Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

epresents Eb. The precise energy required to dislodge a core electron from a metal (i.e., Eb) will
be a function of the oxidation state and covalency of the metal. X-ray absorption studies that
focus on the location of the absorption edge are referred to as XANES (X-ray absorption near
edge structure). In the energy region immediately after the absorption edge, X-ray absorption
increases and decreases with a periodicity that represents the wave functions of the ejected
electrons and the constructive and destructive interference with the wave functions of the nearby
atoms. X-ray absorption studies used to investigate the periodicity of the absorption after Eb are
referred to as EXAFS (extended X-ray absorption fine structure). Since the electron
configuration of a Pb atom will be directly governed by its speciation (e.g., Pb bound to organics,
Pb adsorbed to oxide surfaces, PbS, etc.) X-ray absorption studies provide a powerful in-situ
technique for determining speciation without some of the problems associated with chemical
extractions (Bargar et al., 1997a,b, 1998).
Manceau et al. (1996) used EXAFS to study soil contaminated by gasoline-derived Pb in
France and found that the Pb was divalent and complexed to salicylate and catechol-type
functional groups of humic substances. He concluded that the alkyl-tetravalent Pb compounds
that were added to gasoline were relatively unstable and will not dominate the speciation of Pb
fallout from the combustion of leaded gasoline. The binding mechanism of Pb to organics is
primarily inner-sphere adsorption (Xia et al., 1997). DeVolder et al. (2003) used EXAFS to
demonstrate that Pb phases were shifting to the relatively insoluble PbS when contaminated
wetland soils were treated with sulfate. Strawn and Sparks (2000) gave evidence that added Pb
was adsorbed directly onto organic matter in an untreated silt loam, but in the absence of organic
matter (treated with sodium hypochlorite) the added Pb appeared to adsorb instead to some form
of SiO2. More recent XAS studies have demonstrated the importance of biomineralization of Pb
in soils by bacteria and nematodes (Xia et al., 1997; Templeton et al., 2003a,b; Jackson et al.,
2005). Templeton et al. (2003a,b) demonstrated that biogenic precipitation of pyromorphite was
the dominant source of Pb uptake by Burkholderia cepacia biofilms below pH 4.5. Above pH
4.5, adsorption complexes began to form in addition to Pb mineral precipitation.
In addition to XAS studies of Pb in environmental samples, numerous experimental-based
XAS studies have documented in detail the coordination environment of Pb adsorbed to Feoxides,
Mn-oxides, Al-oxides, and clay minerals (Manceau et al., 1996, 2000a,b, 2002; Bargar
et al., 1997a,b, 1998, 1999; Strawn and Sparks, 1999; Trivedi et al., 2003; Chen et al., 2006).
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