Biotic ligand model of the acute toxicity of metals. 1. Technical Basis

Abstract

The biotic ligand model (BLM) of acute metal toxicity to aquatic organisms is based on the idea that mortality occurs when the metal-biotic ligand complex reaches a critical concentration. For fish, the biotic ligand is either known or suspected to be the sodium or calcium channel proteins in the gill surface that regulate the ionic composition of the blood. For other organisms, it is hypothesized that a biotic ligand exists and that mortality can be modeled in a similar way. The biotic ligand interacts with the metal cations in solution. The amount of metal that binds is determined by a competition for metal ions between the biotic ligand and the other aqueous ligands, particularly dissolved organic matter (DOM), and the competition for the biotic ligand between the toxic metal ion and the other metal cations in solution, for example, calcium. The model is a generalization of the free ion activity model that relates toxicity to the concentration of the divalent metal cation. The difference is the presence of competitive binding at the biotic ligand, which models the protective effects of other metal cations, and the direct influence of pH. The model is implemented using the Windermere humic aqueous model (WHAM) model of metal-DOM complexation. It is applied to copper and silver using gill complexation constants reported by R. Playle and coworkers. Initial application is made to the fathead minnow data set reported by R. Erickson and a water effects ratio data set by J. Diamond. The use of the BLM for determining total maximum daily loadings (TMDLs) and for regional risk assessments is discussed within a probabilistic framework. At first glance, it appears that a large amount of data are required for a successful application. However, the use of lognormal probability distributions reduces the required data to a manageable amount.

Temporal assessment of copper speciation, bioavailability and toxicity in UK freshwaters using chemical equilibrium and biotic ligand models: Implications for compliance with copper environmental quality standards

“…This is in accordance with the commonly reported effects of these major cations on metal toxicity to aquatic organisms and probably reflect the competition between these ions and the free copper ion for binding on the cell surfaces (Playle et al, 1993;Campbell, 1995;Di Toro et al, 2001;De Schamphelaere and Janssen, 2002). Occupation of the sites by calcium and/or magnesium would result in normal membrane function, while occupation of the sites by copper would disrupt function as toxicity is considered to occur when a critical proportion of the sites are occupied by copper.…”

“…This is considered a much better approach than using conventional relationships based only on hardness (e.g. by Di Toro et al, 2001).…”

Evaluating the effects of pH, hardness, and dissolved organic carbon on the toxicity of aluminum to freshwater aquatic organisms under circumneutral conditions

“…Within the range of pH 6 to 8, however, Al exists primarily as insoluble forms of Al(OH) 3 , and at the time the US Environmental Protection Agency (USEPA) AWQC was prepared (1987), the available toxicity data under these conditions were relatively limited. Furthermore, the potential effects of hardness or dissolved organic carbon (DOC) complexation on Al toxicity were not considered even though they exert significant effects on the toxicity of most metals [2][3][4]. Although the effects of hardness cations and DOC on Al toxicity are well understood under acidic pH conditions [5,6], their effects have not yet been rigorously evaluated under more circumneutral conditions.…”

Modeling toxicity of binary metal mixtures (Cu2+–Ag+, Cu2+–Zn2+) to lettuce, Lactuca sativa, with the biotic ligand model

“…According to the BLM concept, ions compete with each other for transport sites at the biotic ligands, and this competition acts as a mechanism for ion-ion interactions [1,2]. This assumption is based on physiological findings indicating that toxic cations, such as Cu 2þ and Ag þ , may inhibit the uptake of Na þ or Ca 2þ for specific binding sites at the fish gill, leading to adverse effects [3][4][5].…”