Which factors influence the immobilization of uranium in soils and sediments?

Abstract

Natural enrichment of uranium in soils and sediments is feasible to study the migration and immobilisation processes of radioactive elements in the geological environment. The investigations covered geological, geochemical, hydrogeological, hydrologic and radiometric aspects. The identification of the available uranium phases and its association with sediment phases was one of the main interests. Main controls in soil-aquifer systems include flow rate and water quality, mineralogical composition and physico-mechanical soil properties, vegetation and microbiological activity. A hydrologic regime with slow flow rates and vertical flow paths facilitates essential contact time between water and solid surfaces. Sufficient contact time (water/solid) allows for chemical retention mechanisms in aquifer material, in soils additionally plant associated and microbiological processes, e. g. bioreduction, biosorption or bioaccumulation. The latter are capable of retaining high U concentrations on the dry mass basis as frequently shown by laboratory investigations. Secondary enrichment of uranium as frequently observed in organic-rich soils and sediments may suggest, that biological processes are fundamental in pre-concentration by means of uranium entrapment. Thereby pore-water depletion of uranium occurs, resulting in retention through plant detritus or microorganisms. At least when the biomass degrades or dies back, the biosorbed or bioaccumulated uranium will be integrated in the soils. Hence, as remobilisation or resuspension are a matter of concern, mainly from laboratory studies it is concluded that uranium reduction and (bio-)mineralisation of U(IV) species are crucial for conservation.

However in field scale, with the help of spectroscopic, wetchemical and mineralogical investigations the presence of a hexavalent immobile uranium phase could be proved. In applying advanced imaging methods (TEM) and microbiological complementary studies (DNA isolation) more detailed conclusions on uranium retention mechanisms can be drawn.