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

Bargar et al. (1997a) showed that Pb can adsorb to FeO6 octahedra on three different types of
sites: on corners, edges, or faces. Ostergren et al. (2000a,b) showed that the presence of
dissolved carbonate and sulfate increased Pb adsorbtion on goethite. The relative fraction of
corner-sharing complexes can be greatly increased by the presence of these ligands, as bridging
complexes between the metal and the corners are formed (Ostergren et al., 2000a,b).
Furthermore, Elzinga and Sparks showed that the mechanism of Pb sorption to SiO2 surfaces is
pH-dependent. In acid environments (pH < 4) Pb adsorbs largely as an inner-sphere mononulear
complex, but as pH increases, Pb sorption increasingly occurs via the formation of surface-
attached covalent polynuclear Pb species.
Recently, Jackson et al. (2005) used microfocused synchrotron-based X-ray fluorescence
(ΦSXRF) to detail the distribution of Pb and Cu in the nematode Caenorhabditis elegans. They
found that, while Cu was evenly distributed throughout the bodies of exposed C. elegans, Pb was
concentrated in the anterior pharynx region. Microfocused X-ray diffraction indicated that the
highly concentrated Pb region in the pharynx was actually comprised of the crystalline Pb
mineral, pyromorphite. The authors concluded that C. elegans precipitated pyromorphite in the
pharynx as a defense mechanism to prevent spreading the toxic metal to the rest of the
organism’s body. They further suggested that, because of the high turnover rate of nematodes,
biomineralization could play an important role in the speciation of Pb in certain soils.
Lead Solid-solution Partitioning
The concentration of Pb species dissolved in soil solution is probably controlled by some
combination of (a) Pb-mineral solubility equilibria, (b) adsorption reactions of dissolved Pb
phases on inorganic surfaces (e.g., crystalline or amorphous oxides of Al, Fe, Si, Mn, etc.; clay
minerals), and (c) adsorption reactions of dissolved Pb phases on soil organic matter. Dissolved
Pb phases in soil solution can be some combination of Pb 2+ and its hydrolysis species, Pb bound
to dissolved organic matter, and Pb complexes with inorganic ligands such as Cl ! and SO4 2! .
Alkaline soils typically have solutions supersaturated with respect to PbCO3, Pb3(CO3)2(OH)2,
Pb(OH)2, Pb3(PO4)2, Pb5(PO4)3(OH), and Pb4O(PO4)2 (Badawy et al., 2002). Pb-phosphate
minerals in particular are very insoluble, and calculations based on thermodynamic data predict
that these phases will control dissolved Pb in soil solution under a variety of conditions (Nriagu,
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