Maghemite (γ-Fe2O3) is a ferrimagnetic Fe oxide commonly found in tropical and subtropical soils, especially in the topsoil where it is usually a product of burning. Isomorphic substitution (IS) of the Fe in maghemite by different metals (mainly Al3+) can modify its mineralogical and chemical attributes, and these modifications may be important to understanding the formation and properties of this mineral in soils and sediments. The objective of this work was to evaluate the crystallochemical alterations of synthetic, Al-substituted maghemites prepared by the precipitation of magnetites from alkaline aqueous media containing FeSO4·7H2O with increasing amounts of Al2(SO4)3·7H2O to obtain hypothetical Al3+ for Fe3+ substitutions ranging from 0.0 to 40.0 mol %. The Al-substituted magnetites were washed and dried, and then heated to 250ºC for 4 h to form yellowish red maghemites that were characterized by total chemical analysis, X-ray diffraction, specific surface area (SSA), mass-specific magnetic susceptibility, infrared spectroscopy, transmission electronic microscopy, and color. Increasing Al3+ substitution to an experimental maximum of 15.9 mol % decreased both the a0 dimension of the cubic unit cell (a0 = 0.8339 − 396.157 × 10−16 Al, r2 = 0.99) and the mean crystallite dimension (MCD = 76.4–3.15Al, r2 = 0.79) of the maghemites. With the decrease in MCD came a more yellowish color, an increase in SSA, and a decrease in crystallinity as measured through extraction of the samples with acid ammonium oxalate. The mass-specific magnetic susceptibility of the maghemites increased with Al3+ substitution up to 5.3 mol % and then decreased with further replacement of Fe by Al. Solid-phase aluminum in excess of 16 mol % substitution appeared to occur as a separate, poorly crystalline phase that was X-ray amorphous.

Due to swelling, smectite minerals are capable of intercalating many organic molecules in their interlayer space. Tetracycline (TC) is a group of antibiotics used extensively in human and veterinary medicine. The great aqueous solubility and long environmental half life of TC mean that the study of interactions between swelling clay minerals and TC are of great importance in TC transport and retention in subsurface soils. In the present study, the intercalation of TC molecules at different levels into smectites was investigated using Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The shift of the FTIR bands of amide I and II in comparison to crystalline TC suggested a strong interaction between the amide groups and the clay surfaces. The band at 1455 cm−1 remained the same after TC intercalation into SAz-1, SWy-2, and SYn-1, suggesting that complexation was not a dominant mechanism of TC uptake by these minerals. With cation exchange as the major mechanism of TC intercalation into these minerals, simultaneous removal of H+ from solution protonated the TC molecules and provided a positive charge to interact with negatively charged mineral surfaces even in neutral to slightly alkaline conditions. The increase in interlayer distance after intercalation by TC, as revealed by XRD, suggested a tilted orientation of the intercalated TC molecules in both twisted conformation in acidic condition and extended conformation in alkaline condition.

Glycerol is a byproduct of biodiesel synthesis by transesterification of triglycerides with short-chain alcohols in the presence of solid base catalysts. Because hydrotalcite is a potentially useful basic catalyst for the transesterification reaction, the interaction of glycerol with hydrotalcite is the focus of this work. Glycerol was intercalated into Mg-Al hydrotalcite with a Mg/Al molar ratio of 4 under elevated temperatures of 4432/503 K in the absence and presence of NaOH. The resulting materials were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, and N2 adsorption. The amount of glycerol incorporated into hydrotalcite increased with increasing temperature of intercalation. However, the brucite-like sheets of hydrotalcite partially decomposed during the intercalation process at the highest temperature. The presence of NaOH stabilized the hydrotalcite layers against decomposition under high temperature (503 K). The intercalated materials exposed large surface areas ranging from 142 to 663 m2 g−1, depending on the preparation conditions. The glycerol-intercalated hydrotalcites were less catalytically active than hydrotalcite with OH− counterions for the transesterification of tributyrin with methanol. The lower reactivity of glycerol-intercalated hydrotalcite was probably the result of a strong interaction between the intercalated glycerolate (HOCH2-CHOH-CH2O−) and the partially decomposed brucite-like sheets.

Bentonite is often proposed as an engineered-buffer material in high-level radionuclide waste-management systems. For effective design of the barrier that will provide protection over the long time periods required, the physical/thermal/chemical processes taking place in the barrier material must be understood thoroughly. These processes, which interact, include the flow of water and gas, the flow of heat, and the transport and reaction of chemical constituents. The purpose of this study was to better understand the processes that occurred in a small-scale experiment within a confined bentonite space. A conceptual and mathematical model (FADES-CHEM) was built in order to simulate the results of an experiment conducted in 2000, and thereby to gain a better understanding of the controlling processes. In that experiment, a block of compacted bentonite was placed in an air-tight cell and subjected, for 6 months, to simultaneous heating and hydration from opposite sides. The bentonite block was then sliced into five sections each of which was then analyzed in order to obtain a series of physicochemical parameters illustrating the changes that had occurred. Before modeling, the chemical composition of the bentonite pore waters was restored in order to account for different processes such as gas outgassing and cell cooling. Modeling indicated that gas-pressure build up was a relevant process when computing the saturation of bentonite, and the computations in the present study suggested that evaporation/condensation processes played a crucial role in the final distribution of the water content. Gas pressure and evaporation/ condensation also affected the geochemical system, and the numerical model developed gives results that were consistent with the experimental values and trends observed. The model reproduced the results obtained and enable use at the repository scale and over longer time frames, provided that adequate data are available.

Many environmental applications in the inorganic remediation field are based on the swelling and ion-exchange capacities of smectites, even though these can be affected by hydrothermal treatment in water and acidic media. Here a systematic study of the properties of layered silicates that could affect their hydrothermal stability at different pH is described: type of layers, octahedral occupancy, layer charge, and origin of the layer charge. The silicates studied were selected on the basis of their different characteristics associated with these properties. Kanemite (1:0 phyllosilicate), kaolinite (1:1 phyllosilicate), and pyrophyllite and talc (2:1 phyllosilicates with no-layer charge) were examined in order to determine the effect of layer structure, whereas the hydrothermal reactivity of silicates with different layer charge was analyzed by comparing the talc-hectorite-Laponite® and talc-saponite-trioctahedral vermiculite series. Samples were treated hydrothermally at 300ºC for 48 h in pure water and in a 0.01 M HNO3 solution and the final products were analyzed by X-ray diffraction, scanning electronic microscopy, and solid-state nuclear magnetic resonance spectroscopy. All layered silicates, except for kanemite, were found to remain intact after hydrothermal treatment in water and acidic media, with only minimal short-range structural changes observed. The extent of the structural changes depended on the octahedral sheet occupancy (greater extent) and the number of isomorphic substitutions (lesser extent), both of which weaken the structure.

LongYan kaolin has a large Fe content which affects the coloring. Bioleaching treatments to remove Fe impurities were conducted here using indigenous dissimilatory Fe(III)-reducing bacteria. The factors that affect bioleaching efficiency include bioleaching time, carbon source, pH, temperature, pulp density, and inoculum density and these were examined. Environmental scanning electron microscopy and X-ray diffraction were used to examine any textural or mineralogical changes at the surface of the kaolin that may have occurred during the bioleaching. Iron impurities in the kaolin were reduced from 0.88% to 0.48% with an increase in the natural whiteness index from 60.8% to 81.5% after 7 days of bioleaching treatment. A granulometric analysis of dispersed kaolin demonstrated that the bioleaching resulted in a decrease in particle size. The results demonstrated that the bioleaching was very effective at improving the quality of the kaolin, where insoluble Fe(III), either adsorbed to the kaolin surfaces or admixed as amorphous forms, was leached out by micro-organisms as water-soluble Fe(II).

High-resolution transmission electron microscopy (HRTEM) images of two dioctahedral micas, celadonite and cis-vacant (cv) illite, were examined in detail to understand the effects of electron radiation on their structures during image acquisition. Celadonite, a dioctahedral mica with Fe and Mg as major octahedral cations, originally has a trans-vacant (tv) octahedral sheet but the contrast in the highresolution transmission electron microscopy (HRTEM) images indicates a different cation distribution in the sheet. Furthermore, the β angle for the 1M polytype derived from the HRTEM images is ~98.5º, which is considerably smaller than that (~100.5º) reported for celadonite. In previous works, cation migration from the tv to cv-like configurations and a decrease in the β angle after dehydroxylation of celadonite/ glauconite by heating were reported. The same phenomenon, dehydroxylation and subsequent cation migration, probably occurs by electron radiation in TEM. However, the new cation-distribution models derived from HRTEM images along the [100] and [110] directions are not in agreement. On the other hand, the contrast in a number of HRTEM images from an illite specimen in which cv-illite is dominant is the same as that for the tv-dioctahedral layer. This result is also interpreted as cation migration accompanied by dehydroxylation in TEM, as reported in heated cv-illite. The increased β angle (~102.5º) from that in the natural state (101.5º) estimated from the HRTEM images also supports this interpretation. This phenomenon is a large obstacle to the investigation of phyllosilicates containing Al-rich cv and Mg,Fe-rich tv 2:1 layers, using HRTEM.

In Sweden and in many other countries, a bentonite barrier will be used in the repository for spent nuclear fuel. In the event of canister failure, colloidal diffusion is a potential, but scarcely studied mechanism of radionuclide migration through the bentonite barrier. Column and in situ experiments are vital in understanding colloid diffusion and in providing information about the micro structure of compacted bentonite and identifying cut-off limits for colloid filtration. This study examined diffusion of negatively charged 2-, 5-, and 15-nm gold colloids in 4-month diffusion experiments using MX-80 Wyoming bentonite compacted to dry densities of 0.6–2.0 g/cm3. Breakthrough of gold colloids was not observed in any of the three diffusion experiments. In a gold-concentration profile analysis, colloid diffusion was only observed for the smallest gold colloids at the lowest dry density used (estimated apparent diffusivity Da ≈ 5 × 10−13 m2/s). The results from a microstructure investigation using low-angle X-ray diffraction suggest that at the lowest dry density used, interlayer transport of the smallest colloids cannot be ruled out as a potential diffusion pathway, in addition to the expected interparticle transport. In all other cases, with either greater dry densities or larger gold colloids, compacted bentonite will effectively prevent diffusion of negatively charged colloids due to filtration.

Fe-reducing micro-organisms can change the oxidation state of structural Fe in clay minerals. The interactions with complex clays and clay minerals in natural materials remain poorly understood, however. The objective of this study was to determine if Fe(III) in loess was available as an electron acceptor and to study subsequent mineralogical changes. The loess samples were collected from St. Louis (Peoria), Missouri, USA, and Huanxia (HX) and Yanchang (YCH), in the Shanxi Province of China. The total Fe concentrations for the three samples was 1.69, 2.76, and 3.29 wt.%, respectively, and Fe(III) content was 0.48, 0.69, and 1.27 wt.%, respectively. All unreduced loess sediments contained Fe (oxyhydr)oxides and phyllosilicates. Bioreduction experiments were performed using Shewanella putrefaciens CN32 with lactate as the sole electron donor and Fe(III) in loess as the sole electron acceptor with and without anthraquinone-2, 6-disulfonate (AQDS) as an electron shuttle. Experiments were performed in non-growth (bicarbonate buffer) and growth (M1) media. The unreduced and bioreduced solids were analyzed by X-ray diffraction, Mössbauer spectroscopy, diffuse reflectance spectroscopy, and scanning electron microscopy/energy dispersive spectroscopy. Despite many similarities among the three loess samples, the extent and rate of Fe(III) reduction varied significantly. In the presence of AQDS the extent of reduction in the non-growth experiment was 25% of total Fe(III) in HX, 34% in Peoria, and 38% in YCH. The extent of reduction in the growth experiment was 72% in HX, 94% in Peoria, and 65% in YCH. The extent of bioreduction was less in the absence of AQDS. Overall, AQDS and the M1 growth medium significantly enhanced the rate and extent of bioreduction. Fe(III) in (oxyhydr)oxides and phyllosilicates was bioreduced. Siderite was absent in control samples, but was identified in bioreduced samples. The present research suggests that Fe(III) in loess sediments is an important potential source of electron acceptors that could support microbial activity under favorable conditions.

The Hercynian metamorphic event is poorly characterized in internal zone complexes of the Betic Cordillera (Spain), as it has been, to a great extent, overprinted by the mineral assemblages formed during the Alpine event. Identification of the signals of the Hercynian episode is easier in series largely unaffected by the Alpine event, such as Intermediate units between the Maláguide and the Alpujárride Complexes, which consist of a set of thrust slices. With the aim of characterizing the Hercynian paragenesis, a detailed comparison of the mineral assemblages of Paleozoic and overlying Triassic sequences, unaffected by the pre-Alpine event, was carried out. Mineral assemblages were characterized by X-ray diffraction, infrared spectroscopy, scanning electron microscopy, electron microprobe, and transmission-analytical electron microscopy. A rapid increase in the illite crystallinity values was observed at the Triassic—Paleozoic transition in the upper tectonic slices. In addition, the diagenetic to anchizonal dickite-, sudoite-, and pyrophyllite-bearing assemblages, characterizing the Triassic rocks, contrast with Paleozoic assemblages consisting of white K-mica ± paragonite + chlorite + mica-chlorite and chlorite-vermiculite mixed layers + garnet, suggesting that this assemblage corresponds to the Hercynian metamorphic event. This assemblage records temperatures on the order of 400ºC and an intermediate pressure regime. Paleozoic rocks contain, in addition, tobelite, which comprises some of the detrital grains and strongly masks the illite crystallinity values. Tobelite has been identified only in the upper thrust slices, suggesting that changes in the detrital input is primarily responsible for the disappearance of tobelite at the transition from the Maláguide to the Alpujárride domain. Tobelite appears finely intergrown with white K-mica and its origin is uncertain. It could have been inherited as tobelite, but a more likely hypothesis is that the intergrowths of white K-mica and tobelite were formed at low temperature from an NH4-bearing mica precursor.

The Ypresian phosphatic series of the Gafsa-Metlaoui basin, southwestern Tunisia, is represented by an alternation of phosphatic levels and interbedded facies, which are composed of marly clay and silica-rich rocks. The present work aimed to clarify the genesis of palygorskite and sepiolite of the interbedded facies and to understand the depositional environment of the phosphatic series. The interbedded facies of the Stah and Jellabia mines were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Energy Dispersive X-ray microanalysis (EDX) of individual constituents and their aggregates. The data obtained indicate that samples are made up of francolite, calcite, dolomite, quartz, feldspars, and clay minerals; the latter consist of palygorskite-sepiolite minerals associated with smectite. Observations by SEM revealed the occurrence of palygorskite and sepiolite as fine and filamentous fibers with thread-like facies and coating dolomite, calcite, and a marly matrix. Such features can be considered as textural evidence of authigenic palygorskite-sepiolite. At the bottom of the Stah section, SEM observations revealed that the fine fibers are more abundant within silica-rich rocks. Silica is commonly available due to bacterial activity saturating its environment with the silicic acid required for the formation of palygorskite-sepiolite. In the interbedded facies of the Jellabia section, the moderate fibrous clay content and the presence of well crystallized dolomite revealed that the shallow-marine water was characterized by high-Mg and low-Si activities.