Francoise Elsass

Colloidal facilitated transfer of metals in soils under different land use

In order to assess the importance of colloids in the metal transfer in soils under varying land use (a podzol under a forest, a cultivated luvisol and a luvisol under a metallophyte grassland), gravitational waters were collected in situ by zero-tension lysimeters. Dissolved and colloidal fractions of metals were separated by ultracentrifugation and colloids were studied by transmission electron microscopy (TEM) coupled with energy dispersive X-ray analysis (EDS). The stability of metal complexes in the collected solutions was operationally defined by an ion exchange method. The results show that in the two luvisols about 75% of Pb was always present in colloidal forms and a substantial fraction of Pb in solution was present as stable complexes and labile complexes with slow dissociation kinetics. Zn and Cd were found mostly in dissolved forms and mainly as free ions or labile complexes in solution. In the podzol, all metals were found in dissolved forms and as free ions or labile complexes. The Zn-bearing colloids were organic (biocolloids) or mineral (smectites, Fe-oxyhydroxides, Ca- or Al-rich phosphates) depending on soil type and land use. The results are discussed in terms of metal mobility to explain the different vertical metal distributions observed in the soil profiles.

A technique for maintaining texture and permanent expansion of smectite interlayers for TEM observations

A process for treating smectite-bearing rock samples that utilizes LR White resin; allows TEM observation of expanded smectite interlayers and therefore preservation of original rock textures. Examples of several lattice fringe images are shown, including: (1) Dioctahedral smectite layers from a shale (1388.9 meter depth, Texas Gulf Coast) give fringes that consistently have spacings of 1.2–1.3 nm, yet duplicate other features previously observed in collapsed samples. (2) Packets of illite layers give fringes with 1.0-nm spacings coexisting with packets of (dominantly) R1 I/S having 2.1-nm lattice fringe spacings in a Gulf Coast shale from 4742.1 m. (3) Rectorite from Garland Co., Arkansas gives 2.3-nm lattice fringes. Samples with wide ranges of I/S ratios and lithologies have been found to be permanently expanded with retention of original textures, commonly leading to unambiguous identification of illite and smectite interlayers in lattice fringe images.

Direction high-resolution transmission electron microscopic measurement of expandability of mixed-layer illite smectite in bentonite rock

Samples of mixed-layer illite/smectite were investigated from a single bentonite bed zoned with respect to expandability from 90 to 30%. Chips of natural rocks were embedded in a resin, using a procedure designed to preserve the original fabric, cut with an ultramicrotome, and observed by high-resolution transmission electron microscopy (HRTEM). These observations confirmed the X-ray powder diffraction (XRD) model of mixed-layer clays, i.e., that illite/smectite grains in natural rocks are built of mixed-layer crystals, from 1 to as many as 15 silicate layers thick (4–6 interlayers per crystal on average). These crystals are present either as individual particles (loose crystals) or, typically, they form nearly parallel face-to-face groupings called here quasi-crystals. Free fundamental smectite and illite particles as defined by Nadeau and coworkers were essentially absent.Illite and smectite interlayer spacings were 10 and 13.5 Å, respectively. Crystal thickness and number of interlayers were measured for 35–100 mixed-layer crystals per sample. Illite/smectite expandabilities were calculated from these data in two ways: either neglecting the crystal edges or accounting for them. The former determinations agree well with XRD estimates of expandability and the latter, with expandabilities calculated from the distributions of fundamental particle thickness measured by a shadowing technique in the TEM. This result explains the systematic discrepancy between XRD and TEM measurements of illite/smectite expandability.

Physical speciation of trace metals in Fe–Mn concretions from a rendzic lithosol developed on Sinemurian limestones (France)

Abstract Because of their high content in heavy metals, Fe–Mn concretions present in soils can be a source of release of trace metals in the environment. Metal-rich concretions were isolated from a top horizon of a Rendzic Lithosol developed on limestones in France. The distribution of metallic elements (Cr, Co, Ni, Cu, Zn, Pb) was studied at different scales: field sampling, isolated concretions, and individual particles. Methods of investigation combined bulk chemical analysis, optical microscopy, X-ray diffraction (XRD) and microanalysis using scanning and transmission electron microscopes. The concretions are constituted by concentric rings and contain quartz, feldspars, kaolinite, mica, goethite, and Ti oxide as crystalline phases. A strong heterogeneity in the chemical composition is evidenced at all different scales. The total composition of different populations of Fe–Mn concretions depends on field sampling. Metals are detected at the micrometer and nanometer scales with variable frequencies, occurrences reflecting the abundance of metal-bearing components and the size of metal clusters. Statistical treatments of microanalytical data show marked trends. A strong positive correlation of Mn (and to a lesser extent of Ca) with Ni, Zn, (and to a lesser extent with Co and Cu) indicates trace metals to be associated with poorly crystallized Mn–Ca-rich areas. The correlation of Pb with Fe and P suggests its incorporation in an Fe-rich phosphate component. No correlation of metals with well crystallized silicates, Fe-sesquioxides or sulfate minerals is established. Cr was diffusely distributed.

TEM-EDX investigation on Zn- and Pb-contaminated soils

Abstract Lead and zinc contaminated soils from a smelter area in the northern part of France have been studied by transmission electron microscopy (TEM). This study was carried out with 4 different soils contaminated by Pb and Zn but with different chemical and physical characteristics. Two soils are tilled and have a neutral or slightly basic pH, one is a wooded soil and the last one is a meadow soil with acidic pH and high total organic content . TEM images of the soil samples have been coupled with focused energy dispersive X-ray (EDX) analyses and chemical mapping on a few micron-sized windows. This study demonstrates that TEM is a particularly efficient method to investigate metal speciation in the fine fractions of the contaminated soils. Zinc could be detected locally in sulphide minerals probably coming from the smelter emissions, but the major phases retaining Zn are Fe-oxyhydroxides and smectites. Lead could be detected in small aggregates which were characterized by EDX and selected area electron diffraction. Their structural formulae correspond to a pyromorphite-like mineral in which Pb is partly substituted by Ca and Na. Pyromorphite is present only in the wooded and meadow soils where it forms partly from amorphous Si-rich phases (slags) coming from the smelter. These results are compared with data previously obtained by spectroscopic methods on the same samples.

PARTICLE SHAPE VERSUS COHERENT SCATTERING DOMAIN OF ILLITE/SMECTITE: EVIDENCE FROM HRTEM OF DOLNA VES CLAYS

Fundamental particle thickness measurements of Dolná Ves hydrothermal illite/smectite (I/S) samples confirmed earlier findings regarding the content of fixed cations in illite interlayers (ca. 0.9/O10(OH)2). The distributions of fundamental particles and mixed-layer crystals of a given sample are internally consistent. In samples dominated by bilayer fundamental particles, mixed-layer crystals most often contain even numbers of layers. The expandabilities measured by XRD are much higher than so-called minimum expandabilities obtained from HRTEM measurements. This discrepancy is explained by assuming that the coherent scattering domains of Dolná Ves clays do not correspond to natural mixedlayer crystals but are thicker, probably due to parallel association of crystals on the oriented XRD slide. This tendency to produce intercrystal contacts is probably related to the unusually large ab dimensions of crystals of Dolná Ves clays.

Characterization of soil clay minerals; decomposition of X-ray diffraction diagrams and high-resolution electron microscopy

Fine clays (<0.1 μm) extracted from an acid soil developed in a granite saprolite from the Massif Central, France, were characterized by X-ray diffraction (XRD) using a curve decomposition program, and high-resolution transmission electron microscopy (HRTEM) associated with a method of impregnation of moist samples. Direct measurement of d-spacings were performed on HRTEM photographs. Decomposition of XRD patterns indicated 5 to 6 different clay phases including chlorite (and/or hydroxy-interlayered vermiculite), vermiculite/smectite, illite/vermiculite and illite/smectite mixed layers. Expandable phases with decreasing layer charge (vermiculite, high- and low-charge smectite) were shown in the clay assemblage. When performed on K-saturated samples subjected to wetting and drying cycles, HRTEM observations were consistent with the XRD results. The major clay mineral phases identified by the decomposition of XRD patterns were also found by direct measurement of d-spacings on HRTEM images. Vermiculite and high-charge smectite appeared to be impregnated with preservation of their initial hydration state, whereas low-charge smectite interlayers were penetrated by the resin molecules during the impregnation procedure. It was concluded that the decomposition of XRD patterns gave a realistic analysis of the clay phases present in a complex soil clay sample, as well as the direct measurement of a limited number (50) of clay crystals on HRTEM images.

Effect of illite particle shape on cesium sorption

Samples containing illite and illite-smectite, having different crystal shapes (plates, “barrels”, and filaments), were selected for sorption experiments with cesium. There is a positive correlation between total surface area and Cs-sorption capacity, but no correlation between total surface area and the distribution coefficient, Kd. Generally Kd increases with the edge surface area, although “hairy” (filamentous) illite does not fit this pattern, possibly because elongation of crystals along one axis reduces the number of specific sorption sites.

Catalytic properties of two pre-cracking matrices: a leached vermiculite and a Al-pillared saponite

Abstract Two clay materials—a leached vermiculite and a saponite pillared with aluminum clusters—have been prepared and characterized as pre-cracking matrices for the conversion of heavy fuels. These two solids present acidic properties and porosity at high temperature (500–550°C). The microactivity test results (MAT) on a very heavy feedstock (carbon Conradson: 2.9) have been compared to those of γ-Al 2 O 3 . Acid-treated vermiculite yields more gasoline and less coke and appears to be a promising active matrix. Leached vermiculite is composed of more or less attacked layers retaining their original platy morphology, and of non-crystalline hydrated silica. The high coke production for Al-pillared saponite is probably related to the Lewis acidic sites and not to the Fe content of the original clay.

Effect of acid mine drainage on the mineralogy of a dystric cambisol

Abstract Two natural soil profiles non-acidified and acidified with acid mine drainage (AMD) were selected for study. Interactions between acid mine drainage and the soil were evaluated by the changes in the mineral composition applying XRD, transmission electron microscopy (TEM), analytical electron microscopy (AEM), and selective area electron diffraction (SAED). The main differences between the acidified and the non-acidified soil were in the presence of jarosite and in the authigenesis of two types of amorphous and/or poorly crystalline matter (Fe-rich; Al, Si-rich). The most frequent Fe-rich matter was identified as ferrihydrite. It appears to be in two morphological varieties: “massive” (composed of spheroidal particles) and “mohair” (composed of layered particles).