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PubMedDate not supplied

Clay-mineral suites, sources, and inferred dispersal routes: Southern California continental shelf.

Clay mineralogy is useful in determining the distribution, sources, and dispersal routes of fine-grained sediments. In addition, clay minerals, especially smectite, may control the degree to which contaminants are adsorbed by the sediment. We analyzed 250 shelf sediment samples, 24 river-suspended-sediment samples, and 12 river-bed samples for clay-mineral contents in the Southern California Borderland from Point Conception to the Mexico border. In addition, six samples were analyzed from the Palos Verdes Headland in order to characterize the clay minerals contributed to the offshore from that point source. The <2 microm-size fraction was isolated, Mg-saturated, and glycolated before analysis by X-ray diffraction. Semi-quantitative percentages of smectite, illite, and kaolinite plus chlorite were calculated using peak areas and standard weighting factors. Most fine-grained sediment is supplied to the shelf by rivers during major winter storms, especially during El Niño years. The largest sediment fluxes to the region are from the Santa Ynez and Santa Clara Rivers, which drain the Transverse Ranges. The mean clay-mineral suite for the entire shelf sediment data set (26% smectite, 50% illite, 24% kaolinite+chlorite) is closely comparable to that for the mean of all the rivers (31% smectite, 49% illite, 20% kaolinite+chlorite), indicating that the main source of shelf fine-grained sediments is the adjacent rivers. However, regional variations do exist and the shelf is divided into four provinces with characteristic clay-mineral suites. The means of the clay-mineral suites of the two southernmost provinces are within analytical error of the mineral suites of adjacent rivers. The next province to the north includes Santa Monica Bay and has a suite of clay minerals derived from mixing of fine-grained sediments from several sources, both from the north and south. The northernmost province clay-mineral suite matches moderately well that of the adjacent rivers, but does indicate some mixing from sources in adjacent provinces.

Aluminum Silicates↗
PubMed2001

Non-aqueous-phase-liquid breakthrough during evaporative drying of clay barriers.

In this study, an attempt has been made to model a real field scenario, whereby an initially almost saturated clay liner in a waste site is gradually drying, due to evaporation at its lower boundary. A detailed conceptual model that deals with the penetration and breakthrough of non-aqueous-phase-liquid (NAPL) in clay liners is introduced. Water content of clay samples was monitored during ambient evaporation through apertures at the base of sample holders. Clay drying rate served as the primary parameter for the NAPL breakthrough study. The interconnection between drying rates, structural damage formation (cracks and suction) and NAPL penetration is especially addressed. The processes taking place in the clay samples during drying appear to be associated with the capillary effects between the different fluid phases in the vicinity of either the NAPL-clay or the clay-air boundaries. A conceptual model of NAPL penetration and breakthrough of the clay layer has been considered, based on both indirect and direct observations of structural damages produced on either clay boundaries. A mutual interaction between these two boundaries is suggested and discussed. NAPL breakthrough is suggested to take place through cracks initiated on the upper soil surface.

Aluminum Silicates↗
PubMed2003

A novel system for reducing leaching from formulations of anionic herbicides: clay-liposomes.

A new approach was developed for reducing leaching of herbicides and contamination of groundwater. Liposome-clay formulations of the anionic herbicides sulfometuron and sulfosulfuron were designed for slow release by incorporating the herbicide in positively charged vesicles of didodecyldimethylammonium (DDAB), which were adsorbed on the negatively charged clay, montmorillonite. Freeze fracture electron microscopy demonstrated the existence of DDAB vesicles and aggregated structures on external clay surfaces. X-ray diffraction results for DDAB with montmorillonite imply the existence of DDAB bilayers with an oblique orientation to the basal plane within the clay interlayer space at adsorbed amounts beyond the cation exchange capacity of the clay. Adding DDAB with sulfometuron or sulfosulfuron to montmorillonite yielded 95% or 83% adsorption of the herbicide at optimal ratios. Liposome-clay formulations exhibited slow release of the herbicides in water. Analytical measurements in soil columns demonstrated 2-10-fold reduction in leaching of the herbicides from liposome-clay formulations in comparison with commercial formulations. Percents of root growth inhibition of a test plant in the upper soil depths were severalfold higher for the liposome-clay formulations than for the commercial ones. Consequently, liposome-clay formulations of anionic herbicides can solve environmental and economical problems by reducing their leaching.

Adsorption↗
PubMed2006

Interactions of DNA with clay minerals and soil colloidal particles and protection against degradation by DNase.

Adsorption, desorption, and degradation by nucleases of DNA on four different colloidal fractions from a Brown soil and clay minerals were studied. The adsorption of DNase I and the structures of native DNA, adsorbed and desorbed, were also investigated by Fourier Transform Infrared (FTIR), circular dichroism (CD), and fluorescence spectroscopy, to determine the protection mechanism of DNA molecules by soil colloids and minerals against enzymatic degradation. Kaolinite exhibited the highest adsorption affinity for DNA among the examined soil colloids and clay minerals. In comparison with organomineral complexes (organic clays), DNA was tightly adsorbed by H2O2-treated clays (inorganic clays). FTIR spectra showed that the binding of DNA on kaolinite and inorganic clays changed its conformation from the B-form to the Z-form, whereas montmorillonite and organic clays retained the original B-form of DNA. A structural change from the B- to the C-form in DNA molecules desorbed from kaolinite was observed by CD spectroscopy and confirmed by fluorescence spectroscopy. The presence of soil colloids and minerals provided protection to DNA against degradation by DNase I. The higher level of protection was found with montmorillonite and organic clays compared to kaolinite and inorganic clays. The protection of DNA against nuclease degradation by soil colloids and minerals is apparently not controlled by the adsorption affinity of DNA molecules for the colloids and the conformational change of bound DNA. The higher stability of DNA seemed to be attributed mainly to the presence of organic matter in the system and the adsorption of nucleases on soil colloids and minerals. The information obtained in this study is of fundamental significance for the understanding of the behavior of extracellular DNA in soil environment.

Adsorption↗
PubMedDate not supplied

Controlling harmful algal blooms through clay flocculation.

The potential use of clays to control harmful algal blooms (HABs) has been explored in East Asia, Australia, the United States, and Sweden. In Japan and South Korea, minerals such as montmorillonite, kaolinite, and yellow loess, have already been used in the field effectively, to protect fish mariculture from Cochlodinium spp. and other blooms. Cell removal occurs through the flocculation of algal and mineral particles, leading to the formation of larger aggregates (i.e. marine snow), which rapidly settle and further entrain cells during their descent. In the U.S., several clays and clay-rich sediments have shown high removal abilities (e.g. > 80% cell removal efficiency) against Karenia brevis, Heterosigma akashiwo, Pfiesteria piscicida and Aureococcus anophagefferens. In some cases, the removal ability of certain clays was further enhanced with chemical flocculants, such as polyaluminum chloride (PAC), to increase their adhesiveness. However, cell removal was also affected by bloom concentration, salinity, and mixing. Cell mortality was observed after clay addition, and increased with increasing clay concentration, and prolonged exposure to clays in the settled layer. Mesocosm, field enclosure, and flume experiments were also conducted to address cell removal with increasing scale and flow, water-column impacts, and the possible benthic effects from clay addition. Results from these studies will be presented, especially those in regards to water quality, seawater chemistry, bottom erodibility and faunal impacts in the benthos. At this time, clay dispersal continues to be a promising method for controlling HABs and mitigating their impacts based on existing information and experimental data.

Aluminum Hydroxide↗
PubMed2005

Nanoalloys based on clays: intercalated semicrystalline blends of syndiotactic polystyrene with poly(2,6-dimethyl-p-phenylene oxide).

Nanoalloys of syndiotactic polystyrene with poly(2,6-dimethyl-p-phenylene oxide) clay were prepared by a solution intercalation technique using 1,1,2,2-tetrachloroethane. A mixed intercalated/exfoliated morphology in the nanoalloys containing organo-clays is evinced by transmission electron microscopy. Thermal stability is found to improve with 5% by weight of organo-clays, but a reverse trend is observed by increasing the concentration of clay. The influence of montmorillonite clays in isothermal melt-crystallization behavior of blends was thoroughly investigated by using differential scanning calorimetry and wide-angle X-ray diffraction. A considerable change in polymorphic behavior of syndiotactic polystyrene is observed by the incorporation of different clays into the blends, and the temperature window of isothermal crystallization for the formation of a-crystals of syndiotactic polystyrene is found to broaden in the presence of organo-clays. In contrast, the pristine clay is found to induce only the beta-crystal of syndiotactic polystyrene at all the crystallization temperatures studied in this work. The amorphous component poly(2,6-dimethyl-p-phenylene oxide) affects the crystallinity of syndiotactic polystyrene adversely and favors the beta-form of syndiotactic polystyrene in blends; a-crystals are observed in the nano-alloys containing 20 wt% of poly(2,6-dimethyl-p-phenylene oxide). The crystalline morphology, as characterized by polarized optical microscopy, clearly indicates rapid formation of the alpha-form induced by the nanoscale dispersion of organo-clays in spherulites of smaller dimension.

Alloys↗
PubMed2005

Sensitivity of the acid-base properties of clays to the methods of preparation and measurement. 1. Literature review.

Measuring and modeling the surface charge of clays, and more especially smectites, has become an important issue in the use of bentonites as a waste confinement material aimed at retarding migration of water and solutes. Therefore, many studies of the acid-base properties of montmorillonite have appeared recently in the literature, following older studies principally devoted to cation exchange. It is striking that beyond the consensus about the complex nature of the surface charge of clays, there are many discrepancies, especially concerning the dissociable charge, that prevents intercomparison among the published data. However, a general trend is observed regarding the absence of common intersection point on raw titration curves at different ionic strengths. Analysis of the literature shows that these discrepancies originate from the experimental procedures for the preparation of the clays and for the quantification of their surface charge. The present work is an attempt to understand how these procedures can impact the final results. Three critical operations can be identified as having significant effects on the surface properties of the studied clays. The first one is the preparation of purified clay from the raw material: the use of acid or chelation treatments, and the repeated washings in deionized water result in partial dissolution of the clays. Then storage of the purified clay in dry or wet conditions strongly influences the equilibria in the subsequent experiments respectively by precipitation or enhanced dissolution. The third critical operation is the quantification of the surface charge by potentiometric titration, which requires the use of strong acids and bases. As a consequence, besides dissociation of surface sites, many secondary titrant consuming reactions were described in the literature, such as cation exchange, dissolution, hydrolysis, or precipitation. The cumulated effects make it difficult to derive proper dissociation constants, and to build adequate models. The inadequation of the classical surface complexation models to describe the acid-base behavior of clays is illustrated by the electrokinetic behavior of smectites, which is independent from the pH and the ionic strength. Therefore, there is still a need on one hand for accurate data recorded in controlled conditions, and on the other hand for new models taking into account the complex nature of the charge of clays.

Journal Article↗
PubMed2006

The influence of montmorillonite clay reinforcement on the performance of a glass ionomer restorative.

OBJECTIVES: A pristine calcium montmorillonite (Ca-MMT) and an organically modified 12-amino-dodecanoicacid treated montmorillonite (ADA-MMT) clay were evaluated to determine the reinforcement effect on the performance of a glass ionomer (GI) restorative ChemFil Superior (Dentsply DeTrey, Kanstanz, Germany) to assess the potential as a posterior filling material. METHODS: Mean compressive fracture strengths and standard deviations were determined for groups of 30 cylindrical specimens with 0.5-2.5 wt.% Ca-MMT and ADA-MMT additions to either the powder or the liquid elements of the GI. Working characteristics were assessed using an oscillating rheometer, the interlayer d-spacings (d(001)) of each MMT clay was determined using X-ray diffraction (XRD) and the set cement structure was determined by scanning electron microscopy (SEM). RESULTS: The addition of up to 1.0 wt.% ADA-MMT resulted in a significant increase in the mean compressive fracture strength of the GI utilising the one-way ANOVA and Tukey test comparisons at P<0.05. When Ca-MMT clay was added to the GI a significant reduction in mean strength was recorded (P<0.05). MMT clay addition to the powder and liquid element of the GI in excess of 1.0 wt.% resulted in no significant differences in working characteristics although increases in setting times were evident for MMT addition up to 1.0 wt.%. XRD patterns identified the d(001) peaks for Ca- and ADA-MMT at 2 theta angles of 8.44 degrees and 5.07 degrees and d(001) spacings of 1.04 and 1.74 nm, respectively. CONCLUSIONS: The increased interlayer d-spacings recorded for ADA-MMT clay demonstrates that the clay had expanded layers which may have provided an increased opportunity for the polyacrylic acid chains of the GI restorative to diffuse into the MMT galleries. It is postulated that the increased interlayer d-spacings for the ADA-MMT clay had a positive reinforcing effect on the GI compared with the Ca-MMT clay where no increased performance was identified. The study has highlighted the potential for increasing the performance of GI restoratives when using an organically modified clay as a reinforcement which could increase the potential of GIs as posterior filling materials.

Aluminum Silicates↗
PubMed1999

Surface geochemistry of the clay minerals.

Clay minerals are layer type aluminosilicates that figure in terrestrial biogeochemical cycles, in the buffering capacity of the oceans, and in the containment of toxic waste materials. They are also used as lubricants in petroleum extraction and as industrial catalysts for the synthesis of many organic compounds. These applications derive fundamentally from the colloidal size and permanent structural charge of clay mineral particles, which endow them with significant surface reactivity. Unraveling the surface geochemistry of hydrated clay minerals is an abiding, if difficult, topic in earth sciences research. Recent experimental and computational studies that take advantage of new methodologies and basic insights derived from the study of concentrated ionic solutions have begun to clarify the structure of electrical double layers formed on hydrated clay mineral surfaces, particularly those in the interlayer region of swelling 2:1 layer type clay minerals. One emerging trend is that the coordination of interlayer cations with water molecules and clay mineral surface oxygens is governed largely by cation size and charge, similarly to a concentrated ionic solution, but the location of structural charge within a clay layer and the existence of hydrophobic patches on its surface provide important modulations. The larger the interlayer cation, the greater the influence of clay mineral structure and hydrophobicity on the configurations of adsorbed water molecules. This picture extends readily to hydrophobic molecules adsorbed within an interlayer region, with important implications for clay-hydrocarbon interactions and the design of catalysts for organic synthesis.

Journal Article↗
PubMed2006

Sorption-desorption behavior of PCP on soil organic matter and clay minerals.

Pentachlorophenol (PCP) contamination is a severe environmental problem due to its widespread occurrence, toxicity and recalcitrance. In order to gain a better understanding of the fate of PCP in soils, the role of the soil organic matter (SOM) and clay minerals in the PCP sorption-desorption was studied on two bulk field soils, two subsoils (i.e., SOM or clay-removed soil) and two artificial soils. The two field soils used were a silty loam from New Mexico (NM) containing 10% clay and a sandy-clay-loam from Colombia (CO) South America comprised of 18% clay minerals. The bulk CO soil containing kaolinite sorbed significantly less PCP than the NM soil. All soils depicted an apparent hysteresis during sorption. The CO bulk and subsoils desorbed 14-20% and 15-26% of the sorbed PCP respectively whereas the NM bulk and subsoils desorbed only 4-12% and 5-16%, respectively. Experiments conducted with pure clay and artificial soils indicated that the expandable clay minerals were key sorbent material. Additional studies to investigate the interaction between SOM and clay minerals are needed to fully understand sorptive phenomena.

Aluminum Silicates↗
PubMed2007

Concentrations of polychlorinated dibenzo-p-dioxins in processed ball clay from the United States.

Processed ball clays commonly used by the ceramic art industry in the United States were collected from retail suppliers and analyzed for the presence and concentration of the 2,3,7,8-Cl substituted polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs/PCDFs). The average PCDD toxic equivalent (TEQ) concentrations of these processed ball clays was approximately 800 pg/g (TEQ-WHO) with characteristic congener profiles and isomer distributions similar to patterns of previously analyzed raw and processed ball clays. The PCDF concentrations were below the average limit of detection (LOD) of 0.5 pg/g. Correlation analyses reveal no significant relationship between total organic carbon (TOC) and either individual, homologues, and total tetra-through octa-chlorinated PCDD congeners, or TEQ concentrations of the processed ball clays. The results are consistent with earlier studies on levels of PCDDs in ball clays. Data from earlier studies indicated that dioxins may be released to the environment during the processing of raw clay or the firing process used in commercial ceramic facilities. The presence of dioxin in the clays also raises concerns about potential occupational exposure for individuals involved in the mining/processing of ball clay, ceramics manufacturing and ceramic artwork.

Aluminum Silicates↗
PubMed2006

Effect of clay content and wetting-and-drying on radiocaesium behaviour in a peat and a peaty podzol.

The interaction of radiocaesium with peat under two moisture regimes was studied in laboratory experiments and by growing ryegrass in pot experiments to simulate changing field moisture conditions. A peat untreated and treated with 5% by weight of clay containing 46% illitic minerals, and a peaty podzol naturally containing 4.5% mineral matter on a dry weight basis were contaminated with (134)Cs and incubated. The soils were exposed to 8 wetting-and-drying cycles or kept constantly wet during 40 days. Extraction of the peat with 1 M CH(3)COONH(4) (pH 7) repeated after each wetting-and-drying cycle indicated increasing (134)Cs fixation with time of incubation. The peat treated with clay showed a much higher (134)Cs fixation than that without clay. The pot experiment with the incubated soils showed a (134)Cs transfer to ryegrass of the same order for the peaty podzol as for the peat treated with clay. For the peat untreated with clay the (134)Cs transfer to ryegrass was much greater. Wetting-and-drying the peat, with or without clay, increased the overall yield of grass and the concentration and uptake of (134)Cs over 5 consecutive harvests. K-fertilisation increased the yield of plant material (except for the peat with added clay), decreased the concentration of (134)Cs, but had no significant effect (p=0.05) on the resultant uptake of (134)Cs. Mixing clay with the surface layer of organic soils appears to be an effective means of decreasing radiocaesium transfer to field crops in fallout situations.

Aluminum Silicates↗
PubMed2002

Heat-shock protein (HSP70) response in the eastern oyster, Crassostrea virginica, exposed to PAHs sorbed to suspended artificial clay particles and to suspended field contaminated sediments.

Sediments are a potentially significant source of pollutants, containing not only organic contaminants but heavy metals as well. The heat shock protein response (HSP70 family) in the eastern oyster exposed to suspended clay particles spiked with polynuclear aromatic hydrocarbons (PAHs) and to suspended field contaminated sediments (SFCS) was investigated. In experiment 1, oysters were exposed to 1.0, 1.5 or 2.0 g suspended clay particles with concentrations of 65.6, 159.0 and 242 micro g PAHs per g of wet clay particles, respectively, and sampled after 40 days. Controls were exposed to 0, 1.0, 1.5, and 2.0 g suspended unspiked clay particles. In experiment 2, oysters were exposed to 0, 1.0, 1.5, and 2.0 g SFCS and the HSP70 expression was determined after 5, 10, 20 and 40 days exposure. Oysters exposed to suspended clay particles spiked with PAHs showed a significant increase in HSP70 levels, while oysters exposed to 1.0, 1.5 or 2.0 g suspended unspiked clay particles did not show changes (P=0.78) in HSP70 levels compared to the group receiving 0 g clay particles. Exposure to the SFCS resulted in a significant increase in HSP70 as a function of exposure (P<0.001) and treatment (P=0.006). The response, however, was not dose dependent. Compared to the control group (0 g SFCS), groups exposed to 1.0, 1.5 and 2.0 g SFCS reached significantly higher levels in HSP70 at 40 days of exposure, with those exposed to 2.0 g SFCS expressing the highest levels. The HSP70 expression for each treatment showed fluctuations at various time intervals. No mortalities were recorded during the exposure experiments. The major contaminants in the SFCS were PAHs, heavy metals and polychlorinated biphenyls (PCBs). These results reveal that exposure to PAHs sorbed to clay particles and to SFCS induced a HSP70 response in the eastern oyster.

Aluminum Silicates↗
PubMed2003

Modelling of silica diffusion experiments with 32Si in Boom Clay.

A mathematical model describing the dissolution of nuclear glass directly disposed in clay combines a first-order dissolution rate law with the diffusion of dissolved silica in clay. According to this model, the main parameters describing the long-term dissolution of the glass are etaR, the product of the diffusion accessible porosity eta and the retardation factor R, and the apparent diffusion coefficient D(app) of dissolved silica in clay. For determining the migration parameters needed for long-term predictions, four Through-Diffusion (T-D) experiments and one percolation test have been performed on undisturbed clay cores. In the Through-Diffusion experiments, the concentration decrease after injection of 32Si (radioactive labelled silica) was measured in the inlet compartment. At the end of the T-D experiments, the clay cores were cut in thin slices and the activity of labelled silica in each slice was determined. The measured activity profiles for these four clay cores are well reproducible. Since no labelled silica could be detected in the outlet compartments, the Through-Diffusion experiments are fitted by two In-Diffusion models: one model assuming linear and reversible sorption equilibrium and a second model taking into account sorption kinetics. Although the kinetic model provides better fits, due to the sufficiently long duration of the experiments, both models give approximately similar values for the fit parameters. The single percolation test leads to an apparent diffusion coefficient value about two to three times lower than those of the Through-Diffusion tests. Therefore, dissolved silica appears to be strongly retarded in Boom Clay. A retardation factor R between 100 and 300 was determined. The corresponding in situ distribution coefficient K(d) is in the range 25-75 cm(3) g(-1). The apparent diffusion coefficient of dissolved silica in Boom Clay is estimated between 2 x 10(-13) and 7 x 10(-13) m(2) s(-1). The pore diffusion coefficient is in the range from 6 x 10(-11) to 1 x 10(-10) m(2) s(-1).

Aluminum Silicates↗
PubMed2001

Clay pica has no hematologic or metabolic correlate in chronic hemodialysis patients.

OBJECTIVE: Clay pica is a form of compulsive ingestion of non-nutritive substances frequently practiced by dialysis patients. Its consequences are unknown. In this study, we evaluated the effect of regular consumption of clay on hematologic and metabolic profiles in hemodialysis patients. DESIGN: A prospective, case-control study with use of structured questionnaire. SETTING: Free-standing hemodialysis units. PATIENTS: One hundred thirty-eight patients on hemodialysis for at least 12 months were interviewed. Thirteen of 138 (9.4%) confessed to clay pica. Ten randomly selected patients with no history of pica served as control. INTERVENTION: Average of all laboratory profiles and interdialytic weight gain (IDWG) over a 3-month period were recorded. Assay of the aluminum (Al), silica (Si), and iron (Fe) content of commercially purchased clay was performed. MAIN OUTCOME MEASURE: Comparison of laboratory profiles and IDWG between cases and control. Estimation of the daily consumption of Al, Fe, and Si from clay and their relationship to the laboratory profiles. RESULTS: There was no statistically significant difference in the levels of Al, albumin, calcium, ferritin, hematocrit, iron saturation, phosphorus, and IDWG between pica cases and control. Iron was significantly higher in pica patients (13.0 +/- 7.9 micromol/L v 7.5 +/- 2.5 micromol/L, P =.04), but potassium was higher among control than pica cases (4.9 +/- 0.7 mmol/L v 4.4 +/- 0.6 mmol/L, P =.07). Estimated metal exposure from daily clay consumption per patient were: Al (1-2 mg), Fe (11-23.5 mg), and Si (2-4.5 g). Multivariate logistic regression analysis failed to show any association between clay consumption and nutrition, anemia, or mineral metabolism (R(2) = 0.0, P =.79). CONCLUSION: Clay pica does not appear to be detrimental to the hematologic and metabolic milieu of hemodialysis patients. The practice should, however, be discouraged, because of potential for ingestion of unknown substances, and reported potential for gastrointestinal complications.

Adult↗
PubMed2003

Reactivity of Fe(II) species associated with clay minerals.

Mineral-bound Fe(II) species represent important natural reductants of pollutants in the anaerobic subsurface. At clay minerals, three types of Fe(II) species in fundamentally different chemical environments may be present simultaneously, i.e., structural Fe(II), Fe(II) complexed by surface hydroxyl groups, and Fe(II) bound by ion exchange. We investigated the accessibility and reactivity of these three types of Fe(II) species in suspensions of two different clay minerals containing either ferrous iron-bearing nontronite or iron-free hectorite. Nitroaromatic compounds (NACs) exhibiting different sorption behavior on clays were used to probe the reactivity of the various types of reduced iron species. The clay treatment allowed for a preparation of nontronite and hectorite surfaces with Fe(II) adsorbed by surface hydroxyl groups at the edge surfaces. Furthermore, hectorite suspensions with additional Fe(II) bound to the ion exchange sites at the basal siloxane surfaces were set up. We found that both structural Fe(II) and Fe(II) complexed by surface hydroxyl groups of nontronite reduced the NACs to anilines. An electron balance revealed that more than 10% of the total iron in nontronite was reactive Fe(II). Fe(II) bound by ion exchange did not contribute to the observed reduction of NACs. Reversible adsorption of the NACs at the basal siloxane surface of the clays strongly retarded NAC reduction, even in the presence of high concentrations of Fe(II) bound by ion exchange to the basal siloxane surfaces. Our work shows that in natural systems a fraction of the total Fe(II) present on clays may contribute to the pool of highly reactive Fe(II) species in the subsurface. Furthermore, this work may help to distinguish between Fe(II) species of different reactivity regarding pollutant reduction. Although structural iron in clays represents only a small fraction of the total iron pool in soils and aquifers, reactive Fe(II) species originating from the reduction of structural Fe(III) in clays may contribute significantly to the biogeochemical cycling of electrons in the subsurface since it is not subject to depletion by reductive dissolution.

Aluminum Silicates↗
PubMed2006

Decomposition of hydrogen peroxide driven by photochemical cycling of iron species in clay.

The reactivity of iron-bearing clays to catalyze the decomposition of hydrogen peroxide (H2O2) under light irradiation was investigated. Free iron oxides and structural iron in clay octahedral lattice are contained simultaneously in montmorillonite K10 (MK10), a representative natural clay mineral. By pretreatment of clay with the particular method, the reactivities of the two kinds of iron species were differentiated. It was found that free iron oxides on clay surface efficiently catalyzed the decomposition of H202 under UV light irradiation but structural iron in the octahedral lattice showed poor reactivity. This was found to result from the difference in production of Fe(II) species under UV irradiation between iron oxides and structural iron. When photoreactive substances such as N,N-dimethylaniline (DMA), rhodamine B (RhB), or malachite green (MG) were introduced, structural iron was found to promote greatly the decomposition of H202. The reduction of clay iron(II) to iron(II) is essential for the decomposition of H2O2, which is achieved by light-induced ligand to metal charge transfer (for iron oxides) or organic matters donating electrons upon irradiation (for structural iron). The light-induced redox cycling of iron did not lead to the release of iron and decomposition of H202 primarily localized on the clay surface. This work implies that iron-bearing clays could be a sink of H2O2 in the environmental system.

Aluminum Silicates↗
PubMed2006

Hydration/expansion and cation charge compensation modulate the Brønsted basicity of distorted clay water.

This letter addresses how iron redox cycling and the hydration properties of the exchangeable cation influence the Brønsted basicity of adsorbed water in 2:1 phyllosilicates. The probe pentachloroethane undergoes facile dehydrochlorination to tetrachloroethene, attributed to increases in the Brønsted basicity of near-surface hydrating water molecules following the reduction of structural Fe(III) to Fe(II). This dehydrochlorination process is studied in the presence of Na(+)- or K(+)-saturated Upton montmorillonite [(Na0.82 (Si7.84 Al0.16)(Al3.10 Fe(3+)0.3 Mg0.66) O20 (OH)4] or ferruginous smectite [(Na0.87 Si7.38 Al0.62)(Al1.08) Fe(3+)2.67 Fe(2+)0.01 Mg0.23) O20 (OH)4]. The effect of iron redox cycling on pentachloroethane dehydrochlorination is studied using reduced or reduced and reoxidized smectite samples saturated with Na+ (fully expanded clay) or K+ (fully collapsed clay). Variations in the clay Brønsted basicity following Na+ -for- K+ exchange are explained by cationic charge compensation or interlayer hydration/expansion imposed by the nature of the exchangeable cation. Inverse relations between K+ fixation and clay water content as well as trends in pentachloroethane transformation indicate that increases in the Brønsted basicity result from increases in the clay hydrophilicity and shifts in the local activity of distorted clay water. Potassium fixation causes partially collapsed smectites bearing low amounts of structural Fe(II) to have a similar reactivity to that of fully expanded smectites (Na+ form) bearing higher amounts of structural Fe(II). In particular, the conversion of up to 80% of the pentachloroethane to tetrachloroethane by K+ -saturated, reoxidized Upton was explained because the fixation of K+ causes nonreversible expansion and incomplete reoxidation of structural Fe(II), which contributes to the stabilization of charge density near sites bearing Fe(II). Higher pentachloroethane conversions by Upton montmorillonite over ferruginous smectite, however, suggest that charge dispersion rather than site specificity contributes predominantly to clay reactivity. Thus, clay interlayer hydration/expansion imposed by the nature of the exchangeable cation alters water dissociation and proton exchange in Fe(II)-Fe(III) phyllosilicates susceptible to iron redox cycling.

Aluminum Silicates↗