Effects of Hydration on the Adsorption of Benzohydroxamic Acid on the Lead-Ion-Activated Cassiterite Surface: A DFT Study

First-Principles Insights into the Selective Separation of MoS42- and WO42-: Crucial Role of Hydration Structures

The selective separation of MoS42- and WO42- using quaternary ammonium salt through solvent extraction or ion exchange methods has been well-established in the metallurgical industry. However, the conventional electrostatic adsorption theory falls short in explaining the separation mechanism. Through first-principles density functional theory (DFT) calculations and newly self-developed deep potential molecular dynamics (DPMD) simulation method, our work first reveals that the disparity in hydration structures of MoS42- and WO42- plays a crucial role in their selective separation. It is proposed that MoS42- and WO42- anions undergo hydration to form [MoS4(H2O)n]2- and [WO4(H2O)n]2-, respectively, facilitated by hydrogen bond (H-bond) interactions. Emphasis is placed on the discrepancy between MoS42- and WO42- in hydration structures by the hydration energy, Hirshfeld charge, evaluation of weak interactions, hydration radius, hydration coordination number, and H-bonds distribution. MoS42- presents a larger first hydration radius and a lower first hydration coordination number due to weaker interactions with H2O, while WO42- is subjected to enhanced hydration shielding, resulting in MoS42- anions being more susceptible to be selectively separated by a quaternary ammonium salt. This insight paves the way for the selective separation of MoS42- and WO42-, further bridging the gap between theory and industry applications.

Facet-dependent adsorption of heavy metal ions on Janus clay nanosheets

Understanding the facet-dependent adsorption behavior and mechanism of heavy metal ions (HMs) on two-dimensional (2D) Janus nanoclays has important implications for the environment and ecosystem but still remains elusive. Herein, ultrathin Janus serpentene (2D serpentine) nanosheets were fabricated via a facile, nontoxic, and residue-free exfoliation strategy. Fabricated serpentene nanosheets exhibited promising Cd(II) and Pb(II) adsorption capacities due to their high surface areas and abundant active sites, approximately four times higher than those of bulk serpentine powders. Interestingly, Cd(II) and Pb(II) adsorption on serpentene nanosheets exhibited a facet-dependent feature, with the adsorption amount on the Mg-OH plane considerably higher than that on the Si-O plane. This facet-dependent adsorption behavior was mainly attributed to the difference in the interaction mechanisms of HMs with the Mg-OH (monodentate inner-sphere complexation) and Si-O (outer-sphere complexation) planes, which was further confirmed via density functional theory calculations. The Cd(II) adsorption on serpentene nanosheets was limited by strong kinetic restrictions (e.g., stronger electrostatic repulsion and higher dehydration energy barrier than that for Pb(II) adsorption). This study provides insights into the facet-dependent adsorption mechanisms of HMs on Janus serpentene nanosheets, which can be extended to other nanoclays used in wastewater treatment and many environmental processes.

Emerging Trends of Computational Chemistry and Molecular Modeling in Froth Flotation: A Review

Froth flotation is the most versatile process in mineral beneficiation, extensively used to concentrate a wide range of minerals. This process comprises mixtures of more or less liberated minerals, water, air, and various chemical reagents, involving a series of intermingled multiphase physical and chemical phenomena in the aqueous environment. Today's main challenge facing the froth flotation process is to gain atomic-level insights into the properties of its inherent phenomena governing the process performance. While it is often challenging to determine these phenomena via trial-and-error experimentations, molecular modeling approaches not only elicit a deeper understanding of froth flotation but can also assist experimental studies in saving time and budget. Thanks to the rapid development of computer science and advances in high-performance computing (HPC) infrastructures, theoretical/computational chemistry has now matured enough to successfully and gainfully apply to tackle the challenges of complex systems. In mineral processing, however, advanced applications of computational chemistry are increasingly gaining ground and demonstrating merit in addressing these challenges. Accordingly, this contribution aims to encourage mineral scientists, especially those interested in rational reagent design, to become familiarized with the necessary concepts of molecular modeling and to apply similar strategies when studying and tailoring properties at the molecular level. This review also strives to deliver the state-of-the-art integration and application of molecular modeling in froth flotation studies to assist either active researchers in this field to disclose new directions for future research or newcomers to the field to initiate innovative works.

Effects of Interfacial Hydroxylation Microstructure on Quartz Flotation by Sodium Oleate

Quartz, a common inorganic nonmetallic mineral, is usually removed or purified by beneficiation, normally flotation. Given the strong polarity of the quartz surface, it is easy to hydrate to form a hydroxylation layer, which makes it impossible to float quartz with sodium oleate (OL) used alone. An ideal flotation method for quartz is preactivation with Ca²⁺, followed by collection with OL. Herein, the effects of surface hydroxylation on the adsorption of the anionic collector OL on the quartz surface before and after Ca²⁺ activation are systematically investigated by density functional theory (DFT) calculations. The results show that the displacement adsorption of surface hydroxyl substituted by OL^- is not feasible in thermodynamics, and the OL^- can only bind to the H atoms of the hydroxylated quartz surface via hydrogen bonds, namely, hydrogen binding adsorption. Due to the electrostatic repulsion and steric hindrance effect induced by the surface hydroxylation structure, the adsorption ability of OL^- on the quartz surface mediated by hydroxyl bridges is very weak, which is insufficient to realize quartz floating. However, Ca²⁺ ions are easily adsorbed on the hydroxylated quartz surface, providing favorable active sites for subsequent adsorption of OL^-, thus becoming a credible solution for the industrial flotation of the strong hydrophilic mineral quartz. These findings shed some new insights for accurately understanding the flotation mechanism of strongly hydrophilic oxide minerals and are beneficial to promoting the development of mineral flotation fundamentals.

Adsorption of Trisiloxane Surfactant for Selective Flotation of Scheelite from Calcite at Room Temperature

The separation and enrichment of scheelite from calcite are hindered by the similar active Ca²⁺ sites of scheelite and the calcite with calciferous gangue. Herein, a novel trisiloxane surfactant, N-(2-aminoethyl)-3-aminopropyltrisiloxane (AATS), was first explored and synthesized and recommended as the collector for the flotation separation of scheelite from calcite. The micro-flotation and mixed binary mineral flotation tests showed that AATS had excellent collection performance for scheelite and high selectivity for calcite within a wide pH range. At the same time, contact angle and zeta-potential measurements, Fourier transform infrared (FTIR) analysis, and density functional theory (DFT) calculations revealed the relevant adsorption mechanism. The contact angle measurement showed that AATS can increase the contact angle of the scheelite surface from 41.7 to 95.8°, greatly enhancing the hydrophobicity of the mineral surface. The results of FTIR analysis and zeta-potential measurement explained that AATS was electrostatically adsorbed on the mineral surface, and DFT calculation further verified that the -N⁺H₃-positive group in AATS was adsorbed on the negatively charged scheelite surface. Therefore, AATS can realize the expectation of high efficiency and selectivity of minerals and enhance the adhesion between the surface of scheelite minerals and bubbles, providing a fresh approach to industrial production.

Selective Separation of Fluorite from Scheelite Using N-Decanoylsarcosine Sodium as a Novel Collector

Fluorite and scheelite, which are strategic calcium-bearing minerals, have similar active sites (Ca2+); as a result, the efficient separation of the two minerals is still one of the world’s most difficult problems in the field of flotation. In this work, N-decanoylsarcosine sodium (SDAA), a non-toxic and low-cost amino acid surfactant, was applied in the flotation separation of fluorite from scheelite for the first time. In the test, single mineral, binary mixed minerals, and actual ore experiments showed that the pre-removal of fluorite from scheelite by reverse flotation can be achieved. The results of adsorption capacity detections, zeta potential tests, and FTIR analysis showed that the negatively charged SDAA prefers to adsorb onto the positively charged fluorite surface due to the electrostatic interaction. The results of crystal chemistry and DFT calculations showed that SDAA has a stronger chemical interaction and more electron transfer numbers to the Ca atom on the fluorite surface and forms a Ca-SDAA complex. Therefore, the significant difference in the adsorption behavior of SDAA on the surfaces of two minerals provided a new insight into the separation efficiency of amino acids and possesses a great potential for industrial application in scheelite flotation.

Discovery of Electronic Structure and Interfacial Interaction Features in Catalytic Activity

The selective transformation of inert bonds (C-H, C-O, C-C, C-F, etc.) via various catalysts is one of the most challenging areas, with applications in organic synthesis, materials science, and biological and pharmaceutical chemistry. The catalytic performance of homogeneous and heterogeneous catalysts can be rationally controlled in two ways: (i) electronic structure modulation of the active site, such as the metal center, ligands, and coordination modes, to improve the catalytic activity and stability and (ii) tuning intermolecular or interfacial interactions to promoting the reaction kinetics by accelerating the transmission of electrons between the catalyst and solvents or support. The rational design of catalysts based on adjustable features, such as metal (monometallic or bimetallic) active sites, crystal phase, ligands, solvents, and supports for inert bond activation under mild conditions remains a challenge. This Perspective summarizes the features of electronic structures, interfacial interactions, and their effects on molecular catalysis, metal-organic frameworks (MOFs), and natural mineral catalysis. The discovery of efficient catalysts could be promoted using machine-learning methods with high-performance descriptors. More attention should be paid to high-throughput quantum-chemical computations and experiments, automatic searches of chemical reaction pathways, and efficient machine-learning or deep-learning methods to accelerate catalyst design and synthesis in the future.

The multiscale solvation effect on the reactivity of β-O-4 of lignin dimers in deep eutectic solvents

Deep eutectic solvents (DESs) emerge as a medium to enhance the depolymerization of lignin. One critical question is how the solvation of lignin in DESs may affect the reactivity of lignin. To shed light on this question, we investigate the solvation of four lignin dimers in three DES solutions using molecular dynamics simulations and quantum mechanical calculations. The four lignin dimers are composed of guaiacyl and syringyl units and are used as the models for lignin. The three DES solutions are composed of choline, Cl^- and three acids: lactic acid, levulinic acid and oxalic acid. We investigate the preferential accumulation of individual DES components in the solvation shells and the exposure area and electrostatic potential of the β-O-4 linkage of the four lignin dimers in the three DESs. The results show that DESs could influence the affinity and nucleophilicity of the β-O-4 linkage through three effects: (1) forming a charged solvation shell, (2) varying the exposure of the β-O-4 linkage and (3) adjusting the electrostatic potential of the β-O-4 linkage. Our simulations indicate a comprehensive and multiscale effect of DESs on lignin decomposition.

Effects of the Goethite Surface Hydration Microstructure on the Adsorption of the Collectors Dodecylamine and Sodium Oleate

Dodecylamine (DDA) and sodium oleate (OL) are commonly used collectors in the reverse flotation and the direct flotation of goethite. However, the flotation mechanisms of DDA and OL on the goethite surface remain unclear. In this study, the first-principles density functional theory calculations were used to reveal the role of the hydration of the goethite surface and its effects on flotation reagents from a microscopic perspective. The calculation results showed that DDA was adsorbed on the surface of goethite by hydrogen bonds in the absence of hydration. However, the existence of the hydration microstructure hindered the formation of hydrogen bonds and made it difficult for DDA to be adsorbed on the goethite surface. In the OL system, oleate ions are chemically adsorbed on the surface Fe sites of goethite in the absence of hydration, while in the presence of hydration, the oleate ions were adsorbed on the H-terminal hydration surface of goethite by hydrogen bonds. This work sheds new light on the roles of the hydration microstructure and the adsorption mechanism of the flotation reagent on the oxide minerals.

Interface Interaction of Benzohydroxamic Acid with Lead Ions on Oxide Mineral Surfaces: A Coordination Mechanism Study

Surface coordination chemistry is important in areas such as adsorption, separation, and catalysts. In this work, surface coordination interactions of benzohydroxamic acid (BHA) with the lead ion [Pb(II)] adsorbed on the cassiterite surface have been investigated by first-principles calculations due to its great significance in froth flotation. Cluster calculations show that BHA possesses the weakest chelation with Pb(II) due to the electron withdrawal ability of the benzyl ring in comparison with other hydroxamic acids. Pb(II) thermodynamically prefers to react with the cassiterite surface rather than BHA. On the other hand, the partial density of states and the atomic overlap populations have consistently verified that the adsorption of BHA results in a better symmetry in electron densities than the hydrated Pb(II). The electron density maps and the electronic localization functions have further visualized the rearrangement of the 6s² lone pair around the lead atom. It can be concluded that the surface coordination mechanisms of Pb(II) on oxide minerals can be attributed to the coordination ability of BHA and the unique electronic structure of Pb(II), which accounts for the reported better flotation performance of the pre-assemble strategy than the pre-activating approach. This work sheds some new light on the unique coordination activation mechanism of metal ions on oxide mineral surfaces. It should be instructive to design and screen new environment-friendly flotation reagents and flotation flowsheets.