste streams.

Reaction mechanisms for methyl isocyanate (CH3NCO) gas-phase degradation

Methyl isocyanate (MIC) is a toxic chemical found in many commercial, industrial, and agricultural processes, and was the primary chemical involved in the Bhopal, India disaster of 1984. The atmospheric environmental chemical reactivity of MIC is relatively unknown with only proposed reaction channels, mainly involving OH-initiated reactions. The gas-phase degradation reaction pathways of MIC and its primary product, formyl isocyanate (FIC), were investigated with quantum mechanical (QM) calculations to assess the fate of the toxic chemical and its primary transformation products. Transition state energy barriers and reaction energetics were evaluated for thermolysis/pyrolysis-like reactions and bimolecular reactions initiated by relevant radicals (•OH and Cl•) to evaluate the potential energy surfaces and identify the primary reaction pathways and products. Thermolysis/pyrolysis of MIC requires high energy to initiate N-CH3 and C-H bond dissociation and is unlikely to dissociate except under extreme conditions. Bimolecular radical addition and H-abstraction reaction pathways are deemed the most kinetically and thermodynamically favorable mechanisms. The primary transformation products of MIC were identified as FIC, methylcarbamic acid, isocyanic acid (isocyanate radical), and carbon dioxide. The results of this work inform the gas-phase reaction channels of MIC and FIC reactivity and identify transformation products under various reaction conditions.

Theoretical Study of the Thermal Decomposition of Urea Derivatives

An extensive theoretical study of the thermal decomposition of alkyl- and phenylureas, which are widely used in the pesticides, pharmaceuticals, and materials industries, has been carried out using electronic structure calculations and reaction rate theories. Enthalpies of formation and bond dissociation energies (BDE) of 11 urea derivatives have been calculated using different levels of theory (CBS-QB3, CCSD(T)/CBS//M06-2X/6-311++G(3df,2pd), and CBS-QM062X) according to the size of the system. Potential energy surfaces for the unimolecular decomposition pathways of these urea derivatives were also systematically computed for the first time. Several pericyclic reactions can be envisaged, as a function of the size and the nature of the N substituents, and all of these pathways were explored. Our calculations show that these compounds are solely decomposed by four-center pericyclic reactions, yielding substituted isocyanates and amines, and that initial bond fissions are not competitive. Based on the set of urea derivatives studied, a new reaction rate rule for their thermal decomposition was defined and involves the nature of the transferred H atom (primary or secondary/alkyl or benzyl) and the nature of the N-atom acceptor (primary, secondary, or tertiary). This new reaction rate rule allows us to determine the product branching ratios in the thermal decomposition of a given urea derivative and its total rate of decomposition. Applications on urea derivatives used in the chemical industry are presented and illustrate the usefulness of this new rate rule that allows to predict the previously unknown thermal decomposition kinetics of a large number of these compounds.

Detailed Kinetic Model for the Thermal Decomposition of Hydrazine Nitrate in Nitric Acid Solution Based on Quantum Chemistry Calculations Combined with the Polarizable Continuum Model

The decomposition mechanism of hydrazine nitrate in nitric acid solutions was investigated using quantum chemistry calculations combined with the polarizable continuum model at the CBS-QB3//ωB97X-D/SMD level of theory. These calculations provided a detailed kinetic model incorporating rate coefficients and thermodynamic data. Rate coefficients were determined using traditional transition state theory, while diffusion-limited reactions were modeled based on the Einstein-Stokes equations. The resulting model comprised the kinetics for 108 reactions and thermodynamic data for 58 species. This model was validated by comparing simulations of the variations in chemical species during the decomposition process to experimental data acquired under isothermal conditions at 100 °C. The model was found to accurately reproduce the concentration changes of N₂H₄, HN₃, and NH₃ and also explained the reaction mechanism. The thermal decomposition was found to proceed via two parallel paths: N₂H₄ + HNO₃ → H₂O + HONO + N₂H₂ and N₂H₄ + HONO → HN₃ + 2H₂O. Following these reactions, a portion of the HN₃ decomposes to produce NH₃ through a multistep process. A sensitivity analysis showed that the rate of decomposition is greatly affected by the pH of the solution.

Catalyzed reaction of isocyanates (RNCO) with water

The reactions between substituted isocyanates (RNCO) and other small molecules (e.g. water, alcohols, and amines) are of significant industrial importance, particularly for the development of novel polyurethanes and other useful polymers. We present very high-level ab initio computations on the HNCO + H₂O reaction, with results targeting the CCSDT(Q)/CBS//CCSD(T)/cc-pVQZ level of theory. Our results affirm that hydrolysis can occur across both the N[double bond, length as m-dash]C and C[double bond, length as m-dash]O bonds of HNCO via concerted mechanisms to form carbamate or imidic acid with ΔH_0K barrier heights of 38.5 and 47.5 kcal mol^-1. A total of 24 substituted RNCO + H₂O reactions were studied. Geometries obtained with a composite method and refined with CCSD(T)/CBS single point energies determine that substituted RNCO species have a significant influence on these barrier heights, with an extreme case like fluorine lowering both barriers by close to 15 kcal mol^-1 and most common alkyl substituents lowering both by approximately 3 kcal mol^-1. Natural Bond Orbital (NBO) analysis provides evidence that the predicted barrier heights are strongly associated with the occupation of the in-plane C-O* orbital of the RNCO reactant. Key autocatalytic mechanisms are considered in the presence of excess water and RNCO species. Additional waters (one or two) are predicted to lower both barriers significantly at the CCSD(T)/aug-cc-pV(T+d)Z level of theory with strongly electron withdrawing RNCO substituents also increasing these effects, similar to the uncatalyzed case. The 298 K Gibbs energies are only marginally lowered by a second catalyst water molecule, indicating that the decreasing ΔH_0K barriers are offset by loss of translational entropy with more than one catalyst water. Two-step 2RNCO + H₂O mechanisms are characterized for the formation of carbamate and imidic acid. The second step of these two pathways exhibits the largest barrier and presents no clear pattern with respect to substituent choice. Our results indicate that an additional RNCO molecule might catalyze imidic acid formation but have less influence on the efficiency of carbamate formation. We expect that these results lay a firm foundation for the experimental study of substituted isocyanates and their relationship to the energetic pathways of related systems.

On the influence of water on urea condensation reactions: a theoretical study

The influence of water molecules on the kinetics of urea condensation reactions was studied with high-level quantum chemical methods and statistical rate theory. The study focuses on the production of biuret, triuret, and cyanuric acid from urea because of their relevance as unwanted byproducts in the urea-based selective catalytic reduction (urea-SCR) exhaust after treatment of Diesel engines. In order to characterize the potential energy surfaces and molecular reaction pathways, calculations with explicitly-correlated coupled-cluster methods were performed. It turned out that the reactions proceed via pre-reactive complexes and the inclusion of one or two water molecules into the condensation mechanisms leads to a decrease of the energy barriers. This effect is particularly pronounced in the production of biuret. Due to the pre-reactive equilibria, the rates of the overall reactions can increase or decrease by incorporating water into the mechanism, depending on the temperature and water concentration. Under the conditions of urea-SCR, the studied reactions are too slow to contribute to the observed byproduct formation.

Kinetics of Aqueous Media Reactions via Ab Initio Enhanced Molecular Dynamics: The Case of Urea Decomposition

Aqueous solutions provide a medium for many important reactions in chemical synthesis, industrial processes, environmental chemistry, and biological functions. It is an accepted fact that aqueous solvents can be direct participants in the reaction process and not act only as simple passive dielectrics. Assisting water molecules and proton wires are thus essential for the efficiency of many reactions. Here, we study the decomposition of urea into ammonia and isocyanic acid by means of enhanced ab initio molecular dynamics simulations. We highlight the role of the solvent molecules and their interactions with the reactants providing a proper description of the reaction mechanism and how the water hydrogen-bond network affects the reaction dynamics. Reaction free energy and rates have been calculated taking into account this important effect.

A simple heuristic approach to estimate the thermochemistry of condensed-phase molecules based on the polarizable continuum model

A simple model based on a quantum chemical approach with polarizable continuum models (PCMs) to provide reasonable translational and rotational entropies for liquid phase molecules was developed.