Kaolinite Surface Research Articles

Study on adsorption-diffusion-seepage behavior of oxygen in coal gangue under the coupling of temperature and pressure

This research delves into the fundamental aspects of oxygen adsorption, diffusion, and seepage within gangue slits, aiming to avert the spontaneous combustion of coal gangue hills. Employing methodologies such as molecular dynamics (MD), Grand Canonical Monte Carlo simulations (GCMC), and the finite element simulation, the intricate behavior of O2 molecules within the coal gangue's slit structure is modeled under varying temperature and pressure conditions. The outcomes evince that augmented pressure facilitates oxygen adsorption while diminishing the diffusion coefficient. Conversely, elevated temperature curtails adsorption but augments the diffusion coefficient. At a pressure of 10 MPa, the system energy of oxygen-coal gangue undergoes the most substantial change (92.18 kcal/mol), and the energy peak exhibits the broadest displacement (0.7 kcal/mol), thus signifying the preeminence of pressure conditions in governing O2 adsorption and diffusion. At temperatures below 423.15 K, O2 molecules and coal gangue primarily undergo physical adsorption, with an amplified exposure of kaolinite (0 0–1) surfaces in coal gangue correlating to intensified adsorption. Within the pressure range of 0.2–10 MPa, the percolation process of oxygen in gangue slits unfolds across four discernible stages, with the structural characteristics of the slit channel, gas diffusion capacity, and oxygen consumption rate of coal gangue exerting a profound influence on the process. Based on these research findings, the reduction of pressure proves to be more efficacious in regulating the spontaneous combustion of coal gangue as compared to temperature control.

Adsorption of Sc on the Surface of Kaolinite (001): A Density Functional Theory Study.

The adsorption behavior of Sc on the surface of kaolinite (001) was investigated using the density functional theory via the generalized gradient approximation plane-wave pseudopotential method. The highest coordination numbers of hydrated Sc3+, ScOH2+, and ScOH2 + species are eight, six, and five, respectively. The adsorption model was based on ScOH2H2O5+, which has the most stable ionic configuration in the liquid phase. According to the adsorption energy and bonding mechanism, the adsorption of Sc ionic species can be categorized into outer layer and inner layer adsorptions. We found that the hydrated Sc ions were mainly adsorbed on the outer layer of the kaolinite (001)Al-OH and (00-1)Si-O surfaces through hydrogen bonding while also being adsorbed on the inner layer of the deprotonated kaolinite (001)Al-OH surface through coordination bonding. The inner layer adsorption has three adsorption configurations, with the lying hydroxyl group (Ol) position having the lowest adsorption energy (-653.32 KJ/mol). The adsorption energy for the inner layer is lower compared to the outer layer, while the extent of deprotonation is limited. This is because the deprotonation of the inner adsorption layer is energetically unfavorable. We speculate that Sc ions species predominantly adsorb onto the surface of kaolinite (001) in an outer layer configuration.

Understanding the Aggregation of Model Island and Archipelago Asphaltene Molecules near Kaolinite Surfaces using Molecular Dynamics.

The solubility of asphaltenes in hydrocarbons changes with pressure, composition, and temperature, leading to precipitation and deposition, thereby causing one of the crucial problems that negatively affects oil production, transportation, and processing. Because, in some circumstances, it might be advantageous to promote asphaltene agglomeration into small colloidal particles, molecular dynamics simulations were conducted here to understand the impacts of a chemical additive inspired by cyclohexane on the mechanism of aggregation of model island and archipelago asphaltene molecules in toluene. We compared the results in the presence and absence of a kaolinite surface at 300 and 400 K. Cluster size analyses, radial distribution functions, angles between asphaltenes, radius of gyration, and entropic and energetic calculations were used to provide insights on the behavior of these systems. The results show that the hypothetical additive inspired by cyclohexane promoted the aggregation of both asphaltenes. Structural differences were observed among the aggregates obtained in our simulations. These differences are attributed to the number of aromatic cores and side chains on the asphaltene molecules as well as to that of heteroatoms. For the island structure, aggregation in the bulk phase was less pronounced than that in the proximity of the kaolinite surface, whereas the opposite was observed for the archipelago structure. In both cases, the additive promoted stacking of asphaltenes, yielding more compact aggregates. The results provided insights into the complex nature of asphaltene aggregation, although computational approaches that can access longer time and larger size scales should be chosen for quantifying emergent meso- and macroscale properties of systems containing asphaltenes in larger numbers than those that can currently be sampled via atomistic simulations.

Study on the Remediation of Pyrene-Contaminated Soil with Surfactants and their Mechanisms

Soil is the main aggregation site of polycyclic aromatic hydrocarbons and an important pathway of migration to other media. In this paper, the adsorption behavior of pyrene and seven different types of surfactants on kaolinite surfaces was studied by molecular dynamics simulation and desorption testing. The molecular dynamics simulation results showed that pyrene was more easily adsorbed on the 001 (-) side of kaolinite. SDBS, SDS, TW80, and TX-100 had strong interactions with pyrene, encapsulating pyrene molecules in aggregates. However, when the concentration of surfactant was too high, the desorption of pyrene molecules on a kaolinite surface will be inhibited. The desorption of pyrene molecules will be inhibited in the presence of BS-12, TW80, and TX-100, while the desorption process can be promoted by using CTAC, DDBAC, SDBS, and SDS as soil remediation agents. The removal rate of pyrene gradually increased with the increase of SDS dosage, while for SDBS, the removal rate showed a trend of first increasing and then decreasing. When the concentration of SDS was 0.014 mol/L, the elution rate of pyrene reached 72.86%. The molecular dynamics simulation results were similar to the desorption test results, verifying the reliability of molecular dynamics simulation. The research results provide theoretical support for the selection of surfactants in the remediation process of pyrene-contaminated soil.

Adsorption behavior of benzene on clay mineral surfaces at different temperatures and air humidity based on molecular simulation

Benzene poses a severe threat to human health due to its prevalence and carcinogenicity. The adsorption behavior of benzene on clay mineral surfaces is significant to its migration and retention in soils. Therefore, it is crucial to understand the mechanism of benzene adsorption behavior under different environmental conditions. In this study, a series of Grand Monte Carlo (GCMC) simulations were performed to obtain benzene isotherms on clay under various temperatures and air humidity. The adsorption mechanisms were investigated via analysis of the distribution of interaction energy, density, and orientation of adsorbed benzene and water. Results show that benzene form multiple layers on mineral surfaces, and surfaces have a great impact on the first layer of adsorbed benzene. The impedance effect of temperature on benzene adsorption is attributed to the temperature-dependent chemical potential of benzene, but not the interaction between benzene and surfaces, which is slightly temperature-reliant. Humidity shows different inhibition effects on benzene adsorption due to different surface hydration behavior and competitive adsorption between water and benzene. Benzene tends to be adsorbed on hollow sites on kaolinite surfaces, which is the same as water. The surface cations on montmorillonite and mica are significant to benzene and water adsorption, and their density distribution causes different competitive adsorption behavior.

Distinct water behaviors at the surfaces of siloxane and gibbsite layers: Response to the concentration and cation type

Gibbsite and siloxane layers are ubiquitous in soil and sediments, however, distinct water behaviors at these surfaces in response to concentration and cation type are still unclear. Molecular method was performed to simulate the aqueous solutions interacting with both the siloxane and gibbsite surfaces of kaolinite. Despite cations were primarily adsorbed on the siloxane surface, they can still regulate the anions adsorption on the gibbsite surface through correlation effect, thereby altering the water behaviors. Cation effect on the atomic density and angle distribution of water molecules was greater at the gibbsite than the siloxane surface, but the concentration effect on these properties was similar. Both concentration and cation effect on hydrogen bonds were more pronounced at the gibbsite surface, however, cation effect was more evident at the siloxane than at the gibbsite surface. The findings are useful in understanding the water availability in soil aggregates containing different clay layers.

Mechanism research on surface hydration of Fe(II)-doped kaolinite, insights from molecular dynamics simulations

To investigate the micro-influence mechanism of Fe(II) doping on the surface hydration of kaolinite, molecular dynamics (MD) simulation method was utilized to simulate the interaction between H2O and Fe(II)-doped kaolinite (referred to as Fe(II)-Kao) surface under different conditions. Results demonstrate that the H2O can be adsorbed on Fe(II)-Kao surfaces, forming a hydration film composed of three H2O layers with a thickness of 8–10 Å. As the thickness of the H2O layer increased on Fe(II)-Kao surfaces, the interfacial effect between H2O and the surface gradually weakened. Furthermore, the interaction between H2O and Fe(II)-Kao (001¯) surface is weaker compared to that on the Fe(II)-Kao (001) surface. Additionally, the presence of Fe(II) doping enhanced the hydration of the kaolinite surface. The micro-influence mechanism of Fe(II) doping on the surface hydration of kaolinite can be attributed to the enhancement of the interface effect between equilibrium cations (such as Na(I)) and the surface, resulting in an increased ion hydration at the kaolinite surface. Overall, Fe(II) doping tends to promote the surface hydration of kaolinite. The elucidated mechanism of how impurity defects affect the surface hydration activity of kaolinite can provide theoretical support for tailings separation and wastewater treatment.

Natural kaolinite modified black titanium dioxide for efficient visible light assisted photocatalysis

A novel black TiO2/kaolinite composite (bTK) was prepared by the hydrothermal method and tested as a photocatalyst. Morphology and structure of samples were systematically characterized by field emission scanning electronic microscopy (FESEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), UV–vis diffuse reflectance spectroscopy (UV–vis DRS) and X-ray photoelectron spectroscopy (XPS). The results showed that black TiO2 (bT) nanoparticles formed point defects were successfully coated as thin layers on the surface of kaolinite. The bTK composite had a narrower band gap and higher specific surface area. XPS results showed that Ti3+ and oxygen vacancies (Vo) existed in bT. The photocatalytic removal performance of bTK composite on methylene blue (MB) was near to 100% in 2 h, which was 32.1% higher than that of bT. Kinetic studies reveal that MB degradation process by the bTK corresponded well to zero-order kinetic.

Nanoscale zerovalent Iron-incorporated kaolinite for hemostatic and antibacterial applications

Nanoscale zero-valent iron (nZVI) was converted into iron oxide and used as an active component in hemostasis, but have to avoid inefficient nano agglomerates and tissue thermal damage. Herein, a nZVI-kaolinite composite was fabricated with kaolinite as the support material through the sodium borohydride reduction method, and its multiple hemostatic mechanisms and broad-spectrum antibacterial activity were studied. Incorporated kaolinite improves the dispersed state in an appropriate spacing against the exothermic oxidation reaction in advance, reducing exothermic side effects within a tissue-tolerable allowance. Regarding the hemostatic properties of nZVI-incorporated kaolinite, the blood coagulation index was 4.10% to 7.30%, and the hemolysis rate in vitro was less than 5%. The hemostatic mechanism originates from negatively charged silanol groups on the kaolinite surface and the iron oxide nanoparticles oxidized from nZVI. In addition, broad-spectrum antibacterial activities were obtained in vitro due to the synergistic effect of nZVI particles and kaolinite, including physical adsorption, exothermic effects, and the generation of reactive oxygen species. This work offered a feasible idea for using nZVI to treat infectious wounds.

Photocatalytic degradation of RhB dye using hybrid nanocomposite BiOCl@Kaol under sunlight irradiation

The development of advanced photocatalytic materials for environmental purposes is among the high research topics. In the present work, a nanocomposite BiOCl@Kaol, made of kaolinite supported Bismuth Oxychloride “BiOCl” photocatalyst, was designed for Rhodamine B dye photodegradation in aqueous solution. The obtained BiOCl@Kaol nanocomposite exhibited high photocatalytic activity in the removal of Rhodamine B dye (RhB) from water, with a degradation efficiency reaching 100 % within 35 min. Furthermore, it was observed that the nanocomposite photocatalyst, BiOCl@0.4Kaol, at optimal Clay/BiOCl ratio of 0.4, exhibited 4.37 times better photocatalytic performance, compared to bare BiOCl. Finally, to elucidate the photodegradation mechanisms, we determined various interactions occurring between BiOCl and kaolinite particles, as well as between RhB molecules and BiOCl@Kaol nanocomposites, using Monte Carlo calculations. It was concluded from Monte Carlo calculations that hydrogen bonds were established between oxygen atoms of BiOCl and H of kaolinite (001) hydroxyl groups upon the adsorption of BiOCl (003) particles on the kaolinite (001) surface, owing to its high negative interaction energy. Meanwhile, van der Waals attraction was also established between the RhB molecules and the BiOCl@Kaol (001) surface.

Investigation of Hydrated Dy(III) and MgSO4 Leaching Agent Ion Adsorption on (001) Surface of Montmorillonite: A Study Using Density Functional Theory

Kaolinite is one of the principal rare earth element (REE) ion-adsorption clays that hosts a wide range of elements, including Dy(III) as a representative example. Ammonium sulfate is a typical salt used to leach REEs. Due to the carbon dioxide emissions which occur during ammonia production, it is urgently necessary to develop low environmental pollution leaching agents that can replace (NH4)2SO4. MgSO4 is regarded as the most promising eco-friendly leaching agent. Herein, the first-principles plane-wave pseudopotential method based on the density functional theory (DFT) was used to investigate the stable adsorption structures of Dy(III) and its hydrated ions, MgSO4 leaching agent ions and the corresponding hydrated ions on the surface of kaolinite, which revealed the adsorption mechanism of Dy(III), Mg(II), and SO42− on the silico–oxygen plane and the aluminum–hydroxyl plane of kaolinite. Based on the research results of the steric hindrance effect of Dy(III) on the silico–oxygen plane and the aluminum–hydroxyl plane of kaolinite, the adsorption of Dy(H2O)103+ was more stable on the silico–oxygen plane. It was easier to leach out Dy(III) with MgSO4, while SO42− tended to interact with the rare earth ions in an aqueous solution. The results provide theoretical guidance for efficient rare earth extraction and obtaining novel efficient leaching agents.

Superhydrophobicity Mechanism and Nanoscale Profiling of PDMS-Modified Kaolinite Nanolayers via Ab Initio-MD Simulation and Atomic Force Microscopy Study.

This study aimed to investigate the superhydrophobic mechanism of kaolinite particles modified with poly(dimethylsiloxane) (PDMS), which has potential as a superior hydrophobic coating. The study employed a combination of density functional theory (DFT) simulation modeling, characterization of the chemical properties and microstructure, contact angle measurements, and chemical force spectroscopy of atomic force microscopy. The results showed successful PDMS grafting onto the kaolinite surface, resulting in micro- and nanoscale roughness and a contact angle of 165°, indicating a successful superhydrophobic effect. The study also identified the mechanism of the hydrophobic interaction through two-dimensional micro- and nanoscale hydrophobicity images, highlighting the potential of this approach for developing new hydrophobic coatings.

Fundamental study of asphaltene cracking and desorption from rock surface due to microwave radiation: A molecular insight into catalytic effect of minerals and asphaltene chemistry

This research aims to investigate the fundamental of asphaltene cracking and desorption during microwave stimulation of an oil well. In this regard, the asphaltene chemistry and the rock mineral type were studied using analytical methods. Two asphaltene samples with different chemistries and origins were adsorbed on the four different rock powders. The asphaltene molecules were characterized using Fourier transform infrared spectroscopy analysis (FTIR), Nuclear magnetic resonance spectroscopy (NMR), Energy-dispersive X-ray (EDS), and elemental analysis. Besides, four different rock powders of carbonate, Bentonite, kaolinite, and sand with identical mesh sizes were used to mimic the various reservoir mineralogy. The rock powders were characterized by FTIR and X-Ray Powder Diffraction (XRD). The asphaltene molecules were adsorbed on the rock powders, and rock sections were provided from the mixture. The molecular changes of asphaltenes on the rock surfaces and desorbed/cracked compounds from the rock surface were respectively addressed by attenuated total reflection spectroscopy (ATR) and Gas chromatography-mass spectrometry (GC-MS). Regarding the ATR technique, the asphaltenes with larger aromatic cores, longer aliphatic chains, and more sulfur compounds had tenser desorption from carbonate and bentonite rock surfaces due to the effect of microwaves. In contrast, the asphaltenes with smaller aromatic cores, and shorter aliphatic chains had tenser desorption from kaolinite and sand rock surfaces. According to the GC-MS analysis, the asphaltene cracking led to producing light aromatic and aliphatic (such as toluene), intermediate aliphatic compounds (such as Hexadecane), and Bis (2-Ethylhexyl) phthalate from the surface of minerals. Based on the ATR spectrums, the microwave radiation of asphaltene-bearing rocks led to the reduction of polar compounds in asphaltenes. This observation achievement was consistent with the contact angle measurements, which showed more hydrophobicity for the microwave-treated rocks.

Effect of salinity, mineralogy, and organic materials in hydrogen wetting and its implications for underground hydrogen storage (UHS)

Hydrogen is a green energy carrier, which appears to reshape the current energy supply chain. To develop a full-scale hydrogen supply chain, underground hydrogen storage has been proposed as a key method. However, one of the key hurdles in UHS is the fluid-rock interactions or hydrogen wetting. The interfacial reactions govern hydrogen wetting in underground reservoirs and thus determine the hydrogen flow and govern the amount of hydrogen trapping in porous media. Although the hydrogen wetting has been gaining attention, how the hydrogen wetting behaves in complicated underground circumstances remains in the shadow.To understand the nano-scale character of hydrogen wetting, a series of molecular dynamics (MD) simulations were performed to analyse the hydrogen distribution on kaolinite surface in function of salinity, mineralogy, and organic materials. We computed density distribution and the radial distribution function (RDF, g(r)) of each species. The results show that salinity can dramatically affect hydrogen distribution on the siloxane surface while negligible effects can be observed on aluminol (gibbsite) surfaces of kaolinite. Furthermore, the hydrogen distribution can be notably shaped by organic materials. Organic materials with acid functional group (–COOH) can generate a more hydrogen wetting surface than base functional group (–NH). This investigation reveals the governing mechanics of hydrogen-brine-kaolinite reactions at molecular scale, unravels the effects of hydrogen-brine-kaolinite reactions in hydrogen wetting and thus provides a paradigm to screen candidate sites for the UHS.

Clay-bacteria interaction: Effect of bacterial cell density on sedimentation behavior and fabric map of kaolinite clay

Clay-bacteria interactions are ubiquitous and the presence of bacteria has a pronounced effect on the mechanical, chemical, and electrical behaviors of clays. However, the role of microscale clay-bacteria interactions to the macro-scale engineering behaviors and properties of clays remains poorly understood. This study presents changes in the microstructure of kaolinite in association with bacteria and the resulting sedimentation behavior of bacterium-associated kaolin suspensions. Sedimentation tests were performed using suspensions composed of kaolin and model bacteria Shewanella oneidensis MR-1 while varying the fluid pH and cell density. The results reveal that kaolinite presents face-to-face aggregated microstructures with the presence of bacteria, attributable to sharing and/or exchange of cations between kaolinite and bacterial cells. Meanwhile, at a pH value lower than the isoelectric point of the kaolinite edge surfaces, the clay-bacteria association appears less pronounced comparing to the higher pH, owing to the competition between negative (kaolinite faces and bacteria surfaces) over positive (kaolinite edges) charges. The fabric change caused by kaolinite-bacteria association accelerates kaolin settling and reduces the final void ratios. A fabric map for bacterium-asssociated kaolinite is suggested based on the sedimentation behaviors and scanning electron microscopy observations. This study advances the understanding of particle-level clay-bacteria interactions and the sedimentation behavior of clays in suspensions with vigorous microbial activities.

Prediction of Adsorption and Diffusion of Shale Gas in Composite Pores Consisting of Kaolinite and Kerogen using Molecular Simulation.

Natural gas production from shale formations is one of the most recent and fast growing developments in the oil and gas industry. The accurate prediction of the adsorption and transport of shale gas is essential for estimating shale gas production capacity and improving existing extractions. To realistically represent heterogeneous shale formations, a composite pore model was built from a kaolinite slit mesopore hosting a kerogen matrix. Moreover, empty slabs (2, 3, and 4 nm) were added between the kerogen matrix and siloxane surface of kaolinite. Using Grand-Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations, the adsorption and diffusion of pure methane, pure ethane, and a shale gas mixture were computed at various high pressures (100, 150, and 250 atm) and temperature of 298.15 K. The addition of an inner slit pore was found to significantly increase the excess adsorption of methane, as a pure component and in the shale gas mixture. The saturation of the composite pore with methane was observed to be at a higher pressure compared to ethane. The excess adsorption of carbon dioxide was not largely affected by pressure, and the local number density profile showed its strong affinity to kerogen micropores and the hydroxylated gibbsite surface under all conditions and pore widths. Lateral diffusion coefficients were found to increase with increasing the width of the empty slab inside the composite pore. Statistical errors of diffusion coefficients were found to be large for the case of shale gas components present at low composition. A larger composite pore configuration was created to investigate the diffusion of methane in different regions of the composite pore. The calculated diffusion coefficients and mean residence times were found to be indicative of the different adsorption mechanisms occurring inside the pore.

Occurrence Characteristics of Water in Nano-Slit Pores under Different Solution Conditions: A Case Study on Kaolinite.

The presence of water in narrow pore spaces affects the occurrence and flow of methane, which in turn affects shale gas production. Therefore, studying the occurrence and distribution characteristics of water is of great significance to predict gas production. Based on molecular dynamics simulations, this study investigated the occurrence characteristics and influencing variables of liquid water in kaolinite nanopores in situ. Owing to its widespread distribution, kaolinite is the most prevalent clay mineral with two surfaces with different characteristics. Three systems of pure water, a CaCl2 solution, and a H2O/CH4 mixed phase were created at varied temperatures (80-120 °C) and pressures (70-120 MPa). The presence of gas and water in the nanopores was investigated thoroughly. The results showed that the adsorption of water on the Al-O octahedral surface of kaolinite was not affected by external conditions under in situ conditions, whereas the adsorption of water on the Si-O tetrahedral surface decreased with increasing temperature, but the change was small. When ions were present in the system, the water capacity decreased. Based on the aforementioned results, external conditions, such as temperature and pressure do not affect the basic state of water. However, if there are more than two fluid types in the system, the adsorption of water on the mineral surface is reduced owing to competitive adsorption. In addition, a CH4-H2O mixed system was simulated, in which methane molecules were distributed in clusters. There are two types of adsorptions in pores: gas-solid interactions and solid-liquid-gas interactions. CH4 molecules are thought to be clustered in water molecules because of the strong hydrogen bonding interactions among the water.

Chemical Transformations of 2D Kaolinic Clay Mineral Surfaces from Sulfuric Acid Exposure.

A combined experimental and computational approach is used to investigate the chemical transformations of kaolinite and metakaolin surfaces when exposed to sulfuric acid. These clay minerals are hydrated ternary metal oxides and are shown to be susceptible to degradation by loss of Al as the water-soluble salt Al2(SO4)3, due to interactions between H2SO4 and aluminum cations. This degradation process results in a silica-rich interfacial layer on the surfaces of the aluminosilicates, most prominently observed in metakaolin exposed to pH environments of less than 4. Our observations are supported by XPS, ATR-FTIR, and XRD experiments. Concurrently, DFT methodologies are used to probe the interactions between the clay mineral surfaces and H2SO4 as well as other sulfur-containing adsorbates. An analysis performed using a DFT + thermodynamics model shows that the surface transformation processes that lead to the loss of Al and SO4 from metakaolin are favorable at pH below 4; however, such transformations are not favorable for kaolinite, a result that agrees with our experimental efforts. The data obtained from both experimental techniques and computational studies support that the dehydrated surface of metakaolin interacts more strongly with sulfuric acid and provide atomistic insight into the acid-induced transformations of these mineral surfaces.

The effects of Fe- and Mn-oxides and imogolite in the presence of kaolinite on organic nitrogen mineralization and soil enzyme activities

ABSTRACT An incubation experiment was conducted to study the effect of clay content and composition on organic nitrogen mineralization. The experiment measured the mineralization of organic nitrogen from alfalfa residues, enzyme activities, and microbial biomass nitrogen in mixtures of sand, kaolinite, and non-layered colloids (NLCs) with sand as a control. The study found that as the contents of kaolinite and NLCs increased, the mineralization of organic nitrogen and enzyme activity decreased, but microbial biomass nitrogen increased. The maximum decrease in organic nitrogen mineralization was 88.6%, and microbial biomass nitrogen increased from 4.7 to 15.5%. The acid and alkaline phosphatase activities also decreased by 86.0 and 93.6%, respectively, with an increase in clay content. The specific surface area (SSA) of the mixtures showed an inverse relationship with enzyme activity and mineralization of organic nitrogen. Inactivation of extracellular enzymes by adsorption on the surfaces of kaolinite and NLCs, and decreased accessibility of organic nitrogen substrate molecules due to adsorption, reduced the mineralization of organic nitrogen. Microbial biomass nitrogen increased as the water holding capacity of the mixtures increased, indicating the importance of water-filled pores in accommodating active microbial biomass and protecting it from desiccation and predators.

First principles investigation on adsorption mechanism of As(OH)3 on the kaolinite (001) surface

The environmental fate of arsenic (As) and arsenic-containing compounds are garnering more attention as a result of the global As contamination. Arsenous acid (As(OH)3) exists as a common arsenic-containing compound in the aqueous phase. Natural clay minerals like kaolinite have the potential to be adsorbents. First-principles calculations based on DFT were used to investigate the adsorption mechanism between As(OH)3 molecules and the kaolinite (001) surface. The two-fold bridge sites of the kaolinite (001) surface were found to be most stable for As(OH)3 adsorption. Three representative stable adsorption configurations, B2, H6, and T3, were selected to analyze. Strong hydrogen bonds formed between As(OH)3 molecules and the kaolinite (001) surfaces made the main contribution to the adsorption. The coverage (Θ) dependence to the adsorption stability was methodically investigated with a wide coverage range of 0 < Θ ≤ 1.0 monolayers (ML). The adsorption energy of As(OH)3 increased in the coverage range of 0 < Θ ≤ 0.5 ML and then decreased in the coverage range of 0.5 < Θ ≤ 1.0 ML. Other properties of the As(OH)3/kaolinite (001) system, including the bonding interaction, atom population/charge, electronic density of states, and lattice relaxation were also discussed and analyzed in detail.