Annett Steudel

Natural mixture of silica and smectite as a new clayey material for industrial applications

Abstract The main physico-chemical properties of a new smectitic clay containing large amounts of amorphous material are reviewed and potential industrial applications of this type of clay are discussed. Due to a 34% amorphous material content (natural silica gel), the investigated clay has very high porosity and can be used as it is or in acid-impregnated form for oil bleaching or phosphate reduction in edible oil. In the field of biodiesel purification, the new clay can be used to remove, in particular, mono-, diglycerides and glycerol. The natural silica-smectite mixture is also suitable as a carrier for liquid ingredients, for example in animal feeds, and might serve as a partial or complete substitute for synthetic precipitated silicas. In the field of bioseparation processes, the clay can be used as an adsorbent for protein separation by means of cation exchange. Due to the suppressed swelling (compared with smectite alone), it can be packed in columns which can be regenerated.

CHANGE OF THE REFRACTIVE INDEX OF ILLITE PARTICLES BY REDUCTION OF THE Fe CONTENT OF THE OCTAHEDRAL SHEET

Sub-micrometer clay particles are of interest in clay-polymer applications, especially when transparency is important. The scattering of light can be reduced by the adjustment of the refractive index (RI) of the clays to that of the matrix. In this study, the RI of sub-micrometer illite particles was changed by treatment with 5 M HCl for treatment times ranging between 2 and 24 h. The dissolution of Fe leads to a decrease in the RI of illite from 1.587 for the unaltered material to 1.502 after 24 h. The layer structure of the illite particles was preserved during the treatment. The RI of the sub-micrometer illite particles was determined by means of a photospectrometer measuring the light intensity passing through suspensions containing the clay particles, with varying refractive indices.

CATION EXCHANGE REACTIONS OF VERMICULITE WITH Cu-TRIETHYLENETETRAMINE AS AFFECTED BY MECHANICAL AND CHEMICAL PRETREATMENT

The cation exchange capacity (CEC) is a characteristic property of expandable clay minerals, such as smectites and vermiculites. The aim of this work was to examine the cation exchange behavior of vermiculite using the Cu-triethylenetetramine (Cu-trien) CEC method and the influence of mechanical and chemical pretreatment, with the ammonium acetate method serving as a reference. The Cu-trien method makes rapid and direct CEC measurements possible. Three different kinds of mill were used to grind a vermiculite sample from Russia, in order to reduce the particle size to <10 µm. The Netzsch CGS 10 dry mill reduced the particle size more effectively than the other grinding methods. Chemical pretreatments were used to remove carbonates, organic matter, Fe oxides, and divalent exchangeable cations from vermiculite samples prior to CEC measurements. Subsamples of ground and chemically pretreated vermiculite samples were saturated with Na, Li, Mg, Ca, and Cu cations to determine the effect of exchangeable cations on measured CEC values. Chemical pretreatment, monovalent cation pretreatment, and 48 h of shaking time were needed to measure vermiculite CEC values effectively using the Cu-trien method.

Mineralogical and physicochemical characterization of a natural bleaching earth containing sepiolite suitable for fast filtration and bioseparation

Abstract A clay from the Mediterranean area classified as natural bleaching earth was comprehensively characterized and the quantitative phase content was determined. Morphology and surface characteristics were determined by environmental scanning electron microscopy (ESEM) and nitrogen adsorption. The BET surface area was >200 m2 g-1 and the average mesopore diameter was >10 nm. Thus the clay is suitable for bioseparation of larger proteins. X-ray diffraction (XRD), X-ray fluorescence (XRF), cation exchange capacity (CEC), layer charge measurement and simultaneous thermal analysis (STA) were used for mineralogical characterization. Determination of the phase content by Rietveld analysis was possible only after Sr2+ saturation, while Rietveld analysis of XRD patterns from Na+-saturated clay resulted in a strong overestimation of the amorphous content and masking of the sepiolite. The clay consists of sepiolite, dioctahedral smectite with high layer charge and low stack height, X-ray amorphous matter (probably mainly SiO2) and accessory feldspars.

Correlation Between the Extent of Catalytic Activity and Charge Density of Montmorillonites

The clay mineral montmorillonite is a member of the phyllosilicate group of minerals, which has been detected on martian soil. Montmorillonite catalyzes the condensation of activated monomers to form RNA-like oligomers. Extent of catalysis, that is, the yield of oligomers, and the length of the longest oligomer formed in these reactions widely varies with the source of montmorillonite (i.e., the locality where the mineral is mined). This study was undertaken to establish whether there exists a correlation between the extent of catalytic property and the charge density of montmorillonites. Charge density was determined by saturating the montmorillonites with alkyl ammonium cations that contained increasing lengths of alkyl chains, [CH₃-(CH₂)(n)-NH₃](+), where n = 3-16 and 18, and then measuring d(₀₀₁), interlayer spacing of the resulting montmorillonite-alkyl ammonium-montmorillonite complex by X-ray diffractometry (XRD). Results demonstrate that catalytic activity of montmorillonites with lower charge density is superior to that of higher charge density montmorillonite. They produce longer oligomers that contain 9 to 10 monomer units, while montmorillonite with high charge density catalyzes the formation of oligomers that contain only 4 monomer units. The charge density of montmorillonites can also be calculated from the chemical composition if elemental analysis data of the pure mineral are available. In the next mission to Mars, CheMin (Chemistry and Mineralogy), a combined X-ray diffraction/X-ray fluorescence instrument, will provide information on the mineralogical and elemental analysis of the samples. Possible significance of these results for planning the future missions to Mars for the search of organic compounds and extinct or extant life is discussed.

CEC AND 7Li MAS NMR STUDY OF INTERLAYER Li+ IN THE MONTMORILLONITE–BEIDELLITE SERIES AT ROOM TEMPERATURE AND AFTER HEATING

The objective of the study was to contribute to the understanding of the influence of the structure and the 2:1 layer dimension of smectites on cation exchange capacity (CEC) reduction and the hydration behavior of Li-saturated smectites after heating. Five montmorillonites extracted from bentonites of different provenance were saturated with Li+ and heated to 300°C. Initial montmorillonites and montmorillonites with reduced layer charge (RCM) were characterized by comprehensive mineralogical analysis supplemented by CEC measurements, surface-area measurements by Ar adsorption, and 7Li, 27Al, and 29Si magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). The CEC of the initial montmorillonites varied between 89 and 130 cmol(+)/kg while the CEC of the RCM prepared at 300°C varied between 8 and 25 cmol(+)/kg. The lateral dimension of the 2:1 layers varied between 70 and 200 nm. The greatest decrease in CEC was observed for the montmorillonite with the largest diameter of the 2:1 layers and the smallest decrease was observed for the montmorillonite with the smallest diameter of the 2:1 layers. 7Li MAS NMR revealed an axially symmetric chemical environment of the hydrated interlayer Li+ with ηΔ = 0 for the chemical shift anisotropy tensor for unheated montmorillonites with >33% tetrahedral layer charge (ξ). The chemical environment is typical of innersphere hydration complexes of interlayer Li+. An axially non-symmetric chemical environment of the interlayer Li+ with ηCS of close to one was observed for all RCM. While the remaining CEC of RCM prepared at 300°C reflected the variable CEC at the edges, and thus the lateral size or aspect ratio of the 2:1 layers, the hydration complex of interlayer Li+ was strongly determined by the isomorphic substitutions in the dioctahedral 2:1 layers.

Simultaneous thermal analysis of different bentonite-sodium carbonate systems: an attempt to distinguish alkali-activated bentonites from raw materials

Abstract Alkali activation with sodium carbonates is a traditional method to improve bentonite properties for a variety of applications. In some applications, natural sodium-rich bentonites are preferred, and custom regulations require proper declaration of Na-rich bentonites, with respect to activation. Consequently, there is need for a method that can unambiguously distinguish between natural and activated Na-rich bentonites. The paper deals with the preparation of several alkali-activated sets, specifically (a) anhydrous Na2CO3 with trace amounts of thermonatrite (Na2CO3 · H2O) and trona (Na3(CO3 )(HCO3 )· 2H2O), hereafter called ASC, (b) mixtures of ASC with CaCO3 , and (c) mixtures of ASC with CaCO3 and a Ca2+-rich bentonite at different moisture contents, to distinguish natural and alkali-activated bentonites by simultaneous thermal analysis (STA) linked with a mass spectrometer for the analysis of evolved gases. STA linked with MS revealed alkali activation of bentonites, even in the presence of CaCO3. The moisture content during activation and storage of activated samples, however, has a strong influence on the detection of activated samples by STA-MS. Uncertainties remain with respect to unknown foreign phase contents of technical ASC used for alkali activation in practice and the influence of carbonates like dolomite or siderite, which are often present in natural bentonites.

Crystal chemistry of Na-rich rectorite from North Little Rock, Arkansas

Abstract The rectorite, a regular mixed layer mineral consisting of dioctahedral swelling and non-swelling 2:1 layers, from North Little Rock, Arkansas, was studied to define the crystal chemistry and structural parameters (e.g. layer charge of the different layers, presence of cis/trans-vacancies). X-ray diffraction, simultaneous thermal analysis coupled with mass spectrometry, X-ray fluorescence and cation exchange capacity are used to characterize this rectorite. The rectorite has a coefficient of variation (CV) of 0.19 and a cation exchange capacity of 60 cmol(+)/kg, as determined by the ammonium acetate method. The mineral is best described as a regular interstratification of brammallite-like and high-charged beidellite-like layers. Dehydration occurs at ≈118°C with a mass loss of 6.77% and dehydroxylation occurs in two steps at 470°C and 588°C with an overall mass loss of 4.67%. Peak decomposition of the mass spectrometer curve of evolved water shows ≈20% peak area with a maximum higher than 600°C, indicating ≈20% cis-vacant layers.