Microbial transformation of arsenic species in municipal landfill leachate

The microbial transformation of arsenic species in municipal landfill leachate (MLL) was investigated with the objective to highlight arsenic transformation in the landfill system. Across the 43 day incubation in MLL, more than 90% arsenate (iAsV) was found to reduce to arsenite (iAsIII) within 20 days, while iAsIII was comparably stable although a fraction of iAsIII was temporarily oxidated to iAsV in the first 3 days. Transformation of monomethylarsonic acid (MMAV) to dimethylarsinic acid (DMAV) in MLL was slow with only 5% MMAV methylated to DMAV after 43 days incubation. A portion of DMAV and MMAV in MLL was demonstrated to transform into thiol-organoarsenic and monomethylarsonous acid (MMAIII), which were identified to include dimethyldithioarsinic acid (DMDTAV), dimethylmonothioarsinic acid (DMMTAV) and monomethyldithioarsonic acid (MMDTAV) by HPLC-ICPMS and LC-ESI-MS/MS. The microbial formation of DMDTAV, DMMTAV and MMDTAV is postulated to relate to hydrogen sulfide generated by bacteria in MLL. Differences in arsenic transformation in sterilised and non-sterilised MLLs demonstrate bacteria play a crucial role in arsenic transformation in the landfill body. This study reveals the complexity of arsenic speciation and highlights the potential risk of forming highly toxic thiol-organoarsenic and MMAIII in the landfill environment.