Uranium sorption on tropical wetland sediments

This paper describes a laboratory study of the interactions between dissolved U and sediments from a tropical wetland in Northern Australia. Sorption experiments were carried out across a wide pH range under well-mixed aerobic conditions. The sediments strongly adsorb U, particularly in the low to neutral pH range. Sorption is affected by several factors, including the pH, amount of U, mass loading of sediment and presence of organic material. The reversibility and kinetics of U sorption were also studied. Adsorption consisted of a fast initial uptake step followed by a slower process. Both adsorption and desorption experiments reached a similar equilibrium position, indicating that sorption reactions were largely reversible. However, the time required for equilibration was much greater than has been reported for similar experimental systems. The presence of sulfate decreased U-sorption, which is attributed to the formation of aqueous complexes between U and sulfate. Isotope exchange experiments showed that a large proportion of the natural U content of the sediments is in labile forms, and is consequently available for leaching. This was confirmed by a mild chemical extraction with Tamm's acid oxalate. Anthropogenic disturbances or other changes may release labile U from wetland sediments. (orig.)