A comparative study of the effect of desferrioxamine B, oxalic acid, and Na-alginate on the desorption of U(VI) from goethite at pH 6 and 25 °C Domenik Wolff-Boenisch * , Samuel J. Traina Sierra Nevada Research Institute, University of California, Merced, CA 95344, USA Received 30 January 2006; accepted in revised form 29 June 2006 Abstract Organic ligands affect the sorption and mobility of radionuclides in soils. Batch desorption experiments on goethite particles reveal the extent of uranyl desorption and hence bioavailability with different organic acids. The desorptive strength increases in the following order: background electrolyte < Na-alginate < desferrioxamine B (DFO-B) < oxalate. The sequence is consistent with decreasing molec- ular size and mass from alginate via DFO-B to oxalate. The concomitant Fe release in the desorption experiments indicates that desorp- tion from goethite and not dissolution of goethite governs the mobility of adsorbed U(VI). A compilation of DFO-B surface excesses on goethite from our experiments together with literature values indicate that DFO-B adsorbs at a constant 3% to the goethite surface. It is surprising that such a small fraction suffices to account for the considerable uranyl desorption and thus remobilization of a radionuclide into solution. Oxalate displays higher surface concentrations but still lower than the determined uranyl surface excess. It follows that based on the high U(VI) stability constants, both organic ligands induce the desorption of uranyl species by increasing the chemical affin- ity of the aqueous phase. In the case of alginate, desorption of uranyl is weak and adsorbed alginate hampers any considerable detach- ment of U(VI) in the presence of the more potent ligands, DFO-B and oxalate. This inhibition is based on biosorption and in this respect polysaccharides in soils may retard and even halt the advance of actinides through the soil column. This hypothesis calls for further stud- ies into the interaction of siderophores and polysaccharides with soil adsorbents and their role in the mobilization of contaminant metals. Ó 2006 Elsevier Inc. All rights reserved. 1. Introduction Uranium is the principal element in the processing and disposal of materials during nuclear energy and nuclear weapons production. It is also the major contaminant in soils and groundwater at sites associated with the afore- mentioned processes. According to MacDonald (1999), the remediation of contamination caused by the manufac- turing of nuclear weapons (e.g. uranium mining) in the United States is the most monumental environmental res- toration task in history. To achieve this objective a compre- hensive understanding of the thermodynamics and kinetics of the physicochemical reactions occurring in the various affected environmental compartments is crucial. Especially the removal and immobilization of radionuclide contami- nants in soils by microbial transformations, sorption and mineralization show the purging potential some natural and engineered microbes may have (Barkay and Schaefer, 2001). Recent studies have focused on the interaction of actinides with siderophores, a group of iron chelators which are produced by bacteria and other microbes to overcome low iron availability. Their principal function is to acquire ferric iron by dissolving poorly soluble iron min- erals and oxides and mediate iron transport and deposition inside the cell (Boukhalfa and Crumbliss, 2002). Although originally synthesized as a means of sequestering ferric iron, siderophores have been shown to complex other metals and radionuclides such as Pu(IV) (Birch and Bach- hofen, 1990; John et al., 2001). In the case of Pu, its com- plexation and subsequent uptake into the bacterial cell 0016-7037/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.gca.2006.06.1565 * Corresponding author. Fax: +1 209 724 4424. E-mail address: dwolff-boenisch@ucmerced.edu (D. Wolff-Boenisch). www.elsevier.com/locate/gca Geochimica et Cosmochimica Acta 70 (2006) 4356–4366