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 an oxidation catalyst for the degradation of methyl orange (MO) dye. This Mn-todorokite mineral has been firstly characterized 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 (E_h > + 400 mV) vs. standard hydrogen electrode performing fast MO degradation (t_1/2 < 1 min). A detailed study using electron paramagnetic resonance spectroscopy revealed that, under oxidative conditions (E_h > + 450 mV), the active Mn centers of todorokite evolve rapidly through Mn³⁺/Mn⁴⁺ states and this is correlated with the fast catalytic degradation of MO. These results suggest Mn-todorokite mineral as an efficient, low-cost, and green catalyst which can be used for industrial and environmental purposes.