Characterisation of natural microporous manganese oxides: The case of todorokite

A natural todorokite reference sample originating from the Montenegro mines, Cuba, was identified by powder X-ray diffraction, whereas details of its composition were investigated using, among others, advanced spectroscopic techniques such as X-ray photoelectron spectroscopy (XPS). The cation exchange capacity (CEC) of this Mn oxide, determined on the basis of the K+ <-> Cs+ exchange method, was found to be rather low (ca. 15 meg / 100 g) compared to the corresponding values of other typical microporous minerals. According to the zeta-potential measurements, the material is negatively charged in aqueous solutions with pH > ca. 2.4 and able to sorb cations. At pH values < ca. 2.4, it is positively charged but considerable amounts of Mn are released from the building framework of the crystal structure. The thermal analyses of the mineral and the following FT-IR investigation showed that the water molecules, initially located into the tunnels, are irreversibly lost at temperatures up to ca. 450 degrees C resulting in dehydrated crystals. In accordance, the maximum specific surface area (SSA), measured at 350 degrees C by means of nitrogen sorption-desorption isotherms (59 m(2)/g), gave additional information on the porosity of the material. The maximum solid acidity determined by temperature programmed desorption (TPD) after thermal pretreatment at 350 degrees C, though low (0.5 mg of ammonia desorbed / g), provided evidence far the absence of strong active sites (such as extended OH groups) even in the dehydrated phase. The data obtained during the present work, based on a reference todorokite sample, indicate that the mineral does nor possess especially interesting sorptive properties compared to other raw natural microporous materials. Therefore, additional research is necessary on certain chemically-modified forms which could exhibit potential favourable properties, suitable for technological and environmental applications.