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

completely extract targeted phases. While chemical extractions provide some useful information
on metal phases in soil or sediment, the results should be treated as “operationally defined,” e.g.,
“H2O2-liberated Pb” rather than “organic Pb.”
Lead forms strong coordination complexes with oxygen on mineral surfaces and organic
matter functional groups (Abd-Elfattah and Wada, 1981), because of its high electronegativity
and hydrolysis constant. Therefore, Pb is generally not readily exchangeable, i.e., the amount of
Pb removed from soils by dilute acid or salts is usually less that 10% (Karamanos et al., 1976;
Sposito et al., 1982; Miller and McFee, 1983; Johnson and Petras, 1998; Bacon and Hewitt,
2005). Lead is typically adsorbed to organic and inorganic soil particles strongly via inner-
sphere adsorption (Xia et al., 1997; Bargar et al., 1997a,b, 1998). Kaste et al. (2005) found that a
single extract of 0.02 M HCl removed 15% or less Pb in organic horizons from a montane forest
in New Hampshire. The fact that relatively concentrated acids, reducing agents, oxidizing
agents, or chelating agents are required to liberate the majority of Pb from soils is used as one
line of evidence that Pb migration and uptake by plants in soils is expected to be low.
Lead that is “organically bound” in soils is typically quantified by extractions that
dissolve/disperse or destroy organic matter. The former approach often employs an alkaline
solution (NaOH), which deprotonates organic matter functional groups, or a phosphate solution,
which chelates structural cations. Extractions used to destroy organic matter often rely on H2O2
or NaOCl. Both organic and mineral horizons typically have significant Pb in this soil phase.
Miller and McFee (1983) used Na4P2O7 to extract organically bound Pb from the upper 2.5 cm of
soils sampled from northwestern Indiana. They found that organically bound Pb accounted for
between 25 and 50% of the total Pb present in the sampled topsoils. Jersak et al. (1997), Johnson
and Petras (1998), and Kaste et al. (2005) selectively extracted Pb from spodosols from the
northeastern United States. Using acidified H2O2, Jersak et al. (1997) found that very little
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