TREATMENT OF ACID MINE DRAINAGE FROM AN URANIUM DEPOSIT BY MEANS OF A NATURAL WETLAND

Acid mine drainage waters generated in the uranium deposit Curilo, western Bulgaria, were treated by means of a natural wetland located in the deposit. The waters had a pH in the range of about 2.7 – 4.1 and contained uranium, radium, some heavy metals (iron, manganese, copper, zinc, cadmium, lead), arsenic and sulphates as main pollutants. The water course through the wetland covered a distance of about 140 m and the water flow rate varied in the range of about 0.3 - 1.5 l/s. The wetland was characterized by an abundant water and emergent vegetation and a diverse microflora. The treatment commenced in May 1998 and by April 2001, pollutant concentrations had decreased below relevant permissible levels for agricultural and industrial use. Removal mechanisms included microbial dissimilatory sulphate reduction and biosorption; uranium precipitated as tetravalent compounds, and heavy metals and arsenic were primarily removed as insoluble sulphides. Organic matter and clay minerals played a significant role in adsorbing radium, while plant retention also contributed. Throughout the process, no negative effects on indigenous plant and microbial communities were recorded.