Abstract No. pan0505 Sorption of Heavy Metal Contaminants onto Hydrated Ferric Oxides: Mechanistic Modeling using X-ray Absorption Spectroscopy

No. pan0505 Sorption of Heavy Metal Contaminants onto Hydrated Ferric Oxides: Mechanistic Modeling using X-ray Absorption Spectroscopy P. Trivedi, D. Sparks (U. Delaware) and K. Pandya (NRL) Beamline(s): X11A Introduction: Iron oxides are ubiquitous in soils and aquatic sediments as discrete particles or coatings on other mineral and organic materials. They play a significant role in controlling the fate of heavy metal contaminants, such as lead, zinc, and nickel in soils and aquatic environments. Hence, to understand the mobility and bioavailability of these metal contaminants, these sorption interactions must be understood as a function of pH, ionic strength, adsorbate concentrations, temperature, competing ions, and the nature of the background electrolyte. Macroscopic isotherm studies demonstrated a linear correlation between the bulk aqueous metal concentration and the sorbed metal concentration suggesting that sorption of these metal ions onto ferrihydrite can be described by one average type of site. Furthermore, temperature studies indicate that these metal cations sorb onto the ferrihydrite surface via endothermic chemical reactions. Interestingly, the modeling results also suggest that these sorption mechanisms are a function of pH. In binary and ternary sorbate systems, the individual maximum sorption capacities of these metal ions were consistent with the ones observed in single metal systems suggesting that metal ions do not compete with each other. Methods and Materials: To validate these mechanistic interpretations, XAS studies were conducted on beamline X-11A at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory where the electron beam energy was 2.528 2.8 GeV with a maximum beam current of 280 mA. All Pb-ferrihydrite sorption samples were studied at the Pb LIII-edge, while all Zn-ferrihydrite sorption samples were studied at the Zn K-edge in fluorescence mode using a Ge solid-state multi-element detector. Additionally, XANES data were acquired on beamline X19A at S and P K-edges for systems involving sulfate and phosphate as background electrolytes. The XAS spectra were analyzed using WinXAS (Version 2.1). To obtain the structural information, the Fourier transforms of the background subtracted, normalized, and averaged XAS spectra were fitted with theoretical models of the reference compounds. Results: The XAS spectra of the sorption samples at various pH and adsorbate concentrations do not resemble those of the corresponding aqueous metal ions, thus confirming that sorbed metal ions do not retain their primary hydration shell upon sorption. At a constant pH, the configuration of the Pb-ferrihydrite sorption complex is invariant of the adsorbate concentration. Interestingly, this sorption mechanism transitions from a mixture of monodentate and bidentate mononuclear sorption complexes at lower pH to predominantly edge-sharing bidentate complexes at higher pH values. In the presence of stronger background ligands such as Cl, SO4, and PO4, Pb(II) ions form metal-bridged ternary sorption complexes with ferrihydrite illuminating the role of these ligands in the interactions of metal contaminants with sorbents such as ferrihydrite. Furthermore, a lack of meaningful Pb-Pb contributions in the sorption complexes confirms the absence of the formation of Pb precipitates on the ferrihydrite surface. The spectroscopic studies also show that Zn(II) chemisorbs to ferrihydrite to form mononuclear, bidentate inner sphere complexes. Upon sorption to ferrihydrite, the aqueous Zn ion loses its octahedral primary hydration shell to form a tetrahedral unit. For pH ≤ 6.5, the local coordination of Zn(II) sorbed onto ferrihydrite did not significantly vary with pH or sorbate/sorbent ratio. There is no evidence for the formation of well-known precipitates of Zn on the surface of ferrihydrite. These results suggest that one average complexation mechanism can describe Zn(II) sorption on ferrihydrite. At higher pH, with an increase in Zn(II) loading, contributions of Zn-Zn interactions become significant suggesting that, besides the mononuclear sorption complexes, Zn(II) ions must also form polynuclear complexes or surface precipitates upon sorption onto ferrihydrite. Similar to Pb(II), Zn(II) ions also form metal-bridged ternary mononuclear bidentate sorption complexes with ferrihydrite in the presence of stronger background ligands such as Cl, SO4, and PO4. Interestingly, the local structures, of sorbed Pb(II) and Zn(II) ions, in binary and ternary systems, did not vary with those found in single metal systems under similar reaction conditions. These results suggest that the metal ions do not interact with each other on ferrihydrite surface. Overall, the information derived from the combination of macroscopic and spectroscopic studies, will aid scientists and engineers in improving the current models that predict and manage the fate of toxic metals in the aquatic and soil environments.