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

techniques can be useful in a study of metal uptake in plants, where transfer takes place
predominately via a solution phase. However, one must keep in mind that they are not
“selective” in metal species, give no particle size information and, above all, these leachable
fractions have never been correlated to bioavailability.
Solution Speciation Using Computer-Based Models. Computer-based models are either
based upon equilibrium constants or upon Gibb’s free energy values in determining metal
speciation from solution chemistry conditions (concentration, pH, Eh, organic complexes,
adsorption/desorption sites, and temperature). Both approaches are subject to mass balance and
equilibrium conditions. These models have undergone a great deal of development in recent
years, as reliable thermodynamic data has become available and can provide some predictive
estimates of metal behavior. A good review of these models and their applications is provided
by Lumsdon and Evans (1995).
Speciation can be controlled by simple reactions; however, in many cases, particularly in
contaminated media, their state of equilibrium and reversibility are unknown. In addition, these
models suffer from other limitations such as a lack of reliable thermodynamic data on relevant
species, inadequacies in models to correct for high ionic strength, reaction kinetics are poorly
known, and complex reactions with co-precipitation/adsorption are not modeled.
The first limitation is perhaps the most significant for contaminated media. For example,
none of the models would predict the common, anthropogenic, Pb phases, i.e., paint, solder,
and slag.
AX7.1.1.4 Biotic Ligand Model
The biotic ligand model (BLM) is an equilibrium-based conceptual model that has been
incorporated into regulatory agencies guidelines (including the EPA) to predict effects of metals
primarily on aquatic biota and to aid in the understanding of their interactions with biological
surfaces (see Annex Section AX7.2.1.3).
Because of assumed similarities in mechanisms of toxicity between aquatic and terrestrial
organisms, it is likely that the BLM approach as developed for the aquatic compartment may also
be applicable to the terrestrial environment. Recent research has been directed toward extending
the BLM to predict metal toxicity in soils (Steenbergen et al., 2005). Steenbergen et al. (2005)
pointed out that, until recently, the BLM concept has not been applied to predict toxicity to soil
AX7-18