Natural cryptomelane and its potential application in the adsorption of heavy metal cadmium

The Xiangtan manganese deposit (XTM) was formerly considered a supergene oxide manganese deposit in South China. Here, we report upon a new identification of naturally outcropping cryptomelane from the XTM, rather than psilomelane and pyrolusite, as it was previously considered. Most crystals in aggregates of XTM cryptomelane are acicular, with a diameter from 20 nm to 120 nm. There are large pseudotetragonal tunnels in the cryptomelane formed by [MnO6] octahedral double chains with an aperture of 0.462 × 0.466 nm2. The importance of our find relates not only to manganese resource prospecting in South China, but also to the application of the octahedral molecular sieve of cryptomelane. On the basis of the study of some mineralogical characteristics, experiments on heavy metal cadmium adsorption have been done. The overall adsorption capacity of cryptomelane for Cd2+ with different electrolytes showed an increase with a pH rise from 3 to 11.5. This work also showed that in addition to an ion exchange with H+, the adsorption of Cd2+ might involve exchange with Mn and K cations in the cryptomelane structure. Isotherms for Cd2+ adsorption in about neutral pH solutions can be satisfactorily represented by the Langmuir expression and the theoretic maximum disposal capacity thus determined is 5.54 mg/g.