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 nm 2 . The importance of our find relates not only to manganese resource prospecting in South China, but also to the appli - cation of the octahedral molecular sieve of cryptomelane. On the basis of the study of some mineralogical char- acteristics, experiments on heavy metal cadmium adsorption have been done. The overall adsorption capacity of cryptomelane for Cd 2+ 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 Cd 2+ might involve exchange with Mn and K cations in the cryptomelane structure. Isotherms for Cd 2+ adsorption in about neutral pH solu- tions can be satisfactorily represented by the Langmuir expression and the theoretic maximum disposal capacity thus determined is 5.54 mg/g.