Clay Edges Research Articles

Preferential interlayer adsorption sites in phyllosilicate clay edge models by molecular dynamics simulation

Phyllosilicate clay minerals have been proposed as a possible buffer material to be used in deep geological repositories containing high-level waste and used nuclear fuel. This work precisely characterizes ion interactions with two types of adsorption sites present in these clays: MgAl’ substitutions and undercoordinated edge surface atoms. A number of unique structural models were considered to represent the diverse local environments that ions in these systems are likely to encounter. Using molecular dynamics simulation with the CLAYFF potential, the spatial distribution, interlayer composition, and residence times of Na+ and Cl− ions as radionuclide analogs in pyrophyllite and montmorillonite clay models were investigated to identify the most favorable conditions for sequestration. The most significant factor impacting ion adsorption was found to be the localization of charge density at substitution sites. In a montmorillonite system in which substitution sites were distributed evenly to produce a low charge density, sequestration performance was found to be comparable to pyrophyllite.

Speciation studies at the Illite - solution interface: Part 2 – Co-sorption of uranyl and phosphate ions

Although it is well known that clays and phosphate ions (hereafter referred to as “P”) can contribute to the long term uptake of uranium (U) in a variety of geochemical systems, work is still needed to document the surface speciation of U(VI) and P sorbed on relevant clay minerals such as illite. Following on from previous work showing strong sorption of phosphate ligands on the surface of a homoionic Na-illite ([1]), we addressed the (competitive/synergistic) mechanisms of (co)sorption of trace levels of uranyl ions (1–10 µM) and phosphate ligands (100 µM) onto this clay, and the identity of the surface species formed, by in situ spectroscopy monitoring of the clay-solution interface along sorption. We also performed complementary batch sorption experiments and electrophoretic mobility (EM) measurements. Macroscopic data indicated a uranyl sorption dependent on pH, aqueous U concentration and clay-to-solution ratio, a signature of P on the U sorption, and a promotion of P sorption with U concentration. Macroscopic and EM data also suggested mostly reversible mechanisms of P and U co-sorption that confer negative charges on the mineral surface and involve several types of uranyl phosphate surface species and/or surface sites present on the clay edges. FTIR interface spectra confirmed that several types of inner sphere uranyl phosphate surface species formed at acidic pH at the illite-solution interface, as a function of U surface coverage and/or reaction time. An inner-sphere uranyl phosphate surface complex, likely with a U-bridging structure, was formed rapidly on high-affinity surface sites existing in limited quantities on the clay edges. Another U-P surface complex (with a possible P-bridging structure) was progressively formed in increasing amounts with U surface coverage and time on low affinity clay edge sites. Finally, a third U-P surface species having an autunite-like structure (probably a U-P polynuclear surface species) was formed on the illite surface at high U concentration (10 µM). The two later species competed with the existence on the illite surface of inner sphere surface complexes and outer-sphere surface complex of phosphate, which were identified in the absence of U. Information on uranyl surface speciation in the presence of phosphate ligands presented here is novel and provides for the first time in situ spectroscopic evidence of synergistic U-P sorption process leading in the formation of several types of uranyl phosphate sorption species on the surface of illite. These results provide a sound basis for better understanding / prediction of U sorption in the environment, including in clayey formations being considered as long term barriers to radionuclide migration in far field of high level radioactive waste repository.

Speciation studies at the Illite - solution interface: Part 1 – Sorption of phosphate ions

Although phosphate ion (hereafter “P”) sorption is known to influence clay surface reactivity and trace metal behavior in the environment, there is still a lack of in situ spectroscopic evidence on the identity of phosphate sorption species formed at the interface between solution and clays with a 2:1 layered structure (2 silica tetrahedral: 1 octahedral layers), which are ubiquitous in soils and clay formations. We studied the surface speciation of phosphate ions by in situ spectroscopic monitoring along P sorption at the interface between a solution and a homoionic Na-illite, as a function of time or aqueous phosphate concentration (20–250 µM) leading to low/moderate P surface coverage. We also combined them with batch sorption experiments and electrophoretic mobility (EM) measurements. Macroscopic data indicated that the percentage of P sorption depends on pH, aqueous phosphate concentration, and the clay/solution ratio of the experiment, i.e., the coverage of the clay surface by P, with a maximal sorption capacity of the clay equal to ca. 6 µmol.g−1 at pH 4. Macroscopic and EM data further suggest that P sorption mechanisms onto illite confer negative charges on the surface and involve several sorption species and/or surface sites present on the clay edges. In situ FTIR spectra showed that, at acidic pH, three types of phosphate surface species exist at the illite-solution interface: an outer-sphere surface complex (OSSC) of phosphate (ΞSOH2+…H2PO4-, with S referring to a sorption site on the clay), an inner-sphere surface complex (ISSC) of phosphate, probably a monodentate binuclear surface complex ((ΞSO)2PO2) formed by surface ligand exchange with high-affinity aluminol sites and, to a lesser extent, low-affinity sites on the clay edges, and, in limited amounts, an Al-phosphate surface precipitate. Phosphate ion sorption occurs through the initial formation of the OSSC of phosphate, which dominates phosphate surface speciation at short reaction times and low P concentrations and transforms over time into the ISSC of phosphate. This study presents the first published in situ surface speciation of phosphate ions sorbed at the interface between a solution and an illite. The evidence of a strong sorption of phosphate ions at this clay surface provides new and sound knowledge to better understand the environmental fate of phosphate ions and trace metals, particularly in agricultural soil - groundwater continua or in clay host rocks considered for high-level radioactive waste disposal.

Surface reconstruction and cleavage of phyllosilicate clay edge by density functional theory

Phyllosilicate materials (e.g., smectite) have been proposed to act as a waste barrier within long-term deep geological repositories. Aside from longevity, these clay materials must also limit the mobility of radionuclides, which is achieved through intermolecular interaction involving clay basal planes and edge surfaces. We aim to contribute to the fundamental knowledge of clay edges by investigating surface reconstruction in a dehydrated context. Using density functional theory, surface reconstruction, charge redistribution, and formation energies have been explored for pyrophyllite edges resulting from thirteen terminations. Three primary reconstruction mechanisms were identified: (1) the displacement of Al into the tetrahedral layer, (2) proton shift, and (3) sub-surface reconstruction involving sub-surface -OH or apical O and surface Si. Reduction in overall relative bond deficiency and charge redistribution to becoming more bulk-like were found to be the main driving force for reconstruction. Formation energies were also determined with dispersion correction for dehydrated and hydrated surfaces. The presence of water terminating agents reduced energetic costs by approximately one order of magnitude and the inclusion of dispersion correction resulted in indistinguishable formation energies between different hydrated surfaces. Linear regression models were also explored to predict surface formation energies based on relative bond deficiency and charge. These models were able to identify the key contributions to the surface energy; however, due to the small dataset they were not found to be predictive.

Molecular dynamics simulation on methane hydrate formation in clay nanopores of edge surfaces

Natural gas hydrates are predominantly buried in clay sediments in natural environments, where are some edge surfaces of clay particles directly contacting the hydrates. However, the exact nature of the interaction between these surfaces and the hydrates, as well as their influence on hydrate formation, remains elusive. Herein, microsecond molecular dynamics simulations have been performed to investigate CH4 hydrates formation in nanopores consisting of clay edge surfaces, to reveal the effects of clay edge surfaces and layer charges. The simulation results show that the clay edge surfaces affect CH4 hydrate formation by changing the distribution of water and CH4 molecules via surface adsorption, mainly ascribed to the different polarities of the groups on the edge surfaces of different clays. The greater the electronegativity of the clay, the stronger the inhibition of CH4 hydrate formation, thus, the electroneutral clays are more beneficial for CH4 hydrate formation than the electronegative clays. Moreover, in the early stage of the simulation, compared with the electronegative clays, the electroneutral clays are more favorable for the diffusion of CH4 molecules from the nanopores into the bulk solution and then promote CH4 hydrate formation. On the other hand, the ions in the solution gradually aggregate together and their distribution becomes denser and more ordered. The edge surfaces of electroneutral clay are more accessible to hydrate solids than electronegative clay. These molecular insights into the formation behavior of CH4 hydrates in clay nanopores consisting of edge surfaces help to understand the formation process of natural gas hydrates in marine sediments.

Np(V) Retention at the Illite du Puy Surface.

In this study, Np(V) retention on Illite du Puy (IdP) was investigated since it is essential for understanding the migration behavior of Np in argillaceous environments. The presence of structural Fe(III) and Fe(II) in IdP was confirmed by Fe K-edge X-ray absorption near-edge structure (XANES) and 57Fe Mössbauer spectroscopy. In batch sorption experiments, a higher Np sorption affinity to IdP was found than to Wyoming smectite or iron-free synthetic montmorillonite. An increase of the relative Np(IV) ratio sorbed onto IdP with decreasing pH was observed by solvent extraction (up to (24 ± 2)% at pH 5, c0(Np) = 10-6 mol/L). Furthermore, up to (33 ± 5)% Np(IV) could be detected in IdP diffusion samples at pH 5. Respective Np M5-edge high-energy resolution (HR-) XANES spectra suggested the presence of Np(IV/V) mixtures and weakened axial bond covalency of the NpO2+ species sorbed onto IdP. Np L3-edge extended X-ray absorption fine structure (EXAFS) analysis showed that significant fractions of Np were coordinated to Fe─O entities at pH 9. This highlights the potential role of Fe(II/III) clay edge sites as a strong Np(V) surface complex partner and points to the partial reduction of sorbed Np(V) to Np(IV) via structural Fe(II).

Chromium(VI) bioreduction by cysteine over clay particle edges: Catalytic mechanisms and influencing factors

Bioremediation is effective for scavenging Cr(VI), one of the eight most toxic chemicals according to the world health organization. This study presents a comprehensive understanding of Cr(VI) bioreduction by cysteine (Cys) over clay minerals that serve as supports, and the central roles of Cys during Cr(VI) bioremediation are verified and unraveled at the molecular level. Impacts of Cys coverage, active sites, and the reducing agent are further discussed. Cr(VI) bioreduction proceeds via competitive mechanisms: Oxidative dehydrogenation (ODH) driven by π-conjugation (C=S) development and SS dimerization (SDM) driven by SS bond formation. The optimal reaction paths for ODH at the Al(III) site, SDM at the Al(III) site, ODH at the Mg(II) site, and SDM at the Mg(II) site have the apparent energy barriers of 124.6, 115.9, 117.3, and 71.9 kJ/mol, respectively. Cr(IV) represents the major product except for SDM at Mg(II) site where Cr(II) dominates. Mechanisms and product distribution are affected by various factors: 1) Increase of Cys coverage triggers Cr(VI) reduction to occur sequentially via Stage 1 of ODH, Stage 2 of ODH, Stage 1 of SDM, and Stage 2 of SDM; 2) Cr(VI) reduction occurs preferentially at the Mg(II) versus Al(III) site, and the conformation transition of CrIV structure promotes reaction and appears for the most preferred paths of the AlIII rather than MgII site; 3) Mechanisms are decided by the Fe(II) content for Fe(II) while by π-conjugation development for alcohols and Cys (ODH): Ethanol is preferred kinetically, while high Cys coverage adopts SDM that lacks for ethanol. Results provide significant new insights for Cr(VI) bioremediation and accelerate catalyst designs for Cr(VI) reduction which conduce to Cr(VI) management and rational utilization of less toxic products.

Understanding the unique geochemical behavior of Sc in the interaction with clay minerals

Abstract Regolith-hosted rare earth elements (REEs) deposits received great attention due to the increasing incorporation of REEs in modern technologies. In lateritic Sc deposits and ion-adsorption deposits (IADs), Sc behaves quite differently from REEs: REEs adsorb as outer-sphere complexes on clay surface in IADs, while Sc could enter the lattice of clay minerals in lateritic Sc deposits. The unique behavior of Sc has not been well understood yet. Here, by using first-principles molecular dynamics techniques, we show that the complexation mechanisms of Y3+ and Sc3+ on clay edge surfaces are distinctly different. Y3+ preferentially adsorbs on Al(OH)2SiO site with its coordination water protonated. Sc3+ is found to behave similarly to other first-row transition metals (e.g., Ni2+) due to its smaller ionic radius and prefers adsorbing on the vacancy site, from where Sc3+ can be readily incorporated in the clay lattice. The H2O ligands of Sc3+ get deprotonated upon complexation, providing new binding sites for further enrichment of Sc3+. These processes prevent Sc3+ from being leached during weathering and lead to the formation of Sc-rich clay minerals found in lateritic deposits. Based on these results, it is revealed that the small ionic radius and high affinity to enter the vacancy on edge surfaces make Sc compatible with clay minerals and are the origin of its unique geochemical behavior.

Molecular-level insights into pH regulation of cation adsorption and exchange at clay particle edges

Clay particle edges are often adsorption and catalytic centers while performances are strongly pH-dependent. Microscopic understanding of adsorption of single (Na+, K+, Cs+, Pb2+) and binary (Na+/Cs+, Na+/K+, K+/Pb2+, Na+/Pb2+) metal ions by montmorillonite particle edges under various pH conditions is addressed by molecular dynamics simulations, and mechanisms regarding how pH regulates adsorption of metal ions, ion-specific effects, impacts of co-ions, thermodynamics and kinetics of ion exchange, and immobilization of heavy metals are unraveled. Under all pH conditions, Na+ and Pb2+ with smaller ionic radii have much higher inner-sphere adsorption densities than K+ and Cs+, and pH elevation generates new adsorption configurations that greatly promote adsorption. The promoting extent is apparently larger for Na+ and Pb2+, while ion-specific sequence (Na+ > K+ > Cs+) remains, which is applicable to single and binary metal ions. Impacts of co-ions onto adsorption rely on their identities and become magnified at higher pH although with consistent trends at all pH conditions; e.g., Na+ always inhibits Pb2+ adsorption while the degree of inhibition is larger due to pH elevation. Whether to promote or suppress adsorption by co-ions can be predicted from relative adsorption preference (Na+ > Pb2+ > K+ > Cs+). Stronger competitive adsorption occurs for binary Na+/Pb2+ than other Aa+/Bb+ ions and for the particle edges than basal and interlayer surfaces. As indicated by selectivity coefficients, montmorillonite particle edges are always Na+-selective, while pH elevation significantly enhances Pb2+ selectivity. Specific adsorption at clay particle edges emerges at lower pH for Na+ than Pb2+ (5.3 vs. 6.9) and increases at higher pH, while the increasing extent is larger for Pb2+, indicating pH elevation favors the immobilization of heavy metals. Results significantly enrich the knowledge about the adsorption/exchange of metal ions and the immobilization of heavy metals by clay minerals.

Molecular dynamics simulation of organic contaminant adsorption on organic‐coated smectite clay

AbstractMolecular dynamics simulations are used to examine the adsorption and aggregation of tyrosine and glutamate molecules on a stack of smectite particles at three different organic loadings. The results reveal a strong affinity of the smectite surface for the organic molecules with the zwitterionic tyrosine molecules coating the exterior surfaces (and entering the interlayer region to a lesser extent), whereas the negatively charged glutamate molecules are generally found near the clay edge sites and in coordination with aqueous and coordinating Ca ions. Additional simulations examine the effects of the tyrosine and glutamate organic coatings on the adsorption of two organic contaminants, dimethyl phthalate (DMP) and perfluorobutanesulfonic acid (PFBS). The addition of these coatings did not prevent the DMP and PFBS molecules from accessing previously identified favorable adsorption domains. An analysis of the adsorption energetics shows an initial decrease in contaminant adsorption relative to pure mineral surfaces as tyrosine and glutamate are introduced to the system, followed by increasing adsorption with increasing organic loadings. Overall, this research advances the mechanistic understanding of the interplay between smectite surfaces, organic coatings, and organic contaminants.

Mechanisms for Cr(VI) reduction by alcohols over clay edges: Reactive differences between ethanol and ethanediol, and selective conversions to Cr(IV), Cr(III) and Cr(II) species

Catalytic reduction by alcohols over clay minerals works efficiently under a wide range of pH and represents an emerging approach to control Cr(VI) contamination. Herein, mechanisms for Cr(VI) adsorption and reduction at clay edges are addressed by dispersion-corrected periodic DFT calculations, considering different active sites, and types (monohydric and polyhydric) and coverage of alcohols. Cr(VI) adsorbs favorably at clay edges, forming direct bonds and strong H-bonds. Mechanisms for Cr(VI) reduction by alcohols are largely determined by π-conjugation development, and efficient conversion conduces to Cr(VI) removal. Cr(II), Cr(III) and Cr(IV) are useful for different purposes, and high selectivity towards these products is realized through rational catalysts design: 1) Cr(IV) dominates at Al3+ site with all ethanol coverage, Al3+ site with high-coverage ethanediol, and Mg2+ site with low-coverage ethanol; 2) Cr(III) dominates at Al3+ and Mg2+ sites with low-coverage ethanediol; 3) Cr(II) dominates at Mg2+ site with high-coverage ethanol or ethanediol. Results agree finely with experimental observations available, and significant new insights have been provided for Cr management and recycling. Detailed electronic structure and vibrational analyses, which can also guide future experimental studies, manifest that Cr(VI) reduction progresses are effectively monitored by ESR and FT-IR techniques.

Anaerobic oxidation of arsenite by bioreduced nontronite

The redox state of arsenic controls its toxicity and mobility in the subsurface environment. Understanding the redox reactions of arsenic is particularly important for addressing its environmental behavior. Clay minerals are commonly found in soils and sediments, which are an important host for arsenic. However, limited information is known about the redox reactions between arsenic and structural Fe in clay minerals. In this study, the redox reactions between As(III)/As(V) and structural Fe in nontronite NAu-2 were investigated in anaerobic batch experiments. No oxidation of As(III) was observed by the native Fe(III)-NAu-2. Interestingly, anaerobic oxidation of As(III) to As(V) occurred after Fe(III)-NAu-2 was bioreduced. Furthermore, anaerobic oxidization of As(III) by bioreduced NAu-2 was significantly promoted by increasing Fe(III)-NAu-2 reduction extent and initial As(III) concentrations. Bioreduction of Fe(III)-NAu-2 generated reactive Fe(III)-O-Fe(II) moieties at clay mineral edge sites. Anaerobic oxidation of As(III) was attributed to the strong oxidation activity of the structural Fe(III) within the Fe(III)-O-Fe(II) moieties. Our results provide a potential explanation for the presence of As(V) in the anaerobic subsurface environment. Our findings also highlight that clay minerals can play an important role in controlling the redox state of arsenic in the natural environment.

Evaluation of component additive modelling approach for europium adsorption on 2:1 clays: Experimental, thermodynamic databases, and models

This study evaluates the component additive approach for Eu adsorption on mixtures of smectite and illite, which are the most common clays used as barriers for contaminant retention in waste repositories. A thorough set of Eu adsorption data for Na-exchanged smectite and illite that encompasses a wide range of pH values, ionic strengths, and Eu concentrations was provided. This database is likely one of the largest sorption databases available for Eu in 2:1 clays, making it appropriate for sorption model calibration.The main adsorption mechanisms considered were surface complexation, on weak and strong clay edge sites, and cation exchange. Further, the role of principal ions, which are naturally leached from clays, as competitive factors for Eu retention, was evaluated in the modelling calculations. The main uncertainties related to the modelling procedures and the use of different thermodynamic data on sorption modelling were outlined.The reactions and parameters successful in modelling Eu adsorption on individual clays were used without any modification to model Eu adsorption on illite/smectite mixtures, wherein only the relative mineral proportions were considered. The fit of the sorption data in the mixed clay system was satisfactory, indicating that, in 2:1 clays, Eu sorption is an additive process, which stresses the predictive capacity of the component additive approach in these systems. This is an important support for assessing the performance of barrier materials for contaminant migration under different geochemical conditions.

Binding of tetracycline on soil phyllosilicates with Cd(II) as affected by pH and mineral type

Antibiotics often coexist with heavy metals in contaminated soils, making their behaviors more complicated than their individual presences. Sorption to soil components deeply affects the mobility, bioavailability, and fate of contaminants. However, limited studies have investigated the cosorption characteristics of tetracycline (TC) and heavy metals to typical soil minerals, particularly phyllosilicates. This study investigates the effect of Cd(II) on TC sorption by kaolinite (Kao, 1:1-type) and montmorillonite (Mont, 2:1-type) via isothermal adsorption, coupled with X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). Isothermal results show that TC sorption on both kaolinite (Kao, 1:1-type) and montmorillonite (Mont, 2:1-type) is little affected by Cd(II) at pH 4.5 but increases significantly at pH 8.5. The overall effect of Cd(II) on TC sorption depends on the intensities of two positive processes, i.e., (1) the formation of soluble Cd-TC complex increases sorption compared to free TC molecules and (2) the formation of a ternary clay-Cd-TC complex at clay variable edge sites increases sorption, and one negative process, i.e., competition of Cd(II) with cationic and zwitterionic TC species at the permanently charged sites decreases sorption. XRD analyses demonstrate that TC intercalates into the interlayer (d001 = 18.7 A) of Mont as one monolayer parallel molecule both with and without Cd(II) but adsorbs predominately on external surface of Kao. FTIR results suggest a flat-lying horizontal configuration of adsorbed TC molecules on clays, accompanied by a strong complexation between the carbonyl and amino groups of TC molecules and the surfaces. Besides, the presence of Cd(II) more or less facilitates this complexation. We propose that Cd(II) can significantly promote TC adsorption on clay minerals at alkaline pH rather than acidic pH. TC and Cd(II) interact intensely and form strong surface complexation on clay surfaces. Hence, coexistence with Cd(II) may lower the mobility and thus the bioavailability of TC in alkaline soils, which provides a new direction for soil remediation by regulating soil pH using alkaline materials.

Nanoscale Forces between Basal Mica Surfaces in Dicarboxylic Acid Solutions: Implications for Clay Aggregation in the Presence of Soluble Organic Acids.

The stability of organomineral aggregates in soils has a key influence on nutrient cycling, erosion, and soil productivity. Both clay minerals with distinct basal and edge surfaces and organic molecules with reactive functional groups offer rich bonding environments. While clay edges often promote strong inner-sphere bonding of −COOH-laden organics, we explore typically weaker, outer-sphere bonding of such molecules onto basal planes and its significance in organomineral interactions. In this surface force apparatus study, we probed face-specific interactions of negatively charged mica basal surfaces in solutions containing carboxyl-bearing, low-molecular-weight dicarboxylic acids (DAs). Our experiments provide distance-resolved, nanometer-range measurements of forces acting between two (001) mica surfaces and simultaneously probe DA adsorption. We show that background inorganic ions display crucial importance in nanoscale forces acting between basal mica surfaces and in DA adsorption: Na+ contributes to strong repulsion and little binding of dicarboxylic anions, while small amounts of Ca2+ are sufficient to screen the basal surface charge of mica, facilitate strong adhesion, and enhance dicarboxylic anion adsorption by acting as cationic bridges. Despite reversible and weak adsorption of DAs, we resolve their multilayer binding via assembly of hydrophobic chains in the presence of Ca2+, pointing the importance of abundant, less reactive basal clay surfaces in organomineral interactions.

Wettability of clay aggregates—A coarse-grained molecular dynamic study

Clay minerals play a vital role in environmental pollution remediation due to their special wettability performance. However, limited by experimental technology and computational costs, wettability mechanism of clay aggregates at mesoscale remains unclear. In this work, the coarse-grained molecular dynamic simulation method has been used as a novel strategy to discover the mesoscale wettability of clay aggregates with water. Through visualization analysis, we have observed that the water droplet firstly got in touch with the clay surface, then some water molecules flowed into clay pores while some adsorbed onto clay platelets edge. With further invasion of water, clay platelets rearranged and reoriented to fit themselves. The clay-water system reached equilibrium under three mechanisms, including normal blocking, non-normal diffusion and interlayer adsorption. The final inhomogeneous equilibrium water droplet had an average contact angle value of 81.5°, which is in good agreement with preceding experimental measurement. These findings help for a better understanding on the wetting property of clay mineral surface and fill a gap of the wettability study on mesoscale level.

Response of tungsten (W) solubility and chemical fractionation to changes in soil pH and soil aging

A thorough understanding of the geochemical behavior of W in soils is crucial for environmental risk assessment. Soil pH is known as master variable of element solubility and bioavailability in soils. Here we report on effects of soil pH (modified by liming and acid – base additions) and soil aging on the environmental availability of W in soil using W solubility and chemical fractionation as indicators. Experimental soils included two naturally acidic soils with contrasting soil texture (SAND, CLAY), at native pH or limed with 2.5% CaCO3, and spiked with increasing concentration of W.Our results showed that W was significantly more labile in alkaline compared to acidic soils, confirming the validity of results of pure-mineral studies for more heterogeneously composed soils. While labile W was generally greater in the SAND compared to the CLAY soil, the reverse trend was observed in the limed soils at the highest W addition (5000 mg kg−1). Combining our results with previous mechanistic reports suggests that clay edge sorption sites significantly contributed to W retention in treatments with low to medium W additions, resulting in lower environmental availability for W in the CLAY soil. At high W concentrations and high pH, the stronger W retention in the SAND was attributed to continuous formation of W surface polymers on the more abundant metal (oxyhydr)oxides, a process that has been previously reported to occur even under alkaline conditions. A first comparison of various soil chemical methods (Bray & AB-DTPA extractions, soil solution centrifugation CL, diffusion-based DGT) to predict W phytoavailability in soil also revealed a strong pH dependency challenging the identification of a suitable method. This study is one of the first demonstrating the pH dependence of W in natural soils and delivers evidence for increased risk of W mobilization in W polluted, alkaline soil environments.

Assessing the wetting state of minerals in complex sandstone rock in-situ by Atomic Force Microscopy (AFM)

Low salinity waterflooding is a low-cost method of enhancing oil recovery although, no consistent concept has been established explaining why some oil-fields show an increase in oil production when the salinity of the injected brine is reduced, while others do not. Various studies were conducted investigating the underlying mechanisms of the ‘low salinity effect’ using different crude oil, brine and rock compositions. Core floods of sandstone rock and analyses of molecular interactions using model systems indicate that clay content may play a dominant role. However, the spatial configuration of the sheet-like clay particles, which may vary from rock to rock, complicate comparisons of these model scenarios with reality.In the present study, we report the development of a pre-screening method using Atomic Force Microscopy (AFM) to assess rock-fluid interactions, which has previously only been used either on artificial model systems or minerals from crushed rock, by exploring the capability to operate in-situ in complex rock without crushing. The orientation of clay particles within a pore of an outcrop sandstone, Bandera Brown, was investigated with AFM and these particles were further assessed for changes in adhesion in brines of differing salinity. The results show a decrease in adhesions between CH3-functionalised AFM tips and the rock surface in low salinity brine, predominantly at the clay edges. This demonstrates that the edges of the clay particles, which may pin the oil phase after wettability alteration and therewith prevent oil from getting produced, lose this capacity when exposed to low salinity brine.

Bisphosphonate nanoclay edge-site interactions facilitate hydrogel self-assembly and sustained growth factor localization

Nanoclays have generated interest in biomaterial design for their ability to enhance the mechanics of polymeric materials and impart biological function. As well as their utility as physical cross-linkers, clays have been explored for sustained localization of biomolecules to promote in vivo tissue regeneration. To date, both biomolecule-clay and polymer-clay nanocomposite strategies have utilised the negatively charged clay particle surface. As such, biomolecule-clay and polymer-clay interactions are set in competition, potentially limiting the functional enhancements achieved. Here, we apply specific bisphosphonate interactions with the positively charged clay particle edge to develop self-assembling hydrogels and functionalized clay nanoparticles with preserved surface exchange capacity. Low concentrations of nanoclay are applied to cross-link hyaluronic acid polymers derivatised with a pendant bisphosphonate to generate hydrogels with enhanced mechanical properties and preserved protein binding able to sustain, for over six weeks in vivo, the localized activity of the clinically licensed growth factor BMP-2.

Innovative Method for Laponite Encapsulation into Polymer Latex Particles by Clay Cluster-Seeded Emulsion Polymerization

We herein report an innovative pathway for the encapsulation of Laponite platelets into polymer nanoparticles via free radical polymerization in heterogeneous aqueous medium. Hydrophobization of the Laponite platelets was performed via double functionalization of the clay basal surfaces and edges by a cationic surfactant and an organosilane, respectively. The hydrophobized platelets were then dispersed in toluene and ultrasonicated with an aqueous solution of an anionic surfactant to form clay-loaded toluene droplets. The droplets were subsequently transformed into clusters by toluene evaporation and finally encapsulated into polymer latex particles using a seeded-emulsion polymerization process. Two different copolymers were synthesized: poly(styrene-co-methyl acrylate), as a model system and poly(vinylidene chloride-co-methyl acrylate), a specialty film-forming copolymer. Stable composite particles with a diameter ranging from 150 to 180 nm were obtained for both copolymers. Transmission electron microscopy analysis showed that the Laponite clay platelets were successfully encapsulated into the polymer latex particles. The films cast from the composite suspensions of poly(vinylidene chloride-co-methyl acrylate)/clay particles showed spherical inclusions of clay tactoids dispersed within the polymer matrix.