Adsorption of hydroxamate siderophores and EDTA on goethite in the presence of the surfactant sodium dodecyl sulfate

摘要

Siderophore-promoted iron acquisition by microorganisms usually occurs in the presence of otherorganic molecules, including biosurfactants. We have investigated the influence of the anionicsurfactant sodium dodecyl sulfate (SDS) on the adsorption of the siderophores DFOB (cationic)and DFOD (neutral) and the ligand EDTA (anionic) onto goethite (α-FeOOH) at pH 6. We alsostudied the adsorption of the corresponding 1:1 Fe(III)-ligand complexes, which are products of thedissolution process. Adsorption of the two free siderophores increased in a similar fashion withincreasing SDS concentration, despite their difference in molecule charge. In contrast, SDS had littleeffect on the adsorption of EDTA. Adsorption of the Fe-DFOB and Fe-DFOD complexes alsoincreased with increasing SDS concentrations, while adsorption of Fe-EDTA decreased. Ourresults suggest that hydrophobic interactions between adsorbed surfactants and siderophores aremore important than electrostatic interactions. However, for strongly hydrophilic molecules, suchas EDTA and its iron complex, the influence of SDS on their adsorption seems to depend on theirtendency to form inner-sphere or outer-sphere surface complexes. Our results demonstrate thatsurfactants have a strong influence on the adsorption of siderophores to Fe oxides, which hasimportant implications for siderophore-promoted dissolution of iron oxides and biological ironacquisition.