The Use of XAFS to Distinguish between Inner- and Outer-Sphere Lead Adsorption Complexes on Montmorillonite

Adsorption mechanisms of Pb on montmorillonite were investigated by conducting equilibrium and X-ray absorption fine structure (XAFS) spectroscopy studies. Data from the batch equilibrium studies indicate that Pb could be adsorbing via two mechanisms, depending on ionic strength. At low ionic strength (I = 0.006 M) Pb adsorption is pH-independent: 97% of the available Pb was removed from solution at pH 4.42 and 100% at pH 8.0. This behavior is consistent with an outer-sphere complexation mechanism. At high ionic strength (I = 0.1 M) Pb adsorption is pH-dependent, suggesting inner-sphere complexation as the adsorption mechanism: 43% of the available Pb was removed from the solution at pH 4.11 and 98.9% at pH 7.83. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy results reveal that in the sample equilibrated at I = 0.006 M, pH 4.48-6.40 the local atomic structure (LAS) surrounding the adsorbed Pb is similar to the LAS surrounding Pb(2+) (aq), confirming that the adsorption mechanism is outer-sphere complexation. In the system equilibrated at I = 0.1 M, pH 6.77 the XANES and EXAFS results show that the LAS surrounding the adsorbed Pb atom is similar to the LAS surrounding reference compounds in which Pb is forming covalent bonds (Pb(4)(OH)(4+)(4) (aq) and a sample of gamma-Al(2)O(3) with Pb adsorbed via inner-sphere complexation). These similarities indicate that Pb is forming inner-sphere complexes on the montmorillonite at this ionic strength and pH. In samples equilibrated at I = 0.006 M, pH 6.77 and I = 0.1 M, pH 6.31 the XAFS results suggest that Pb is forming both inner- and outer-sphere adsorption complexes. This observation could not be distinguished by making macroscopic observations only. Thus, the results of this study reveal important information on Pb sorption behavior on clays and also provides insights into the use of XAFS to determine sorption mechanisms. Copyright 1999 Academic Press.