David Koloušek

Characterization of activated Cu/Mg/Al hydrotalcites and their catalytic activity in toluene combustion

Five hydrotalcite-type compounds containing different amounts of Cu2+ and Mg2+ cations were prepared by coprecipitation method (the molar ratio Cu2+:Mg2+:Al3+ in brucite-like layers varied as follows: 0:4:2, 1:3:2, 2:2:2, 3:1:2, and 4:0:2). The products were characterized by XRD, TG and DTA measurements. The increasing content of Cu2+ decreased both the dehydration and decomposition temperatures. Extrudates prepared from the solids were calcined at different temperatures and the properties of the resulting materials (surface area, pore volume, TPR, surface basicity and acidity) were examined. Maximum of the Mg/Al hydrotalcite surface area was observed at 400°C, basicity at 450°C and acidity at 550°C. Incorporation of Cu2+ into brucite-like layers caused a substantial decrease in both basicity and acidity. Acidity rose with increasing amounts of copper in the layers and basicity went through a local minimum. An increase in the amount of magnesium in the precursors manifested itself in a decreased reducibility of the Cu2+ component. The oxidation activity in the toluene combustion increased with increasing amounts of Cu2+ in the solids and corresponded with the amount of easily reducible copper component.

Modified aluminosilicates as low-cost sorbents of As(III) from anoxic groundwater

The utilization of low-grade clay materials as selective sorbents represents one of the most effective possibilities of As removal from contaminated water reservoirs. The simple pre-treatment of these materials with Fe (Al, Mn) salts can significantly improve their sorption affinity to As oxyanions. The natural kaolin calcined at 550 degrees C (mostly metakaolin) and raw bentonite (mostly montmorillonite) pre-treated with Fe(II), Fe(III), Al(III) and Mn(II) salts were used to remove of As from the model anoxic groundwater with As(III) concentration about 0.5 and 10 mg L(-1). All the pre-treating methods were appropriate for bentonite; the efficiency of As(III) sorption varied from 92 to >99%, by the sorption capacity higher than 4.5 mg g(-1). In the case of metakaolin, Fe(II)- and Mn(II)-treatments proved the high sorption efficiency (>97%), while only <50% of As was removed after Fe(III) and Al(III) pre-treatment. The sorption capacities of treated metakaolin ranged from 0.1 to 2.0 mg g(-1).

Thermal behaviour of synthetic pyroaurite-like anionic clay

Thermal behaviour of synthetic pyroaurite-like anionic clay with molar ratio Mg/Fe=2 was studied in the range of 60-1100°C during heating in air. TG/DTA coupled with evolved gas analysis, emanation thermal analysis (ETA), surface area measurements, XRD, IR and Mössbauer spectroscopy were used. Microstructure changes characterized by ETA were in a good agreement with the results of surface area measurements and other methods. After the thermal decomposition of the pyroaurite-like anionic clay, which took place mainly up to 400°C, a predominantly amorphous mixture of oxides is formed. A gradual crystallization of MgO (periclase) and Fe2O3 (maghemite) was observed at 400-700°C by XRD. The MgFe2O4 spinel and periclase were detected at 800-1100°C. The spinel formation was also confirmed by Mössbauer spectroscopy.

Interface Induced Growth and Transformation of Polymer-Conjugated Proto-Crystalline Phases in Aluminosilicate Hybrids: A Multiple-Quantum 23Na–23Na MAS NMR Correlation Spectroscopy Study.

Nanostructured materials typically offer enhanced physicochemical properties because of their large interfacial area. In this contribution, we present a comprehensive structural characterization of aluminosilicate hybrids with polymer-conjugated nanosized zeolites specifically grown at the organic-inorganic interface. The inorganic amorphous Al-O-Si framework is formed by alkali-activated low-temperature transformation of metakaoline, whereas simultaneous copolymerization of organic comonomers creates a secondary epoxide network covalently bound to the aluminosilicate matrix. This secondary epoxide phase not only enhances the mechanical integrity of the resulting hybrids but also introduces additional binding sites accessible for compensating negative charge on the aluminosilicate framework. This way, the polymer network initiates growth and subsequent transformation of protocrystalline short-range ordered zeolite domains that are located at the organic-inorganic interface. By applying an experimental approach based on 2D (23)Na-(23)Na double-quantum (DQ) MAS NMR spectroscopy, we discovered multiple sodium binding sites in these protocrystalline domains, in which immobilized Na(+) ions form pairs or small clusters. It is further demonstrated that these sites, the local geometry of which allows for the pairing of sodium ions, are preferentially occupied by Pb(2+) ions during the ion exchange. The proposed synthesis protocol thus allows for the preparation of a novel type of geopolymer hybrids with polymer-conjugated zeolite phases suitable for capturing and storage of metal cations. The demonstrated (23)Na-(23)Na DQ MAS NMR combined with DFT calculations represents a suitable approach for understanding the role of Na(+) ions in aluminositicate solids and related inorganic-organic hybrids, particularly their specific arrangement and clustering at interfacial areas.

STABILITY OF IRON IN CLAYS UNDER DIFFERENT LEACHING CONDITIONS

The iron chemistry of aluminosilicates can markedly affect their adsorption properties due to possible changes in surface charge upon exposure to a variety of processes in the environment. One of these processes is chemical leaching, but to date little has been reported on the susceptibility of structural Fe to chemical leaching. The purpose of the current study was to determine the effects of solution pH on the stability of structural Fe in kaolinites, illite, and bentonite and the potential for formation of ancillary (oxyhydr)oxides. Structurally bound Fe does not participate in sorption properties but Fe that is released and phase transformed during leaching could take part in adsorption processes and form complexes and/or covalent bonds via Fe ions. Five different Fe-bearing clay minerals were treated in 0.5 M and 2 M HCl, distilled H2O, 0.1MKCl, and 0.5MKHCO3 for 24 h. The amount of Fe leached varied from 10 μg g-1 (for 0.1 M KCl) to 104 μg g-1 (for 2 M HCl) depending on the leaching agents. Acidic and water treatments indicated a relative independence of leached Fe on the initial Fe content in the clay and, conversely, a heavy dependence on the crystallinity of initial Fe phases. Well crystallized Fe(III) was stable during the leaching process, while poorly crystallized and amorphous Fe(III) phases were less stable, forming new ion-exchangeable Fe3+ particles. Under alkaline conditions, no relation between Fe crystallinity and mobility was found. The structural and surface changes resulting from leaching processes were identified by equilibrium adsorption isotherms. In kaolinite, the specific surface area (SBET) and porosity changed independently of Fe leaching due to the stability and crystallinity of Fe. In bentonite, the number of micropores was reduced by their partial saturation with Fe3+ particles caused by poorly crystallized and more reactive Fe forms during the leaching process. Potential phase transformations of Fe were characterized by the voltammetry of microparticles; well crystallized Fe(III) oxides remained stable under leaching conditions, while poorly crystallized and amorphous Fe(III) phases were partially dissolved and transformed to reactive Fe3+ forms.

Removal of arsenate and antimonate by acid-treated Fe-rich clays

Iron impurities in clays degrade the quality in many aspects, but available Fe oxides can significantly improve adsorption affinity of clays to anionic particles. Two natural Fe-rich clays (kaolin and bentonite) were treated in 0.5 M HCl (pH = 1.1) and 0.15 M (COOH)2 (pH = 1.2), and then used to adsorb AsV/SbV oxyanions from model solutions. After acid leaching, the equilibrium sorption capacities (qmax) increased from 2.3 × 10-3 to 39.2 × 10-3 mmol g-1 for AsV and from 2.4 × 10-3 to 40.1 × 10-3 mmol g-1 for SbV, more than doubling the adsorption yields (≈95%) of both oxyanions. Leaching in 0.5 M HCl enhanced both AsV and SbV adsorption, whereas leaching in 0.15 M (COOH)2 mainly improved the adsorption of SbV. Bentonite, which contained fewer crystalline forms of Fe, exhibited better sorption properties for both oxyanions. The leaching of Fe followed first-order kinetics, whereas the adsorption of AsV/SbV followed second-order kinetics. Acid leaching of Fe-rich clays can be used for the preparation of highly selective anionactive sorbents.