Siderophore Adsorption to and Dissolution of Smectite Clays.

Although many studies have quantified the dissolution of iron from kaolinite clays and iron oxides by siderophores, little work has been done determining the siderophore-mediated dissolution of smectite clays found in temperate soils.  Using the Cu-Chromeazurol S (CAS) method outlined by Shenker (1995) and Flame Atomic Absorption Spectroscopy (FAAS), we determine the level of sorption to and iron release from several nontronite and low-iron montmorillonite clays.  We have found that for a given concentration of the high iron clay, SWa-1, clay particle fractions of a lesser Stokes diameter—and, hence, greater edge-to-basal surface areas—adsorb greater amounts of desferrioxamine B indicating that the siderophore releases iron primarily from clay particle edges.  No such pattern was determined for the low-iron montmorillonite, STx-1, where modest increases in edge-to-basal surface area would have less effect on iron release. Results quantifying the effect of charge-reduction, redox cycling, and alteration induced by low molecular weight organic and humic acids reveal further details on how various bacterial and fungal siderophores release iron from smectite clays.