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

Abstract

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.

Biological Low-pH Mn(II) Oxidation in a Manganese Deposit Influenced by Metal-Rich Groundwater

“…Previous works have also revealed that romanechite is likely to be one of the biogenic Mn oxides in nature (73)(74)(75)(76)(77). Larger scale field studies revealed birnessite and todorokite as the main species of the autochthonous Mn minerals in the Mn deposit at this site (1,78), and these minerals are often biogenic origin (73). Birnessite is a phyllomanganate with a ϳ10-Å d-spacing and intends to be transferred to todorokite, which has the same structure as romanechite (79)(80)(81).…”

“…Furthermore, the fine Liesegang ring-like structure of the autochthonous Mn oxides might suggest a progressive mineralization process over the years, concomitantly with the dynamic balance of Mn(II) concentration found in the groundwater-affected porewater of the subsoil of the former uranium leaching heap (1). A slow mineralization process was explained by a slow activity of microorganisms in this oligotrophic ecosystem which are present in very low abundance compared to surface soils (5) and their low basal respiration rates (26).…”

“…Elevated concentrations of dissolved and colloidal Mn and Fe enriched in the acidic and metals contaminated groundwater (26) was speculated as one of the plausible sources for metals that led to the development of a secondary mineral deposit below the heap through sediment capillary effects (1). The strata-like secondary mineral deposit in the Pleistocene sediments was approximately near the raised groundwater table after pumping ceased and is comprised now of three different mineralized layers ( Fig.…”

“…1): (i) a silty sand layer consisting of Fe oxy/hydroxides with yellow-reddish color at the base of the strata (approximately Ϫ64 cm from the surface, referred to here as the "bottom layer"); (ii) a condensed, gray-to-black manganiferous sandy layer closely above the groundwater table (around Ϫ56 cm from the surface, referred to here as the "Mn layer") with a thickness up to 13.5 cm but unevenly distributed within the former mining area; and (iii) a 0.1-m thick, yellow-to-gray oxic layer with 75.5% sand and 24.3% silt (around Ϫ47 cm from the surface, referred to here as the "top layer"). The Mn layer is crumpled at the contact point with the bottom layer and has an average Eh of 650 Ϯ 39 mV reflecting the favorable oxidizing conditions within the Mn layer (1). Metals such as Cd, Ni, Co, and Zn are conspicuously enriched and attenuated due to the strong scavenging of Mn oxides in the Mn layer (26).…”

“…B iogenic Mn oxides contribute to the natural attenuation of metals, providing a potentially ecologically and economically friendly strategy for the in situ stabilization of metal pollutants in natural ecosystems and anthropogenic settings (1,2). The pH of environments that would be good targets for Mn oxide-mediated attenuation, e.g., drainage basins, mine tailings, and leaching heaps, is normally acidic due to the aerobic oxidation of sulfidic minerals (3).…”