Acid mine waters from the Ronneburg uranium mine in Thuringia (Germany) are partly enriched in uranium and other heavy metals and can be high in magnesium and sulphate. The discharge of mine waters as a consequence of the flooding of the underground operations might therefore cause contamination of ground and surface waters in the surrounding areas. In this study the chemical composition of selected mine waters was determined and different approaches, which might be used for an in-situ treatment of acid mine waters were investigated in a laboratory study.
The influx of seepage waters from uranium and sulphide bearing waste rock heaps results in highly contaminated mine waters in parts of the Ronneburg mine (Hähne, 1992). Due to this effect very acid (pH ª 2.5) and oxidizing mine waters occur in the area Lichtenberg. The investigated water samples are strongly enriched in magnesium (up to 2.8 g/L) and sulphate (up to 11 g/L) and contain high levels of uranium (up to 2.5 mg/L) and other heavy metals like iron (up to 240 mg/L), manganese (up to 25 mg/L), nickel (up to 7.5 mg/L), zinc (up to 6.5 mg/L), cobalt (up to 1.5 mg/L), and copper (up to 1 mg/L) as well as aluminium (up to 30 mg/L).
As a first approach, the mine water was neutralized by alkaline addition, which results in a strong decrease of uranium and heavy metal contents due to the precipitation of amorphous iron- and aluminium-hydroxides with increasing pH and coprecipitation and/or subsequent sorption of contaminants. At neutral pH, the contents of dissolved uranium, nickel, zinc, cobalt, and copper are 60 to 90 % lower compared to the original mine water. In addition, the use of fly ash from lignite-burning power plants for neutralization of acid mine water was investigated. Batch experiments show that the mine water can be neutralized by additon of about 2.5 g/L of fly ash. Again, the precipitation of iron- and aluminium-hydroxides with increasing pH and sorption or coprecipitation of heavy metals, including uranium, results in a significant decrease of contaminant levels. Similar results can be obtained by reaction of mine waters with fly-ash cement.
A further option for an in-situ treatment of acid mine waters and reduction of contaminant levels was the addition of scrap iron. The general idea was that the oxidation of metallic iron in contact with acid mine waters should cause the reduction of U6+ to U4+ and the subsequent precipitation of uranium. Therefore different amounts of scrap iron, filings and powdered iron were added to the mine water to investigate the presumed changes in the chemical composition. The addition of metallic iron results in a relatively fast decrease of the redox potential and increase in pH, although significant changes can only be achieved by addition of high amounts of scrap iron (e.g. 100 g/L) or by increasing the surface area, i.e. by using filings or powdered iron.
The contents of dissolved iron in the water samples increases to up to 380 mg/L, due to the dissolution of metallic iron, whereas the contents of aluminium, zinc, nickel, cobalt, and copper decrease strongly with the increase in pH. The effect of redox- an pH-changes on the uranium contents in the mine water treated with metallic iron is comparatively small. Even at neutral pH and Eh below -250 mV the water contains still about 1.6 mg/L uranium. Calculations of predominant dissolved uranium species and saturation indices of uranium minerals using the geochemical model WATEQ4F (Ball and Nordstrom, 1991) suggest that the water samples treated with metallic iron are strongly supersaturated with respect to uraninite and sometimes also in respect to pitchblende. The amount of supersaturation up to several orders of magnitude indicates that the uranium contents in the samples are not limited by the precipitation of uraninite. The results of the geochemical modeling and the small decrease of the uranium contents of the mine waters after treatment with metallic iron, compared to neutralization by alkaline addition or treatment with fly ash can be explained by (i) kinetic inhibition of the reduction of U6+ to U4+ even under reducing conditions at neutral pH, (ii) kinetic inhibition of uraninite precipitation, and (iii) the lack of freshly precipitating iron-hydroxides under reducing conditions which can otherwise serve as sorbents for uranium.
Ball, J.W. & Nordstrom, D.K., USGS Open File Rep. 91-183, 1-189 (1991).
Hähne, R., Wasser, Luft, Boden 6/92, 24-26 (1992).