Structural and Morphological Characterization of the PP-0559 Kaolinite from the Brazilian Amazon Region

Abstract

Caulinita natural proveniente da região amazônica (PP-0559) foi caracterizada morfologica e estruturalmente com o objetivo de contribuir para o entendimento do papel da caulinita em reações redox no meio ambiente. Através de microscopia eletrônica de transmissão e difração de elétrons, confirmou-se a estrutura triclínica em cristais orientados, com diâmetros entre 0,2 e 2 mm e cerca de 50 nm de espessura. Os cristais anisotrópicos após orientação foram estudados por ressonância paramagnética eletrônica em função da direção do campo magnético aplicado. Foram estudadas as principais linhas da impureza de Fe +3 substitucional e centros paramagnéticos induzidos por radiação, possíveis centros oxidantes da caulinita.We report a structural and morphological characterization of the natural kaolinite PP-0559, from the Brazilian Amazon region, with the aim to contribute to the understanding of this clay mineral in environmental redox reactions. The triclinic structure was confirmed by transmission electron microscopy and electron diffraction studies. It exhibits oriented crystals, with diameters between 0.2 and 2 mm and about 50 nm of thickness. The anisotropic crystals after orientation were studied by electron paramagnetic resonance as a function of the applied magnetic field. The main EPR absorption lines of the substitutional Fe +3 impurity and radiation induced paramagnetic centers were studied as possible oxidizing centers of kaolinite. Keywords: kaolinite, clay minerals, EPR spectroscopy IntroductionClay minerals are the main colloidal soil fraction. Their physico-chemical properties are fundamentally influenced by their composition and surface reactivity. They present acidic catalytic and redox properties that promote the polymerization of organic residual substrates which strongly contribute to their humification. 1 The mechanisms promoting organo-mineral complex formation are very important also in the maintenance of a micronutrient reservoir for crops. These clay mineral systems are therefore important for agricultural activities in tropical countries. 2 Stable defects in the kaolinite structure, which are detectable through electron paramagnetic resonance spectroscopy (EPR), have been associated with their habilities to polymerize amines and amino acids. 3 Thus, the comprehension of the structural and surface properties of clay minerals is fundamentally important to various scientific enterprises, such as soil and environmental sciences, as well as special advanced materials.Kaolinite, Al 2 Si 2 O 5 (OH) 4 is a dioctahedral layered hydrated aluminosilicate of the 1:1 type with two distinct interlayer surfaces. One side of the layer is gibbsite-like with aluminum atoms coordinated octahedrically to corner oxygen atoms and hydroxyl groups. The other side of the layer is constituted by a silica-like structure, where the silicon atoms are coordinated tetrahedrically to oxygen atoms (Figure 1). The adjacent layers are linked by hydrogen bonding involving aluminol (Al-OH) and siloxane (Si-O) groups. These bonding f...

EPR and semi-empirical studies as tools to assign the geometric structures of FeIII isomer models for transferrins

“…The spectrum of A presents the sharp signal at g 1 ≅ 4.1 and a shoulder at g 2 ≅ 9.1, as expected for the middle and lower Kramer's doublet transitions of a rhombically distorted high-spin Fe III complex (E/D ≅ 0.33), ground state 6 S. 45,46 The spectrum of B, in addition to the peak at g 1 ≅ 4.2 and the shoulder at g 2 ≅ 9.0, also shows a set of peaks at g 1 ' ≅ 3.0, g 2 ' ≅ 3.6 and g 3 ' ≅ 5.1, which has been assigned to weak resonances of the ground Kramer's doublets in an axial symmetry (E/D ≅ 0.22). 11,17,47,48 The same behavior has been observed for other octahedral Fe III complexes 18 and for transferrins. 11 In transferrins this may be due to the slight distortions in the Fe III coordination spheres of the N-and C-lobes.…”

Comportamento térmico da caulinita hidratada

“…A caulinita possui uma cor branca amarelada, uma elevada ordem estrutural e está contaminada com uma pequena concentração de titânio estrutural e ferro estrutural e não estrutural. O nível de impurezas situa-se em torno de 3%, como determinado por experimentos de fluorescência de raios-X e ressonância paramagnética eletrônica 30,31 . Não existe nenhuma impureza cristalina que possa ser detectada, já que nos experimentos de difratometria de raios-X somente reflexões correspondentes à caulinita foram observadas.…”

Proposal of an EPR based method for pollution level monitoring in mangrove sediments

“…This region of the EPR Fe(III) spectrum is called of diluted dominion. 10 All sediment samples showed a wide line of absorption ( Figure 5) that in the case of the non-polluted samples are extended practically over all the magnetic field range used. These wide lines occur due to the presence of Fe III oxides, Fe III -containing clay minerals or other Fe III species in relatively great concentration on the samples.…”

“…These wide lines occur due to the presence of Fe III oxides, Fe III -containing clay minerals or other Fe III species in relatively great concentration on the samples. [9][10][11]13,14 Fe III occurring in this domain is referred to as concentrated or "clustered" Fe III ions domain. EPR spectra for sediment samples BCS-NP and BMS-NP suggest that the origin of these wide lines is due to a large concentration of Fe III ions forming complexes with constituents of the inorganic-organic matter, besides iron oxides.…”

“…That technique may provide useful information about oxidation states, modes of coordination and geometry of the ligand sites. [9][10][11] If the sample place is polluted, the action of microorganisms in oxidizing the contaminants is very intense. The level of semiquinone resulting from the reduction of quinone moieties is high and that of Fe III species will be lower, as can be seen by means of the scheme presented above.…”