Showing papers in "Clays and Clay Minerals in 1988"

Pentachlorophenol sorption by organo-clays

Abstract: Several clay organic complexes were prepared by placing organic cations on the exchange sites of smectite clays and studied as sorbents for pentachlorophenol (PCP) The organic cations used ranged from very hydrophobic in nature (eg, dioctadecyldimethyl+ (DODMA+)- and hexadecyltrimethyl+ (HDTMA+)-ammonium) to those having minimal hydrophobic properties, such as tetramethylammonium+ (TMA+) In general, the more hydrophobic the cation on the smectite the greater the uptake of PCP from water For the very hydrophobic clays (DODMA+- and HDTMA+-smectite) the uptake of PCP was via non-polar interactions between the alkyl (eg, -C18) groups on the organic cation and PCP In a mechanistic sense, this interaction appeared to be similar to a partitioning process between water and the organic phase of the clay-organic complex The organic phases of DODMA+-smectite were about 10 times more effective than the organic matter of natural sediments for removing PCP from water For those organo-clays containing small organic cations (eg, TMA+), the organic phase consisted of separate small organic moieties, such as the methyl group This phase did not act as an effective partitioning medium despite a significant carbon content, and very little PCP was taken up Results from this study suggest the possibility of treating soils and subsurface materials with large hydrophobic organic cations to enhance the sorptive properties of these natural materials

Visible spectroscopy of methylene blue on hectorite, laponite B, and barasym in aqueous suspension

Abstract: The absorption spectra of methylene blue ion exchanged on hectorite, Laponite B, Barasym, or sepiolite in dilute aqueous suspensions show the presence of the monomer, the protonated monomer, the dimer, and the trimer. Due to conformational differences, the absorption band maximum of the monomer with respect to its maximum in aqueous solution is red shifted when it is adsorbed on the external surface and blue shifted when it is adsorbed on the interlamellar surface. The availability of the interlamellar surface for methylene blue as a function of the type of clay and/or the counterion present thereby can be probed. The results indicate that 0.6-0.7% of the cation-exchange capacity of Barasym consists of acid sites capable of protonating methylene blue. Counterions of low hydration energy were found to induce a small number of similar sites in hectorite and Laponite B; hence, these sites must be situated on the external surface. Dimers formed on external surfaces show one absorption band. Dimers formed on the interlamellar surface of hectofite yield spectra having two absorption bands. The trimer was formed only at the external surface. With increasing loading of the clays, the spectra of methylene blue showed metachromasy. The metachromatic behavior can be fully explained by dye aggregation, which is the result of its concentration on the surface. No 7r-electron interaction with the surface oxygens need be invoked.

New Data and a Revised Structural Model for Ferrihydrite

Abstract: Synthetic 2-line and 6-line ferrihydrite samples prepared from ferric nitrate solutions have the bulk compositions Fe4(O,OH,H2O)12 and Fe4,6(O,OH,H2O)12, respectively. The composition depends on crystal size, which averages 20 A for 2-line and 35 A for 6-line ferrihydrite. X-ray absorption edge spectra indicate the presence of tetrahedral Fe3+, a conclusion supported by heating experiments which show the development of maghemite after heating to 300°C in the presence of N2, followed by the formation of hematite at higher temperatures. These two reactions are recorded on differential thermal analysis traces by exotherms at 350° and 450°C. Transmission electron microscopy shows that 2-line ferrihydrite has no Z-axis regularity but does show hexagonal 2.54-A lattice fringes. Six-line ferrihydrite forms faceted crystals having 9.4-A c-parameter only detectable in dark field. In bright field, 2.54-A lattice fringes indicate greater atomic regularity than in 2-line ferrihydrite. Analysis of the X-ray powder diffraction pattern of 6-line ferrihydrite suggests a structure based on double-hexagonal close-packed oxygens, containing 36% Fe in tetrahedral sites. Selective chemical dissolution, surface area measurements, and magnetic susceptibility are consistent with the recorded properties of ferrihydrite.

Transmission electron microscope study of biotite weathering

Abstract: Transmission electron microscopy suggests that biotite transforms to vermiculite primarily by direct structural modification, involving the replacement of K+ by hydrated interlayer cations, and only minor reorganization of the 2:1 layer. A second relatively uncommon mechanism appears to involve redistribution of components from two biotite sheets to form a single vermiculite layer. Distortion of the surrounding structure initially inhibits growth of vermiculite in the surrounding biotite, and promotes the propagation of vermiculite layers in opposite directions. This phenomenon may contribute to the development of relatively regular, widely spaced interstratifications of biotite and vermiculite. Additional components and space are provided by the dissolution of biotite where access of solutions is greater. During weathering, biotite and vermiculite become increasingly replaced by kaolinite, which crystallizes epitactically onto existing layers, and goethite, which develops from a poorly crystalline iron oxyhydroxide precursor to form oriented laths. In areas parts of strongly weathered samples kaolinite and goethite appear to develop in proportions consistent with a reaction that conserves both Al and Fe.

Analysis and Implications of the Edge Structure of Dioctahedral Phyllosilicates

Abstract: Crystal growth theory was applied to describe edge sites of phyllosilicates. Three face config- urations were found to exist. One face has one tetrahedral site per tetrahedral sheet and two octahedral one-coordinated sites per crystallographic area ac sin/3, where a and c are layer dimensions and/3 is the angle between them. The other two faces are similar except that they have one less octahedral site which is replaced by one Si'v-O-A1 vt site in this same ac sin/3 area. A transfer of bonding energy from the remaining octahedral site to the Si'v-O-A1 w site is believed to neutralize all edge charge on faces containing these latter sites at normally encountered pHs (pH 3-9). A similar charge rearrangement along the edges results in an apparent decrease in the permanent charge of the mineral with an increase in edge area. On the basis of such an analysis, lath-shaped illite can be described as a very fine grained dioctahedral mica in which the apparent deficient occupancy of the octahedral sheet, presence of excess water, and measurable cation-exchange capacity may in part be the result of a large ratio of edge area to total volume, with no other chemical or structural change in the mica layers. The increasing importance of edge charge relative to layer charge produces erroneous formulae for 2:1 phyllosilicates in very fine grained samples containing fewer than 2 of 3 octahedral sites occupied by cations, on the basis of a 22-charge half cell.

Chromate adsorption by kaolinite

Abstract: Chromate (CrO4 2-) adsorption was investigated on kaolinite (0.2-2 ~m) saturated with NaC104 over a range of pH. Adsorption increased with decreasing pH because of protonation of chromate and/ or variable charge sites on kaolinite. Chemical pretreatment to remove noncrystalline and crystalline oxide contaminants affected the magnitude ofCrO42- adsorption, but not the pH range over which CrO42- adsorbed. Chromate adsorption at different sorbate and sorbent concentrations increased below the pHz~ for the kaolinite edge, suggesting the formation of weak surface complexes. If frO4 2- and SO42- were present at equal concentration (5.0 x 10 -7 M), the two solutes sorbed independently, suggesting binding to separate sites. The presence of excess SO42- (5.0 x 10 -4 M), however, unexplainably enhanced CrO42- adsorption. The adsorption of both chromate and sulfate can be described in terms of a site-binding model of the kaolinite edge, in which the edge is viewed as composite layers of AI and Si oxide. Surface complexation constants for CrO4 2- on kaolinite were similar to those for alumina, pointing to the im- portance of A1-OH edge sites in chromate adsorption.

Verification of the triclinic crystal structure of kaolinite

Abstract: On the basis of neutron diffraction studies, the two inner-hydroxyl ions in highly ordered kaolinite were recently shown to be differently oriented. One of the inner-hydroxyl ions points generally toward a hole in the octahedral sheet and the other toward a hole in the tetrahedral sheet. These orientations and the locations of the other atoms in the primitive triclinic unit cell have now been determined for a sample of Keokuk kaolinite with improved precision compared with that reported earlier. Rietveld structure refinement was carried out for the entire crystal structure simultaneously (99 atom positional and 17 other parameters) with each of two newly collected sets of high-resolution neutron powder dif- fraction data. The different orientations of the inner-hydroxyl ions are the most marked evidence that the unit cell is not C centered. The positions of the inner-surface hydrogen atoms provide further evidence in that all differ from a C-centered relationship by six to eight estimated standard deviations in their y coordinates. The cell is, therefore, not centered. The space group is P1.

X-ray diffraction identification of the polytypes of mica, serpentine, and chlorite

Abstract: Six standard polytypes of micas exist that have c-periodicities between 1 and 6 layers; these can be subdivided into groups A and B according to whether the octahedral cations are in the same set of positions in every layer or alternate regularly between I and II possible sets of positions (alternate slant directions), respectively. Correspondingly, 12 standard polytypes exist for 1:1 layers of the serpentine-type structures, which are divisible into four groups A-D. Groups A and B differ as in the micas, as do groups C and D. Groups A and B have ±a/3 interlayer shifts, whereas groups C and D have zero or ±b/3 interlayer shifts. Six structural types of semi-randomly or regularly stacked 1-layer chlorites exist that differ in the slant of the interlayer sheet (I or II) and its position (a or b) relative to 6-member rings in adjacent 2:1 layers. Criteria have been established that permit the X-ray diffraction identification of the above-mentioned groups as well as the individual polytypes within each group. These criteria involve inspection of the strong X-ray diffraction reflections of index k = 3n for identification of the groups and of the generally weaker k ≠ 3n reflections for identification of the polytypes (assuming indexing on orthohexagonal axes throughout for convenience). In all hydrous phyllosilicates the octahedral cations and anions repeat at intervals of b/3 and thus contribute strongly to X-ray diffraction reflections of index k = 3n. The intensities of these reflections identify the two mica groups, the four serpentine-type groups, and the six chlorite groups. The periodicities of these reflections along Z* is that between identical octahedral sheets. The basal oxygen atoms do not repeat at intervals of b/3 and are a primary contributor to the intensities of k ≠ 3n reflections. The period along Z* for these reflections is that between identical basal oxygen planes, and the periodicity plus the symmetry identify the individual trioctahedral polytypes. For dioctahedral polytypes of the kaolin-group minerals and chlorites, the position of the vacant octahedral site must be considered also. These general principles can be illustrated especially well by single-crystal precession photographs and extrapolated to powder photographs.

Model for competitive adsorption of organic cations on clays

Abstract: The adsorption on montmorillonite of two monovalent organic cations, methylene blue (MB) and thioflavin T (TFT), was studied in four different situations: (1) separate adsorption of MB or TFT; (2) competitive adsorption of TFT and Cs; (3) competitive adsorption of the two organic cations from their equimolar solutions; and (4) adsorption of TFT on a clay whose cation-exchange capacity (CEC) had been previously saturated with MB. MB and TFF adsorbed to as much as 120% and 140% of the CEC, respectively. Cs did not appear to compete with TFT for the adsorption sites of the clay. TFT molecules adsorbed more strongly than those of MB and displaced them from the clay surface. A model was developed to evaluate the strength of the clay-organic cation interactions. The specific binding of the cations to the negatively charged surface, determined by solving the electrostatic equations, appears to account for adsorption exceeding the CEC and formation of positively charged complexes, which are due to non-coulombic interactions between the organic ligands. The charge reversal predicted by the model beyond the CEC of the clay was confirmed by microelec- trophoretic experiments. Particles in a sample of montmorillonite loaded with 50 meq TFT/100 g clay moved to the positive electrode, whereas in samples containing the two dyes, MB and TFT, coadsorbed at a total concentration of 100-120 meq/100 g clay, the particles moved to the negative electrode. Binding coefficients describing the formation of neutral and charged complexes of TFT and the clay were larger than those for MB and the clay, thereby explaining the preferential adsorption of TFT observed exper- imentally. The binding coefficients for the formation of neutral complexes of either MB and TFT and the clay were more than six orders of magnitude larger than those previously reported for inorganic

Modified clays for the adsorption of environmental toxicants; binding of chlorophenols to pillared, delaminated, and hydroxy-interlayered smectites

Abstract: Due to their unique polarity, pore-size distribution, and high surface areas, pillared and de- laminated clays are potentially useful materials for the adsorption of environmenlal toxicants. To deter- mine their properties for adsorption of chlorinated phenols, alumina-pillared montmorillonite (APM), chromia-pillared montmorillonite (CPM), and alumina-delaminated Laponite (ADL) were reacted with aqueous pentachlorophenol (PCP) solutions in batch equilibrium experiments. An hydroxy-A1 Laponite (HAL) in which the Na + exchange ions were replaced by ions of the type AlI304(OH)(24+x)(H20)(I2 .)(7- was included in the study. With ADL as the adsorbent, the extent of PCP adsorption increased with decreasing pH, and then became constant at pH _< pKa. Thus, the neutral phenol was preferred over the phenolate form. Binding of neutral PCP at pH 4.7 to all adsorbents never reached saturation values, and the loadings achieved were limited by the water solubility of the adsorbate. Among the pillared and delaminated clays investigated, ADL exhibited the largest capacity for physical adsorption of PCP at pH 4.7. Differences in the PCP binding capacities for APM, CPM, and ADL suggested that adsorption was dependent on the pore structure and surface composition of the modified clay adsorbent, not on surface area alone. HAL exhibited quantitative uptake of PCP at the 8 #mole/g level, indicating that a chemi- sorption mechanism may operate for PCP binding to this adsorbent. Adsorption of 3-chlorophenol, 3,5- dichlorophenol, and 3,4,5-trichlorophenol by ADL at pH 7.4 increased as the degree of hydrophobicity and chlorination of the phenol increased; hence, the binding capacity was not limited by the molecular size of the adsorbate. In contrast to the adsorption properties observed for pillared, delaminated, and hydroxy-interlayered clays, Na+-montmoriUonite and Na+-Laponite did not adsorb PCP from aqueous solution.

Transformation of Trace Element-Substituted Maghemite to Hematite

Abstract: X-ray powder diffraction (XRD), transmission electron microscopy, infrared spectroscopy, differential thermal analysis, and surface area (BET) measurements were employed to investigate the transformation of microcrystalline maghemite to hematite. At 500°C pure maghemite was completely altered to hematite in 3 hr, whereas maghemites containing small amounts (

Near-infrared reflectance spectra of mixtures of kaolin-group minerals: use in clay mineral studies

Abstract: Near-infrared (NIR) reflectance spectra for mixtures of ordered kaolinite and ordered dickite have been found to simulate the spectral response of disordered kaolinite. The amount of octahedral vacancy disorder in nine disordered kaolinite samples was estimated by comparing the sample spectra to the spectra of reference mixtures. The resulting estimates are consistent with previously published estimates of vacancy disorder for similar kaolin minerals that were modeled from calculated X-ray diffraction patterns. The ordered kaolinite and dickite samples used in the reference mixtures were carefully selected to avoid undesirable particle size effects that could bias the spectral results. NIR spectra were also recorded for laboratory mixtures of ordered kaolinite and halloysite to assess whether the spectra could be potentially useful for determining mineral proportions in natural physical mixtures of these two clays. Although the kaolinite-halloysite proportions could only be roughly estimated from the mixture spectra, the halloysite component was evident even when halloysite was present in only minor amounts. A similar approach using NIR spectra for laboratory mixtures may have applications in other studies of natural clay mixtures.

Evidence for the formation of interlayer polyacrylonitrile in kaolinite

Abstract: A kaolinite-polymer intercalation complex was apparently formed for the first time by the polymerization of acrylonitrile between the kaolinite layers. A kaolinite-ammonium acetate intercalation complex was dispersed in acrylonitrile monomer. The monomer was apparently incorporated between the layers by displacing intercalated ammonium acetate. After the removal of excess monomer, the intercalation complex was heated to cause polymerization. The resulting kaolinite-polyacrylonitrile (PAN) intercalate showed a basal spacing of ∼ 13–14 A. On heating the complex at 220°C for 1 hr in air, the spacing decreased slightly. The hydrogen bond between the hydroxyls of kaolinite and probably the C≡N group of PAN was not affected after heating at 220°C. Even after heating at 400°C, the layers expanded. Because the starting kaolinite-ammonium acetate intercalation complex decomposed at a much lower temperature, these observations strongly suggest the presence of PAN between the layers.

Hypothetical structures of magadiite and sodium octosilicate and structural relationships between the layered alkali metal silicates and the mordenite- and pentasil-group zeolites

Abstract: Hypothetical model structures for magadiite and sodium octosilicate, based on the structure of the zeolite dachiardite, are proposed that consist of layers of 6-member tings of tetrahedra and blocks containing 5-member rings attached to both sides of the layers. The infrared (IR) and nuclear magnetic resonance spectra of magadiite and sodium octosilicate have features in common with spectra of zeolites in the ZSM-5 and mordenite groups. A peak at 1225 cm l in the IR spectra of magadiite and sodium octosilicate is characteristic of zeolites containing 5-member rings, such as ZSM-5- and mordenite-type zeolites. The defect structures of pentasil zeolites may therefore be akin to layered alkali metal silicates containing zeolite-like domains, in which part of the silanol groups from adjacent silicate layers are condensed (cross-linked) forming siloxane linkages.

Modification of the porous structure and surface area of sepiolite under vacuum thermal treatment

Abstract: Modifications of the external surface area and the two types of microporosity of sepiolite (structural microporosity and inter-fiber porosity) were examined as a function of the temperature of a vacuum thermal treatment to 500°C. The methods used included: reciprocal thermal analysis, N2 and Ar low-temperature adsorption microcalorimetry, gas adsorption volumetry (for N2, Ar, and Kr at 77 K and CO2 at 273 and 293 K), water-vapor adsorption gravimetry, and immersion microcalorimetry into liquid water at 303 K. If the sample was not heated >100°C, only 20% of the structural microporosity was available to N2, whereas 52% was available to CO2 at 293 K. In both experiments, the channels filled at very low relative pressures. At >350°C, the structure transformed to anhydrous sepiolite, which showed no structural microporosity. The inter-fiber microporosity decreased from 0.031 to 0.025 cmVg (as seen with N2), and the external specific surface area decreased from 120 to 48 m2/g. The water adsorption isotherms showed a lower and lower affinity of the external surface of fibers for water as the temperature of thermal treatment increased. The thickness of the bound water on the external surface was estimated to be ≤ 3.5 monolayers, i.e., less than 10 A.

Analytical electron microscopy and the problem of potassium diffusion

Abstract: Diffusion of K during analytical electron microscopy (AEM) results in anomalously low count rates for this element. As the analysis area and specimen thickness decrease, count rates become dispro- portionally lower. Adularia and muscovite show different diffusion profiles during AEM; for muscovite a strong dependence of diffusion on crystallographic orientation has been observed. Conditions giving rise to reliable chemical data by AEM are the use of a wide scanning area (>800 x 800 A) and/or large beam size to reduce the effect of diffusion of alkali elements, a specimen thickness greater than about 1000 ~, constant instrument operating conditions, and the use of a homogeneous, well-characterized standard sample. The optimum thickness range was obtained by determining the element intensity ratio vs. thickness curve for given operating conditions. The standard and unknown should have a similar crystal structure and, especially for strongly anisotropic minerals such as phyllosilicates, a similar crys- tallographic orientation with respect to the electron beam.

Preparation of Delaminated Clay Having a Narrow Micropore Distribution in the Presence of Hydroxyaluminum Cations and Polyvinyl Alcohol

Abstract: The pillaring of Na-montmorillonite with cationic oligomers of hydroxyaluminum (COHA) in the presence of an aqueous solution of polyvinyl alcohol resulted in the formation of a clay having a large surface area and pore volume. The pore-size distribution determined from a N2 adsorptiordde- sorption hysteresis was narrow and centered at about 25 ~. The peak width at half height in the distribution curve was < 5 /~. As a result of delamination, the layer structure of the prepared clay was found from X-ray powder diffraction measurements to be lost. Short-range ordering, however, still existed in this delaminated clay, because exchangeable cations in the montmorillonite completely exchanged with A13+, a requisite step for pillaring. The order of adding the starting materials (Na-montmorillonite, polyvinyl alcohol, and COHA) greatly affected the surface area and the pore volume of the delaminated clay. Two orders of addition (Na- montmorillonite, then COHA, then polyvinyl alcohol; and COHA, then Na-montmorillonite, then poly- vinyl alcohol) gave no measurable surface area and pore volume. Two other orders of addition (polyvinyl alcohol, then COHA, then Na-montmoriUonite; and polyvinyl alcohol, then Na-montmorillonite, then COHA) gave surface areas of 107 and 160 mVg and pore volumes of 0.13 and 0.29 cm3/g, respectively. The amounts of the COHA solution and polyvinyl alcohol added greatly influenced the surface area and pore volume of the delaminated clay. Both properties increased monotonically with increasing amount of added polyvinyl alcohol, and increased to a maximum and then decreased with increasing amount of added COHA solution. The maximum surface area of the prepared delaminated clay was 330 m2/g.

Sintering of montomorillonites pillared by hydroxy-aluminum species

Abstract: The sintering of montmorillonites pillared by hydroxy-Al species was investigated by several techniques. The change of the microporosity of the pillared interlayer clay with temperature shows that sintering was mainly due to the loss of microporosity. On calcination of the parent clay at 760°C, the X-ray powder diffraction pattern did not change; the 060 line shifted only slightly from 8.988 to 9.017 A. The infrared (IR) spectrum suggested some dehydroxylation, but the structure appears to have been preserved. On calcination of the pillared clay at >700°C, the intensity of the 001 line decreased steadily, the 060 line broadened, and the b parameter decreased slightly from 8.988 to 8.928 A. Fe3+ ions apparently occupied only one site after calcination at 300°C and two sites after calcination at 700°C. Smaller isomer shifts and higher quadrupole splittings in the Mossbauer spectra suggest that calcination produced more covalent bonding of Fe3+ and a highly distorted structure. The IR spectrum was significantly modified by calcination of the sample, and the 935- and 1125-cm−1 bands disappeared and the 1035-cm−1 band broadened. These results suggest that at 750°C. The pillaring-induced decrease of thermal stability of the clay was likely due to the introduction of protons during the first step of the preparation. The thermal stability of these Al-pillared interlayer complexes is comparable to that reported for hydroxy-silicoaluminum montmorillonites and fluorhectorites.

Distribution of Ca and Na Ions in Dioctahedral Smectites and Interstratified Dioctahedral Mica/Smectites

Abstract: The structures of 11 smectites and 2 interstratified mica/smectites containing mainly Ca 2. and Na § as exchangeable cations in variable ratio were studied under a relative humidity of about 40%. Observed X-ray powder diffraction patterns were compared with diagrams calculated from interstratifi- cation models containing 15.2-~ Ca-smectite layers (C), 12.4-/k Na-smectite layers (N), and 10.0-~ mica layers (M) in various combinations. The smectites appear to be a random interstratification of component layers C and N, and display a tendency towards segregation. One of the interstratified minerals studied is a regular interstratification of C and M layers; the other is a regular interstratification of C, N, and M layers, in which N and C layers are randomly distributed in equal proportion and show a tendency towards segregation.

Components of surface free energy of some clay minerals

Abstract: The wetting contact angle was measured for water drops settled on the surface of pressed discs of kaolinite, alumina, bentonite, marble, montmorillonite, and quartzite immersed in hexane, octane, dodecane, cis-decalin, and air. Minimum and maximum values of the contact angle were obtained for the given systems of solid-water drop-hydrocarbon, depending on the manner of disc preparation. Using both minimum (θmin) and maximum (θmax) values of the contact angle, values of the dispersion component (γsd) of surface free energy of these solids were calculated from the equation which was derived on the basis of an equilibrium state of the system solid-water drop-hycrocarbon for two different hydrocarbons. The values of γsd for kaolinite, alumina, bentonite, marble, montmorillonite, and quartzite obtained from θmin are 83.5, 98.1, 98.9, 80.2, 95.9, and 89.7 mJ/m2, and from θmax are 73.1, 85.0, 84.4, 75.8, 85.5, and 75.5 mJ/m2. These values for marble and quartzite are similar to those in the literature (marble = 67.7 mJ/m2; quartzite = 71.3 and 76.0 mJ/m2). The values of the dispersion components of surface free energy for marble and quartzite covered with a water film (γsfd) were found to be: 41.8, 36.9; 49.2, 42.5; 49.6, 42.2; 40.2, 38.1; 48.1, 42.8; and 44.9, 38.0 mJ/m2, respectively. Values of γsfd for kaolinite, bentonite, and montmorillonite agreed well with those obtained from hydrocarbon adsorption isotherms determined by differential thermal analysis (35.5, 36.5, and 37.4 mJ/m2). Using values of γsfd and contact angles measured in the system solid-water drop-air, the nondispersion component of the surface free energy of solids with adsorbed water film (γsfn) was calculated from the modified Young equation. The values of γsfn for kaolinite and quartzite are as follows: 55.8, 69.0; 85.6, 94.0; 52.1, 75.0; 64.7, 68.9; 54.9,71.3; and 59.2,74.4 mJ/m2. The values of the nondispersion components determined for kaolinite, bentonite, and montmorillonite agreed well with those obtained by differential thermal analysis (67.6, 78.3, and 65.5 mJ/m2, respectively). Further, based on the assumption that the adsorbed water film decreased the surface free energy of these solids by the value of the work of spreading wetting, the nondispersion component (γsn) of the surface free energy of the solids was calculated to be: 86.9,129.6; 169.5, 187.7; 67.1, 144.8; 117.5, 129.3; 83.0, 135.7 and 100.2, 143.4 mJ/m2. These calculated values of the nondispersion component of marble and quartzite surface free energy agree with those obtained from adsorption isotherms determined by chromatographic and differential thermal analysis (marble = 103.8, 106.4; quartzite = 112, 115, 153.6 mJ/m2).

Electron transfer processes between hydroquinone and iron oxides

Abstract: The kinetics of hydroquinone oxidation by aqueous suspensions of pure hematite and goethite- ferrihydrite mixtures at pH 6.0, 7.4, and 9 was studied using an on-line analysis system. The electron transfer between hydroquinone and the Fe oxides was monitored by UV-visible and electron spin reso- nance spectroscopy. The adsorption of organics on the Fe oxide surface was detected by Fourier-transform infrared spectroscopy. For different Fe oxides, a higher surface area was correlated with a greater oxidizing ability and greater adsorption of organics, suggesting that the oxidation reaction was a surface process. A reversal of the initially rapid redox reaction was found in this system, suggesting a delayed release of Fe E§ into solution as the reduction of the Fe oxide proceeded. Redox potential calculations confirmed the thermodynamic favorability of the reaction reversal. A distribution of the reduced state over neigh- boring Fe atoms on the oxide surface probably was responsible for the initial suppression of Fe 2+ release into the aqueous phase. Based upon these observations and detection of the semiquinone radical as an intermediate of hydroquinone oxidation, an inner-sphere one-electron transfer mechanism for the oxi- dation of hydroquinone at the oxide surface is proposed. itored the concentration ofp-benzosemiqui none anion radical, a suggested precursor of humic substances, by electron spin resonance of the solution phase through- out the reaction period. This radical, which is stable only at high pH, was found to persist in the Mn oxide suspensions at pH 6. The presence of the radical sug- gested that the reduction of the Mn oxide involved a one-electron transfer process. In the present work, the oxidation reaction of hydroquinone by Fe oxide is re- ported in the expectation that Fe oxides, while less powerful oxidizing agents than Mn oxides, are more important in a practical sense because of their wide- spread occurrence in soils.

Microstructures, formation mechanisms, and depth-zoning of phyllosilicates in geothermally altered shales, Salton Sea, California

Abstract: Scanning, transmission, and analytical electron microscopy studies of shales from the Salton Sea geothermal field revealed that phyllosilicates progress through zones of illite-muscovite (115°−220°C), chlorite (220°−310°C), and biotite (310°C) These phyllosilicates occur principally as discrete, euhedral to subhedral crystals which partly fill pore space The structural and chemical heterogeneity, which is typical of phyllosilicates in shales subject to diagenesis, is generally absent Textures and microstructures indicate that the mineral progression involves dissolution of detrital phases, mass transport through interconnecting pore space, and direct crystallization of phyllosilicates from solution Phyllosilicate stability relations indicate that either increase in temperature or changing ion concentrations in solutions with depth are capable of explaining the observed mineral zoning Textural and compositional data suggest that the observed mineral assemblages and the interstitial fluids approach equilibrium relative to the original detrital suites The alteration process may have occurred in a single, short-lived, episodic hydrothermal event in which the original detrital phases (smectite, etc) reacted directly to precipitate illite, chlorite, or biotite at different temperatures (depths) without producing intermediate phases

Chemical Weathering of Crystalline Rocks in the Catchment Area of Acidic Ticino Lakes, Switzerland

Abstract: Atmospheric acidic deposition introduces hydrogen ions to terrestrial and aquatic ecosystems, which become partially neutralized by chemical weathering. In the southern Alps of Switzerland, small catchments containing little or no soil and lacking carbonate minerals represent sensitive hydrological settings in which the relationship between alteration of granitic gneiss by acid deposition and the resulting composition of lake waters can be studied. Transmission and scanning electron microscopy, coupled with X-ray powder diffraction of lake sediments from such areas showed mainly unaltered minerals from parent rocks and no secondary silicate minerals. Element mapping indicated noncrystalline aluminum hydroxide as a product of the chemical weathering of silicates. Noncrystalline iron hydroxide was also observed. Mass balance calculations and the stoichiometry of suitable chemical reactions representing the weathering processes were used to derive a plausible reaction sequence on the interaction of the predominant reactive rock minerals with acid precipitation that accounted for the measured chemical composition of the acid lakes.

Relative Solubility of Corundum, Gibbsite, Boehmite, and Diaspore at Standard State Conditions1

Abstract: The solubilities of HCl-treated samples of corundum, gibbsite, boehmite, and diaspore in aqueous solutions at 298 K and one atmosphere pressure were determined from undersaturated and supersaturated initial conditions. Solution characteristics at apparent equilibrium were measured and used to calculate equilibrium constants (Kr) for mineral dissolution reactions: $$\begin{array}{l} A{l_2}{O_3}\left( {corundum} \right) + 3{H^ + } = A{l^3} + 1.5\,{H_2}O,\\ Al{\left( {OH} \right)_3}\left( {gibbsite} \right) + 3{H^ + } = A{l^3} + 3\,{H_2}O,\\ and\\ AlOOH\left( {boehmite\,or\,diaspore} \right) + 3{H^ + } = A{l^3} + 2{H_2}O, \end{array}$$ Assuming solid phase and water activities of unity, log Kr = 3pH - pAl3+ for all three equations. The calculated log Kr values were: 9.01 ± 0.05 (corundum), 7.76 ± 0.14 (gibbsite), 7.49 ± 0.09 (boehmite), and 6.75 ± 0.24 (diaspore), indicating that the relative thermodynamic stabilities under the experimental conditions were: corundum < gibbsite < boehmite < diaspore. The gibbsite value agreed well with that determined independently by another research group using acid-treated subsamples of the same source mineral (7.70 ± 0.02). The calculated Gibbs free energies of formation (kJ/mole) were: -1587.4 ± 2.1 (corundum), -1156.7 ± 1.6 (gibbsite), -921.0 ± 1.5 (boehmite), and -925.1 ± 2.0 (diaspore). The Δ G f o values of the acid-treated minerals were 1.8 to 12.4 kJ/mole more negative than values recently compiled for untreated mineral samples. This result is attributed to the removal of reactive surface coatings that isolated the bulk mineral phases from the solution phase.

Infrared studies of surface acidity and reversible folding in palygorskite

Abstract: The infrared absorption spectra of a palygorskite sample from Caceres, Spain, showed two previously unreported bands in the OH-stretching region at 3420–3440 and 3220–3230 cm−1 after evacuation at 90°–230°C. These bands, which reached maximum intensity after the sample was heated at 150°C, were assigned to OH in the $$\begin{array}{*{20}{c}} H \\: \\ {Si - O - Si\,and\,} \\ \end{array}\begin{array}{*{20}{c}} H \\: \\ {Si - O - A1} \\ \end{array}$$ groups, respectively. To characterize the nature of these OH groups, pyridine was adsorbed on the sample. The resultant spectra suggest that at 150°C the palygorskite folded and OH groups protonated, resulting in the formation of a deformed pyridinium ion between 150° and 290°C. A high concentration of thermally stable Lewis-acid sites on the surface of the palygorskite was also noted.

Transformation of hausmannite into birnessite in alkaline media

Abstract: The transformation of hausmannite (Mn3O4) into a K-bearing, 7-A phyllomanganate (K-birnessite) in KOH was followed using X-ray powder diffraction and transmission electron microscopy. The transformation involved dissolution of Mn3O4 followed by reprecipitation of the 7-A phase. The rate-determining step was the dissolution of Mn3O4. The reaction was accelerated by increasing the pH and/or the temperature of the system. K-birnessite precipitated initially as thin, irregular plates and films that gradually recrystallized to thicker, more structured plates and laths. A pseudohexagonal unit cell with a0 = 2.87 A and c0 = 7.09 A was found for this phase. Synthetic K-birnessite was stable in KOH at 70°C for many months. In neutral to slightly acidic media it converted rapidly to Mn7O13•5H2O, and in more acid media, it dissolved and reprecipitated as γ-MnO2. The replacement of K+ by Na+ was not achieved. Jacobsite and magnetite also underwent a dissolution/reprecipitation transformation in KOH.

Photostabilization of a nitromethylene heterocycle insecticide on the surface of montmorillonite

Abstract: The photochemical stability of the insecticidal compound tetrahydro-2-(nitromethylene)-2H- 1,3-thiazine (NMH) adsorbed on montmorillonite (Mont), in the presence or in the absence of a second organic chromophore was studied. Two different organic dyes were investigated as possible stabilizers of NMH: the divalent cation methyl green (MG) and the monovalent cation thioflavin T (TFT). Samples of free NMH and of the adsorption complexes Mont-NMH, Mont-MG-NMH, and Mont-TFT-NMH were exposed to direct sunlight, and the residual insecticidal activity was estimated. Some photostabili- zation of the pesticide adsorbed to the clay was observed. The highest degree of photoprotection was achieved in samples containing 0.5 mmole of TFT and 0.2 mmole of NMH/g clay. Increasing the load of TFT to 0.8 mmole/g clay resulted in a complete loss of photostabilization. The interactions of the organic molecules at the clay surface were studied by UV-VIS absorption and Fourier-transform infrared spectroscopy. For the Mont-NMH and Mont-MG-NMH complexes, the observed photostabilization is probably due to clay-NMH interactions. In the Mont-TFT-NMH complex specific interactions between the cationic dye and the pesticide molecules probably occurred as well.

Stacking order in a 14.30-Aa Mg-vermiculite

Abstract: The stacking order of a 14.30-A Mg-vermiculite from Santa-Olalla, Spain, has been determined from Weissenberg photographs. The results prove that the Mg-vermiculite structure is not a 2-layer polytype structure, but a semi-ordered one. Because the structure is semi-ordered, its resolution needed a dual approach: (1) a direct approach using an electron density projection along the y axis in conjunction with a one-dimensional electron-density projection onto the z axis; and (2) an indirect approach in which the observed intensities along the (0,k) and (1,k) reciprocal rods were compared to the calculated intensities given by model defect structures. The semi-ordered structure of the Mg-vermiculite results from ±b/3 shifts in the stacking of the silicate layers. The shifts are randomly either along +b or -b.

Determination of surface free energy of kaolinite

Abstract: The adsorption of n-octane and water vapor on natural kaolinite was measured. From ad- sorption isotherms film pressures were determined which were then used to calculate the dispersion and nondispersion components of the kaolinite surface free energy. In addition, thermodesorption of water from the kaolinite surface was determined. These results suggest that physically adsorbed water remained on the kaolinite surface, even at temperatures as great as 125"(2. Therefore, experimentally determined dispersion and nondispersion components appear to relate to the surface precovered with a film of water. These values are: ~gw) = 34.4 mJ/m 2 for dispersion interactions and ~gw) = 60.2 mJ/m 2 for nondispersion interactions. Assuming a kaolinite surface precovered with a film of water, which decreased the free energy by the work of spreading, the following components of the energy for the bare surface were calculated: ~s = 67.6 mJ/m 2 and 3'~ = 103.4 mJ/m 2, for dispersion and nondispersion components, respectively.