A relevant discovery of the last decade is the identification of the photochemical activity of the kaolinite-type minerals. The activity depends on several surface properties, such as mineral composition, morphology, lattice impurities and structural order. Preparation of the activated surface can be made by intercalation, exfoliation, mechanochemical activation, acid treatment, thermal treatment. The process of intercalation is a well-known way for a long while to produce delaminated and finally exfoliated structure from double-layered minerals. Coordination of octahedral Al atoms can be reduced by means of mechanochemical activation (i. e. dry grinding) and/or thermal treatment. The (hydrochloric) acid treatment alters not only the acid-base properties of the surface, but has effect on the mineral composition also. In this study the influence of the mentioned surface modification methods is shown on kaolins from different deposits with varying structural order, mineral and lattice impurities. The modified surface is investigated with thermal analysis (TG-DTG, CRTA), XRD, solid phase NMR and IR spectroscopic techniques. Photocatalytic activity is tested in liquid phase with oxalic acid test molecule during 365nm irradiation. Applying and combining various surface modification methods it is possible to enhance the natural kaolin-based materials in order to develop photocatalysts.
Dynamic and controlled-rate thermogravimetric analyses have been carried out on acid-treated (11 and 5.8 M HCl), highiron-content kaolinites as potential photocatalysts. The mineral contaminants were determined by XRD, while the defect sites of reduced coordination number obtained by surface treatments were identified with 27 Al MAS NMR spectroscopy. Upon heating, water is evolved from the surface-treated samples in three main stages: (1) removal of adsorbed water up to ~ 200 °C, (2) goethite dehydroxylation between 200 and 350 °C and (3) dehydroxylation of the clay in the 300-700 °C temperature range. Identification of water released from the above mass loss steps is difficult due to the significant overlap of steps 2 and 3, as well as to the presence of coordinated water at broken edges and defect sites (-OH 2 + groups). As a result, the thermal behavior of surface-treated kaolinites should be taken into account both in the preparation of hybrids/composites and in the acid-base characterization of the catalytic surface.
Clay minerals are natural, abundant and widely used industrial raw materials. Kaolinite is a 1:1 type, layered phylloaluminosilicate, constituted by Si 4+ -centered tetrahedral (T) and Al 3+ -centered octahedral (O) layers. Kaolinite has a strong potential as innovative, environmental-friendly photocatalyst, due to its not yet understood photocatalytic activity. Photocatalytic investigations require clean samples, free of mineral contaminants. Natural kaolinite is obtained through mining of kaolin. The mineral composition and varying properties of kaolin significantly influence their catalytic activity, and therefore pose an adverse impact on their catalytic investigations. Laboratory synthesis of kaolinite offers a way to obtain kaolinite with the desired properties and purity. In the present work, the laboratory synthesis and evaluation of a synthetic kaolinite and its TiO 2 nanocomposites are reported. The hydrothermal synthesis method was chosen to minimize pollutants. The effect of the applied acid concentration and liquid phase ratio were investigated. The synthesized kaolinites were characterized by XRD, FTIR-ATR, TG/DTG/DTA. The mineral composition, the presence and crystallinity (Hinckley, Stoch, Range-Weiss indices) of kaolinite were determined by XRD. Fourier transform infrared spectroscopy was utilized to identify kaolinite vibrations. Thermal stability, mineral purity and dehydroxilation was determined by TG/DTG/DTA. The morphology and elemental composition maps of the best sample was investigated by TEM-EDX. Sol-gel method and thermal treatment were used to prepare synthetic kaolinite-TiO 2 nanocomposites with varying surface concentrations of TiO 2 . The composites were characterized by XRD and FTIR-ATR. The photocatalytic activity of the samples were investigated by the aqueous degradation of an oxalic acid test compound upon 365nm UV irradiation.
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