REDUCING BEHAVIOUR OF NATURAL ORGANIC MATTER AND ITS INFLUENCE ON THE URANIUM MOBILITY IN FLOODED MINES

Large underground mines are beiing flooded by surface and groundwater during the restoration of the East German uranium mining sites. The mine shafts and galleries are heavily reinforced with wood, and the flooding process isolates the wood from exposure to air. In conjunction with the wood degradation, this may cause potentially reducing conditions.

To study this effect, redox potential measurements and redox titrations with potassium hexacyanoferrate(III) were carried out on hydrothermal wood leachates, lignin and humic acid using a platinum electrode in combination with a silver-silver chloride electrode in a cell with liquid junction. The redox potentials of spruce lignin and humic acid, extrapolated to pH 0 and infinite dilution, were determined to be 570"5 mV for both with a negative pH gradient of 54.0"1 mV/pH. The potential indicates that the organic matter produced by the wood degradation can reduce U(VI) to U(IV). We identified U(IV) as the reduction product by spectrophotometry of the U(IV)-arsenazo(III) complex . The capacity for lignin to reduce U(VI) in-creases from pH 5 to 8 from 1.3@10-2 to 0.5 µeq/g, respectively. For comparison, iron(III) was reduced at pH 4.5 with a capacity of 0.15 meq/g.

We also studied how reducing conditions were generated by the decay of wood in a highland bog . We measured the redox potential in the field as a function of depth, and found that the redox potential de-creased by more than 800 mV from the surface to the depth of one meter. In comparison, the redox potential in deeper layers of the flooded mines was diminished by about 700 mV. This is ascribed to the oxidation of sulfidic and arsenic min-erals, especially of pyrite, markasite and arsenopyrite. This lowering of the redox potential causes the UO22+ to be immobilized as UO2. Our results show that the degradation of the large quantities of wood in the mines also contribute to establish, maintain and enhance reducing conditions and, hence, to immobilize significant parts of the 100 tons of urani-um and 50 tons of arsenic that are dissolved in Saxony=s largest underground mine at Schlema.