Comparison of the Effects of Hydroxamate Siderophores on Lead and Cadmium Adsorption to Kaolinite

Siderophores are Fe(III)-specific organic ligands biosynthesized by microbes in Fe-deficient environments. They also complex other metals, such as Pb and Cd, influencing metal mobility. This study compared the effects of siderophores desferrioxamine-B (DFO-B), desferrioxamine-D (DFO-D1), desferrioxamine-E (DFO-E), as well as siderophore-like ligand acetohydroxamic acid (aHA) on Pb and Cd (except for DFO-E) adsorption to kaolinite (KGa-1b) at pH 4.5 to 9, in 0.1 M NaClO4, at 22oC, in the dark. At pH > 6, all of the ligands inhibited Pb adsorption with inhibition increasing in the order aHA < DFO-D1 < DFO-B < DFO-E. At lower pH, the presence of DFO-B slightly enhanced Pb adsorption. These ligands also inhibited Cd adsorption at pH > 8, but had no effect at low pH. Modeling the systems with PHREEQC suggested that the siderophores in solution out-competed the kaolinite surface for the metal cations, thus causing decreased adsorption at high pH.