Kaolinite Edge Research Articles

Nanoplastics enhance the stability of kaolinite and affect the sorption of Pb2+ in aquatic environments

The heteroaggregation of nanoplastics with minerals has been widely investigated. However, few studies have focused on the interaction between similar negatively charged clay minerals and nanoplastics, especially how their interactions affect the migration of heavy metals. This study investigated heteroaggregation of kaolinite (a model of natural mineral) with polystyrene nanoplastics (PSNPs) and the effect of heteroaggregates on lead (Pb2+) sorption. Dynamic light scattering (DLS) measurement highlighted that PSNPs owned higher colloidal stability than kaolinite in aquatic environments. Due to the surface charge heterogeneity of kaolinite, the heteroaggregation behavior between kaolinite and PSNPs was highly pH-dependent. At pH 3.0 and 6.0, kaolinite particles with positively charged edge surfaces favored the association of PSNPs. PSNPs were found to be primarily attached on the kaolinite edge (010) surface, followed by Al-terminated (001) and Si-terminated (001¯) surfaces based on density functional theory (DFT) calculations. At pH 10.0, they had negligible interaction due to the strong electrostatic repulsion. Interestingly, kaolinite carried more negative charges after heteroaggregation with PSNPs, resulting in slower edge-to-face homoaggregation and higher dispersibility, and thus increasing the sorption capacity for Pb2+. This investigation provided a new understanding of nanoplastics interactions with negatively charged minerals and highlighted the vital role of nanoplastics in mineral stability and contaminant migration.

Water thin films on kaolinite gibbsite and edge surfaces and their effects on surface wettability in relation to geological carbon sequestration

Geological carbon sequestration (GCS) is a promising method to alleviate CO2 emission, while CO2 structural trapping is one of main GCS mechanisms, in which the storage capacity is strongly dependent on CO2-water–rock contact angle. Although CO2-water-kaolite contact angle on basal surfaces has been studied, the knowledge about kaolinite edge surface wettability remains unknown. In this work, we use molecular dynamics (MD) simulations to study CO2-water-kaolinite contact angle on kaolinite gibbsite and edge surfaces under a typical GCS condition (330 K and 200 bar). The common belief is that surface wettability in presence of water is determined by surface hydroxyl (–OH) group density. While the surface –OH group density of kaolinite edge surface is much lower than that of gibbsite surface, both gibbsite and edge surfaces are strongly water-wet. Edge surface is better hydrated than gibbsite surface as both silanol and aluminol groups can form hydrogen bonding with water molecules thanks to large effective water accessible volume around them, while pocket water can further anchor water film. Therefore, the atomic-level surface characteristics dictate interfacial water structures which determine CO2-water-kaolinite contact angle. Our study provides important insights into the effect of surface heterogeneity on interfacial water structures, and kaolinite gibbsite and edge surface wettability which is crucial to the optimization of GCS processes.

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.

ANISOTROPY IN THE ADSORPTION OF H2O ON KAOLINITE SURFACES

Adsorption capacities of kaolinite's edge and basal surfaces may differ. Тhese features determine the nature of its adsorption behavior while remaining poorly studied. At the same time, the anisotropy of the edge surface stipulates the wide application of kaolinite in geochemical, environmental, and technological fields. In this work, ab initio calculations have been employed to determine the effect of kaolinite surface anisotropy on its adsorption capacity. The mechanism has been studied for binding water molecules to the most stable kaolinite surfaces, namely, (001), (001¯), (010), and (110). It has been found that the most energetically advantageous adsorption configurations of water molecules arise on edge surface (010), with adsorption energy being ~four times higher than that on basal surface (001¯). The observed energy difference is due to the fact that hydroxyl groups are involved in the adsorption mechanism in addition to surface oxygen atoms. The absence of these groups on the basal surface significantly reduces the probability of hydrogen bonding. The results obtained are in agreement with the data from published studies.

Effect of kaolinite edge surfaces on formation of Tb3+-doped phosphor by solid-state reaction.

This study assessed the effect of kaolinite edge surfaces on solid-state reactions. Specifically, Tb3+-doped metastable CaAl2Si2O8 showing green phosphorescence was prepared via a solid-state reaction between expanded kaolinite, a methoxy-modified kaolinite, having Tb3+ ions adsorbed on its edge surfaces and CaCO3. This material cannot be obtained by the conventional grinding of kaolinite, CaCO3 and Tb2O3, indicating that the use of kaolinite edge surfaces is advantageous as a means of achieving certain solid-state reactions.

Effects of clay minerals on the transport of nanoplastics through water-saturated porous media

Clay minerals are important constituents of porous media. To date, only little is known about the transport and retention behavior of nanoplastics in clay-containing soil. To investigate the effects of clay minerals on the mobility of nanoplastics in saturated porous media, polystyrene nanoplastics (PS-NPs) were pumped through columns packed with sand and clay minerals (kaolinite and illite) at different pH and ionic strengths (IS). Mobility of PS-NPs decreased with increasing clay content attributed to physical straining effects (smaller pore throats and more complex flow pathways). Variations in pH and IS altered the surface charges of both PS-NPs and porous media and thus affecting the interaction energy. An increase of IS from 10 mM to 50 mM NaCl decreased the maximum energy barrier and secondary minimum from 142 KBT to 84 KBT and from −0.1 KBT to −0.72 KBT, respectively. Thus, the maximum C/C0 ratio decreased from ~51% to ~0% (pH 5.9, 3% kaolinite). Among the two clay minerals, kaolinite showed a stronger inhibitory effect on PS-NPs transport compared to illite. For instance, at the same condition (3% clay content, pH 5.9, 10 mM NaCl), the (C/C0)max of PS-NPs in kaolinite was ~51%, while for illite, it was ~77%. The difference in transport inhibition was mainly attributed to amphoteric sites on the edges of kaolinite which served as favorable deposition sites at pH 5.9 (pHpzc-edge is ~2.5 for illite and ~6.5 for kaolinite). Besides, the morphology of kaolinite was more complex than illite, which may retain more PS-NPs in kaolinite. Results and conclusions from the study will provide some valuable insights to better understand the fate of NPs in the soil-aquifer system.

Will coupling low salinity water and steam flooding for heavy oil affect the rock properties of sandstone reservoirs? An experimental and simulation study

Abstract Recently Al-Saedi and Flori et al. (2018d) studied the potential of low salinity alternating steam flooding (LSASF) in laboratory sandstone cores, and the results were promising for increasing heavy oil recovery [1]. In this study, we investigate the effect of LSASF on rock properties for the Bartlesville Sandstone Reservoir. These core samples contain high viscous oil (600 cp) with an average permeability of 80 mD. Combining low salinity (LS) water flooding and steam flooding is a novel idea that takes advantage of the relative strengths of both methods. To investigate the proposed new method and the role of LSASF, core flooding experiments, contact angle measurements, spontaneous imbibition tests, zeta potential tests, and reactive transport model were performed. The laboratory experiments showed that optimum recovery is achieved by diluting the FW 40 times and using the same water in a shorter steam cycle. Permeability measurements before and after core flooding revealed that the higher the salinity, the more permeability decreased. Steam can enhance the performance of LS water by reducing precipitation, increasing permeability, and dissolving minerals, and in turn, increasing oil recovery. Flooding the core with both LS water and steam resulted in more water wetness in the sandstone cores. Comparing the cores that were flooded with only LS water with the cores flooded with both LS water and steam, lower water wetness was obtained for the cores that were flooded only with LS water. The results were deliberated concerning wettability change processes by LS Water and steam. The zeta potential tests confirmed that as the salinity decreases, the zeta potential decreases too. Also, measuring contact angle before and after core flooding resulted in more water wetness. Spontaneous imbibition tests were in line with all other tests. The reactive transport modeling confirmed the crucial role of diluting FW in decreasing the number of the most effective kaolinite edges Si O and Al O , which explains why lowering the salinity gives additional oil recovery.

Enrichment mechanisms of lithium in the No. 6 coal seam from the Guanbanwusu Mine, Inner Mongolia, China: Explanations based on Li isotope values and density functional theory calculations

High concentration of Li, associated with coal, has been found in the Guanbanwusu coal Mine, Inner Mongolia, China. Earlier studies suggested that the lithium came from the Yinshan Paleoland during the peat-forming stage. The results of Li isotope determinations and calculations based on density functional theory (DFT) indicated that the lithium in the peat was released into the pore water and adsorbed by kaolinite edge surfaces (octahedral layers and/or tetrahedral layers) in the subsequent diagenesis and coalification. Alternatively, the Li of the partings, the floor, and the roof of the No. 6 coal seam also likely originated from the underlying bauxite of the Benxi Formation. Specifically, we determined the Li isotopic fractionations produced in chelation by small carboxylate (Δ7/6Liorganolithium-aq = −0.35‰), in adsorption onto kaolinite edge surfaces (Δ7/6Lioctahedral layer-aq = 4.36‰, Δ7/6Litetrahedral layer-aq = −4.53‰), and during incorporation into the lattices (Δ7/6LiAl2-aq = −8.2‰ and Δ7/6LiAl1-aq = −11.9‰). Combining these results with the δ7Li of 3.37 ± 0.13‰ for the moyite of the Yinshan Paleoland, we deduced a theoretical δ7Li range for the No. 6 coal seam and the partings, 2.85–7.38‰, exactly covering the determined δ7Li values of the coal and the partings. However, only the partings plotted in the theoretical δ7Li range of 1.13–5.66‰ assuming the bauxites (with δ7Li 1.30 ± 0.03‰) as the lithium source. During weathering and deposition, conversion of structurally incorporated Li within the grain boundaries of polycrystalline minerals of the bauxites to adsorbed Li was inferred to explain the Li isotope excursion in the latter case.

Investigation of Smart Waterflooding in Sandstone Reservoirs: Experimental and Simulation Study Part 2

Summary In a previous work (Al-Saedi et al. 2018c), we studied the effect of mineral composition of cores (using synthetic columns with varying mineralogy) on low-salinity (LS) waterflooding, and we presented a reactive-transport model (RTM) for the water/rock interactions. The results showed that kaolinite has the strongest effect and then quartz because of the high kaolinite surface area, and the most effective complexes were >SiOH (hydroxylated Si), >AlO– (aluminum oxide complex on quartz surface), and >SiO– (silicon mono oxide complex on quartz surface). In this paper, we use the same Bartlesville Sandstone cores (constant mineralogy) for all cases to investigate the effect of water chemistry on water/rock interactions during seawater and smart waterflooding of reservoir sandstone cores containing heavy oil. Oil recovery, surface-reactivity tests, and multicomponent reactive-transport simulation using CrunchFlow (Steefel 2009) were conducted to better understand smart waterflooding. Bartlesville Sandstone cores were saturated with heavy oil and connate formation water. Secondary waterflooding of these cores with formation water (FW) at 25°C resulted in an ultimate oil recovery of approximately 50% original oil in place (OOIP) for all reservoir cores in this study. FW salinity was 104,550 ppm. FW was diluted twice to obtain Smart Water 1 (SMW1). SMW2 was similar to SMW1 but depleted in divalent cations (Ca2+ and Mg2+). SMW3 was also similar to SMW1 but depleted in Mg2+ and SO42−, whereas SMW4 was the same as SMW1 but Ca2+ was diluted 100 times. Seawater (SW) salinity was 48,300 ppm, which is close to the SMW salinity (52,275 ppm). No oil recovery was observed during SMW1 flooding, whereas softening SMW1 (SMW2) resulted in a significant additional oil recovery of OOIP. Depleting Mg2+ and SO42− resulted in additional oil recovery but less than in SMW2. Diluting Ca2+ 100 times was the second-best scenario, after depleted Ca2+ in SMW2. The results of this study showed that the more diluted Ca2+ is in the injected brine, the more additional oil recovery that can be obtained, although the other divalent/monovalent cations/anions were increased or decreased or even depleted. Additional reservoir cores were allocated for surface-reactivity tests. The absence of an oil phase allows us to isolate the important water/rock reactions. The Ca2+, Mg2+, and SO42− effluents for all cores were matched using CrunchFlow, and then further investigations of the water/rock interactions were conducted. The RTM showed that decreasing the Mg2+ concentration will decrease the number of the most effective kaolinite edges Si-O− and Al-O−, but was not as pronounced as that in the presence of Ca2+, which explains why lowering the Mg2+ concentration gives lower additional oil recovery and why lowering the Ca2+ concentration gives higher additional oil recovery.

Kaolinite under pressure at audio frequency range and its electrical features

ABSTRACT Electrical features of a humid kaolinite specimens was measured at various pressures and at various frequencies . Surface impedance is supposed on kaolinite particle surfaces (from humidity), chemical reaction and charge transport along surface hydroxyls of kaolinite edges besides the permanent negative charges. Classic mixture laws do not support the interpretation of electrical properties, especially, at high pressures. Accordingly, interpretation was made with percolation threshold theories. Pressure increase causes decrease in pore spaces and accordingly increases possible connections of pore distances filled with humidity to total pore spaces after applying pressure. With pressure increase, the pore voids between particles decrease and water conducting paths increase. Also, dielectric constant and conductance increase with increase of pressure. On the other hand, at a certain pressure, at low frequencies, a noticeable increase of capacitance with pressure increase, that tends to reach constant value, for relatively high pressures. The diffusion and interfacial mechanisms was suggested to interpret samples with increase of pressure and frequency. The latter mechanism acts simultaneously with surface chemical reaction mechanism (Debye behavior) to give a depressed arc (relaxation time distribution). Finally, the pressure and frequency are effective factors on electrical properties.

Ab Initio Calculations of Partial Charges at Kaolinite Edge Sites and Molecular Dynamics Simulations of Cation Adsorption in Saline Solutions at and above the pH of Zero Charge

Molecular dynamics simulations are used to study adsorption of cations on the (010) kaolinite edge surface at and above the pH of zero charge. The cation solutions are highly concentrated and inclu...

Geochemistry of sillimanite-magnetite-kaolinite of metasomatic rocks of the island of Great Tyuters (gulf of Finland, Russia)

Among the secondary quartzites of the island of the Great Tyuters there are extended linear zones with a capacity of up to 3 meters, clearly visible in detailed satellite images. They are composed of sillimanite-magnetite-kaolinite metasomatites discovered for the first time. The formation of metasomatites is associated with fragmentation of quartzites and the subsequent introduction of fluids of a substance saturated with various components. Metasomatites are enriched with Al2O3 (16–23 wt.%), in contrast to 2–5 wt. per cent of this oxide is present in their substrate – quartzite. In metasomatic rocks dominated by iron oxide, quartzite – ferrous. The rocks do not contain any alkaline and alkaline-earth elements. They are enriched with Zr (146–199 g/t) and a number of other elements, including impurities and rare earth, among which prevail Ce (34 g/t) and La (17 g/t). REE of both breeds is characterized by a small and close degree of fractionation ((La/Yb)n=6.55 and 6.17). Metasomatites on a set of minerals do not differ from quartzites, but differ in their quantitative ratios and composition. Of particular interest is the magnetite of metasomatites. It contains many quartz inclusions with kaolinite edges. Quartz inclusions, in turn, contain inclusions of titanium magnetite crystals. In the formation of metasomatites, there were several stages of the metasomatic process with a changing composition of fluids and different acidity-alkalinity of the mineral formation medium. The development of metasomatites in linear zones, the disintegration of their substrate, the high content of kaolinite – all this gives them a similarity with weathering cows. However, the same set of minerals in metasomatite and its substrate, the complete absence of sulfides and sulfur, the presence of magnetite with double mineral inclusions – such features distinguish these rocks from the classical weathering crust.

Selective adsorption of soil humic acid on binary systems containing kaolinite and goethite: Assessment of sorbent interactions

Inorganic colloids are ubiquitous in soils and coexist as simple mixtures or as strong associations of particles. The coexisting inorganic colloids with a variety of active sites can selectively adsorb humic substances and thus affect the availability and mobility of organic matter. To investigate the selective adsorption of organic matter by coexisting inorganic colloids, humic acid (JGHA) extracted from soil, a kaolinite and goethite synthetic association (KGA) and a kaolinite and goethite mixture (KGM) were prepared and studied. With the adsorption expressed in μmol m−2, the adsorption affinity of JGHA on KGA (initial slope of isotherms) was slightly higher than that on KGM, although KGM could adsorb more JGHA molecules at larger JGHA concentrations. At pH 4, JGHA was adsorbed on the kaolinite edges and goethite through electrostatic attraction and ligand exchange, and on the kaolinite siloxane plates through hydrophobic interaction. At pH 6 and 8, the adsorption occurred only on the kaolinite siloxane plates and goethite. The overall average adsorption Gibbs energies were moderately negative, and comprised a strongly negative average overall molar entropy and a negative average overall molar entropy contribution. Thus, overall JGHA adsorption on KGA and KGM was enthalpy controlled. The large exothermic enthalpy gains and entropy losses were a result of electrostatic attraction and ligand exchange. The entropy gains because of hydrophobic attraction between JGHA and the kaolinite siloxane plates are masked by the larger entropy loss of adsorption on the hydrophilic surfaces. The fractions of JGHA with molar mass >~12.9 kDa (1 Da = 1 g mol−1) were adsorbed selectively by KGM, whereas the fractions <~12.9 kDa were selectively adsorbed by KGA. High molar mass and hydrophobic fractions adsorb selectively on the kaolinite siloxane plates, whereas low molar mass fractions with hydrophilic groups adsorb selectively on goethite and the kaolinite edges.Highlights Selective adsorption behaviour of HA on binary systems containing kaolinite and goethite. High molar mass and hydrophobic HA fractions selectively adsorb on kaolinite siloxane plates. Low molar mass and hydrophilic HA fractions selectively adsorb on kaolinite edges and goethite. Binding affinity of HA to KGA is stronger than to KGM; mineral associations are more beneficial for fixing OM in soil.

Structure of Hydrated Kaolinite Edge Surfaces: DFT Results and Further Development of the ClayFF Classical Force Field with Metal–O–H Angle Bending Terms

Structure of Hydrated Kaolinite Edge Surfaces: DFT Results and Further Development of the ClayFF Classical Force Field with Metal–O–H Angle Bending Terms

Defects in structure as the sources of the surface charges of kaolinite

The sign and magnitude of the surface charge govern the particle interactions and hence determine the physical properties of dispersions. The present work is to evaluate the distortions, isomorphic and broken surface bond as the sources of the permanent or variable surface charges of kaolinite. The theoretical permanent charge density (σcc) was calculated by XRF. The experimental permanent charge density (σp) and variable charge density (σv) were obtained by the method of potentiometric titration. The degree of broken bonds (Db, b) of Al–O–Al and Si–O–Si at the side of kaolinite structure were characterized by the numbers and strength of surface hydrogen in 1H MAS NMR, absorbance of Al–OH or Si–OH in FTIR and edge fractures in FESEM. The surface area was measured by N2 adsorption–desorption. The defects such as basal z corrugation (∆z), tetrahedral rotation angles (θ) and octahedral flattening angles (ψ) were obtained by the XRD rietveld refinement. The negative σcc or σp of three samples KA, KB and KC are 0.012, 0.007 and 0.009C·m−2 or 0.24, 0.19 and 0.22C·m−2, respectively. Db, b of three samples are Db, b-KA>Db, b-KC>Db, b-KB. The σv depend on the degree of the broken bonds at edges of kaolinite and can be affected by pH significantly. The negative variable charge density at alkaline condition decreased with the increase of the degree of broken bonds at the side of kaolinite structures. Surface charges of kaolinite come from the defects such as isomorphic substitutions and broken bonds Al–O–Al and Si–O–Si. The distortions in structure e.g. ∆z, θ and ψ may not directly create the surface charges.

Binding sites of chlorpheniramine on 1:1 layered kaolinite from aqueous solution

Interactions between chlorpheniramine (CP), an antihistamine drug used to treat allergy, and kaolinite in aqueous solution were investigated under batch studies and molecular simulations. The CP adsorption was relatively fast with a large rate constant. The CP adsorption capacity on kaolinite was 25mmol/kg, about the same magnitude of the cation exchange capacity of kaolinite. Molecular dynamic simulation showed that the edges of kaolinite were responsible for the uptake of CP, while a net repulsive interaction between the basal plane and CP molecules was obtained. As the broken bond effect of kaolinite was strongly affected by solution pH via protonation–deprotonation of kaolinite edges, a higher CP adsorption was achieved under neutral to weak alkaline solution. It was the charge density, rather than the surface area, that ultimately controlled the amount of CP adsorption on kaolinite.

Surface force measurements at kaolinite edge surfaces using atomic force microscopy

Fundamental results obtained from research on the properties of the edge surfaces of kaolinite particles (∼500nm) are reported. Of particular significance was the development of the experimental protocol. Well-ordered kaolinite edge surfaces were prepared as an epoxy resin sandwich structure having layered kaolinite particles in the center of the epoxy resin sandwich. Images of the sectioned kaolinite edge surfaces were examined by atomic force microscopy (AFM), and the average thickness of kaolinite particles in this study was determined to be 38.3nm±11.7nm. Furthermore, the surface charge of the kaolinite edge surfaces was evaluated with a super sharp Si tip. The point of zero charge (PZC) of the kaolinite edge surface was determined to be below pH 4, in contrast to the traditional view that the edge surfaces of kaolinite particles may carry a positive charge at pH 4. This lower PZC of the kaolinite edge surface was attributed to the lack of isomorphous substitution in the silica tetrahedral layer when compared to the PZC for the muscovite edge surface. Our results are consistent with the particle aggregation and flotation behavior of kaolinite, and should provide the basis for improved flotation strategies leading to the efficient recovery and utilization of mineral and energy resources.

A 1H and 27Al NMR investigation of yttrium(III) and europium(III) interaction with kaolinite

Sorption of the trivalent metals Eu3+ and Y3+ on a natural kaolinite mineral was investigated with solid-state 1H and 27Al NMR. The metal ion concentration in the samples was varied between 4×10−6M and 8×10−5M, while the kaolinite concentration and ionic strength with respect to Na+ were kept constant at 5g/l and 1mM, respectively. The aqueous samples were equilibrated for at least six days at pH8.0±0.1 before the mineral was separated from the supernatant, recovered, and dried. Metal ion addition to the kaolinite samples had no influence on recorded 27Al spectra, which show a center peak at 6.15ppm corresponding to octahedrally coordinated aluminum in the gibbsite-like sheets of kaolinite. The 27Al measurements were, however, imperative to perform as potential mineral dehydroxylation was monitored from the 27Al spectra before and after drying treatment and normalization of 1H spectra was based on the acquired 27Al data. The attachment of Y3+/Eu3+ on the kaolinite surface was shown to reduce 1H resonances with chemical shifts at 2.3–3.5ppm, 1.6, 1.3, 0.9 and 0.7–0.3ppm. The loss of proton signal could be attributed to metal ion attachment to isolated Al–OH groups on the kaolinite edge surfaces and possibly to bridging Al–OH–Al hydroxyls on the gibbsite-like basal plane of the mineral.

Acidity of edge surface sites of montmorillonite and kaolinite

Acid–base chemistry of clay minerals is central to their interfacial properties, but up to now a quantitative understanding on the surface acidity is still lacking. In this study, with first principles molecular dynamics (FPMD) based vertical energy gap technique, we calculate the acidity constants of surface groups on (010)-type edges of montmorillonite and kaolinite, which are representatives of 2:1 and 1:1-type clay minerals, respectively. It shows that Si–OH and Al–OH2OH groups of kaolinite have pKas of 6.9 and 5.7 and those of montmorillonite have pKas of 7.0 and 8.3, respectively. For each mineral, the calculated pKas are consistent with the experimental ranges derived from fittings of titration curves, indicating that Si–OH and Al–OH2OH groups are the major acidic sites responsible to pH-dependent experimental observations. The effect of Mg substitution in montmorillonite is investigated and it is found that Mg substitution increases the pKas of the neighboring Si–OH and Si–OH2 groups by 2–3pKa units. Furthermore, our calculation shows that the pKa of edge Mg–(OH2)2 is as high as 13.2, indicating the protonated state dominates under common pH. Together with previous adsorption experiments, our derived acidity constants suggest that Si–O– and Al–(OH)2 groups are the most probable edge sites for complexing heavy metal cations.

Atomic scale structures of interfaces between kaolinite edges and water

This paper reports the atomic scale structures of kaolinite edge surfaces in contact with water. The commonly occurring edge surfaces are investigated (i.e. (010) and (110)) by using first principles molecular dynamics (FPMD) technique. For (110)-type edge surface, there are two different surface topologies (denoted as (110)-1 and (110)-2) and they are considered separately.By using constrained FPMD technique, the free-energy changes for the leaving processes of water ligands of O-sheet Al cations have been calculated and thus, the coordination states of those Al cations are determined. The results show that for (010) and (110)-2 edges, both the 5 and 6-fold coordination states are possible whereas for (110)-1 edges, only the 6-fold states are stable. Based on the analyses of H-bonding structures at the interfaces, the surface acid/base reactive sites are illustrated. (1) T-sheet groups are Si–OH, behaving as both proton donors and acceptors. (2) Bridging oxygen At (010) and (110)-2 edges, these sites are inaccessible from the water and thus, they are not effective reactive sites. At (110)-1 edges, the bridging oxygen atoms are proton accepting sites. (3) O–sheet groups At (010) and (110)-2 edges, for 6-fold Al cases, the active surface groups are Al–(OH)(OH2) and for the 5-fold Al cases, the active surface groups are Al–(OH). At (110)-1 edges, the active site is Al–OH2. This study provides fundamental structural properties for understanding the interface chemistry of widely occurring 1:1 type phyllosilicates.