Clay-organic Interactions Research Articles

Evolution of surface acidity during smectite illitization: Implication for organic carbon cycle

The clay-organic interactions in marine sediments and mudstones have long been concerned and play key roles in preservation of organic matter (OM). As chemically active mineral, smectite could transform into illite naturally, however, how this process affects stability and cycle of OM remains unclear. Here, we find that solid acidity varies greatly in terms of amount, type, and strength during illitization. The amount and strength of Brønsted (B) acid sites are enhanced markedly and interlayer space remains expandable in randomly interstratified illite/smectite, which enables immediate contact of active acid sites and adsorbed OM. In this case, the promotion of clay minerals on OM evolution would be greatly enhanced. Different from the traditional views on the effects of clay-OM interactions on organic carbon cycle, which include adsorptive protection and desorption, a new notion of clay mineral reactor is proposed which allows in-situ decomposition or degradation of OM.

Montmorillonite-Hydrochar Nanocomposites as Examples of Clay–Organic Interactions Delivering Ecosystem Services

AbstractClay–organic interaction is an important natural process that underpins soil ecosystem services. This process can also be tailored to produce clay–organic nanocomposites for industrial and environmental applications. The organic moiety of the nanocomposites, typically represented by a toxic surfactant, could be replaced by hydrochar formed from biomolecules (e.g. glucose) via hydrothermal carbonization. The effect of montmorillonite (Mnt) and glucose dosage on hydrochar formation, however, has not been clarified. In addition, the mechanisms by which Mnt-hydrochar nanocomposites (CMnt) can detoxify and remove carcinogenic Cr(VI) from aqueous solution are not well understood. In the current study, research milestones in terms of clay–organic interactions are summarized, following which the synthesis and characterization of CMnt for Cr(VI) adsorption are outlined. Briefly, 1 g of Mnt was reacted with 75 mL of glucose solution (0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 mol L−1) by hydrothermal carbonization at 200°C for 16 h. The resultant CMnt samples were analyzed for chemical composition, functional groups, morphological features, and Cr(VI) adsorptive properties. Mnt promoted the conversion of glucose to hydrochars, the particle size of which (~80 nm) was appreciably smaller than that formed in the absence of Mnt (control). Furthermore, the hydrochars in CMnt had an aromatic structure with low hydrogen substitution and high stability (C/H atomic ratio 0.34–0.99). The weakened OH (from hydrochar) and Si–O–Si stretching peaks in the Fourier-transform infrared (FTIR) spectra of CMnt are indicative of chemical bonding between Mnt and hydrochar. The CMnt samples were effective at removing toxic Cr(VI) from acidic aqueous solutions. Several processes were involved, including direct reduction of Cr(VI) to Cr(III), complexation of Cr(III) with carboxyl and phenolic groups of hydrochar, electrostatic attraction between Cr(VI) and positively charged CMnt at pH 2 followed by indirect reduction of Cr(VI) to Cr(III), and Cr(III) precipitation.

Early formation and taphonomic significance of kaolinite associated with Burgess Shale fossils

Abstract The role of minerals in Burgess Shale–type fossilization is controversial, particularly that of the clay mineral kaolinite. Kaolinite may have formed on carcasses or attached to them as they decayed, stabilizing organic matter. Alternatively, kaolinite may have formed during metamorphism, playing no role in the preservation of soft tissues. Evaluating the formation and taphonomic role of kaolinite is difficult, because the mineralogy of Burgess Shale–type fossils is incompletely known. We used in situ selected-area X-ray diffraction to constrain the mineralogy of fossils from the classic Burgess Shale Formation in British Columbia, Canada. Fossils can be distinguished from the matrix that surrounds them by the presence of dolomite, kaolinite, and pyrite. Chlorite may be more abundant in the matrix. The preferential survival of kaolinite in association with fossils provides evidence of early diagenetic clay-organic interactions that protected the clay from metamorphic transformation. Kaolinite likely played a crucial role in fossilization, inhibiting the growth of heterotrophic bacteria and aiding polymerization of soft tissue biomolecules. This may result in biases in soft-tissue preservation to areas and times where kaolinite was prevalent.

Microcosmic gas adsorption mechanism on clay-organic nanocomposites in a marine shale

Marine sourced clay minerals and organic matter enrichment as clay-organic nanocomposites within shale has attracted attention because of their changeable surface areas, nanoporosity, and methane adsorbability. Here marine shale samples were used to investigate the characteristics of clay-organic nanocomposites and their impact on gas adsorption response. The results show that organic nanolayers mainly occur as nanometer intercalations with clay nanolayers. High-resolution scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images provide direct evidence of the parallel aligned clay-organic nanocomposites with an approximate thickness of each organic and clay nanolayer and the positive correlations between their morphology, occurrence, and relative content. However, such clay-organic interactions are significantly damaged by structural deformation, accompanied by a weakening in gas adsorption capacity. We suggest that the naturally occurring structures that hydrocarbon molecules are attached or resided in fine-grained marine shales are predominantly associated with clay-organic nanocomposites, which play an important role as adsorption carriers. Moreover, the clay-organic association may be an effective mechanism of gas adsorption. These observations and investigations could challenge the common perception that discrete kerogen and clay minerals are the major components for gas adsorption.

Specific polarizability of sand-clay mixtures with varying ethanol concentration.

We utilise a concept of specific polarizability (c s ), represented as the ratio of mineral-fluid interface polarization per pore-normalised surface area S p , to demonstrate the influence of clay-organic interaction on complex conductivity measurements. Complex conductivity measurements were performed on kaolinite- and illite-sand mixtures as a function of varying ethanol (EtOH) concentration (10% and 20% v/v). The specific surface area of each clay type and Ottawa sand was determined by nitrogen-gas-adsorption Brunauer-Emmett-Teller method. We also calculated the porosity and saturation of each mixture based on weight loss of dried samples. Debye decomposition, a phenomenological model, was applied to the complex conductivity data to determine normalised chargeability (m n ). Specific polarizability estimates from previous complex conductivity measurements for bentonite-sand mixtures were compared with our dataset. The c s for all sand-clay mixtures decreased as the EtOH concentration increased from 0% to 10% to 20% v/v. We observe similar c s responses to EtOH concentration for all sand-clay mixtures. Analysis of variance with a level of significance α = 0.05 suggests that the suppression in c s responses with increasing EtOH concentration was statistically significant for all sand-clay mixtures. On the other hand, real conductivity showed only 10% to 20% v/v changes with increasing EtOH concentration. The c s estimates reflect the sensitivity of complex conductivity measurements to alteration in surface chemistry at available surface adsorption sites for different clay types, likely resulting from ion exchange at the clay surface and associated with kinetic reactions in the electrical double layer of the clay-water-EtOH media. Our results indicate a much larger influence of specific surface area and ethanol concentration on clay-driven polarization relative to changes in clay mineralogy.

Probing the nanoscale architecture of clay minerals

AbstractIn recent years, experimental and theoretical methods have provided new insights into the size, shape, reactivity, and stability of clay minerals. Although diverse and complex, the surface chemistry of all clay minerals is defined spatially on a common scale of nanometres. This review is organized around the nanoscale architecture of clay minerals examined at several different length scales. The first, and perhaps most important, is the length scale associated with Hbonding in clay minerals. Hbonding interactions define the size and shape of 1:1 phyllosilicates and dominate the surface chemistry of many clay minerals. Structural and surface OHgroups contained within and on the surface of clay minerals provide a type of ‘molecular reporter group’ and are sensitive to subtle changes in their local environment. Examples of OH-reporter group studies in clay minerals, and the spatial scales at which they provide diagnostic information, are examined. The second length scale considered here is that associated with clay–water and clay–organic interactions. Inorganic and organic solutes can be used to explore the surface chemistry of clay minerals. Similar to the use of reporter groups, molecular probes have diagnostic properties that are sensitive to changes in their molecular environment. Clay–water interactions occur at a length scale that extends from the size of the H2O molecule (~0.3 nm) to the larger scales associated with clay-swelling (>10 nm). Similarly, clay–organic interactions are also defined, in part, on the basis of their molecular size, in addition to the type of chemical bonding interactions that take place between the organic solute and the clay surface. Examples illustrating the use of clay–water and clay–organic solute interactions as molecular probes are presented. The largest scale to be considered is that of the particles themselves, with scales that approach micrometres. Recent developments in the synthesis and characterization of ultrathin hybrid films of clay minerals provide complementary information about the nature and distribution of active sites on clay minerals, as well as providing new opportunities to exploit the surface chemistry of clay minerals in the design of functional materials.

Molecular simulations to estimate thermodynamics for adsorption of polar organic solutes to montmorillonite

SummaryIdeally, one could use molecular mechanics or quantum mechanics to predict the magnitude of organic solute adsorption from water to soil minerals. Reproduction and/or prediction of mineral and interfacial structures remains challenging, but calculation of meaningful energy relations through computational chemistry techniques is even more difficult than structural calculations. This paper attempts to define the necessary and relevant components for an overall scheme that allows translation of computed interaction enthalpies to experimental adsorption enthalpies and vice versa. While the scheme could be applied to quantum calculations, we test it for the possibility of using empirical molecular mechanics to estimate relative energies for the adsorption of non‐ionic organic solutes in clay mineral‐water‐solute systems. We used molecular dynamics’ simulations to estimate relative clay–organic interaction enthalpies for a series of nitro‐aromatic solutes and hydrated, K‐saturated montmorillonite, for comparison with experimental adsorption isotherm data for the same clay‐nitroaromatic systems. The trend of computed interaction enthalpies (e.g. −234 ± 17 kJ mol−1 for trinitrobenzene and −154 ± 16 kJ mol−1 for p‐nitrobenzene) agreed modestly well with the trend of adsorption maxima from the experiments. Furthermore, we developed several variants on a thermodynamic cycle framework for comparing computed interaction energies with experimentally determined adsorption enthalpies. The algorithms, which include estimates for enthalpy changes both in bulk solution and in the clay interlayer, show promise: for p‐dinitrobenzene and for 1,3,5‐trinitrobenzene, the overall predicted adsorption enthalpies (e.g. −13 ± 22 and −67 ± 23 kJ mol−1, respectively) were in modest agreement with experiments (−18 ± 1 and −28 ± 4 kJ mol−1, respectively). We discuss shortcomings of the methods, in hopes of encouraging better estimates for the various energy terms, improvement of the algorithms, and more valid comparisons between quantum mechanical or molecular mechanical interaction energies and experimental enthalpies.

Thermodynamics of organic cation exchange selectivity in smectites

AbstractThe selectivities of clay minerals for larger organic cations over smaller ones have been attributed to favorable clay-organic interactions in clay interlayers and to hydrophobic effects resulting from (partial) dehydration of organic cations in the clay interlayers, but the magnitudes of these energy components have not been estimated. The objective of this study was to differentiate and quantify the contributions of clay-phase and aqueous-phase energy changes to the overall thermodynamics of cation exchange, and thereby to determine which forces control the general selectivity of smectites for organic cations. We compiled literature measurements and estimates for the free energies of overall cation exchange reactions and also for the free energies of organic cation hydration. Our study suggests that organic cation-exchange thermodynamics can be broken into three classes: (1) For two organic cations with identical head-groups, the difference in their cation exchange selectivities is driven almost quantitatively by the difference in their free energies of hydration. Here, the mechanism for organic cation selectivity is almost pure hydrophobic expulsion of the larger cation from water. The clay interlayer simply behaves like a subaqueous phase into which the least hydrophilic organic cations partition and the essentials of such cation exchange selectivity can be explained without any favorable clay-organic interactions. (2) For two organic cations with rather different head-groups, the difference in their cation exchange selectivities is just a small percentage of the difference in their free energies of hydration. This indicates that the clay phase interacts much more strongly with the cation having the smaller head-group, as might be expected on the basis of simple electrostatics. Here, the clay has an intrinsic strong preference for the cation with smaller head-group yet ‘selects’ for the cation with larger head-group because the aqueous-phase preference for the cation with smaller head-group is even stronger than the clay preference. (3) When the clay is already substantially loaded with organic cations, then van der Waals forces apparently can play a significant role in determining organic cation exchange selectivity differences.

Interlayer Expansion and Mechanisms of Anion Sorption of Na-montmorillonite Modified by Cetylpyridinium Chloride: A Monte Carlo Study

To study the change of interlayer structure of a Wyoming-type Na-montmorillonite as a result of the replacement of interlayer Na+ ions by cetylpyridinium (CP+) ions, a series of NPT Monte Carlo simulations of the clay mineral with different contents of CP+, Na+, Cl- ions and water in its interlayer space is carried out. In agreement with conclusions from experimental studies, the simulations show that the CP+ ions form monomolecular, bimolecular, and pseudotrimolecular layers with increasing interlayer contents. Calculated potential energies reveal that clay-organic interactions are stronger than organic-organic interactions in CP+-modified montmorillonite, which is in conformity with observations of earlier thermogravimetric experiments. The simulation results indicate that the pseudotrimolecular arrangement of CP+ ions is a prerequisite for the experimentally observed interlayer sorption of inorganic anions. Furthermore, in the interlayer space with a pseudotrimolecular layer, chloride ions favor the formation of pairs with inorganic rather than organic cations. On the basis of these findings and available experimental data, we propose that the interlayer sorption of inorganic anions from the pore space of an organically modified montmorillonite may occur not only in pairs with organic cations, as suggested earlier, but also in pairs with inorganic cations, which represents a so-far unconsidered and maybe more important mechanism of anion sorption on clay minerals.

Quantification of Soil Aggregates Treated with Bituminous Emulsion and Other Organic Amendments by Using Scanning Electron Microscope

ABSTRACT Several organic amendments have been used for recovering soil physical properties that become worse when losses in soil organic matter take place. Bituminous emulsion has proved to be quite effective for improving aggregates, but the mechanisms involved are still unknown. This research aims to extend current knowledge about bituminous emulsions by using scanning electron microscopy in the quantification of aggregates. Two different soils (clay and sandy-loamy) were amended with emulsion bituminous and other organic amendments. Total and partial numbers (different-size fractions) of aggregates after 60-days of treatment, obtained by scanning electronic microscopy (SEM), were compared to aggregates percentages obtained by analytical methods. Water-soluble and EDTA-extractable fractions of the elements most likely involved in the aggregation processes were also determined. Better results in aggregate stability were obtained in the clay soil amended, which is probably due to a greater clay-organic matter interaction in this soil. While treatments having bituminous emulsion increased aggregate size and water-stability in both soils, treatments having organic matter caused formation of small low-stable aggregates in sandy soil and formation of stable aggregates in clay soil as showed the number of large aggregates, mean diameter, and total aggregated area. Bituminous emulsion seems to exert two effects in both soils: an hydrophobic effect which increases aggregate stability, and a colligating effect, favored by the clay content, which increases aggregate size. Organic matter seems to act by forming new aggregates from silt + clay particles, which are joined together by cation bridges.

Organo-Clay Complexes and Interactions, edited by S. Yariv and H. Cross. Marcel Dekker New York, (2002), viii + 688 pages. [ISBN 0 8247 0586 6]. Price $195.

This book contains 11 chapters, four of which are written or co-written by Shmuel Yariv. Despite the number of pages, it does not offer a comprehensive review and the editors acknowledge this in the preface before expressing the intention to remedy this in a second volume. From my own perspective the major omissions are in the areas of kaolin-organic interactions and the use of organoclays as adsorbents and barrier materials. However, given the enormous number of publications that have emerged in recent years in both these areas, it is perhaps understandable. Moreover, given the amount of (sometimes repetitious) attention to alkylammonium-exchanged clays in this volume it is surprising that little mention is made of their importance in clay-based nanocomposites. The book opens with a chapter devoted to the ‘Structure and Surface Acidity of Clay Minerals’ by S. Yariv and K.H. Michaelian (35 pages, 44 references). Twenty pages are devoted to the structure of 1:1 and 2:1 layer silicates and are used as a prelude to a well-structured discussion of the water present in the gallery and how it is influenced by the type of exchangeable cation and the locus of isomorphous substitution. There is also a discussion of the surface acidity/basicity of clays and how it is affected by the occupancy of the octahedral sheet. The second chapter ‘Introduction to Organo-Clay Complexes and Interactions’ by S. Yariv (72 pages, 188 references) positions clay-organic interactions with respect to their role in fixation and/or transport of organic matter in the environment, industry and …

Effects of iron oxidation states on the surface and structural properties of smectites

Abstract The oxidation state of iron (Fe) in the crystal structure of smectite clay minerals profoundly alters their physical-chemical properties. Among the properties affected are layer charge, cation exchange and fixation capacity, swelling in water, particle size, specific surface area, layer stacking order, magnetic exchange interactions, octahedral site occupancy, surface acidity, and reduction potential. Also affected is the surface chemistry of the clay, which alters clay–water and clay–organic interaction mechanisms. Rates and extents of degradation of pesticides are increased in the presence of reduced smectites compared to oxidized and reduced-reoxidized counterparts. A hypothesis regarding the mechanism for Fe reduction in clay minerals was first developed in 1963, and subsequent modifications have been proposed periodically through the present time. Recent studies clearly reveal that the process of Fe reduction involves more than the mere transfer of an electron to octahedral Fe(III) in the clay crystal. Ancillary reactions occur that produce significant structural modifications, some of which are reversible and others of which are not. Such changes in the crystal-chemical environment of structural Fe are thought to play a dominant role in altering the clay surface chemistry.

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.

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.

FT-IR and FT-Raman spectroscopic investigations of adsorption of 2,2′-biquinoline by smectite group clay minerals from Anatolia

The sorption of 2,2′-biquinoline onto natural and ion-exchanged montmorillonite (Fe-, Co- or Cu-montmorillonite) and saponite from Anatolia have been investigated using FT-IR and FT-Raman Spectrometry. The intercalation of 2,2′-biquinoline within natural and ion-exchanged montmorillonite and saponite has been shown by X-ray diffraction to increase the interlayer spacing. The difference of basal spacing of air-dried clays and biquinoline treated ones shows monolayer arrangement. Raman spectroscopy was particularly useful for investigation of clay-organic interaction. Vibrational spectroscopy indicates that intercalated 2,2′-biquinoline molecules are coordinated to exchangeable cations (directly and indirectly through water bridges) and/or Lewis acid sites or as bidentate ligand.

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.

Secular distribution of Burgess‐Shale‐type preservation

Burgess-Shale-type preservation is defined as a taphonomic pathway involving the exceptional organic preservation of non-mineralizing organisms in fully marine siliciclastic sediments. In the Phanerozoic it occurs widely in Lower and Middle Cambrian sequences but subsequently disappears as a significant taphonomic mode. The hypothesis that this distribution derives solely from a secular increase in the depth of bioturbation is falsified: low bioturbation indices do not prevent the rapid enzymatic degradation of organic structure, nor do they account for the conspicuous absence of comparable preservation during the Vendian. An earlier, Late Riphean (ca. 750–850 Ma), interval of enhanced organic-walled fossil preservation suggests a long-term recurrence in Burgess-Shale-type taphonomy that is independent of metazoan activity. A model based on the potentially powerful anti-enzymatic and/or stabilizing effects of clay minerals on organic molecules is proposed to account for Burgess-Shale-type preservation. Long-term changes in average clay mineralogies and the ocean chemistry that determines their interaction with organic molecules are likely to have induced the pronounced secular distribution of these fossil biotas, while regional variations in tectonism, weathering, etc., explain their non-uniform geographic distribution; the close correlation between exceptional, organic-walled fossil preservation and volcano-genic sedimentation in Tertiary lake deposits provides a compelling analogue. Recognition of a temporal control on Burgess-Shale-type preservation constrains the evolutionary scenarios that can be drawn from such biotas; significantly, neither the initial rate of appearance, nor the ultimate fate of Burgess-Shale-type taxa can be directly assessed. □Taphonomy, exceptional preservation, organic preservation, fossil Lagerstatten, Burgess Shale, clay mineralogy, clay-organic interactions, secular change, Cambrian, Proterozoic.

Controlling the destructive effects of clay-organic liquid interactions by application of effective stresses

Controlling the destructive effects of clay-organic liquid interactions by application of effective stresses

Organic preservation of non-mineralizing organisms and the taphonomy of the Burgess Shale

Organic preservation of non-mineralizing animals constitutes an important part of the paleontological record, yet the processes involved have not been investigated in detail. Organic-walled fossils are generally explicable as a coincidence of original, relatively recalcitrant, extra-cellular materials and more or less anti-biotic depositional circumstances. One of the most pervasive natural inhibitors of biodegradation results from substrate and enzyme adsorption onto, and within, clay minerals; such interactions are likely responsible for many of the organic-walled fossils preserved in clastic sediments. Close examination of the fossilLagerstätteof the Burgess Shale (Middle Cambrian, British Columbia) reveals that most of its so-called soft-bodied fossils are composed of primary (although kerogenized) organic carbon. Their preservation can be attributed to pervasive clay-organic interactions as the organisms were transported in a moving sediment cloud and buried with all cavities and spaces permeated with fine grained clays. The organic-walled Burgess Shale fossils were studied both in petrographic thin section and isolated from the rock matrix, following careful acid maceration. Isotopic analysis of bulk organic and carbonate carbon yielded values consistent with a normal marine paleoenvironment. Anatomical and histological consideration of the enigmatic Burgess wormAmiskwiasuggest that it may in fact be a chaetognath, while the putative chordatePikaiaappears not to be related to modern cephalochordates.

Luminescence in coal and its relation to clay minerals

AbstractRoom-temperature dehydration of aqueous coal slurries, coal extraction residue, or coal ash results in spontaneous luminescence. The phenomenon is characterized by a monotonic decay in photon activity which precedes a sharp rise in photon release. A maximum is reached within several minutes but subsequent decay is slower and photon emission continues for an extended period of time. The magnitude of photon activity is severely limited by the thickness of the strongly absorbing coal slurry. Prior removal of mineral matter from the coal appears to curtail the dehydration-induced phenomena, suggesting that clays or clay-organic interactions are responsible. Data also suggest that air is required as a medium for discharging the excited species.