Smectite Suspensions Research Articles

Oxidative alteration of ferrous smectites and implications for the redox evolution of early Mars.

Surface conditions on early Mars were likely anoxic, similar to early Earth, but the timing of the evolution to oxic conditions characteristic of contemporary Mars is unresolved. Ferrous trioctahedral smectites are the thermodynamically predicted products of anoxic basalt weathering, but orbital analyses of Noachian-aged terrains find primarily Fe3+-bearing clay minerals. Rover-based detection of Fe2+-bearing trioctahedral smectites at Gale Crater suggest that ferrous smectites are the unoxidized progenitors of orbitally-detected ferric smectites. To assess this pathway, we conducted ambient-temperature oxidative alteration experiments on four synthetic ferrous smectites having molar Fe/(Mg+Fe) from 1.00 to 0.33. Smectite suspension in air-saturated solutions produced incomplete oxidation (24-38% Fe3+/ΣFe). Additional smectite oxidation occurred upon re-exposure to air-saturated solutions after anoxic hydrothermal recrystallization, which accelerated cation and charge redistribution in the octahedral sheet. Oxidation was accompanied by contraction of the octahedral sheet (d(060) decreased from 1.53-1.56 Å to 1.52 Å), consistent with a shift towards dioctahedral structure. Ferrous smectite oxidation by aqueous hydrogen peroxide solutions resulted in nearly complete Fe2+ oxidation but also led to partial Fe3+ ejection from the structure, producing nanoparticulate hematite. Reflectance spectra of oxidized smectites were characterized by (Fe3+,Mg)2-OH bands at 2.28-2.30 μm, consistent with oxidative formation of dioctahedral nontronite. Accordingly, ferrous smectites are plausible precursors to observed ferric smectites on Mars, and their presence in late-Noachian sedimentary units suggests that anoxic conditions may have persisted on Mars beyond the Noachian.

Fe(II) Interactions with Smectites: Temporal Changes in Redox Reactivity and the Formation of Green Rust.

In this study, temporal changes in the redox properties of three 0.5 g/L smectite suspensions were investigated-a montmorillonite (MAu-1) and two nontronites (NAu-1 and NAu-2) in the presence of 1 mM aqueous Fe(II) at pH 7.8. X-ray absorption spectroscopy revealed that the amount of Fe(II) added quantitatively transformed into chloride-green rust (Cl-GR) within 5 min and persisted over 18 days. Over the same time, the reduction potential of all three suspensions increased by 50 to 150 mV to equilibrate at approximately -100 mV vs SHE. The reduction of a model organic contaminant, 4-chloronitrobenzene (4-CINB), also became increasingly slower over time with virtually no 4-CINB reduction being observed after 18 days. There was a strong correlation between reduction potential and the quantity of 4-ClNB reduced by MAu-1, although other factors were likely involved in the decreased redox reactivity observed in the nontronites. It is hypothesized that the temporal increase in reduction potential results from clay mineral dissolution resulting in increased Fe(III) contents in the Cl-GR. These results demonstrate that long-term studies are recommended to accurately predict contaminant management strategies.

Temperature‐dependent residual shear strength characteristics of smectite‐bearing landslide soils

AbstractThis paper presents experimental investigations regarding the effect of temperature on the residual strength of landslide soils at slow‐to‐moderate shearing velocities. We performed ring‐shear tests on 23 soil samples at temperatures of 6–29°C. The test results show that the shear strength of smectite‐rich soils decreased when temperatures were relatively low. These positive temperature effects (strength losses at lower temperatures) observed for smectite‐bearing soils are typical under relatively slow shearing rates. In contrast, under relatively high shearing rates, strength was gained as temperature decreased. As rheological properties of smectite suspensions are sensitive to environmental factors, such as temperature, pH, and dissolved ions, we inferred that temperature‐dependent residual strengths of smectitic soils are also attributed to their specific rheological properties. Visual and scanning electron microscope observations of Ca‐bentonite suggest that slickensided shear surfaces at slow shearing rates are very shiny and smooth, whereas those at moderate shearing rates are not glossy and are slightly turbulent, indicating that platy smectite particles are strongly orientated at slow velocities. The positive temperature effect is probably due to temperature‐dependent microfriction that is mobilized in the parallel directions of the sheet structure of hydrous smectite particles. On the contrary, the influence of microviscous resistance, which appears in the vertical directions of the lamination, is assumed to increase at faster velocities. Our results imply that if slip‐surface soils contain high fractions of smectite, decreases in ground temperature can lead to lowered shear resistance of the slip surface and trigger slow landslide movement.

Synthesis and Characterization of Heterogeneous Catalysts and Comparison to Iron-ore

The placement of metal oxide pillars between clay mineral layers modifies their physicochemical properties, including surface area, acidity, and catalytic activity. Aluminum is the most commonly used pillar cation, but the use of Fe offers a distinct opportunity to expand the range of catalytic behavior. The purpose of this study was to prepare Al-Fe-pillared smectite and to characterize the resulting materials. Al-Fe-pillared clay was synthesized from Tunisian clay precursors according to a common procedure: grinding, sieving, Na exchange, pillaring, drying and calcinations. Smectite suspension was mixed with different pillaring solutions containing Al and Fe oligomers with Fe/ (Al+Fe) percent ratios: 1; 5; 10 and 50%. Other types of material were investigated in this work; natural iron-ore was defined and characterized as heterogeneous catalysis. The structural and textural properties of synthesis and natural catalysis have been determined by X-ray diffraction, nitrogen adsorption-desorption isotherms, Transmission Electron Microscopy (MET), X-ray fluorescence, CEC and infrared spectroscopy.

How the hydro-gel flocculation microstructure changes

Abstract Microstructural investigations were conducted to better understand the poor settling and dewatering of a clay mineral-rich slurry. Investigations of the flocculated microstructure in a 2.5 wt% smectite clay suspension in a moderately salty aqueous solution were conducted with the aid of a synchrotron-powered transmission X-ray microscope and a cryo-transmission electron microscope. These investigations enabled the study of the clay particles’ mutual configuration in three-dimensional arrangements within natural water environments without drying the samples. The observed microstructure was similar to that observed in Wyoming smectite suspensions. A transition was observed from the “net of flakes” in monovalent cations in solution to the “Bander” model, when bivalent cations were present in the solution. As these structural models differ significantly from each other, the physical properties of the resulting gel also demonstrated remarkable diversities. These findings may play a crucial role in understanding smectite gel behaviour and its aggregating nature, which are of interest across many industries and are important in designing the most suitable technology to deal with colloidal stability.

Nano Dimensional Hybrid Organo-clay Langmuir-Blodgett Films

Clay mineral particles are interesting nanosized building blocks due to their high aspect ratio and the chemical properties. The main interest in this nanosized building blocks results essentially from the colloidal size and the permanent structural charge of the particles. Smectites or swelling clay minerals are naturally occurring nanomaterials that can be fully delaminated to elementary clay mineral platelets in dilute aqueous dispersion. This dilute aqueous smectite suspensions are well suited to convert into functional nanofilms. The functionalization is performed by ion exchange reaction with amphiphilic molecules carrying the desired functionality, such as chirality, two photon absorption, energy transfer, optical nonlinearity and magnetism, which are due to the nature of the amphiphilic cations and to the organization of both the amphiphilic molecules and the elementary clay mineral platelets. Controlling the structure of materials at the nanometre scale is of fundamental importance for tailoring such materials properties. Langmuir Blodgett films are known for a high degree of organization of organic molecules. Fundamental research into the organization of molecules at clay mineral platelets is necessary to optimize the materials for specific applications. This paper gives an overview of organo-clay hybrid Langmuir-Blodgett nano films. Keywords: Langmuir-Blodgett, J-aggregate, Fluorescence resonance Energy Transfer (FRET), Oragano-clay.

Therapeutic Effect of Dioctahedral Smectite on Diarrhea Caused by E. coli and Salmonella in Calves

The objectives in the present study were to evaluate the therapeutic effect of dioctahedral smectite(smectite) against calf diarrhea caused by pathogenic E. coli and/or Salmonella typhimurium. Of this study, twenty calves (aged 2-3 months) with diarrhea were used to estimate the efficacy of smectite on calf diarrhea with 20% smectite suspension in PBS. Calves were administered with 10 ml smectite suspension three times a day after feeding, and fecal samples were collected at the gate of treatment and on 1st, 2nd, 3rd, 4th, and 5th day after administration. On the 5th day after treatment of smectite suspension, the treated group significantly reduced diarrhea index compared to control group (P<0.001). The number of pathogenic E. coli in feces of the treated group was significantly decreased compared to each control group from on the 2nd day after treatment (P<0.001), and that of Salmonella typhimurium was significantly decreased from on the first day after treatment (P<0.05). From results of the present study, 20% smectite suspension had therapeutic effect on diarrhea caused by E. coli and/or Salmonella in calves.

The Effect of Smectite on the Corrosion of Iron Metal

AbstractThe combination of zero-valent iron (ZVI) and a clay-type amendment is often observed to have a synergistic effect on the rate of reduction reactions. In the present study, electrochemical techniques were used to determine the mechanism of interaction between the iron (Fe) and smectite clay minerals. Iron electrodes coated with an evaporated smectite suspension (clay-modified iron electrodes, CMIEs) were prepared using five different smectites: SAz-1, SWa-1, STx-1, SWy-1, and SHCa-1. All the smectites were exchanged with Na+ and one sample of SWy-1 was also exchanged with Mg2+. Potentiodynamic polarization scans and cyclic voltammograms were taken using the CMIEs and uncoated but passivated Fe electrodes. These electrochemical experiments, along with measurements of the amount of Fe2+ and Fe3+ sorbed in the smectite coating, suggested that the smectite removed the passive layer of the underlying Fe electrode during the evaporation process. Cyclic voltammograms taken after the CMIEs were biased at the active-passive transition potential for varying amounts of time suggested that the smectite limited growth of a passive layer, preventing passivation. These results are attributed to the Brønsted acidity of the smectite as well as to its ability to sorb Fe cations. Oxides that did form on the surface of the Fe in the presence of the smectite when it was biased anodically were reduced at a different electrochemical potential from those that form on the surface of an uncoated Fe electrode under otherwise similar conditions; this difference suggested that the smectite reacted with the Fe2+ formed from the oxidation of the underlying Fe. No significant correlation could be found between the ability of the smectite to remove the Fe passive film and the smectite type. The results have implications for the mixing of sediments and Fe particles in permeable reactive barriers, underground storage of radioactive waste in steel canisters, and the use of smectite supports in preventing aggregation of nano-sized zero-valent iron.

Fate of L-DOPA in the Presence of Inorganic Matrices: Vectorization or Composite Material Formation?

Novel bionanocomposite materials were successfully prepared using 2:1 phyllosilicates and an amino acid, L-DOPA. A systematic study of DOPA interactions with these smectites and with a nonporous amorphous silica (Aerosil) used for comparison was undertaken by combining macroscopic information, including XRD, thermogravimetric analysis, and proton relaxation times in the synthesis supensions, with microscopic observations and several spectroscopic techniques: vibrational (transmission mode IR and photoacoustic spectroscopies), solid-state 13C NMR, and XPS. A significant variety of behaviors is revealed. DOPA interacts very weakly with Aerosil, although it contributes to the structuring of the solution with the appearance of a population of bound water molecules. In contrast, the weakly basic conditions in smectite suspensions catalyze oxidation by dissolved O2 to indolic species and polymerization to pseudomelanin. With saponite, structural characterization reveals that L-DOPA zwitterions are accommodated vertically in the interlayer region as a monolayer of partially superimposed species. In addition, TEM indicates that lamellar deposits of pseudomelanin grow epitaxially on the saponite layers. With Laponite, the oxidation reactions seem to be somewhat modified by the specific catalytic effect of edge groups, and the organic species cause a strong structuring of the dispersion with gelification. The interesting chemistry taking place in these systems has implications for both DOPA vectorization and nanocomposite materials synthesis.

Kaolinite flocculation induced by smectite addition – A transmission X-ray microscopic study

The influence of smectite addition on kaolinite suspensions in water was investigated by transmission X-ray microscopy (TXM) and Scanning Electron Microscopy (SEM). Sedimentation test screening was also conducted. Micrographs were processed by the STatistic IMage Analysing (STIMAN) program and structural parameters were calculated. From the results of the sedimentation tests important influences of small smectite additions to about 3 wt.% on kaolinite suspension flocculation has been found. In order to determine the reason for this smectite impact on kaolinite suspension, macroscopic behaviour micro-structural examination using Transmission X-ray Microscope (TXM) and SEM has been undertaken. TXM & SEM micrographs of freeze-dried kaolinite–smectite suspensions with up to 20% smectite showed a high degree of orientation of the fabric made of highly oriented particles and greatest density when 3 wt.% of smectite was added to the 10 wt.% dense kaolinite suspension. In contrast, suspensions containing pure kaolinite do not show such platelet mutual orientation but homogenous network of randomly oriented kaolinite platelets. This suggests that in kaolinite–smectite suspensions, smectite forms highly oriented basic framework into which kaolinite platelets may bond in face to face preferential contacts strengthening structure and allowing them to show plastic behaviour which is cause of platelets orientation.

Influence of smectite suspension structure on sheet orientation in dry sediments: XRD and AFM applications

The structure-building phenomena within clay aggregates are governed by forces acting between clay particles. Measurements of such forces are important to understand in order to manipulate the aggregate structure for applications such as dewatering of mineral processing tailings. A parallel particle orientation is required when conducting XRD investigation on the oriented samples and conduct force measurements acting between basal planes of clay mineral platelets using atomic force microscopy (AFM). To investigate how smectite clay platelets were oriented on silicon wafer substrate when dried from suspension range of methods like SEM, XRD and AFM were employed. From these investigations, we conclude that high clay concentrations and larger particle diameters (up to 5 μm) in suspension result in random orientation of platelets in the substrate. The best possible laminar orientation in the clay dry film, represented in the XRD 0 0 1/0 2 0 intensity ratio of 47 was obtained by drying thin layers from 0.02 wt.% clay suspensions of the natural pH. Conducted AFM investigations show that smectite studied in water based electrolytes show very long-range repulsive forces lower in strength than electrostatic forces from double-layer repulsion. It was suggested that these forces may have structural nature. Smectite surface layers rehydrate in water environment forms surface gel with spongy and cellular texture which cushion approaching AFM probe. This structural effect can be measured in distances larger than 1000 nm from substrate surface and when probe penetrate this gel layer, structural linkages are forming between substrate and clay covered probe. These linkages prevent subsequently smooth detachments of AFM probe on way back when retrieval. This effect of tearing new formed structure apart involves larger adhesion-like forces measured in retrieval. It is also suggested that these effect may be enhanced by the nano-clay particles interaction.

Smectite suspension structural behaviour

Smectite suspensions, at low solids contents, are known to be naturally high in volume with diverse structural properties. The changing structural properties of smectite aqueous suspensions in the absence and presence of calcium ions were investigated using an acoustosizer and an advanced cryo-SEM technique to further understand and thereby control their environmental impact. In the absence of Ca(II) ions, smectite particles are present as a colloidally stable sol due to electrical double layer repulsion of the negatively charged platelets. The smectite network is observed to be extended throughout the suspension via clay platelets networking with an edge–edge (EE) orientation due to high basal surface repulsion. After the initial addition of Ca(II) ions, the smectite negative zeta potential reduces and the smectite platelets coagulate forming 2 µm aggregates. The platelets are randomly orientated, lettuce-like, coagulated aggregates with a high presence of both edge–edge (EE) and edge–face (EF) orientations. After equilibration, the smectite platelets forming an orientated honeycomb cellular structure comprised of face–face (FF) multiply sheet aggregates. The voids in the cellular structure are larger than prior to Ca(II) addition, measured at 2–8 µm. The changing structural properties of a smectite suspension in the absence and presence of Ca(II) greatly influence smectite stability and in turn, mineral processing and/or environmental management. Adequate time is required to allow suppression of the initial swelling of the smectite, full Ca(II) exchange and platelet aggregation.

Sorption of uranium (VI) on homoionic sodium smectite experimental study and surface complexation modeling

This paper is an extension of a previous paper where the natural and purified clay in the homoionic Na form were physico-chemically characterized ( doi:10.1016/j.clay.2008.04.014). In this study, the adsorption behavior of U (VI) on a purified Na-smectite suspension is studied using batch adsorption experiments and surface complexation modeling (double layer model). The sorption of uranium was investigated as a function of pH, uranium concentration, solid to liquid ratio, effect of natural organic matter (NOM) and NaNO 3 background electrolyte concentration. Using the MINTEQA2 program, the speciation of uranium was calculated as a function of pH and uranium concentration. Model predicted U (VI) aqueous speciation suggests that important aqueous species in the [U (VI)] = 1 mg/L and pH range 3–7 including UO 2 2+, UO 2OH +, and (UO 2) 3(OH) 5 +. The concentration of UO 2 2+ decreased and that of (UO 2) 3(OH) 5 + increased with increasing pH. The potentiometric titration values and uptake of uranium in the sodium smectite suspension were simulated by FITEQL 4.0 program using a two sites model, which is composed of silicate and aluminum reaction sites. We compare the acidity constants values obtained by potentiometric titration from the purified sodium smectite with those obtained from single oxides (quartz and α-alumina), taking into account the surface heterogeneity and the complex nature of natural colloids. We investigate the uranium sorption onto purified Na-smectite assuming low, intermediate and high edge site surfaces which are estimated from specific surface area percentage. The sorption data is interpreted and modeled as a function of edge site surfaces. A relationship between uranium sorption and total site concentration was confirmed and explained through variation in estimated edge site surface value. The modeling study shows that, the convergence during DLM modeling is related to the best estimation of the edge site surface from the N 2-BET specific surface area, SSA BET (thus, total edge site concentrations). The specific surface area should be at least 80–100 m 2 /g for smectite clays in order to reach convergence during the modeling. The range of 10–20% SSA BET was used to estimate the values of edge site surfaces that led to the convergence during modeling. An agreement between the experimental data and model predictions is found reasonable when 15% SSA BET was used as edge site surface. However, the predicted U (VI) adsorption underestimated and overestimated the experimental observations at the 10 and 20% of the measured SSA BET, respectively. The dependence of uranium sorption modeling results on specific surface area and edge site surface is useful to describe and predict U (VI) retardation as a function of chemical conditions in the field-scale reactive transport simulations. Therefore this approach can be used in the environmental quality assessment.

Interaction between smectite and bacteria: Implications for bentonite as backfill material in the disposal of nuclear waste

This study presents experimental results on the interaction between smectite clays (nontronite and MX80 bentonite) and the facultative anaerobic, heterotrophic Shewanella putrefaciens in two types of conditions: i) batch experiments with low solid to liquid ratios and agitated oxygenated conditions and ii) reaction-cell experiments with high solid to liquid ratios in anaerobic, confined volume conditions. The former was chosen to study the ability of S. putrefaciens to live on smectite as the only substrate and the latter was designed to simulate more compacted subsurface environments of underground repository waste sites. Bacterial cell counts in the batch experiments reveal the prolonged survival of S. putrefaciens in the smectite suspension compared to standard laboratory culture media. In the case of nontronite, variations in solution chemistry indicate bacterial consumption and/or partial binding of cations. Microscopic investigations show associated biofilm-smectite aggregates and Si-rich gels produced by the partial dissolution of clay mineral grains. In contrast, the MX80 bentonite was not seen to be chemically affected by bacterial activity in batch cultures. However, the confined volume experiments, using reaction-cell X-ray diffraction combined with peak calculations (CALCMIX), do indicate that S. putrefaciens has a pronounced effect on the water content of compacted MX80 bentonite. The presence of these bacteria enhances both the amount of adsorbed interlayer water and the available pore space. The anaerobic conditions were also favourable for accessory phase dissolution (notably calcite) and synchronous precipitation of lepidocrocite related to bacterially induced changes in pH and Eh. The varied response of the two studied clays to the presence of bacteria is attributed largely to the materials composition. The interlayer Ca of nontronite facilitates bacterial attachment to surfaces and Fe(III) provokes the production of chelators that enhance mineral dissolution. Although MX80 bentonite is less affected by bacterially enhanced dissolution, it is more sensitive to microstructural changes. Mechanisms involve aggregation of Na-smectite particles in voids created by cell lyses, the initial production of biofilm and the pH and Eh dependent dissolution and precipitation of accessory minerals. This investigation highlights the importance of including bacteria-mineral studies in assessing the safety issue of underground disposal of nuclear waste material.

Assessing the Redox Reactivity of Structural Iron in Smectites Using Nitroaromatic Compounds As Kinetic Probes

Structural Fe(II) in clay minerals is an important source of electron equivalents for the reductive transformation of contaminants in anoxic environments. We investigated which factors control the reactivity of Fe(II) in smectites including total Fe content Fe(II)/total Fe ratio, and excess negative charge localization using 10 nitroaromatic compounds (NACs) as reactive probe molecules. Based on evidence from this work and previous spectroscopic studies on Fe redox reactions in iron-rich smectites, we propose a kinetic model for quantifying the reactivity, abundance, and interconversion rates of two distinct Fe(II) sites in the minerals' octahedral sheet. Excellent agreement between observed biphasic NAC reduction kinetics and model fits points toward existence of two types of Fe(II) sites exhibiting reactivities that differ by 3 orders of magnitude in iron-rich ferruginous smectite (SWa-1) and Olberg montmorillonite. Low structural Fe content, as found in Wyoming montmorillonite (SWy-2), impedes the formation of highly reactive Fe sites and results in pseudo-first order kinetics of NAC reduction that originate from the presence of a single type of Fe(II) species of even lower reactivity. Similar correlations of one-electron reduction potentials of the NACs vs their second order reduction rate constants for all smectite suspensions suggest that contaminant-Fe(II) interactions were identical in all smectite minerals.

Role of Smectite Quasicrystal Dynamics in Adsorption of Dinitrophenol

Understanding sorption mechanisms and processes is critical for predicting the fate of organic pollutants in soils and for development of effective remediation strategies for contaminated soils. A novel suspension x‐ray diffraction (XRD) technique was used to elucidate the influence of the size and swelling of K‐ and Ca‐smectite quasicrystals in aqueous suspensions on adsorption of 4,6‐dinitro‐o‐cresol (DNOC). X‐ray diffraction patterns of randomly oriented smectite quasicrystals in aqueous suspensions were compared with diffraction patterns for air‐dried and oven‐dried oriented films of the same smectites. The XRD patterns reveal that the phenolate form of DNOC, which is predominant in solution with pH ≥ 4.4, is primarily adsorbed on external surfaces of K‐smectite quasicrystals in aqueous suspensions and may enter the interlayers as a K‐DNOC complex when K‐smectite quasicrystals coalesce. In Ca‐smectite suspensions with pH &gt; 4.4, the phenolate form of DNOC is adsorbed only on external surfaces; however, DNOC may become entrapped between substacks within Ca‐smectite quasicrystals as the smectite suspension dries to form an oriented film. The neutral form of DNOC (pH ≤ 4.4) was adsorbed in the interlayers of a low‐charge‐density Ca‐smectite in aqueous suspension but not in the interlayers of the high‐charge‐density Ca‐smectite, apparently due to steric restrictions. Clearly, XRD analysis is a valuable tool for gaining insight into interactions between smectites and organic molecules, but caution must be used in extrapolating data based on XRD patterns of air‐dried and oven‐dried smectite films to processes that may occur in aqueous smectite suspensions.

Effects of poly(ethylene oxide) adsorption on the dispersion of smectites

The adsorption of polymers on clay is important in many applications. However the mechanisms of poly(ethylene oxide) (PEO) adsorption on smectite and its effect on suspension rheology are not well elucidated at present. The aim of this study was to investigate the mechanisms responsible for PEO sorption on smectite and how this impacts on dispersive behavior of smectite suspension. The results indicated that the hydrophobic interaction between CH 2 CH 2 groups and siloxane surface is the major driven force for PEO adsorption on smectite. In addition, PEO adsorbed preferentially on the surface of low-charge smectite and the delamination of low-charge smectite in water was enhanced due to PEO adsorption presumably due to the hydrophilic ether oxygen of adsorbed PEO.

Geochemical Modulation of Bioavailability and Toxicity of Nitroaromatic Compounds to Aquatic Plants

Nitroaromatic compounds (NACs) are prominent soil and sediment contaminants that are strongly adsorbed by smectites at extents that depend on hydration properties of the exchangeable cation. Potassium smectites adsorb nitroaromatics much more strongly than calcium smectites, so that adjustment of K+ versus Ca2+ occupation on cation exchange sites in smectites can be used to modulate the retention and release of nitroaromatics. We suggest that this modulation can be used to advantageously manage the bioavailability and toxicity of NACs during bioremedation. We have measured the toxicity of 2,4-dinitrotoluene (2,4-DNT) to duckweed grown in smectite suspensions and utilized Ca2+/K+ exchange to retain or release 2,4-DNT. Retention by potassium smectite reduced bioavailability and hence toxicity to duckweed. Addition of Ca2+ to replace K+ by ion exchange released adsorbed 2,4-DNT, which is toxic to duckweed. So smectites can be used to sequester or release 2,4-DNT predictably and provide means to control bioavailability and environmental toxicity.

Influence of layer charge on swelling of smectites

Six separate processes control the swelling of smectites saturated with alkali and alkaline earth cations in aqueous systems. The basic mechanism and forces controlling each of the processes are different. Crystalline swelling occurs between smectite layers within quasicrystals and involves the intercalation of zero to four discrete layers of water molecules. A balance between strong electrostatic-attraction and hydration-repulsion forces controls crystalline swelling. The extent of crystalline swelling decreases with increasing layer charge. Double-layer swelling occurs between quasicrystals. An electrostatic repulsion force develops when the positively charged diffuse portions of double layers from two quasicrystals overlap in an aqueous suspension. Layer charge has little or no direct effect on double-layer swelling. The break up and formation of quasicrystals is a dynamic process that controls the average size of quasicrystals in an aqueous smectite suspension. As layer charge increases, quasicrystals tend to become larger and more stable. In smectite suspensions with more than one type of exchangeable cation, the cations can demix (e.g., Na and Ca may be segregated in different interlayer regions) due to a complex feed-back between cation exchange selectivity and crystalline swelling. Demixing influences the breakup and formation of quasicrystals because quasicrystals preferentially cleave along interlayers dominated by alkali cations. Increasing layer charge increases selectivity for alkaline earth cations relative to Na or Li, and hence reduces the breakup of quasicrystals. Co-volume swelling is an entropy driven process caused by restrictions on the rotational freedom of suspended quasicrystals. Brownian swelling is also an entropy driven process resulting from random thermal motion of suspended colloids. There is no reason to believe that layer charge directly influences either co-volume or Brownian swelling. Macroscopic measures of swelling (e.g., change in total volume or water content) necessarily measure the combined effect of all swelling processes occurring within the system.

The Effect of Microbial Fe(III) Reduction on Smectite Flocculation

AbstractThis study was undertaken to investigate the changes in flocculation properties of Fe-rich smectite (nontronite, NAu-1) suspensions, including settling velocity, aggregate size and floc architecture associated with microbial Fe(III)-reduction in the smectite structure. The dissimilatory Fe-reducing bacterium Shewanella oneidensis MR-1 was incubated with lactate as the electron donor and structural Fe(III) as the sole electron acceptor for 3, 12, 24 and 48 h in an anaerobic chamber. Two controls were prepared; the first was identical to the experimental treatments except that heat-killed cells were used (non-reduced control), and the second control was the same as the first except that the incubation was carried out in an aerobic environment. The extent of Fe(III) reduction for the 48 h incubation was observed to reach up to 18%. Neither the non-reduced control nor the aerobically inoculated sample showed Fe(III) reduction. Compared with the non-reduced control, there was a 2.7 μm increase in mean aggregate size and a 30-fold increase in average settling velocity in the bioreduced smectite suspensions as measured using a Micromeritics Sedigraph®. The aerobically inoculated smectite showed a similar aggregate-size distribution to that of the non-reduced control. Significant changes in physical properties of smectite suspensions induced by microbial Fe(III) reduction were measured directly using transmission electron microscopy. The floc architecture of bioreduced smectite revealed less open structures compared to those of a non-reduced control. The aspect ratio (thickness/length) of individual smectite particle increased from 0.11 for the non-reduced control to 0.18 on average for the bioreduced smectite suspensions. The effects of pH on the clay flocculation were minimal in this study because the value of pH remained nearly constant at pH = 7.0–7.3 before and after the experiments. We therefore suggest that the increase in net negative charge caused by microbial Fe(III) reduction significantly promoted clay flocculation by increasing the electrochemical attraction in the smectite suspensions.