Phyllosilicate Clay Research Articles

On the Role of the Interlayer Interactions in Atomistic Simulations of Kaolinite Clay

A systematic simulation study was performed to investigate the interlayer interactions in a 1:1 layered phyllosilicate clay, kaolinite. Atomistic simulations with classical realistic force fields (INTERFACE and ClayFF) were used to examine the influence of the individual non-bonded interactions on the interlayer binding in the kaolinite model system. By switching off selected pairwise interactions in the applied force fields (leaving the intralayer interactions intact), it was confirmed that the tetrahedral–octahedral-type pairwise interactions held the kaolinite plates together and that interlayer hydrogen bonding, modeled by Coulombic forces, played a dominant role in this. Furthermore, it was observed that the number of hydrogen bonds formed had a significant influence on the basal spacing, and thus there was a striking change in the layer–layer interaction strength when there were only two kaolinite plates in the system, rather than several plates, as in real kaolinite particles. Contrary to expectations, the dispersion forces of the studied force fields alone were found to be strong enough to hold the kaolinite plates together.

Significant effect of salinity on zinc adsorption on tropical coastal and floodplain soils

AbstractRising sea levels due to climate change are causing increased salinisation of low‐lying coastal and floodplain soils, and the impact of this process on the bioavailability of plant nutrients needs to be understood as mitigation strategies are adapted. Zinc (Zn) is an element of particular importance due to its function as a micronutrient for plants including rice and other staple foods. In the current study, our aim was to investigate the effects of salinisation on zinc adsorption onto soils representing at‐risk coastal and floodplain environments, addressing in particular our knowledge gap concerning the roles that solution chemistry and soil composition play. To this end, we conducted batch adsorption experiments in the laboratory and ran geochemical models in saline solutions up to 0.7 mol L−1 ion strength incorporating both (i) a multi surface model (MSM) for surface reactions containing three phases, that is iron hydroxides, organic matter and phyllosilicate clays, and (ii) aqueous‐phase complexation to dissolved organic and inorganic ligands. Surface reactions were modelled using the diffuse double layer model, the NICA–Donnan model and an ion exchange model using the Gaines–Thomas convention. We combined the experimentally determined mass composition of surface phases with generic modelling parameters taken from the literature. We first show that increasing salinity enhances the formation of aqueous Zn‐chloride complexes in the presence of dissolved organic matter and bicarbonate, thereby decreasing the availability of free Zn2+ and supressing the partitioning of zinc to the adsorbed phase. We demonstrate using batch adsorption experiments with a calcareous hydraquent and a tropaquept, that salinity decreases zinc adsorption strongly in the pH range between 3 and 6. Satisfactory agreement between experiments and model calculations was achieved with root‐mean‐square errors ranging for different salinities between 2.88% and 2.92% for the hydraquent and between 4.59% and 2.74% for the tropaquept soil. Model predictions of adsorption were slightly inferior at low salinity for the hydraquent soil and at high salinity for the tropaquept soil, pointing possibly to an incomplete geochemical model or to a need to parametrise surface adsorption models at higher ionic strengths. Present surface models have been largely parametrised at lower ionic strength. We lastly apply the MSM to examine zinc adsorption in five endoaquepts soils, representing soil series from Bangladesh. We show that increasing salinity decreases zinc adsorption to the soil organic matter and the clay fractions. We conclude from our findings that increased soil salinity due to rising sea levels and climate change will have a significant impact on zinc cycling and possibly other micronutrients in areas where coastal soils and floodplain soils overlap, such as deltas and estuaries. In particular, we predict a decrease in zinc adsorption in acidic to neutral soils. The availability of zinc for biouptake through the roots of crop plants including rice will be significantly disturbed following salinisation, most likely affecting crop production. Our study demonstrates the potential that geochemical modelling combined with experimental data has to improve our capability to assess the effects of salinity due to rising seawater levels in vulnerable regions of the world.

Removal of an organophosphate insecticide from aqueous media using phyllosilicate clay: multivariable optimization, nonlinear kinetic modelling and thermodynamic study

The excessive and irrational use of organophosphate insecticides in our lives has resulted in a serious environmental problem, necessitating the proposal of novel and cost-effective plans to remove these organophosphate insecticides from agricultural or industrial wastewater. In this approach, we investigated the use of phyllosilicate clay as a low-cost and environmentally friendly adsorbent to eliminate an organophosphate insecticide (Diazinon DZN) from an aqueous medium. To achieve an adequate elimination of this insecticide from contaminated water, three main parameters including clay dose ( D c ) (0.1–1.5 g L−1), medium pH (5–9), and initial DZN concentrations ( C DZN ) (55–40 mg L−1) were optimized using a multivariable optimization based on Box-Behnken Design (BBD). The response surface method (RSM) and composite desirability function technique were used to find the ideal conditions for the three input variables and the adsorption amount ( Q e ) was selected as the output variable. The higher adsorption amount ( Q max ) was achieved with an adsorbent amount of 1.5 g L−1 in an acidic medium (pH = 5) at room temperature and a Diazinon initial dose of 25.85 mg L−1, with a maximum adsorbent loading of 12.20 mg g−1. The nonlinear modeling study showed that the adsorption kinetics of Diazinon follows the pseudo-second-order model. The thermodynamic study shows that the DZN removal process is exothermic, spontaneous, and of a stable configuration.

Formation of mineral-associated organic matter in temperate soils is primarily controlled by mineral type and modified by land use and management intensity.

Formation of mineral-associated organic matter (MAOM) supports the accumulation and stabilization of carbon (C) in soil, and thus, is a key factor in the global C cycle. Little is known about the interplay of mineral type, land use and management intensity in MAOM formation, especially on subdecadal time scales. We exposed mineral containers with goethite or illite, the most abundant iron oxide and phyllosilicate clay in temperate soils, for 5 years in topsoils of 150 forest and 150 grassland sites in three regions across Germany. Results show that irrespective of land use and management intensity, more C accumulated on goethite than illite (on average 0.23 ± 0.10 and 0.06 ± 0.03 mg m-2 mineral surface respectively). Carbon accumulation across regions was consistently higher in coniferous forests than in deciduous forests and grasslands. Structural equation models further showed that thinning and harvesting reduced MAOM formation in forests. Formation of MAOM in grasslands was not affected by grazing. Fertilization had opposite effects on MAOM formation, with the positive effect being mediated by enhanced plant productivity and the negative effect by reduced plant species richness. This highlights the caveat of applying fertilizers as a strategy to increase soil C stocks in temperate grasslands. Overall, we demonstrate that the rate and amount of MAOM formation in soil is primarily driven by mineral type, and can be modulated by land use and management intensity even on subdecadal time scales. Our results suggest that temperate soils dominated by oxides have a higher capacity to accumulate and store C than those dominated by phyllosilicate clays, even under circumneutral pH conditions. Therefore, adopting land use and management practices that increase C inputs into oxide-rich soils that are under their capacity to store C may offer great potential to enhance near-term soil C sequestration.

Terra Rossa Soil as an Adsorbent to Remove Hexavalent Chromium in Aqueous Solutions

Kaolinite is a phyllosilicate clay of type 1:1 which is usually associated with iron oxides in nature. The clay is abundant in Terra Rossa soils which is located in the Gunungkidul, karst area. Based on the structure and nature of it’s the charge, it has the potential to adsorb anionic heavy metals, especially hexavalent chromium. Heavy metal chromium in the form of hexavalent is very dangerous for human health and the metal is mostly produced from the waste of leather tanning factories that are widely available in Yogyakarta. The purpose of this study was to investigate the possibility of using Terra Rossa soil as an adsorbent to remove heavy metal Cr (VI) from aqueous solution. The experiment was carried out by mixing the adsorbent with a dose of: 1 – 4.5 grams of fine soil, Φ 0.5 mm into a 50 ml solution containing 10 ppm chromium with pH 5 and 3. Stirred at 200 rpm for 2 hours, then centrifuged 3500 rpm for 20 minutes. The filtrate was given phosphoric acid and diphenyl carbazide and then measured using a spectrophotometer with a wavelength of 540 nm. The results showed that the average percentage of chromium removal at pH 5 = 30%, while pH 3 = 49%. The adsorbent absorption (mg/g) decreased in line with increasing adsorbent dose. Absorption graphs of Freundlich and Langmuir models in pH3 solution show R2=0.9264 and 0.9361 respectively, while in pH5 solution the R2 values are 0.3752 and 0.3461, respectively. Adsorption capacity according to Freundlich = 1.7331 mg/g, while according to Langmuir = 5.28 mg/g with adsorption site affinity 0.1894 L/mol.

Water Adsorption on Kaolinite Basal and Edge Surfaces

The sequence of water adsorption is significant to understand the mechanism of clay-water interactions on clay mineral surfaces. Kaolinite is a typical non-expansive phyllosilicate clay, and its water adsorption is generally recognized to occur on the basal surfaces of aluminum-silicate particles, whereas edge surface adsorption is prevalently overlooked due to its complexity despite its potential large surface area available for adsorption. In this study, we used molecular dynamics and metadynamics simulation to quantitatively assess the free energy of water adsorption, viz., matric potential, on kaolinite for four types of external surfaces, namely, a basal silicon-oxygen (Si-O) surface, a basal aluminum-oxygen (Al-O) surface, and edge surfaces with deprotonation and protonation. The results show that edge surfaces exhibit adsorption sites that are more active with the lowest matric potential of -1.86 GPa, lower than that of basal surfaces (-0.92 GPa), due to protonation and deprotonation processes of the dangling oxygen. The adsorption isotherm from 0.2% of relative humidity (RH) was measured and analyzed using an augmented Brunauer-Emmet-Teller model to separate the edge and basal surface adsorption, further verifying that edge surface adsorption may prevail in kaolinite and occur earlier than base surface adsorption in RH less than 5%.

Chemical weathering intensity controls the accumulation of nickel in rice (Oryza sativa L.) cultivated in basalt-derived paddy fields

Globally, a high concentration of nickel (Ni) in agricultural soil is closely related to basalt weathering. Rice grains readily accumulate Ni, however, the evolution of Ni distribution and mobility in paddy fields with basalt weathering remain unclear. This study investigated the distribution, geochemical fraction, and mobilization of Ni in basalt bedrock-paddy soil-rice plant continuums under contrasting climates through a combination of field surveys, chemical extractions, X-ray diffraction, and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analyses. Results show that the soils developed in tropical climates were more enriched in Ni than those in temperate climates. The concentration of Ni in rice grains from the studied temperate regions (3.71 ± 2.01 mg/kg) was higher than that in the tropical regions (0.57 ± 0.37 mg/kg). The concentration of Ni bound to Fe (hydro)oxides in paddy soils from temperate regions was five times higher than that in tropical regions. In contrast, the concentration of Ni found in phyllosilicate clays (mainly smectite/illite) in paddy soils from the temperate regions was only ∼1/4 of that of clay (mainly kaolinite) in the tropical paddy soils, and the ratio of Ni concentration in phyllosilicate clays to highly reactive Fe (hydro)oxides in paddy topsoil in temperate regions was one order of magnitude less than that in tropical regions, which contributes to the low bioavailability of Ni in tropical paddy soils. Furthermore, the accumulation of Ni in rice grains was insensitive to soil pH in tropical regions but showed significant negative correlations with the degree of chemical weathering (chemical index of alteration and index of laterisation) in temperate and tropical regions. These observations support the hypothesis that chemical weathering and the dissolution–recrystallization of Fe (hydro)oxides induced by frequent redox fluctuations in paddy soils cause Ni repartition between Fe (hydro)oxides and phyllosilicate minerals, which influence Ni geochemical cycling and bioavailability, and fundamentally regulate Ni accumulation in rice. This study elucidates the mechanism and controlling factors underlying Ni partition in the basalt-paddy soil-rice plant continuum under distinct climates, providing new insights into the understanding of geochemical behavior and environmental fate of Ni.

The Structure of Ordered Mesoporous Materials Synthesized from Aluminum Phyllosilicate Clay (Bentonite).

This paper reports the synthesis and structural analysis of mesoporous silica materials with the use of aluminum phyllosilicate clay (bentonite) as an alternative silica source. In the proposed synthesis, bentonite, as natural aluminosilicate, was used instead of commercially available and quite expensive tetraethyl orthosilicate (TEOS) silica source. The objective of the research study was to determine the effect of aluminum loading in the mesoporous silica body for ordering structure, porosity, and potential sorption capacity to thorium ions. The unique direction developed in this procedure is focused on preparing advanced materials from natural sources with their own desired functionality and general availability. The applied procedure based on the classic, one-step synthesis of SBA-15 silicates was modified by gradually increasing the bentonite amount with simultaneous reduction of the TEOS content. The structural and morphological characterization, as well as evaluation of the porous structure of the obtained materials, was performed using powder wide-angle X-ray diffraction (XRD), small-angle scattering (SAXS), transmission and scanning electron microscopy (TEM, SEM), low-temperature nitrogen adsorption-desorption methods and potentiometric titration. The new, cost-effective composites for the removal of Th(IV) ions are proposed. The synergistic effect of expanding the porous surface using bentonite as a silica precursor and the presence of thorium-binding groups (such as Al2O3) is indicated.

Formation mechanism of wear-resistant composite film by Span 80-decorated halloysite nanotubes

Phyllosilicate clay mineral, despite being a high-performance and low-cost lubricant additive, faces the problems of possible biotoxicity and inconsistent mechanisms. Halloysite nanotube (HNT) clays have attracted much research attention because of their good biocompatibility and potential film-forming properties. The tribological properties of Span 80-decorated HNTs dispersed in a base oil at various concentrations were investigated using a pin-on-disk tribometer. The results show that at low concentrations (0.1–0.6 wt%), the modified HNTs, which have higher friction coefficients, exhibited excellent anti-wear performance, especially at 0.6 wt% concentration, at which the wear was readily reduced by 46.5%. The outstanding wear resistance is attributed to the formation of a composite tribofilm on the surface. The tribofilm comprises an outer layer of soft amorphous carbon film and an inner hard transition layer containing Al2O3, SiOx, and iron oxide. In contrast, at high HNT concentrations (0.8–1.0 wt%), the friction coefficient decreased by 13.5% attributed to the conversion of adhesive wear to abrasive wear by unfilmed halloysites. Further experiments demonstrate that the key factor to the formation of the composite biofilm is the crushing of halloysite, which is also affected by the load. These findings provide new insights into the tribological mechanism of phyllosilicates as potential eco-friendly additives.

Acid-in-Clay Electrolyte for Wide-Temperature-Range and Long-Cycle Proton Batteries.

Proton conduction underlies many important electrochemical technologies. A family of new proton electrolytes is reported: acid-in-clay electrolyte (AiCE) prepared by integrating fast proton carriers in a natural phyllosilicate clay network, which can be made into thin-film (tens of micrometers) fluid-impervious membranes. The chosen example systems (sepiolite-phosphoric acid) rank top among the solid proton conductors in terms of proton conductivities (15 mS cm-1 at 25°C, 0.023 mS cm-1 at -82°C), electrochemical stability window (3.35V), and reduced chemical reactivity. A proton battery is assembled using AiCE as the solid electrolyte membrane. Benefitting from the wider electrochemical stability window, reduced corrosivity, and excellent ionic selectivity of AiCE, the two main problems (gassing and cyclability) of proton batteries are successfully solved. This work draws attention to the element cross-over problem in proton batteries and the generic "acid-in-clay" solid electrolyte approach with superfast proton transport, outstanding selectivity, and improved stability for room- to cryogenic-temperature protonic applications.

Reversible Intercalation of Li Ions in an Earth-Abundant Phyllosilicate Clay.

The phyllosilicate family of clays is an intriguing collection of materials that make ideal models for studying the intercalation of alkali ions due to their layered topology and broadly tunable composition space. In this spirit, we present a hydrothermal method to prepare a layered iron phyllosilicate clay, Fe2Si4O10(OH)2, and an evaluation of its electrochemical performance for the (de)insertion of Li ions. Through careful structural refinement, we determined that this iron clay contains a 2:1 stacking sequence, which is directly analogous to the widely studied mineral montmorillonite, with the crystallites adopting a platelike morphology. Cyclic voltammetry and galvanostatic cycling reveal reversible insertion of lithium into the interstitial layers via a solid solution mechanism. Comparison of ion (de)intercalation with reports on other clay systems like muscovite, KFe2.75Si3.25O10(OH)2, which features a rigidly bound interlayer cation, demonstrates that controlling the net charge on the layers with phyllosilicate minerals is a route to enabling reversible cationic intercalation within the structure.

Vibrational Sum Frequency Generation Spectroscopy of Surface Hydroxyls on Nickel Phyllosilicate Nanoscrolls.

Phyllosilicate clays are layered structures with diverse nanoscale morphology depending on the composition. Size mismatch between the sheets can cause them to form nanoscrolls, a spiral structure with different inner and outer surface charges. The hydroxyls on the exposed surface of the nanoscrolls determine the adsorption properties and hydrophilicity of the surface. Vibrational sum frequency generation (VSFG) spectroscopy was applied to study the surface hydroxyls of nickel phyllosilicate (Ni3Si2O5(OH)4), revealing three distinct in-phase OH-stretch modes: 3642, 3645, and 3653 cm-1. The relative signs of the peaks allow their assignment as "outward" and "inward" pointing hydroxyls on the opposite sides of the scrolled sheet, consistent with the crystal structure. Orientational analysis of polarization-selected VSFG spectra is consistent with a broad (140-164°) step-function distribution of the OH-stretch tilt angles, which we attribute to the uncompensated portion of the scroll rolled more than a whole number of full turns.

Redox transformation of structural iron in nontronite induced by quinones under anoxic conditions

In natural anoxic subsurface environments, the geochemical cycles of iron are largely associated with the migration and transformation of organic matter. Intensive attention has been paid to the redox interaction of organic matter with aqueous Fe and iron (hydr)oxides. Whereas, the abiotic redox cycling of structural Fe in clay minerals induced by quinones has not been well understood. In this study, we selected nontronite (NAu-2) as a model Fe-bearing phyllosilicate clay mineral and 1,4-hydroquinone (H2Q)/1,4-benquinone (BQ) as a model quinone couple. Our results show that the structural Fe(III) in NAu-2, with tetrahedral Fe(III) priority, can oxidize H2Q into BQ, and octahedral Fe(II) in NAu-2 can reduce BQ to H2Q, with semiquinone radicals (SQ−) as intermediate. The extent of the redox reactions depends on the reduction potential difference between NAu-2 and H2Q/BQ. However, a fraction of Fe(II)-Fe(III)-OH and Fe(II)-Fe(II)-OH groups in the octahedral sheet are difficult to be oxidized by BQ, because the reduction potential gradient decreases to a low level as the reaction proceeds. And the structure of NAu-2 can only partially restored upon re-oxidation with tetrahedral Fe(III) irreversibility. Output of this study replenishes the understanding regarding redox cycling of structural Fe in clay minerals induced by quinones.

A one‐pot biosynthesis of an aerogel composite based on attapulgite clay/bacterial cellulose to remove Pb 2+ ion

A one‐pot biosynthesis of an aerogel composite based on attapulgite clay/bacterial cellulose to remove Pb ²⁺ ion

Switchable Bioelectrocatalysis of Glucose Oxidase Immobilized into Multilayers with Lamellar Nanoparticles of Amino-Functionalized Magnesium Phyllosilicate Clay

Lamellar organic clay consisting of amino-functionalized magnesium phyllosilicate (AMPC) with controllable positive charges was successfully prepared by simple one-pot synthesis. Then, the positively charged AMPC was assembled layer by layer with negatively charged glucose oxidase (GOx) enzyme into {AMPC/GOx} n multilayer films on pyrolytic graphite (PG) electrode surface, where n is the number of the bilayers. The {AMPC/GOx} n film electrodes demonstrated sensitive and reversible pH-responsive switchable behavior toward the cyclic voltammetric (CV) responses of the ferrocenedicarboxylic acid (Fc(COOH)2) probe in solution. The CV peak currents were strongly enhanced at pH 5.0 and were significantly reduced at pH 9.0. The pH-sensitive behavior of the system originated from the electrostatic interactions between the nanolayered films and Fc(COOH)2 in solution. This property was applied to realize pH-triggered electrocatalytically oxidation of glucose by GOx enzyme in the {AMPC/GOx} n films. This model system demonstrates the great potential perspective of the fabrication of novel multiple factor-switchable biosensors with immobilized enzymes.

The dissolution of simulant UK Ca/Zn-modified nuclear waste glass: the effect of increased waste loading

The influence of increasing the waste loading of Ca- and Zn-containing MW high-level-waste glass, from 20 to 35 wt%, on the chemical durability was investigated using 112-day PCT-B tests at 90 °C in ultra-high-quality water. During the initial stage of dissolution, from 0 to 3 days, increased waste loadings corresponded to reduced boron dissolution rates. By 112 days, however, there was no difference in the normalised mass loss of boron as a function of waste loading, within experimental uncertainty. In comparison to literature values obtained for traditional MW high-level-waste glasses, containing no Ca and Zn, the boron dissolution rates of CaZn MW glasses in the residual stage of dissolution, > 28 days, were greater by at least an order of magnitude. Analysis of secondary phases, combined with geochemical modelling of the solutions, indicate that the dual formation of Mg- and Zn-smectite phyllosilicate clays, potentially saponite (Ca0.2Mg3(Si,Al)4O10(OH)2·4H2O) and sauconite (Na0.3Zn3(Si, Al)4O10(OH)2·nH2O), respectively, may be the cause of this enhanced dissolution.

Effect of suspension conductivity and fines concentration on coarse particle settling in oil sands tailings

AbstractSedimentation of coarse particles in mature fine oil sands tailings (OST) was studied by varying the fines concentration and aqueous phase conductivity to determine under which conditions the coarser particles will settle and when they will not. An OST sample was desalinated, separated into finer and coarser fractions, then recombined for settling tests. The isolated finer fraction was predominately phyllosilicate clays (>87 wt%) while the coarser fraction was mostly quartz (<77 wt%). From optical backscattering (OBS) height scan measurements, complete sedimentation of the coarse particles was observed at low conductivity values and fines concentrations, but a sufficient increase in suspension conductivity or fines concentration caused a reduction in coarse particle settlement. Samples with lower fines concentration, 0.5 wt% and 2.4 wt%, exhibited coarse settlement over a wider range of conductivity values; whereas samples with higher fines concentration, 5.9 wt% and 8.2 wt%, exhibited decreased sedimentation even at low suspension conductivity values. At similar conductivity values and fines concentrations, increased sedimentation of pure quartz was observed compared to the coarse OST particles. This difference in settling behaviour was partially attributed to the presence of residual organics on the surface of the OST coarse particles. In addition to typical metallurgical assay methods (x‐ray diffraction and scanning electron microscopy), a synchrotron‐source computed tomography scan of the untreated OST sample was obtained to visualize the distribution of the fine, coarse, and fluid phases within the sample.

Iron and aluminum association with microbially processed organic matter via meso-density aggregate formation across soils: organo-metallic glue hypothesis

Abstract. Global significance of iron (Fe) and aluminum (Al) for the storage of organic matter (OM) in soils and surface sediments is increasingly recognized. Yet specific metal phases involved or the mechanism behind metal–OM correlations frequently shown across soils remain unclear. We identified the allocation of major metal phases and OM to density fractions using 23 soil samples from five climate zones and five soil orders (Andisols, Spodosols, Inceptisols, Mollisols, Ultisols) from Asia and North America, including several subsurface horizons and both natural and managed soils. Each soil was separated into four to seven density fractions using sodium polytungstate with mechanical shaking, followed by the sequential extraction of each fraction with pyrophosphate (PP), acid oxalate (OX), and finally dithionite–citrate (DC) to estimate pedogenic metal phases of different solubility and crystallinity. The concentrations of Fe and Al (per fraction) extracted by each of the three reagents were generally higher in meso-density fractions (1.8–2.4 g cm−3) than in the lower- or higher-density fractions, showing a unique unimodal pattern along the particle density gradient for each soil. Across the studied soils, the maximum metal concentrations were always at the meso-density range within which PP-extractable metals peaked at 0.3–0.4 g cm−3 lower-density range relative to OX- and DC-extractable metals. Meso-density fractions, consisting largely of aggregated clusters based on SEM observation, accounted for on average 56 %–70 % of total extractable metals and OM present in these soils. The OM in meso-density fractions showed a 2–23 unit lower C : N ratio than the lowest-density fraction of the respective soil and thus appeared microbially processed relative to the original plant material. The amounts of PP- and OX-extractable metals correlated positively with co-dissolved C across the soils and, to some extent, across the density fractions within each soil. These results led to a hypothesis which involves two distinct levels of organo-metal interaction: (1) the formation of OM-rich, mixed metal phases with fixed OM : metal stoichiometry followed by (2) the development of meso-density microaggregates via “gluing” action of these organo-metallic phases by entraining other organic and mineral particles such as phyllosilicate clays. Given that OM is mainly located in meso-density fractions, a soil's capacity to protect OM may be controlled by the balance of three processes: (i) microbial processing of plant-derived OM, (ii) dissolution of metals, and (iii) the synthesis of organo-metallic phases and their association with clays to form meso-density microaggregates. The current hypothesis may help to fill the gap between well-studied molecular-scale interaction (e.g., OM adsorption on mineral surface, coprecipitation) and larger-scale processes such as aggregation, C accrual, and pedogenesis.

Development of martian regolith and bedrock simulants: Potential and limitations of martian regolith as an in-situ resource

The last decade of Martian surface exploration has increased knowledge of the composition and variability of minerals present in the regolith and bedrock. This informs planning for manned missions to Mars for which these surface materials may be an important in-situ resource in agricultural applications. Soil on Mars is known to contain the majority of planet essential nutrients, but many questions of both the benefits (e.g. bioavailability of present nutrients) and limitations (e.g. extent of toxins) of Martian soil as a plant growth medium remain unanswered. It may be necessary to augment Mars soil with other regolith and bedrock materials (e.g. phyllosilicates, carbonates) to produce a medium appropriate for plant growth. A better understanding of the feasibility of using Mars soil as a plant growth medium could be accomplished with methods common to agricultural practices on Earth (e.g. biofertilization) using appropriate regolith and bedrock simulants as a plant growth medium. The study herein discusses the development of five new regolith and bedrock simulants. These simulants are designed to reflect several mineralogical (e.g. phyllosilicate clays) and chemical characteristics of various Mars surface materials that are most relevant to establishing the potential and limitations of these materials as in-situ resources for agricultural applications. The use of simulants provides a test bed for studies examining the potential of Mars' regolith and bedrock in agricultural applications, understanding data gaps surrounding these materials that will need further characterization, and methods to then conduct needed characterization of true Martian-regolith and bedrock in-situ. The simulants discussed herein are analyzed by X-ray diffraction using Reitveld refinement, X-ray fluorescence, V/NIR reflectance spectroscopy, and 57Fe Mössbauer spectroscopy and then compared to equivalent published data available from Mars rover and orbiter missions. The resulting simulants are chemically and mineralogically comparable to Martian regolith and bedrock and reflect data particularly relevant for growing plants on Mars. The developed simulants are relevant for analogue plant growth studies and fertility measurements that may inform hypothesis, habitat design, and mission plans which involve using Martian soil as a plant growth medium.

Multiscale analysis and experimental validation of the effective elastic modulus of epoxy-dioctahedral phyllosilicate clay composite

In this research, developed finite element codes were used to study the effective elastic modulus and stress-strain distribution profiles of epoxy resin filled with 6 wt. % microparticles of kaolinite. The random distribution of the particles was microstructurally regenerated with Digimat MSC software and random sequential algorithm codes in epoxy matrix. Stochastic representative volume element models of the composites were developed and analyzed under periodic boundary conditions. For validation, the predicted result by finite element analysis was compared with that of Mori-Tanaka's mean field homogenization scheme, selected micromechanical models and experiment. All the results indicated that 6 wt. % of kaolinite microparticles can improve the elastic modulus and load-bearing capacity of epoxy resin with <5 % error between predicted and actual results. The microstructure, phase identification and chemical characterization of the composite were also studied with scanning electron microscopy, x-ray diffraction spectroscopy and energy-dispersive x-ray spectroscopy, respectively. In addition, the particle size and distribution of the kaolinite in the epoxy matrix were experimentally investigated.