Metal toxicity of water may be defined as the dissolved metal concentration per unit volume of water. Various models have been developed to predict acute metal toxicity of water. Following are the most used and most satisfactory models used for acute metal toxicity prediction:
To calculate water effect ratio first of all one solution is prepared in laboratory with known toxicity and average range of pH, dissolved oxygen, temperature, and specific conductivity is calculated. After that a water sample is collected from a site whose water effect ratio has to be calculated. Metal toxicity of the site water is calculated and the values of test from lab sample and site sample are used to calculate the ratio.
Free ion activity model is based on the fact that there is always equilibrium between free metal ions in a solution. This model uses Ion characteristics to predict the relative toxicity effects of metal ions. Most ion characteristics that are useful in predictive modeling of metal toxicity reflect the binding tendencies of metals to ligands (Christopher P. Tatara, Michael C. Newman, John T. McCloskey, Phillip L. Williams).
The concentration of metal ions in a solution depends upon various factors such as temperature and chemistry of water. Free ion activity model predicts the metal toxicity by directly measuring the metal ion concentration in water. This model is based on the assumption that organism only responds to the free-metal ion concentration in solution, regardless of the nature of the metal complexes present in water (J. Phycol, 2005).
For example in case of AgCl(s) Ag+(aq) + Cl-(aq) if the Cl- ion increase that that can be consumed by Ag+ ions the concentration of Cl- will increase in water and if the concentration of Cl- ions is lesser than that of Ag+ there will be more free Ag+ cations in the water. And if the extra Ag+ ions are not consumed by other anions available in water the Ag metal toxicity of water will increase.
Biotic Ligand Model
The Biotic Ligand Model (BLM) use metal speciation and the protective effects of competing cations (+ve metal ions) to predict metal binding at a surface with possibility of acute metal toxicity (e.g. gill of a fish). A legand may be defined as chemical structures that bind with another chemical or metal.
It is an effective and widely used model to determine acute metal toxicity of dissolved metals. It relies on the mathematical integration of interaction of trace metal or solution phase ligands with biotic ligands i.e. the living organism exposed to the toxicity. This model analyses the ligand's interaction with living beings (biotic ligands). Biotic legand model predict the toxicity of dissolved metal according to the quantity of metal deposited on the receptor living being. Following diagram shows conceptual biotic legand model.
Fig: Conceptual Biotic Legand Diagram
Biotic legand model of toxicity prediction clearly shows the effect of water chemistry variation on the toxicity of metal. Water chemistry which is characterized by the dissolved hydrogen, oxygen, and carbon-di-oxide