Biotic Ligand Model, a Flexible Tool for Developing Site-Specific Water Quality Guidelines for Metals

Abstract

The biotic ligand model (BLM) is a mechanistic approach that greatly improves our ability to generate site-specific ambient water quality criteria (AWQC)for metals in the natural environment relative to conventional relationships based only on hardness. The model is flexible; all aspects of water chemistry that affect toxicity can be included, so the BLM integrates the concept of bioavailability into AWQC--in essence the computational equivalent of water effect ratio (WER) testing. The theory of the BLM evolved from the gill surface interaction model (GSIM) and the free ion activity model (FIAM). Using an equilibrium geochemical modeling framework, the BLM incorporates the competition of the free metal ion with other naturally occurring cations (e.g., Ca2+, Na+, Mg2-, H+), togetherwith complexation by abiotic ligands [e.g., DOM (dissolved organic matter), chloride, carbonates, sulfide] for binding with the biotic ligand, the site of toxic action on the organism. On the basis of fish gill research, the biotic ligands appear to be active ion uptake pathways (e.g., Na+ transporters for copper and silver, Ca2+ transporters for zinc, cadmium, lead, and cobalt), whose geochemical characteristics (affinity = log K, capacity = Bmax) can be quantified in short-term (3-24 h) in vivo gill binding tests. In general, the greater the toxicity of a particular metal, the higher the log K. The BLM quantitatively relates short-term binding to acute toxicity, with the LA50 (lethal accumulation) being predictive of the LC50 (generally 96 h for fish, 48 h for daphnids). We critically evaluate currently available BLMs for copper, silver, zinc, and nickel and gill binding approaches for cadmium, lead, and cobalt on which BLMs could be based. Most BLMs originate from tests with fish and have been recalibrated for more sensitive daphnids by adjustment of LA50 so as to fit the results of toxicity testing. Issues of concern include the arbitrary nature of LA50 adjustments; possible mechanistic differences between daphnids and fish that may alter log K values, particularly for hardness cations (Ca2+, Mg2+); assumption of fixed biotic ligand characteristics in the face of evidence that they may change in response to acclimation and diet; difficulties in dealing with DOM and incorporating its heterogeneity into the modeling framework; and the paucity of validation exercises on natural water data sets. Important needs include characterization of biotic ligand properties at the molecular level; development of in vitro BLMs, extension of the BLM approach to a wider range of organisms, to the estuarine and marine environment, and to deal with metal mixtures; and further development of BLM frameworks to predict chronic toxicity and thereby generate chronic AWQC.

Critical Limits for Hg(II) in soils, derived from chronic toxicity data

“…Ligand Model (Niyogi and Wood, 2004;Thakali et al, 2006), in that chemical speciation is regarded as the key to metal interaction with organisms, and thereby to toxicity. The free metal ion concentration is central to this idea, but competition with other cations, notably H + , is also taken into account, hence the pH term in the CLF.…”

Relative importance of direct and trophic uranium exposures in the crayfish Orconectes limosus: Implication for predicting uranium bioaccumulation and its associated toxicity

“…In parallel with the use of the biotic ligand model [9], the intention of the present study is to express the chronic toxic effects as a function of accumulated dose. This is referred to as the critical body residues approach [7,[10][11][12][13][14], which focuses on the characterization of metal distribution in organisms.…”

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].…”

“…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]. Furthermore, the assumption potentially allows taking into account interactions between different metal ions in assessment of mixture toxicity [1,6,7]. In particular, it is possible to predict how different metals interact with one another if their stability constants are known.…”