Toxicity, of metal ions

Wastewaters generated from manufacturing plants that produce or use inorganic chemicals vary considerably, depending on raw materials, type of process, and the end product, among others. A screening program is often conducted to determine the presence, concentration, and toxicity of metal ions in such wastewaters. The minimum detection limits for the toxic metals are presented in Table 22.1. 

S.J. Stohs and D. Bagchi, Oxidative mechanisms in the toxicity of metal ions. Free Radical Biol. Med. 18, 321-336 (1995). 

List some mechanism that cells can use to combat the toxicity of metal ions ... 

TABLE 2.2 Relationship between the Toxicity of Metal Ions and Outer d-Shell Electrons 1... 

Bioavailability and toxicity of metal ions in aqueous systems are often proportional to the concentration of the free metal ion and thus decrease upon complexation. However, there are some metal compounds more dangerous than the metallic element itself (e.g., mercury vs. methyl mercury). 

Castranova V, Bowman L, Wright JR, et al. 1984. Toxicity of metallic ions in the lung Effects on alveolar macrophages and alveolar Type II cells. J Toxicol Environ Health 13 845-856. 

It has been found, further, that the above condition for the toxicity of metallic ions extends also to the toxicity of other derivatives (Maxted and Moon, 26) of the metals. This extension is of importance as confirmatory evidence for the part played by the structure of the d band in determining the presence or absence of toxicity, since there occur, in the metallic ions themselves, unoccupied s and p levels which have been left vacant by the loss of valency electrons as a result of ionization. Accordingly, in the case of the ions, the possibility cannot entirely be ruled out of some occupation of these vacant s and p levels, for instance by a relatively small excitation, by lower-level electrons. So long as such an effect is possible, the dependence of the strong chemisorptive bond on a suitable d-shell occupation—with the inference that these d electrons are in some way involved in the chemisorptive bonding— is not entirely clear, since the promoted electrons would also be available for taking part in the bond. If however metallic compoimds are taken in which the s and p levels, in place of being vacant as in the ions, are occupied by electrons which are already concerned in stable bond formation with another element, the possibility of the above effect vanishes. As an example, the toxicity of tetramethyl lead, with all four of its s and p orbitals already taken up in bond formation with carbon, i.e.,... 

McCloskey, J.T., M.C. Newman, and S.B. Clark. 1996. Predicting the relative toxicity of metal ions using ion characteristics Microtox bioluminescence assay. Environ. Toxicol. Chem. 15 1730-1737. 

Walker, J.D., M. Enache, and J.C. Dearden. 2007. Quantitative cationic-activity relationships for predicting toxicity of metal ions from physicochemical properties and natural occurrence levels. QSAR Combin. Sci. 26 522-527. 

Another electrochemical parameter, the standard reduction-oxidation potential (AEP) represents the absolute difference in electrochemical potential between an ion and its first stable reduced state, or in other terms, the ability of an ion to change its electronic state. This parameter is seldom used alone in the studies concerning the toxicity of metal ions, but is usually combined with AN/AIP (where AN=atomic number, AIP=the difference in the ionization potential (in eV) between the actual oxidation number (O.N.) and the next lower one (O.N.-1) or with log AN/AIP (Kaiser 1980 Kaiser 1985 McCloskey et al. 1996 Enache et al. 2003). 

The combined use of the two indices has been applied in QSAR studies to predict the relative toxicity of metal ions (McCloskey et al. 1996) or to predict biosorption capacity (Can and Jianlong 2007). 

Shaw, W.H.R. 1954b. Toxicity of cations toward living systems. Science 120(3114) 361-363. Shaw, W.H.R., and B. Grushkin. 1957. The toxicity of metal ions to aquatic organisms. Arch. Biochem. Biophys 67(2) 447-452. 

The references are summarized with emphasis on QSARs, as they relate to bioavailability, bioconcentration, biosoption, binding, or toxicity of metal ions. 

Enache, M., P. Paht, J.C. Dearden, and N.W. Lepp. 2000. Correlation of physico-chemical parameters with toxicity of metal ions to plants. PestManag Sci 56 821-824. 

Turner, J.E., M.W. Wilhams, K.B. Jacobson, and B.E. Hingerty. 1985. Correlations of acute toxicity of metal ions and the covalent/ionic character of their bonds. In QSAR in Toxicology and Xenobiochemistry, edited by M. Tichy, 171-178. Amsterdam Elsevier. 

The European Chemicals Agency s metal-specific tools were presented to a February 2, 2012 workshop (http //echa.europa.eu/documents/10162/17098/01Jl metal-specific tool use communication d2 lrws 20120203 en.pdf). While the tools did not include QSARs, appendix R.7.13-2 Environmental risk assessment for metals and metal compounds, discusses the possible use of QSARs for predicting the toxicity of metal ions (http //guidance.echa.europa.eu/docs/guidance document/information requirements r7 13 2 en.pdf). The following quote is from page 41 ... 

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