Thermospectroscopic study of the adsorption mechanism of the hydroxamic siderophore ferrioxamine B by calcium montmorillonite

The behavior of iron-chelating agents in soils is highly affected by interactions with the solid phase. Still this aspect is frequently ignored. In this research the adsorption of the siderophore ferrioxamine B by Ca-montmorillonite, as a free ligand (desferrioxamine B, DFOB) and as a complex with Fe ³⁺ (ferrioxamine B, FOB), was studied, using thermo X-ray diffraction (thermo-XRD) in the temperature range 25-360°C and thermo-FTIR spectroscopy in the temperature range 25-170°C. The effect of pH (4-7.5) on the adsorption was examined. Extensive use of curve-fitting analysis was required due to significant overlapping of the characteristic absorption bands of the various functional groups. Thermo-XRD analysis showed that both DFOB and FOB penetrated into the interlayer space of Ca-montmorillonite. FTIR results indicated strong interactions of DFOB within the interlayer, which involved all functional groups (NH ₃ ⁺, secondary amide groups, and hydroxamate groups). In contrast, the folded Fe complex of FOB retained its molecular configuration upon adsorption, and the basal spacing of the clay increased correspondingly. FOB interacted in the interlayer space of the clay, mainly through the NH of the secondary amide groups and NH ₃ ⁺, while the functional groups bound to the central Fe cation remained unchanged. The suspension pH had no significant effect on both DFOB and FOB adsorption at the examined range. Adsorption protected the adsorbates from thermal degradation compared to the nonadsorbed samples up to 105°C. At 170°C both DFOB and FOB were already partially degraded, but to a lesser extent than the nonadsorbed samples. Degradation of the molecules occurred mainly through the hydroxamic groups, which constitute the Fe-chelating center in the hydroxamic siderophore.