AN INVESTIGATION OF THE FEASIBILITY OF BIOMETHYLATION FOR GROUP IV METALS AND COMPOUNDS
Methylmetal compounds are more toxic than their inorganic counterparts. The process of environmental methylation of some toxic metals and metalloids has been briefly reviewed. Group IV metals (Sn, Pb) have found widespread industrial use in recent years. Use of lead and organolead chemicals has declined but tin and organotin chemicals production has been increasing. Their methyl derivatives are of comparable toxicity but long terra toxicological studies for tin do not exist. Possible methylation of trimethyllead species in environmental sediment samples was attributed by other workers to a dismutation reaction of the bis trimethyllead sulphide formed by the reaction of trimethyllead species in the presence of biologically produced free sulphide ions. Preparation of bis trimethyltin sulphide, and reaction with artificial daylight radiation proved that the same applies to trimethyltin species. Tetraraethyltin and dimethyltin sulphide trimer were the products of the reactions. Dismutation of trimethyl tin species on the oxo-ligand tropolone suggests that oxygen and sulphur ligands in the environment will play a part in the volatilisation and cycling of tin in the environment. From incubations of deuterated trimethyltin species in a number of sediment samples only fully deuterated tetramethyltin species were detected by ms, suggesting that tetramethyltin can only be produced from dismutation of bis-trimethyltin sulphide. Computer simulation of the ms of methylated, deuterated trimethyltin and mathematical treatment of its ms breaking pattern in the presence of fully deuterated tetramethyltin , set a precedent for future use of deuterated trimethyltin species in such incubation studies. Reactions of lead and tin compounds in aqueous abiotic system with naturally occurring carbonium ion donors like iodomethane, dimethyl sulphide, trimethyl sulphonium iodide and N-trimethyl glycine produced partially methylmetal species but not in all cases. Tetramethylmetal species could only be detected species but not in all cases. Tetramethylmetal species could only be detected from reactions of metals (Sn°, Pb^) with iodomethane. All reactions proceed initially by formation of an oxidative addition methyl product and its dis mutation or disproportionation to methyl metal species with a larger number of methyls attached to it. Methylation of triraethylmetal with methyl cobalamin could not be detected in kinetic experiments or experiments with more concentrated solutions. Reactions of divalent metal salts in aqueous abiotic systems resulted in detection of three monomethyltin hydroxo-chloride complexes from stannous chloride and methylcobalamin under acidic and oxidising conditions. Incubation of tin and lead compounds in numerous environmental sediment samples and some fulvic and humic acid extracts were not successful in producing methylmetal products. Optimum conditions for such experiments were set. Analytical methods for speciation and detection of methylmetal products (Sn, Pb) in the environment have been briefly reviewed and methods suitable for the requirements of this study have been modified and developed. Results from the present work suggest that environmental methylation of tin and lead can only be effected by dismutation reactions or chemically by carbonium ion donors present in the environment. Evidence of widespread biological methylation was not obtained although specific microorganisms not widely available may be able to effect such a process.
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