Metal Exposure and Transformation

Development of the Speciation-Based Metal Exposure and Transformation Assessment Model (META4)... 

Martin, J. L. Medine, A. J. META4 - Metal Exposure and Transformation Assessment Model, Model Documentation for Version 3, December, 1998. 

The continuous availability of trillions of independent microreactors greatly multiplied the initial mixture of extraterrestrial organics and hydrothermal vent-produced chemicals into a rich variety of adsorbed and transformed materials, including lipids, amphiphiles, chiral metal complexes, amino add polymers, and nudeo-tide bases. Production and chiral amplification of polypeptides and other polymeric molecules would be induced by exposure of absorbed amino adds and organics to dehydration/rehydration cydes promoted by heat-flows beneath a sea-level hydro-thermal field or by sporadic subaerial exposure of near-shore vents and surfaces. In this environment the e.e. of chiral amino adds could have provided the ligands required for any metal centers capable of catalyzing enantiomeric dominance. The auto-amplification of a small e.e. of i-amino adds, whether extraterrestrially delivered or fluctuationally induced, thus becomes conceptually reasonable. 

These examples show that also in the case of single chemicals, such as pesticides and metals, exposure assessment should not only focus on the parent chemical but also include the metabolites and transformation products produced either in the environment or upon biotransformation in the organism. 

This is a safe, low cost, and more convenient approach as it does not involve special instrumentation, poisonous intermediates, and the growing rate can also be easily controlled. The utility of this process is underlined by the fact that even after continuous exposure to the toxic metal ions, the fungus readily grows and transforms the toxic conditions to nontoxic by reducing Cd to CdS without the use of any external source of sulfur. Another important, potential benefit of the process described is the fact that the semiconductor CdS nanoparticles, which are quite stable in solution, are synthesized extracellularly in large quantities. This is therefore, a very important advantage over other biosynthetic methods where the nanoparticles are entrapped within the cell matrix in limited quantity whereby an additional processing is required to release them from the matrix. 

The assay system described in the present study should be useful in screening potential metal carcinogens for their ability to induce transformation. However, further work is required to demonstrate and validate that each of the various metals induce neoplastic transformation in tissue culture. A statistically significant number of transformed colonies induced by exposure of cells to each of the metal compounds must be cloned and derived into cell lines. The tumorigenic potency of these various cell lines must be tested in nude mice. 

The human health and environmental factors are then multiplied by the exposure potential which includes parameters expressing biological oxygen demand half-life, hydrolysis half-life and an aquatic bioconcentration factor. It is felt that this system is probably one of the better impact assessment systems available today because it assigns impact values based on quantitative scientific data rather than subjective concern over a chemical which is often based on perception rather than scientific data. On the other hand, the bioaccumulation and persistence factors have already been shown to be not particularly relevant to metals per se. In the future, alternative evaluation systems such as solubility and transformation characteristics of metals and metal compounds, and models such as the biotic ligand model will be found to be much more appropriate for evaluating the human health and environmental impacts of battery metals. 

Developing an objective assessment of the hazard that copper poses to humans and the environment depends on an intimate understanding of the bioavailability. Bioavailability, which is defined as the extent to which the metal is taken up by the organism upon exposure, depends on the species of the metal or metallo complex and/or how easily it can be transformed to a more or less bioavailable species. The key components of the environmental risk assessment paradigms include problem formulation, analysis (which includes both exposure and effects analysis) and risk characterization (WHO 1998). 

Cobalt is a brittle, hard metal, resembling iron and nickel in appearance. It has a metallic permeability of about two-thirds that of iron. The transformation is slnggish and accounts in part for the wide variation in reported data on physical properties of cobalt. Exposure to cobalt (metal fumes and dust) should be limited to 0.05 mg/m (8-hour time-weighted average in a 40-hour week). 

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