Ferruginous Smectite Research Articles

Controlled rate thermal analysis of nontronite

The thermal decomposition of a series of nontronites and a ferruginous smectite has been studied using a combination of high-resolution thermogravimetric analysis (TGA) with a controlled heating rate and the mass spectrometry (MS) of evolved water vapour. The temperature–mass spectrum curve follows the differential TGA (DTGA) curve with precision, thus providing a second means of calculating the percentage weight loss. Three dehydration steps were observed in the controlled rate thermal analysis (CRTA) experiment at around 86, 117 and 245 °C depending on the nontronite sample. Two overlapping dehydroxylation steps were observed at approximately 350 and 400 °C with a weight loss ratio of 1:2. A comparison is made between the between the results obtained from the dynamic DTGA experiment and the CRTA experiment.

Dechlorination of pentachloroethane by commercial Fe and ferruginous smectite

Short-term experiments were conducted to investigate the effect of a commercial Fe and an iron-bearing clay mineral, ferruginuous smectite (SWa-1), on the degradation of pentachloroethane (PCA). After 3 h of contact time, SWa-1 catalyzed PCA dehydrochlorination to tetrachloroethene (PCE, 65% conversion), whereas commercial Fe promoted PCA stepwise dechlorination via dehydrochlorination (≈40% conversion) and subsequent PCE hydrogenolysis to trichloroethene (TCE). The addition of unaltered SWa-1 to commercial Fe led to a complete inhibition on TCE production, whereas the addition of reduced SWa-1 barely resulted in a 30% decrease.

Fate of atrazine and alachlor in redox‐treated ferruginous smectite

AbstractThe oxidation state of structural iron (Fe) in clay minerals exerts a large influence on clay surface chemistry and may affect the adsorption and degradation of pesticides in the environment. This effect, however, has been little investigated. In the present study, herbicides atrazine and alachlor were reacted with ferruginous smectite (sample SWa‐1) in its oxidized, reduced (either chemically or bacterially), and reduced–reoxidized states. In some experiments the herbicide was labeled with 14C. Gas chromatography/mass spectrometry (GC‐MS) was also used to detect alachlor degradation products. Compared to oxidized clays, reduction by both chemical and microbial treatments decreased the concentration of both herbicides in the surrounding solution. Reoxidized clay exhibited behavior similar to the oxidized clay. Hydrolysis–dechlorination of atrazine occurred in the presence of chemically reduced SWa‐1, and GC‐MS analysis of alachlor revealed at least 14 degradation products after treatment with reduced clay and only two with the oxidized clay. Interaction of atrazine and alachlor with the clay may be through a H bond with the waters of hydration surrounding interlayer cations, the extent of which should increase with increasing acidity; but under reduced conditions, the validity of this model is unclear. Reduction of structural Fe may affect pH‐dependent phenomena in two ways: The increased surface charge density increases the number of hydrated interlayer cations, thereby enhancing surface acidity, and increased electron density at basal surface oxygens increases their Brønsted basicity. Atrazine could, therefore, adsorb and/or degrade through either acid or alkaline hydrolysis pathways. Increased reduction potential of the reduced clay surfaces may also promote degradation.

Near-infrared spectroscopic study of nontronites and ferruginous smectite

The existence of life on planets such as Mars depend upon the presence of water. This water may not necessarily be as liquid or crystalline water but may be as interlayer water such as is found in smectitic clays. One group of smectites, relevant to the search for interplanetary life are those which have a high iron content, known as nontronites. Near-IR reflectance spectroscopy has been used to show the presence of water and hydroxyl units in these minerals. Three near-IR spectral regions are identified, (a) the high frequency region between 6400 and 7400 cm −1 attributed to the first overtone of the hydroxyl stretching mode; (b) the 4800–5400 cm −1 region attributed to water combination modes; and (c) the 4000–4800 cm −1 region attributed to the combination of the stretching and deformation modes of the FeFeOH units of nontronite. Two types of hydroxyl groups were identified using near-IR spectroscopy, hydroxyl units coordinated to the iron; and hydroxyl groups from water in the nontronite structure. The first hydroxyls are characterised by several bands, firstly in the 7055–7098 cm −1 region assigned to the first overtone of the AlFeOH stretching unit, secondly in the 6958–6878 cm −1 region attributed to the FeFeOH unit. The overtone of the hydroxyl stretching frequency of water was observed at around 6800 cm −1. These observations show that nontronites can be a source of water that may support life.

Sorption and Abiotic Redox Transformation of Nitrobenzene at the Smectite–Water Interface

The effect of the redox state of structural Fe on the surface reactivity of iron-bearing phyllosilicates in aqueous suspension was investigated using a molecular probe. For this purpose the structural Fe in montmorillonite and ferruginous smectite was chemically reduced by sodium dithionite in citrate–bicarbonate (CB) buffer solution under N2 at 70°C, with the excess reactants removed by washing and centrifugation. Nitrobenzene was chosen as an electron acceptor probe to react with unaltered and chemically reduced smectites. Nitrobenzene was transformed into aniline only in the presence of the reduced smectites. This abiotic reductive transformation depended on the concentration of the electron acceptor in solution and the total accessible structural FeII in smectites. As much as 40% of the crystal layer structural FeII of the reduced smectites was oxidized to FeIII; these electrons migrated to the surfaces/edges and then were transported into the adjacent aqueous layer. No significant effect of the oxidation state of structural Fe on the sorption of nitrobenzene on smectites was observed, but the reduced smectites sorbed less aniline than the unaltered smectites. The electronic structure and molecular geometry of the probe were modified within the smectite–water interface. Reducing structural Fe in smectites perturbed the surface reactivity.

Methylene Blue Interactions with Reduced-Charge Smectites

AbstractThe objective of this work was to prepare series of reduced-charge materials from different parent Li-saturated dioctahedral smectites, to investigate the effects of temperature, chemical composition and charge location in smectites on the charge reduction and to characterize reduced-charge smectites (RCSs) using methylene blue (MB) adsorption. The layer charge decrease, induced by Li fixation, is correlated with the trends in the spectra of MB-RCS dispersions in the visible region (VIS) spectra. Distribution of the negative surface charge of the clay minerals controls the distance between the adsorbed MB cations and thus affects the formation of MB dimers and higher agglomerates. Because each form of MB (monomer, dimer, higher agglomerate, J-aggregates) absorbs light at a different wavelength, the VIS spectra of MB depend sensitively on the charge density at the clay surface. Both cation exchange capacity (CEC) values and spectra of MB-clay dispersions clearly detect extensive reduction of the layer-charge density in reduced-charge montmorillonites (RCMs) upon Li-thermal treatment. The extent of charge reduction depends on the temperature of the thermal treatment, as well as on the octahedral charge of the montmorillonite. Reduction of the layer charge proceeds to a much lesser extent for smectites with mainly tetrahedral charge and high Fe content (Fe-rich beidellite and ferruginous smectite). Both CEC data and MB spectra detect only a slight decrease of the layer charge density, which relates to the low octahedral charge of these minerals. Following heating at higher temperatures (120-160°C), slightly higher Li fixation is indicated by CEC values; however, no charge reduction is confirmed by MB spectra. Release of protons accompanying Li+ fixation in Fe-rich smectites heated at 180 or 200°C was detected in the spectra of MB-clay dispersions and confirmed by potentiometric titrations.

Abiotic transformation of perchloroethylene in homogeneous dithionite solution and in suspensions of dithionite-treated clay minerals.

The reductive dechlorination of perchloroethylene (PCE) in homogeneous solutions of dithionite and at the surfaces of dithionite-citrate-bicarbonate (DCB) treated ferruginous smectite and Na-montmorillonite was studied. Transformation products of PCE identified in dosed dithionite-treated samples included TCE, DCE, 1,1,2-trichloroethane (TCA), 1,1-dichloroethane (DCA), chloroacetylene, acetylene, ethene, and ethane. The decomposition of dithionite to sulfate yielded both protons and electrons necessary for hydrodechlorination (hydrogenolysis) of PCE. Dithionite treatment of the Fe-poor Na-montmorillonite enhanced reductive dechlorination of PCE relative to dithionite-treated Fe-rich ferruginous smectite, within the range of 11.5-137.8 mM dithionite. For the same dithionite concentration, the kinetics of the heterogeneous reactions of PCE was generally faster than that of the homogeneous reaction, and higher concentrations of TCE were measured in the heterogeneous reactions. Interestingly, increases in the mass of the clay minerals used, the Fe2+ content in the clay mineral structure, or the dithionite concentration used did not necessarily enhance the abiotic transformation of PCE, as would otherwise be predicted. The most efficient reductive dechlorination of PCE was observed with 0.5% clay (m/v) treated with 34.5 mM dithionite buffered at pH 8.5. The solid-state transfer of electrons to surfaces and edges, rather than the redox capacity, limited the dechlorination of PCE by reduced ferruginous smectite and/or suspensions containing a higher clay mass. The greater reactivity of dithionite-reduced montmorillonite than similarly treated ferruginous smectite is attributed to (i) the well-documented layer collapse and aggregation of chemically reduced clays that increases with the clay's iron content, (ii) the location of solid-phase Fe2+ in the reduced clay mineral and whether it is accessible or inaccessible for reaction with PCE at the mineral edges and surfaces where the reactions are thought to occur, and (iii) the greater swellability of montmorillonite versus ferruginous smectite. The faster dechlorination rate of PCE observed with dithionite-reduced Fe-poor montmorillonite than similarly reduced iron-rich ferruginous smectite suggests that the use of dithionite barriers for in-situ treatment of chlorinated solvent plumes should not be limited to aquifers with Fe-rich sediments.

Transformation of chlorinated aliphatic compounds by ferruginous smectite.

A series of chlorinated aliphatic compounds (RCI, including carbon tetrachloride (PCM), 1,1,1-trichloroethane (TCA), 1,1,2,2-tetrachloroethane (TeCA), pentachloroethane (PCA), hexachloroethane (HCA), trichloroethene (TCE), tetrachloroethene (PCE), trichloronitromethane (chloropicrin, CP), and trichloroacetonitrile (TCAN)) was reacted with ferruginuous smectite (sample SWa-1 from The Source Clays Repository), SWa, in aqueous suspension under anoxic conditions. Compounds highly polarizable or sharing substituents that facilitate charge delocalization adsorbed faster by reduced (SWa-R) than by unaltered (SWa-U) clay, indicating stronger dipole--dipole interactions between the substituents and the clay surface and/or hydrating water molecules. The reduction of the clay accelerated RCI adsorption up to 100-fold. Incubations with SWa-R promoted RCI reduction (CP, TCAN) or dehydrochlorination (TeCA and PCA). The reduction of structural Fe catalyzes the transformation of RCI via Brønsted and Lewis-basic promoted pathways. This study indicates that oxidation state of the structural Fe in SWa greatly alters surface chemistry and has a large impact on clay-organic interactions.

Fate of atrazine and alachlor in redox-treated ferruginous smectite.

The oxidation state of structural iron (Fe) in clay minerals exerts a large influence on clay surface chemistry and may affect the adsorption and degradation of pesticides in the environment. This effect, however, has been little investigated. In the present study, herbicides atrazine and alachlor were reacted with ferruginous smectite (sample SWa-1) in its oxidized, reduced (either chemically or bacterially), and reduced-reoxidized states. In some experiments the herbicide was labeled with 14C. Gas chromatography/mass spectrometry (GC-MS) was also used to detect alachlor degradation products. Compared to oxidized clays, reduction by both chemical and microbial treatments decreased the concentration of both herbicides in the surrounding solution. Reoxidized clay exhibited behavior similar to the oxidized clay. Hydrolysis-dechlorination of atrazine occurred in the presence of chemically reduced SWa-1, and GC-MS analysis of alachlor revealed at least 14 degradation products after treatment with reduced clay and only two with the oxidized clay. Interaction of atrazine and alachlor with the clay may be through a H bond with the waters of hydration surrounding interlayer cations, the extent of which should increase with increasing acidity; but under reduced conditions, the validity of this model is unclear. Reduction of structural Fe may affect pH-dependent phenomena in two ways: The increased surface charge density increases the number of hydrated interlayer cations, thereby enhancing surface acidity, and increased electron density at basal surface oxygens increases their Brønsted basicity. Atrazine could, therefore, adsorb and/or degrade through either acid or alkaline hydrolysis pathways. Increased reduction potential of the reduced clay surfaces may also promote degradation.

Effects of chemical composition and temperature of heating on the infrared spectra of Li-saturated dioctahedral smectites. (II) Near-infrared region

AbstractDiffuse reflectance spectra in the near-infrared region are presented for Li-saturated smectites heated up to 200°C. The smectites included three montmorillonites, an Fe-beidellite and a ferruginous smectite. Unheated samples showed a complex band near 7060 cm-1 assigned to the first overtone of the OH-stretching mode of structural hydroxyl groups and bound H2O. The OH combination bands appeared in the 4600-4300 cm-1 region. Spectra of heated montmorillonites showed an upward shift and splitting of the OH-overtone band into two components near 7170 and 7110 cm-1. The presence of a band near 7170 cm-1, assigned to the overtone of the AlMgLiOH-stretching vibration, implied that local trioctahedral domains were created upon Li(I) fixation in the previously vacant octahedral positions of montmorillonites. The OH combination bands were shifted to higher frequencies and a new band appeared near 4472 cm-1 in the spectra of montmorillonites heated to temperatures in excess of 130°C. No features indicating Li(I) in the structure of Fe-beidellite or ferruginous smectite were observed in the NIR spectra.

Effects of chemical composition and temperature of heating on the infrared spectra of Li-saturated dioctahedral smectites. (I) Mid-infrared region

AbstractInfrared spectroscopy in the mid-IR region was used to follow the structural changes occurring in five Li-saturated dioctahedral smectites upon heating. The smectites included three montmorillonites, an Fe-beidellite and a ferruginous smectite. Fixation of Li+ ions in the structure even upon heating at 120°C caused the appearance of an AlMgLiOH-stretching band near 3670 cm–1 in the spectra of all three montmorillonites. This band confirmed the presence of Li(I) in the previously vacant octahedral positions in montmorillonites. No similar band was observed in the spectra of ferruginous heated smectites with prevailing tetrahedral charge. A gradual upward frequency shift and decrease in intensity of the AlAlOH-bending band showed that Li(I) present in the hexagonal cavities causes pronouncedperturbation of this OH-bending mode. The Li(I) present in the octahedral sheets causes small perturbations of the OH-bending mode near 850 cm–1 and activation of a new OH-bending mode near 803 cm–1. Reversible changes in the positions of the stretching Si–O and bending OH bands in the spectra of Fe-beidellite and a ferruginous smectite proved that Li was present in these minerals primarily in the hexagonal holes of the tetrahedral sheets.

Vibrational spectroscopy of ferruginous smectite and nontronite

A comparison is made between the Raman and infrared spectra of ferruginous smectite and a nontronite using both absorption and emission techniques. Raman spectra show hydroxyl stretching bands at 3572, 3434, 3362, 3220 and 3102 cm −1. The infrared emission spectra of the hydroxyl stretching region are significantly different to the absorption spectrum. These differences are attributed to the loss of water, absent in the emission spectrum, the reduction of the samples in the spectrometer and possible phase changes. Dehydroxylation of the two minerals may be followed by the loss of intensity of the hydroxyl stretching and hydroxyl deformation frequencies. Hydroxyl deformation modes are observed at 873 and 801 cm −1 for the ferruginous smectite, and at 776 and 792 cm −1 for the nontronite. Raman hydroxyl deformation vibrations are found at 879 cm −1. Other Raman bands are observed at 1092 and 1032 cm −1, assigned to the SiO stretching vibrations, at 675 and 587 cm −1, assigned to the hydroxyl translation vibrations, at 487 and 450 cm −1, attributed to OSiO bending type vibrations, and at 363, 287 and 239 cm −1. The differences in the molecular structure of the two minerals are attributed to the Al/Fe ratio in the minerals.

Trifluralin Degradation under Microbiologically Induced Nitrate and Fe(III) Reducing Conditions

Trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine] ranks among the most commonly used herbicides in the United States. The compound persists under most environmental conditions, yet it is rapidly transformed under certain anaerobic conditions. In this study, the fate of trifluralin in anoxic environments and the contribution of Fe(II) to its anaerobic degradation have been investigated. Trifluralin was rapidly degraded under anaerobic conditions in a range of soils representing typical agricultural usage in the Midwest. The presence of nitrate or oxygen suppressed trifluralin degradation. Degradation rate increased under iron-reducing conditions, and the addition of trifluralin appeared to promote reoxidation of extractable Fe2+. Transformation of trifluralin under iron-reducing conditions apparently involved the soil solid phase and was not limited by bioavailability. In a soil-free aqueous system, no reaction of trifluralin with dissolved Fe2+ was detected in the presence or absence of kaolinite clay under anoxic conditions. Reduced but not oxidized or reoxidized forms of purified ferruginous smectite (sample Swa-1) catalyzed rapid transformation (72% of applied in 30 h) of trifluralin to polar products with a concomitant reoxidation of structural Fe in the clay. Results indicate that, as for other nitroaromatics, trifluralin is subject to reaction with Fe(II) associated with the minerals in anoxic environments.

Magnetic properties of oxidized and reduced smectites

Four smectites with different total Fe contents (two nontronites, one ferruginous smectite, and one montmorillonite) were reduced to obtain a range of Fe(II)/Fe(III) ratios and their magnetic properties measured with a SQUID (superconducting quantum interference device) as a function of applied magnetic field strength at 5 K and as a function of temperature in a field of 0.1 T. The unaltered nontronite and ferruginous smectite specimens showed antiferromagnetic coupling, whereas the coupling in the reduced samples was ferromagnetic; the paramagnetic Curie temperature increased with increasing Fe(II) content. Data collected after cooling samples in both the presence and absence of an external magnetic field of 0.1 T showed that at low temperatures the reduced (ferromagnetic) nontronite and ferruginous smectite samples exhibit a memory effect of previous magnetic field exposure consistent with superparamagnetic or spin glass behavior. The superparamagnetic/ferromagnetic transition temperature, T f , increased linearly with increasing Fe(II) content for each of the nontronites, but the relationship between T f and Fe(II) content differed for different clays, thus demonstrating that T f is sensitive to isomorphous substitutions in the clay structure. The montmorillonite was paramagnetic in both oxidized and reduced forms.

Dehydration and dehydroxylation of nontronites and ferruginous smectite

Differential thermal analysis and thermogravimetry have been used to compare the dehydration and dehydroxylation behaviour of a series of nontronites including two newly discovered nontronites from Uley, South Australia. A comparison is made with the thermal behaviour of ferruginous smectite. Three dehydration endotherms were found at around 60°C, 100°C and 160°C. A strong dehydroxylation endotherm for all samples was found at ∼430°C. The nontronites from Hohen-Hagen and Garfield show well-defined steps for dehydroxylation whereas the two Uley nontronites showed continuous dehydroxylation. The intensities of the infrared spectra of the hydroxyl-stretching region showed excellent correlation with the nontronite dehydroxylation endotherms. The infrared spectra of the water bending region showed two bands centred at ∼1630 and 1670 cm −1. These bands are attributed to adsorbed water and coordinated water.

Adsorption Kinetics of Pentachloroethane by Iron-Bearing Smectites

AbstractThe oxidation state of structural Fe greatly alters surface chemistry, which may have a large influence on clay-organic interactions. The effect of structural-iron oxidation state on chlorinated hydrocarbons at the clay-water interface was examined. Pentachloroethane (5CA) was reacted with oxidized, reduced, and reoxidized forms of three different smectites: montmorillonite, ferruginous smectite, and nontronite in aqueous suspension under controlled-atmosphere conditions. Pentachloroethane was found to adsorb at different rates for the three smectites. A series of 5CA-adsorption rate constants in the presence of these clays showed a strong correlation with the Fe(II) content of the clay (r2 = 0.98). The clay surface behaves as a Brønsted base and promotes 5CA dehydrochlorination. The adsorption kinetics at the clay-water interface were described by the formation of a precursor complex prior to 5CA dehydrochlorination.

Transformation of Chloropicrin in the Presence of Iron-Bearing Clay Minerals

Chloropicrin (trichloronitromethane, CP) reacted with reduced ferruginous smectite (SWa-1) in aqueous suspension. SWa-1 promoted CP-dechlorination to produce dichloro-(DCNM) and chloronitromethane (CNM) with yields of up to ∼80% within 30 min. The simultaneous formation of DCNM and CNM suggests that CP and SWa-1 surface participate in two electron-transfer pathways. A series of CP-adsorption and dechlorination rate constants in the presence of SWa-1 showed a strong correlation with the Fe(II) content of SWa-1 (r 2 = 0.96 and 0.98, respectively), {Fe(II)}. This study indicates that the oxidation state of structural Fe greatly alters surface chemistry and has a large impact on electron transfer during clay-organic interactions.

Partial Stabilization of Fe(II) in Reduced Ferruginous Smectite by Li Fixation

AbstractPartial stabilization of Fe(II) in chemically reduced smectite, which normally readily undergoes reoxidation in air, was achieved. The purpose of this study was to determine if Fe(II) can be stabilized in reduced smectites by Li fixation upon heating. More than 80% of total Fe in ferruginous smectite SWa-1 was reduced to Fe(II) using the citrate-bicarbonate-dithionite (CBD) method while purging the clay dispersion with N2. The reduced smectite was Li-saturated, washed free of excess ions, freeze-dried, and heated in N2 atmosphere at 260°C for 24 h to produce Li-fixation. This treatment caused partial stabilization of Fe(II) in the clay structure. Chemical analysis, Mössbauer spectroscopy, and Fourier transform infrared (FTIR) spectroscopy proved that <20% of total Fe was Fe(II) after reoxidation with oxygen in a water dispersion, a treatment which causes complete reoxidation of Fe(II) in reduced Na-smectites. Decomposition of the OH-stretching band evident in the IR spectra indicated migration of Li into the vacant octahedra. Some of the OH groups in the reoxidized smectite were found in local trioctahedral configurations, associated with the AlFe(II)Li or Fe(III)Fe(II)Li groupings of central atoms in the octahedral sheet.

Characterization and Catalytic Activity of Acid-Treated, Size-Fractionated Smectites

Five layered silicates in which the octahedral sheet contained differing amounts of Al, Mg, Fe, and Li were acid leached using acid concentrations and treatment temperatures selected to produce materials in which the octahedral sheet was depopulated in a controlled, stepwise (yet comparable) manner. SAz-1 and JP are dioctahedral smectites with octahedral compositions rich in Mg and Al, respectively. SWa-1 is a ferruginous smectite and ST an iron-rich beidellite. The fifth mineral was a trioctahedral hectorite which contains almost exclusively octahedral Mg. The Bronsted acidity and catalytic activity of the resulting materials were highest for the samples prepared with the mildest acid treatments but decreased as the octahedral sheet became increasingly depleted. Only the hectorite exhibited no catalytic activity despite the proven existence of Bronsted acid sites. The elemental composition of the starting material did not appear to make a significant contribution to the catalytic activity for the chosen ...

Redox Characterization of the Surfaces of Seven Iron-Bearing Minerals: Use of Molecular Probes and UV-Visible Spectroscopy

AbstractRedox properties of iron-bearing mineral surfaces may play an important role in controlling the transport and transformation of pollutants into ground waters. Suspensions of seven iron-bearing minerals were reacted with pH and redox indicators under anaerobic conditions at the pH of the natural suspension. The responses of the indicators to the mineral surfaces were monitored by UV-visible spectroscopy using a scattered transmission technique. The Hammett surface acidity function (Hs) and the surface redox potential (Ehs) of these iron-bearing minerals were measured. These measured values were used to calculate Eh values for the seven minerals: goethite = +293 mV; chlorite = +290 mV; hematite = +290 mV; almandite = +282 mV; ferruginous smectite = +275 mV; pyrite = +235 mV; and Na-vermiculite = +223 mV. Calculated surface redox potentials of minerals are different from their potentials measured by platinum electrode in bulk suspensions. UV-visible spectroscopy provides a quick and non-destructive way of monitoring organic probe response at the mineral surface.