The uptake of uranium by organic substances in a peat bog environment on a granitic bedrock

Abstract

The contents of uranium in the Brocken granite are bound to mineral phases like biotite, apatite, sphene and zircon. Due to the weathering during humid climatic conditions, U is primarily mobilized as UO22+ from the bedrock; thus the U concentration in the Brocken granite has diminished from about 14 ppm to 2–3 ppm in the weathered parts. The surface waters of the investigated area show oxidative and acid conditions. These facts support a mobilisation of U.The bog waters transport U mainly as dissolved uranyl fulvate, but a steady feed of rainwater (rainfall rate: more than 1500 mm/a) prevents a higher U concentration in the aqueous environment.In the peats of the high bog plateau and in the phytogenic sediments of Lake Oderteich there exist average U concentrations of between 3 and 4 ppm. In the sediment samples from the bottom of Lake Oderteich, the U concentration (2.5–5.4 ppm) correlates positively with the portion of sedimentary material extractable in alkaline solution. Thus U is to be associated with the contents of humic and fulvic acids.The sample material was fractionated into humic acids, fulvic acids and insoluble residues. Chemical investigations show that being soluble in water, the isolated organic acids are acid derivates of carbohydrates.Experiments in enrichment of U by these fractions show that the functional acid groups of the organic substances take up equivalent quantities of uranyl ions by ion-exchange mechanisms, depending on the pH value. A content of U of 47.5 wt.% was found as a maximum result in humic acid with about 6 meq. total acidity.