Natural Mn-todorokite as an efficient and green azo dye-degradation catalyst

A natural Mn mineral, i.e., todorokite [(Ca,Na,K)X(Mn4+,Mn3+)6O12•3.5H2O], has been collected in the Apulia region, south of Italy, and evaluated as oxidation catalyst for degradation of methyl orange (MO) dye. This Mn-todorikite 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. Catalytic dye-degradation data show that this Mn-todorokite can operate under strongly oxidizing potentials (Eh >+400 mV) vs. standard hydrogen electrode performing fast MO degradation (t1/2 <1 minute). A detailed study using Electron Paramagnetic Resonance spectroscopy revealed that, under oxidative conditions (Eh >+450 mV), the active Mn-centers of todorikite evolve rapidly through Mn3+/Mn4+ states and this is correlated with the fast catalytic degradation of MO. These results suggest Mn-todorikite mineral as an efficient, low-cost and green catalyst which can be used for industrial and environmental purposes.