Influence of microbial hydroxamate siderophores on Pb(II) desorption from α-FeOOH

Siderophores are low-molecular weight organic molecules secreted by plants and micro-organisms in response to Fe stress. With stability constants commonly exceeding 10(30), siderophores are considered to have higher affinities for Fe(III) than for any other major or trace element dissolved in soil solution. However, several siderophores have affinities for trace metals that approach those for Fe(III), and certain actinides form siderophore complexes of surprisingly high stability. The purpose of this study was to examine the role of hydroxamate siderophores in controlling Pb sorption to an Fe(III) oxide adsorbent. Goethite [alpha-FeOOH], prepared by standard methods and identified by X-ray diffraction, gave a specific surface of 36 m(2) g(-1) as determined by N-2 multipoint BET analysis. Adsorption experiments were performed aseptically using a batch method with a goethite concentration of 1.0 g l(-1) and an ionic strength of 0.01 M NaClO4. Soluble Pb and Fe were measured between pH 3 and 8 by first adding Pb (10 muM) and then siderophore (10, 20, or 40 muM) to the goethite suspension. Three hydroxamate siderophores were employed: desferrioxamine B (DFB), ferrichrome (FC), and rhodotorulic acid (RA). Following 20 h reaction, Pb and Fe in solution were measured by ICP-MS and ICP-AES, respectively. The efficacy of siderophore-mediated Pb desorption varied with siderophore type and generally increased with pH and siderophore/Pb molar ratio. Desferrioxamine B, at pH 6.5 and a DFB/Pb molar ratio of 4, solubilised nearly 25% of the total sorbed Pb. In the presence of 10 muM FC, Pb adsorption largely mimicked that for the siderophore-free system, whereas significant amounts of Pb were desorbed with 20 muM FC at pH > 5.5. The dihydroxamate siderophore, RA, was the least effective Pb chelator, requiring 20 muM to desorb detectable amounts of Pb. (C) 2003 Elsevier Ltd. All rights reserved.