Free ion activity model

The most widely used model in environmental studies is the free ion activity model (FIAM or FIM) which postulates that the uptake is dependent on the bulk activity of free M (i.e. cjy, as a practical simplification) [2,67], rather than to the total metal concentration [2,5,66,68,69]. This has led to recognition of the... 

Campbell, P. G. C. (1995). Interactions between trace metal and aquatic organisms a critique of the free-ion activity model. In Metal Speciation and Bioavailability in Aquatic Systems, eds. Tessier, A. and Turner, D. R., Vol. 3, IUPAC Series on Analytical and Physical Chemistry of Environmental Systems, Series eds. Buffle, J. and van Leeuwen, H. P., John Wiley Sons, Ltd, Chichester, pp. 45-102. 

Brown, P. L. and Markich, S. J. (2000). Evaluation of the free ion activity model of metal-organism interaction extension of the conceptual model, Aquat. Toxicol., 51, 177-194. 

Parker, D. R. and Pedler, J. F. (1997). Reevaluating the free-ion activity model of trace metal availability to higher plants, Plant and Soil, 196, 223-228. 

BINDING TO BIOLOGICAL LIGANDS AND FREE ION ACTIVITY MODEL (FIAM)... 

Hassler, C. S. and Wilkinson, K. J. (2003). Failure of the biotic ligand and free-ion activity models to explain zinc bioaccumulation by Chlorella kesslerii, Environ. Toxicol. Chem., 22, 620-626. 

The aim of this study was to investigate the effect of high but environmentally realistic concentrations of humic acid on the relative long-term accumulation of Cd in the freshwater mussel D. polymorpha under controlled laboratory conditions. We assessed whether the long-term uptake of Cd is in agreement with the free-ion activity model and if Cd accumulation is related to the Cd -jon activity in the water. 

Campbell, P.G.C. (1994) A critique of free ion activity model. In Metal Speciation and Bioavailability (eds Tessier, A. and Turner, D.). Lewis Publishers, CRC Press, Boca Raton, FL. 

Campbell PGC. 1995. Interactions between trace metals and aquatic organisms a critique of the free-ion activity model. In Tessier A, Turner DP, editors. Metal speciation and bioavailability in aquatic systems. Chichester (UK) John Wiley, p 45-102. 

Past studies have shown a decrease in the toxicity of metals in the presence of organic matter, due to the binding of natural ligands that are believed to bind metals and reduce the concentration of free ionic species. The free ion activity model proposes that the free metal ion (as opposed to total metal ion concentration) is the dominant species available for organisms, and assumes that the colloidally complexed species should be less available. 

Ion selective electrodes Measurement of the free metal ion is an important goal of metal speciation studies in natural waters, because of the tenet of the free ion activity model, that bioavailability is proportional to the free metal ion concentration. Ion selective electrodes respond to the free metal ion activity. A range of ion selective electrodes are available, but only the copper electrode has sufficient sensitivity for use in measurement at realistic environmental concentrations in natural waters. The correct use of the copper ion selective electrode remains a matter of debate, especially its calibration at the very low free metal ion concentrations using metal ion buffers and the applicability of this procedure to natural samples. 

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). 

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