The use of isotopically enriched tin tracers to follow the transformation of organotin compounds in landfill leachate

• Isotopically enriched Sn tracers were used to follow transformation of OTC in landfill leachate.
• Biomethylation process resulted in formation of toxic methyltins from inorganic Sn.
• Bacterial methylation processes were observed from both Sn2+ and Sn4+ ionic species.
• Hydrolysis of Sn2+ and Sn4+ controlled the extent of methyltin formation in leachate.
• Biodegradation of TBT and abiotic degradation of DBT lead to less toxic Sn species.

Landfill leachates are an important pool of organotin compounds (OTCs). Several studies have been performed on the occurrence of OTCs in landfill leachates, but only a few of them report degradation and biomethylation processes by bacteria. In the present study transformation of OTCs in landfill leachate was investigated under simulated landfill conditions over a time span of six months. The degradation and biomethylation processes of OTCs were followed by the use of isotopically enriched tin tracers, namely 117Sn-enriched tributyltin (TBT), 119Sn-enriched dibutyltin (DBT), 117Sn-enriched SnCl2, 117Sn-enriched SnCl4 and a 119Sn-enriched butyltin mix containing TBT, DBT and monobutyltin (MBT). Transformation of OTCs in spiked leachates was followed at m/z of the enriched spikes and at m/z 120, which allowed simultaneous observation of the transformation of OTCs in the leachate itself and of the added spike. In parallel, these processes were also monitored in a non-spiked leachate sample at m/z 120. Quantification of OTCs was performed by gas chromatography – inductively coupled plasma mass spectrometry (GC-ICP-MS). To discriminate the biotic and abiotic transformations of OTCs and inorganic tin species, sterilization of leachate was also performed and data compared to non-sterilized samples. During the course of the experiment the microbial degradation of TBT was clearly manifested in Sn-enriched spiked leachate samples, while abiotic pathway of degradation was observed for DBT. Biomethylation process was also observed in the leachate spiked with Sn-enriched Sn2+ or Sn4+, in concentrations close to those found for total tin in landfill leachates. Monomethyltin (MMeT) was formed first. Stepwise alkylation resulted in dimethyltin (DMeT) and trimethyltin (TMeT) species formation. Hydrolysis of Sn2+ and Sn4+ species was found to be a limiting factor which controlled the extent of methyltin formation. The results of the present investigation importantly contribute to a better understanding of the processes that OTCs undergo in leachates, and provide useful information to managers of landfills in taking measures necessary to prevent the release of toxic methyltin species to the nearby environment.

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