Fast formation of supergene Mn oxides/hydroxides under acidic conditions in the oxic/anoxic transition zone of a shallow aquifer.

Extensive uranium mining in the former German Democratic Republic (GDR) in eastern Thuringia and Saxony took place during the period of 1946-1990. During mining activities, pelitic sediments rich in organic carbon and uranium were processed and exposed to oxygen. Subsequent pyrite oxidation and acidic leaching lead to partial contamination of the area with heavy metals and acid mine drainage (AMD) even few years after completion of remediation. One of those areas is the former heap Gessen (Ronneburg, Germany) were the residual contamination can be found 10 m under the base of the former heap containing partly permeable drainage channels. Actually, in such a system, a rapid but locally restricted mineralization of Mn oxides takes place under acidic conditions. This formation can be classified as a natural attenuation process as certain heavy metals, e.g., Cd (up to 6 μg/g), Ni (up to 311 μg/g), Co (up to 133 μg/g), and Zn (up to 104 μg/g) are bound to this phases. The secondary minerals occur as colored layers close to the shallow aquifer in glacial sediments and could be identified as birnessite and todorokite as Mn phase. The thermodynamic model shows that even small changes in the system are sufficient to shift either the pH or the Eh in the direction of stable Mn oxide phases in this acidic system. As a consequence of 9-15-year-long formation process (or even less), the supergene mineralization provides a cost-efficient contribution for remediation (natural attenuation) strategies of residual with heavy metals (e.g., Cd, Co, Ni, Zn) contaminated substrates.