Testing natural Mn-todorikite as an efficient, low-cost and environmental-friendly catalyst

Manganese (Mn) oxide minerals occur in a wide variety of geological settings. Besides being major components of Mn nodules that pave huge areas of the ocean floor, Mn oxide minerals are ubiquitous in soils and sediments and participate in a variety of chemical reactions affecting groundwater and bulk soil composition. Todorokite [(Ca,Na,K)X(Mn4+,Mn3+)6O12·3.5H2O] is one of the major Mn minerals identified in ocean Mn nodules, but it is also a major mineral in the oxidized zones of many terrestrial Mn deposits. As todorokite shows large zeolite-like tunnels, there has been considerable interest in producing synthetic analogues for possible utilization as catalysts or molecular sieves. A natural Mn-todorokite mineral collected in the Apulia region (Italy) has been evaluated as oxidation catalyst for degradation of methyl-orange (MO) dye. This mineral has been firstly characterised by X-ray diffraction, wavelength dispersive X-ray fluorescence, BET, scanning electron microscopy, attenuated total reflectance Fourier transform infrared spectroscopy and thermogravimetry. The catalytic dye-degradation data show that this Mn-todorokite can operate under strongly oxidizing potentials >+400 mV vs. standard hydrogen electrode performing fast MO degradation (t1/2 <5 minutes). A detailed study using Electron Paramagnetic Resonance spectroscopy revealed that, at Eh >+450 mV, the active Mn-centers of Mn-todorikite evolve rapidly through Mn3+/Mn4+ states and this is correlated with the rapid catalytic degradation of MO. These results suggest Mn-todorikite mineral as an efficient, low-cost and environmental-friendly catalyst which can be used for industrial purposes.