Chemistry Reference
In-Depth Information
6
QSARs versus BLM
6.1 INTRODUCTION
In Chapter 1 it was noted that the biotic ligand model (BLM) estimates equilib-
rium concentrations or activities of dissolved metals species that can compete for
and adsorb to ligands on biological surfaces. Chapter 5 provided detailed infor-
mation about quantitative structure-activity relationships (QSARs) for metal ions.
The purpose of this chapter is to distinguish between QSARs and the BLM for
metals.
6.2 BLM
The technical basis for the BLM was described by DiToro et al. (2001). Its application
to acute aquatic copper toxicity was described by Santore et al. (2001). Paquin et al.
(2002) provided a historical overview of the BLM, while the final section of this
chapter provides a list of 175 references describing BLM studies published from
2001 to 2011.
The BLM concept is a permutation of the free ion activity model (FIAM) (e.g.,
see Campbell 1995). As a primary distinction, the FIAM predicts metal uptake by
aquatic organisms by assuming that metal internalization is related to the free metal
ion activity in the bulk solution; however, the BLM assumes that the association
of metals with biotic ligands of the organism determines the toxic effects (Zeng
et al., 2009). Additional differences between the FIAM and BLM were discussed by
Hassler et al. (2004).
The aim of the BLM is to explain and predict the effects that metal speciation in
aqueous media has upon metal toxicity to aquatic organisms. The term
biotic ligand
refers to a specific biological receptor of an organism that interacts with the metal
ions in aqueous media. This interaction is similar to those involving any other type
of chemical ligand. It is considered in certain organisms such as fish that the biotic
ligand would be Na
+
and K
+
channel proteins of gills, which have a role in regulating
the ionic composition of blood. Therefore, in more explicit terms, the biotic ligand
is a biological receptor.
The BLM for acute toxicity to aquatic organisms is based on the hypothesis that
the mortality through intoxication occurs if the metal-biotic ligand complex reaches
or exceeds a critical concentration. This threshold is directly linked to the quantity
of solvated metal ions available in water. It follows that metal bioavailability in water
can be influenced by different chemical factors, such as (a) the agonist or antagonist
competition with other cations for interaction with the biotic ligand, (b) the action of
anions present in water, and (c) the action of some organic chemicals that can form
metal complexes, and in so doing, prevent or influence metal binding to the biotic
ligand.
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