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Siderophores are released by microorganisms to ensure their iron nutrition through the dissolution of iron(III) oxides. These compounds can also chelate divalent heavy metal ions and may therefore affect heavy metal sorption and mobility in soils. Using model systems, we studied the influence of the hydroxamate siderophore desferrioxamine B (DFOB) on the adsorption of Cu, Zn, and Cd on Na-montmorillonite and kaolinite. In the absence of heavy metals, DFOB was adsorbed onto montmorillonite much stronger than onto kaolinite. In montmorillonite suspensions, heavy metal sorption was enhanced by DFOB due to electrostatic interactions. Only at high pH values, a mobilizing effect of DFOB was observed for Zn and Cd. In kaolinite suspensions, DFOB drastically diminished heavy metal sorption. Surface complexation modeling revealed that the different effects of DFOB on the metal sorption by kaolinite and montmorillonite can be explained by the large difference in their permanent negative surface charge responsible for the attraction of the positively charged metal DFOB complexes. The effect of DFOB on the adsorption of heavy metal ions on montmorillonite and kaolinite decreased in the sequence Cu > Zn > Cd in accordance with complex stability constants. Introduction Mobility and toxicity of heavy metals in soil are governed by various parameters including pH and the content of clay minerals and organic matter. Complex formation with naturally occurring or synthetic complexing agents may enhance metal mobility. For this reason, adding organic ligands as mobilizing agents has been proposed as a means to increase heavy metal availability and plant uptake in soil remediation by phytoextraction using metal accumulating plants. In some studies, synthetic ligands such as EDTA were used (1). Chelating agents that occur naturally in the rhizosphere as exudates of plants and microorganisms are of particular interest because they are biodegradable and in general nontoxic to soil biota. In addition, leaching of contaminants can be kept to a minimum since mobilization would occur in the immediate surroundings of the roots of metal accumulating plants. Chelation may also help to increase the phytoavailability of essential trace elements under conditions of deficiency. Zn deficiency for example is probably the most important heavy metal micronutrient problem in crop growth (2). Siderophores are natural complexing agents, which occur especially in the rhizosphere (3, 4). They are exudated by the majority of soil microorganisms under iron deficiency to enhance Fe(III) availability and uptake (5). In soils, a wide range of siderophore-to-metal ratios is found. For a bacterial microniche, Hersman et al. (6) estimated siderophore concentrations in soil solutions to range from tens of micromoles to a few millimoles per liter. The stability constants for 1:1 Fe(III)-siderophore complexes are typically around 10 30 . Siderophores form stable complexes also with divalent heavy metal cations but exhibit lower stability constants than with Fe(III). Nevertheless, heavy metals influence siderophore production, and vice versa siderophores influence heavy metal toxicity (7-9). Chemical structures were analyzed (10, 11), and complex stability constants for Fe(III) and other metal complexes are known for a number of siderophores In this work, a model system for a soil was used. It consisted of the siderophore desferrioxamine B (DFOB, Experimental Section Reagents. Na-montmorillonite (SWy-2, Crook County, Wyoming, U.S.A.) and kaolinite (KGa-2, Warren County, Georgia, U.S.A.) were obtained from the Clay Minerals Society, Columbia, MO. Both clay minerals were fractionated to obtain the particle fraction e2 µm for montmorillonite and e1 µm for kaolinite. The clay minerals were washed three times with 1 M NaCl and with NANOpure water as many times * Corresponding author phone: ++41 1 6336009; fax: ++41 1 6331122; e-mail: furrer@ito.umnw.ethz.ch