Colloid-borne Uranium in Mine Waters

High concentrations (≥ 1 g/l) of ultra fine Fe oxyhydroxy sulphate colloids (< 5 nm) were found in acid rock drainage solutions. The near-neutral “bulk waters” of mines such as tunnel waters, on the other hand, contain lower concentrations (about 1 mg/l) of particles of 100 to 300 nm in size which consist of Fe and Al compounds. The uranium(VI) is normally not colloid-borne in these two water types, either due to the high acidity (acid rock drainage) or due to the formation of dissolved carbonato complexes („bulk waters“). However, most of the U(VI) is colloid-borne in the pH range 4 to 6, a pH range that must be passed through during the flooding of a mine. Scavenging by particles and colloid aggregation and sedimentation can significantly immobilize the U(VI) in this pH range (natural attenuation).