Free ion activity

Here m>u and m>UCd++ are molal concentrations of the unoccupied and occupied sites, respectively, and aCd++ is the activity of the free ion. Activity coefficients for the surface sites are not carried in the equation they are assumed to cancel. Equilibrium constants reported in the literature are in many cases tabulated in terms of the concentrations of free species, rather than their activities, as assumed here, and hence may require adjustment. 

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)... 

The behaviour of natural ligands has been discussed in Section 4.3.3. In addition to the direct effect of complexation that is related to a decrease in the free ion activity, it has been shown that some ligands, in particular the HS, can be sorbed directly to biological surfaces, in the presence or absence of the trace metal [228,229]. This result is likely due to the fact that HS and similar macromolecules contain hydrophobic moieties that facilitate their adsorption to the plasma membrane and cell wall [157,230,231]. Because adsorption is expected to occur primarily with sites that are independent of the transporters,... 

Most laboratory measurements of trace metal uptake are performed by manipulation of the metal and chelator concentration, and therefore it is often impossible to distinguish between a thermodynamic and a kinetic dependence on the free-ion activity. In fact, only limited work has tested, in detail,... 

When biological uptake does not perturb the external medium, then /int can be given by equation (35). As discussed above, this limiting condition is assumed to occur in both the free-ion activity and biotic ligand models. When Ka[M] < 1, then (cf. equation (7)) ... 

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. 

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