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

1974; Ruby et al., 1994). However, certain chelating agents, such as dissolved organic matter,
can prevent the precipitation of Pb minerals (Lang and Kaupenjohann, 2003).
Using a combination of desorption experiments and XAS, EXAFS, and XANES, Rouff
et al. (2006) found that aging of Pb-calcite suspensions resulted in changes in the solid-phase
distribution of Pb. Increased sorption time can reduce trace metal desorption by enhancing the
stability of surface complexes (Rouff et al., 2005) or by mechanisms involving microporous
diffusion (Backes et al., 1995), recrystallization-induced incorporation into the solid phase
(Ainsworth et al., 1994), and formation and stabilization of surface precipitates (Ford et al.,
1999). For adsorption of Pb to hydrous Fe oxides, goethite, and Pb-contaminated soils, aged
samples show Pb to be reversibly bound, suggesting Pb adsorption primarily to the substrates’
surfaces (Rouff et al., 2006). However, for Pb adsorption to calcite aging played a significant
role due to detection of multiple sorption mechanisms, even for short sorption times (Rouff et al.,
2006). pH also played a role in Pb sorption. Over long sorption periods (60 to 270 days), slow
continuous uptake of Pb occurred at pH 7.3 and 8.2. At pH 9.4, no further uptake occurred with
aging and very little desorption occurred. These results show the importance of contact time and
pH on Pb solid-phase partitioning, particularly in geochemical systems in which calcite may be
the predominant mineralogical constituent.
Soil solution dissolved organic matter content and pH typically have very strong positive
and negative correlations, respectively, with the concentration of dissolved Pb species (Sauvé
et al., 1998, 2000a, 2003; Weng et al., 2002; Badawy et al., 2002; Tipping et al., 2003). In the
case of adsorption phenomena, the partitioning of Pb 2+ to the solid phase is also controlled by
total metal loading, i.e., high Pb loadings will result in a lower fraction being partitioned to the
solid phase. Sauvé et al. (1997, 1998) demonstrated that only a fraction of the total Pb in
solution was actually Pb 2+ in soils treated with leaf compost. The fraction of Pb 2+ to total
dissolved Pb ranged from