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 meq / 100 g) compared to the corresponding values of other typical microporous minerals. According to the ζ-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 °C resulting in dehydrated crystals. In accordance, the maximum specific surface area (SSA), measured at 350 °C by means of nitrogen sorption-desorption isotherms (59 m2/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 °C, though low (0.5 mg of ammonia desorbed / g), provided evidence for 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 not 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.