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

soils from shooting ranges and paint spills is commonly found as PbO and a variety of Pb
carbonates (Vantelon et al., 2005; Laperche et al., 1996; Manceau et al., 1996). Atmospherically
delivered Pb resulting from fossil fuel combustion is typically introduced into terrestrial
ecosystems as Pb-sulfur compounds and Pb oxides (Olson and Skogerboe, 1975; Clevenger
et al., 1991; Utsunomiya et al., 2004). After deposition, Pb species are likely transformed.
Although the specific factors that control the speciation of anthropogenic Pb speciation in soils
are not well understood, there are many studies that have partitioned Pb into its different
geochemical phases. A thorough understanding of Pb speciation is critical in order to predict
potential mobility and bioavailability (see Section AX7.1.1).
Selective chemical extractions have been employed extensively over the past 20 years for
quantifying amounts of a particular metal phase (e.g., PbS, Pb-humate, Pb-Fe/Mn-oxide) present
in soil or sediment rather than total metal concentration. Sometimes selective chemical
extractions are applied sequentially to a particular sample. For example, the exchangeable metal
fraction is removed from the soil using a weak acid or salt solution (e.g., BaCl2), followed
immediately by an extraction targeting organic matter (e.g., H2O2 or NaOCl), further followed by
an extraction targeting secondary iron oxides (e.g., NH2OH·HCl), and finally, a strong reagent
cocktail (e.g., HNO3-HCl-HF) targets primary minerals. Tessier et al. (1979) developed this
technique. More recently, this technique has been modified and developed specifically for
different metals and different types of materials (Keon et al., 2001). Alternatively, batch-style
selective chemical extractions have been used on soils and sediments to avoid the problems
associated with nonselective reagents (Johnson and Petras, 1998). Selective extractions can be a
relatively rapid, simple, and inexpensive means for determining metal phases in soils and
sediments, and the generated data can be linked to potential mobility and bioavailability of the
metal (Tessier and Campbell, 1987). However, some problems persist with the selective
extraction technique. First, extractions are rarely specific to a single phase. For example, while
H2O2 is often used to remove metals bound to organic matter in soils, others have demonstrated
that this reagent destroys clay minerals and sulfides (Ryan et al., 2002). Peroxide solutions may
also be inefficient in removing metals bound to humic acids, and in fact could potentially result
in the precipitation of metal-humate substances. In addition to the nonselectivity of reagents,
significant metal redistribution has been documented to occur during sequential chemical
extractions (Ho and Evans, 2000; Sulkowski and Hirner, 2006), and many reagents may not
AX7-25