Adsorption of organic molecules on mineral surfaces studied by first-principle calculations: A review

Temperature effects and molecular insights towards the optimization of polyvinyl alcohol as adsorbent of organic pollutants from aqueous solution

One of the easier methods of wastewater treatment is adsorption due to its simplicity in implementation, environmental friendliness, and economic feasibility. Polyvinyl alcohol (PVA) looks promising as an adsorbent due to its biocompatible, non-toxic, water-soluble and eco-friendly nature. The investigation of PVA for its potential in the adsorption of pollutants has been reported in many studies but the mechanistic understanding of the adsorption is poor. The present study used a theoretical approach through density functional theory and Monte Carlo with molecular dynamics simulations to investigate the adsorption mechanism behaviors of model organic molecules (bromothymol blue (BTB), methylene blue (MB), metronidazole (MNZ) and tetracycline (TC)) on PVA surface. The quantum chemical calculations result showed that with the increase in PVA chains (2, 4, 8, 16, and 32 units), the zero-point energy decreases (from -308.79 to -4922.93 kcal/mol) while the dipole moment increases (from 4.37 to 87.52 Debye). Temperature effect on the PVA chain structures showed the same trends for all the chain units and with the increase in temperature (50-600 K), there are no appreciable changes in zero-point energy, enthalpy energy increases while Gibbs free energy decreases. Considering PVA-pollutant complexes, the effects of temperature on the structures showed that there are no appreciable changes in the zero-point energy, Gibbs free and thermal energies increase with an increase in temperature while the kinetic rate of reactions decreases with an increase in temperature. The enthalpy of the reaction showed different trends with antibiotic and dye complexes. In all the thermodynamic properties investigated and the rate of reaction, the order of affinity of the pollutants with PVA followed TC > MNZ > MB > BTB. Monte Carlo simulation was used to investigate the adsorption behavior of the pollutants on the surface of PVA. The negative adsorption energies (-366.56 to -2266.81 kcal/mol) in terms of affinity towards the pollutants on the surface of PVA followed the sequence TC > MNZ > BTB > MB and the molecular dynamic simulation results followed the same order. The obtained results give valuable insights into the mechanism and performance of PVA as an adsorbent. Most of these computational observations are in good agreement with the available experimental results.

Progress in the Computer-Aided Analysis in Multiple Aspects of Nanocatalysis Research

Making the utmost of the differences and advantages of multiple disciplines, interdisciplinary integration breaks the science boundaries and accelerates the progress in mutual quests. As an organic connection of material science, enzymology, and biomedicine, nanozyme-related research is further supported by computer technology, which injects in new vitality, and contributes to in-depth understanding, unprecedented insights, and broadened application possibilities. Utilizing computer-aided first-principles method, high-speed and high-throughput mathematic, physic, and chemic models are introduced to perform atomic-level kinetic analysis for nanocatalytic reaction process, and theoretically illustrate the underlying nanozymetic mechanism and structure-function relationship. On this basis, nanozymes with desirable properties can be designed and demand-oriented synthesized without repeated trial-and-error experiments. Besides that, computational analysis and device also play an indispensable role in nanozyme-based detecting methods to realize automatic readouts with improved accuracy and reproducibility. Here, this work focuses on the crossing of nanocatalysis research and computational technology, to inspire the research in computer-aided analysis in nanozyme field to a greater extent.

What Determines the Low-Friction Mechanism of the Silicon-Doped Diamond-like Carbon Film in a Water Environment: An Atomic-Level Understanding

It is widely acknowledged that doping silicon can significantly enhance the friction performance of diamond-like carbon (DLC) films in a water environment. However, the mechanism of low friction caused by doped silicon is still highly controversial. Therefore, this article compares the interface interaction between DLC and Si-DLC films in a water environment through first-principles calculations of physisorption and chemisorption effects. The results indicate that water molecules are predominantly chemically adsorbed rather than physically adsorbed on the Si-DLC surface. Further study reveals that when OH-termination is formed on the Si-DLC surface, water molecules are predominantly physically adsorbed rather than chemically adsorbed on the Si-DLC hydroxylation surface. Consequently, a more stable hydration layer is formed on the surface through the hydrogen bond network formed by Si-OH groups, ultimately leading to lower friction. Moreover, molecular dynamics simulations further suggest that the lower friction coefficient of Si-DLC films in a water environment may be due to more water molecules at the friction interface and fewer interface covalent bonds. In short, the low-friction coefficient of the Si-DLC film in a water environment may be caused not only by the chemisorption of water molecules on its surface but also by the physisorption of water molecules on the Si-DLC film after surface hydroxylation.

A critical review and bibliometric analysis of methylene blue adsorption using leaves

The rapid development of the industrial world causes wastewater containing dyes to continue to increase. Even in recent years, the food, textile, cosmetic, plastic, and printing industries have developed the use of dyes. Methylene blue (MB) is one of the cationic dyes widely used in dyeing silk, wood, and cotton because of its absorbency and good fastness to materials. The adsorption process is the best technique and preferred in removing dyes from wastewater due to excellent selectivity, high efficiency from high-quality treated effluent, flexibility in design, and simplicity. Therefore, there is a growing interest to identify low-cost alternative adsorbents that have reasonable adsorption efficiency, especially natural materials such as leaves. In this study, research on MB adsorption using leaves was analyzed using bibliometric analysis. Information of bibliometric is extracted from the Scopus database with the keyword "Methylene Blue", "Adsorption or Desorption", and "Leaves or leaf". The results showed that India, Desalination and Water Treatment, and SASTRA Deemed University were the country, journal, and institution that contributed the most publications on this topic. Therefore, it is expected that with the use of bibliometrics, the use of leaf-based MB adsorption processes in their potential for MB dye removal can be investigated especially for large-scale development.

A multi-scale assessment of the impact of salinity on the desorption of chromate from hematite: Sea level rise implications

The solubility and transport of Cr(VI) is primarily controlled by adsorption-desorption reactions at the surfaces of soil minerals such as iron oxides. Environmental properties such as pH, ionic strength, and ion competition are expected to affect the mobility and fate of Cr(VI). Sea level rise (SLR), and consequent seawater intrusion, is creating a new biogeochemical soil environment at coastal margins, potentially impacting Cr(VI) retention at contaminated sites. We employed in-situ ATR-FTIR spectroscopy and DFT calculations to investigate at the molecular level the adsorption of Cr(VI) on the hematite surface and its desorption by sulfate, as a function of pH and ionic strength. We further used a batch experiment to assess Cr(VI) desorption at varying artificial seawater (ASW) concentrations. IR results demonstrate the complexity of Cr(VI) adsorption, showing a combination of monodentate inner-sphere complexation at high pH and dichromate outer-sphere (∼75%) at low pH. The Cr(VI)-complexes exhibited desorption induced by increasing pH values (58% of desorption) and sulfate competition (∼40% desorption). ASW desorbed ∼20% more Cr(VI), even at just 1% concentration. Our findings provide insight into Cr(VI)-adsorption complexation that controls the retention and remobilization of Cr(VI) on Fe-oxide minerals. The results point to an elevated risk of Cr(VI) mobilization in contaminated soils affected by SLR.

Sodium removal from bauxite desilication product (sodalite) aided by chelating effects of inorganic and organic acids

Dealkalization is a prerequisite to converting bauxite residue into non-hazardous materials that can be used for various upcycling applications. Structural alkali (Na⁺) lodged inside the densely packed aluminosilicate-cages of sodalite, the dominant desilication product from refining alumina, is a common culprit in the persistence of strong alkalinity of bauxite residue. The present study unravelled chemical and mineralogical processes involved in sodalite dealkalization, driven by organic and inorganic acids. These acids have different H⁺ dissociation coefficients and their anions have different chelation abilities with surface metal atoms of aluminosilicate minerals. The efficacy of sodium removal by exposure to the acids was found not only dependent on the acid strength (pK_a), but also on the chelating property of dissociated conjugate anions. Following an initial H⁺-Na⁺ exchange, Na⁺ removal from sodalite was correlated with partial hydrolysis of aluminosilicate network and resultant chelating reactions with acid anions. The selection of organic and inorganic acids whose conjugate bases possess good chelating capability in the pH buffer zone 7-9 (e.g., oxalate or phosphate), would provide significant aid to the dealkalization process. The findings in this study are crucial in understanding the conversion of bauxite residue into a soil-like growth media (technosol) for sustainable mined land rehabilitation.

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.

Research on the remediation of cesium pollution by adsorption: Insights from bibliometric analysis

While nuclear energy with zero carbon emissions will continue to occupy an indispensable position in future scenarios for power generation, the proper disposal of nuclear waste is still highly challenging in many countries. Adsorption is currently one of the primary methods used for removal of cesium from wastewater. However, no available literature has systematically summarized advances and outlooks on the adsorptive removal of cesium, and research issues such as relevant adsorption mechanisms remain largely unexplored. In this study, a bibliometric analysis was used to quantitatively analyze 10141 publications in the Web of Science Core Collection that were published from 1900 to 2022. Current publication trends and active countries, most influential authors and institutions, journal distribution, and research hotspots and trends were reviewed and summarized. The results for the conceptual structure and evolution of investigations in this field showed three distinct periods of rapid development in recent decades. The first period concerned the scope, degree, and influences of pollution by cesium and the development of natural adsorbents. The second period included the exploration and verification of adsorption mechanisms, the fabrication and optimization of new materials, and the application of density functional theory for chemical calculations. The third period involved the development of more advanced biodegradable, nanoscale and synthetic materials with great potential for use as adsorbents as well as advances in engineering applications. Notably, the study showed that it is necessary to further enhance application-driven laboratory investigations. Future directions for research were proposed, such as the investigation of complex adsorption mechanisms, development of new materials, and engineering applications of materials developed in the laboratory. The findings will provide valuable insights and serve as a reference for researchers and policymakers as they address the adsorptive remediation of cases of pollution by cesium.

Molecular Modeling in Anion Exchange Membrane Research: A Brief Review of Recent Applications

Anion Exchange Membrane (AEM) fuel cells have attracted growing interest, due to their encouraging advantages, including high power density and relatively low cost. AEM is a polymer matrix, which conducts hydroxide (OH-) ions, prevents physical contact of electrodes, and has positively charged head groups (mainly quaternary ammonium (QA) groups), covalently bound to the polymer backbone. The chemical instability of the quaternary ammonium (QA)-based head groups, at alkaline pH and elevated temperature, is a significant threshold in AEMFC technology. This review work aims to introduce recent studies on the chemical stability of various QA-based head groups and transportation of OH- ions in AEMFC, via modeling and simulation techniques, at different scales. It starts by introducing the fundamental theories behind AEM-based fuel-cell technology. In the main body of this review, we present selected computational studies that deal with the effects of various parameters on AEMs, via a variety of multi-length and multi-time-scale modeling and simulation methods. Such methods include electronic structure calculations via the quantum Density Functional Theory (DFT), ab initio, classical all-atom Molecular Dynamics (MD) simulations, and coarse-grained MD simulations. The explored processing and structural parameters include temperature, hydration levels, several QA-based head groups, various types of QA-based head groups and backbones, etc. Nowadays, many methods and software packages for molecular and materials modeling are available. Applications of such methods may help to understand the transportation mechanisms of OH- ions, the chemical stability of functional head groups, and many other relevant properties, leading to a performance-based molecular and structure design as well as, ultimately, improved AEM-based fuel cell performances. This contribution aims to introduce those molecular modeling methods and their recent applications to the AEM-based fuel cells research community.

Perchlorate adsorption onto epichlorohydrin crosslinked chitosan hydrogel beads

Perchlorate (ClO₄^-) in water is an emerging contaminant that threatens human health by inhibiting the uptake of iodine in the thyroid gland. Biopolymer adsorbents including chitosan hydrogel beads (CSBs) have attracted increasing attentions in water treatment for their low costs, ease in preparation, and environmental friendliness. However, the adsorption capacity for ClO₄^- by several crosslinked CSBs has been shown to be low. To overcome this, epichlorohydrin (ECH) crosslinked CSBs (ECH-CSBs) that preserved -NH₂ functional groups as potential sites for adsorption are synthesized and characterized, followed by batch adsorption experiments to evaluate adsorption and desorption reactions. The point of zero charge is determined to be 5.1 ± 0.1. Both XPS spectra and DFT calculations support that electrostatic interaction between ClO₄^- and protonated -NH₃⁺ functional groups is responsible for adsorption that reaches a capacity of 63.4 to 76.3 mg/g between pH of 4.0-10.0 at 303.15 K that follows Langmuir isotherm. ECH crosslinking also enhances hydrophilicity of CSBs to allow for increased adsorption for ClO₄^-. Adsorption of ClO₄^- (10 and 100 mg/L) follows a pseudo-first order kinetics with equilibrium time of 2-6 h but is limited by intra-particle diffusion. Anions common in natural waters exhibit interference effects due to similar electrostatic attraction mechanism, thus HCO₃^- and SO₄^2- with high abundance in natural waters need pre-treatment. Regeneration of the adsorbents to 100% of its adsorption capacity by rinsing with 0.1 M NaOH is demonstrated for 12 cycles due to complete desorption of ClO₄^- via electrostatic repulsion, assuring reusability.

Morphology dependent interaction between Co(II)-tetraphenylporphyrin and the MgO(100) surface

Porphyrins are key elements in organic–inorganic hybrid systems for a wide range of applications. Understanding their interaction with the substrate gives a handle on structural and electronic device properties. Here we investigate a single transition-metal porphyrin, namely Co(ii)-tetraphenylporphyrin (CoTPP), on the MgO(100) surface and the effect of multilayer film formation within hybrid density-functional theory and many-body perturbation theory. We focus on the relevant adsorption sites, simulate their photoemission spectra as a key fingerprint and compare with experiments on MgO(100) films on Ag(100). While we find only weak interaction between the cobalt centre and terrace sites on the MgO(100) surface, a strong interaction manifests itself with the low-coordinated sites. This leads to distinct features in both the valence and core-level regions of the electronic structure, as observed in the ultraviolet and X-ray photoemission spectra, corroborated by simulated spectra and calculated cobalt core-level shifts. Our work thus demonstrates the relevance of morphology-related low-coordinated sites and their properties in the adsorption of CoTPP on the MgO(100) surface.

The adsorption of Co-tetraphenylporphyrin at relevant low-coordinated sites on MgO(100) shows distinct features from terrace-site and multilayer films in the near-valence and corelevel regions of the electronic structure.

Research on the Mechanism of Interaction between Styrene-Butadiene-Styrene (SBS) and Asphalt Based on Molecular Vibration Frequency

Based on the four-component theory of asphalt, molecular models of the saturate, aromatic, resin, and asphaltene were constructed, respectively. The styrene–butadiene–styrene (SBS) polymer was used as the modifier. Using density functional theory (DFT) to study the effect of SBS on the molecular vibration of each component of asphalt, the vibration spectrums and binding energy of the systems composed of SBS and each component molecule of asphalt were calculated. Prepared SBS modified asphalt and measured Fourier transform infrared spectroscopy (FTIR) before and after the experiment. The results show that after SBS was added to asphalt, no chemical reaction occurred, and the system was mainly physical blending. The vibrational peak intensity of SBS and the light components of asphalt (saturate and aromatic) is stronger than that of SBS and the heavy components of asphalt (resin and asphaltene). The interaction strengths of asphalt components and polybutadiene (PB) blocks, polystyrene (PS) blocks of SBS are different. The binding energy of SBS and the saturate is the lowest and the bonding of the system is weakest. The bonding of the systems of SBS and the aromatic, resin, asphaltene is stable, and the stability of these systems are all stronger than that of SBS and the saturate.

Configurations of lead(II)-benzohydroxamic acid complexes in colloid and interface: A new perspective

Lead(II)-benzohydroxamic acid (Pb-BHA) complex collectors perform well with respect to scheelite flotation, and, due to their structure, they are widely used for industrial purposes. This paper examines the controversial issue of whether "O, O" five-membered ring or "N, O" four-membered ring complexes are formed when BHA coordinates with Pb ions, with their structure being comprehensively studied from the aspect of colloid and interface science. The configurations of Pb-BHA complexes are examined in a solution and on a mineral surface with experimental and computational methods. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) revealed that the five-membered ring is the dominant form of Pb-BHA complexes in a solution, whereas four-membered ring complexes are the stronger electron acceptor of the two. Moreover, XPS and time-of-flight secondary ion mass spectrometry (TOF-SIMS) confirmed that the four-membered ring complexes are stable with respect to being adsorbed on the scheelite surface. Therefore, although the four-membered ring is not as stable as the five-membered ring in a solution, it offers advantages with respect to adsorption on an electron-rich mineral surface during short-flotation processes.

Selective and enhanced adsorption of the monosubstituted benzenes on the Fe-modified MCM-41: Contribution of the substituent groups

The Fe-modified spherical meso-silica MCM-41 was synthesized via the base precipitation with Fe³⁺/urea, and the structure was characterized. Especially, the selective and enhanced adsorption characters and mechanism of the monosubstituted benzenes were investigated. The results showed that Fe modification increased the specific surface area of MCM-41 and retained the mesopore structure. Importantly, adsorption of the monosubstituted benzenes indicated that the adsorption behavior of the monosubstituted benzenes on the Fe-modified MCM-41 (Fe-MCM-41) was a monolayer adsorption on the heterogeneous surfaces, and it showed great selective adsorption towards aniline, and the maximum adsorption capacity of the Fe-MCM-41 towards aniline was 17.5 and 7.9 times of nitrobenzene and phenol. Additionally, the adsorption process and the isotherm of aniline conformed to the pseudo-second order kinetic mode and the Langmuir mode. The maximum adsorption capacity of the Fe-MCM-41 and the pure MCM-41 towards aniline were 17.9 and 1.9 mg g^-1, which indicated that the Fe modification significantly enhanced the adsorption capacity of MCM-41 towards aniline. Mechanism analysis reveals that the selective adsorption of the monosubstituted benzenes was attributed to the electron donating/withdrawing capacity of the substituent groups on benzene ring. Due to the electron withdrawing capacity of O atom, the exposed Fe atom of the ferric oxide loaded in the Fe-MCM-41 gave a strong electrophilic surface, which electrostatically interacted with the electron donating group (amino) in aniline.