Molecular Mechanisms of Lead Binding to Ferrihydrite–Bacteria Composites: ITC, XAFS, and μ-XRF Investigations

Binding of Pb(II) to ferrihydrite-Bacillus subtilis composites formed in the presence of bacterial cells were investigated through macroscopic and microscopic techniques. Diffuse layer model (DLM) fitting and isothermal titration calorimetry (ITC) analysis indicated that the hydroxyl group played a key role in Pb(II) sorption onto composites by masking reactive sites, such as carboxyl and phosphoryl groups of bacterial cells. Negative enthalpy (from -39.29 to -57.87 kJ mol^-1) and positive entropy (from 135.61 to 193.47 kJ mol^-1) of Pb(II) sorption onto composites revealed that inner-sphere complexes formed through exothermic reactions and was driven by both entropy and enthalpy. Spatial distribution of these inner-sphere species at varied Pb(II) loading demonstrated that interactions between Pb(II) and bacterial cells preceded that of mineral components in composites, using microfocus X-ray fluorescence spectroscopy (μ-XRF) maps and microfocus X-ray absorption near edge structure (μ-XANES) spectra. Combined with bulk Pb L_III-edge X-ray absorption fine structure (XAFS) spectrum, we inferred that mononuclear bidentate edge-sharing hydroxyl-Pb complexes, monodentate mononuclear carboxyl-Pb and phosphoryl-Pb complexes predominantly contributed to Pb(II) inner-sphere binding with mineral and bacterial fractions in composites, respectively. The molecular binding mechanisms obtained in this study provide further insight into the sequestration and migration of toxic metals in natural environments.