Speciation and microbial transformation of chromium, arsenic and mercury in municipal landfill leachate

摘要

Transformation of heavy metals in landfill environment is currently not regulated by waste disposal guidelines. Chromium, arsenic and mercury speciation were investigated in municipal landfill leachate (MLL) to evaluate microbial transformation of the heavy metals in the landfill system. MLL is a complex matrix that contains multiple inorganic and organic contaminants, as well as a consortium of bacteria in an environment of changing pH and redox potential. Interplay among these variables provides an in-depth understanding of the interactive roles played by bacteria and chemical and physical characteristics of MLL in transforming metal species in landfill environment.While hexavalent chromium (CrVI) was chemically reduced to the less toxic trivalent chromium through a shuttle mechanism that entailed a microbial reduction of ferric iron, arsenate (iAsV) was found to reduce to the more toxic arsenite (iAsIII) in MLL. Different reaction profiles of CrVI and iAsV in sterilised MLL demonstrated that these reactions are principally redox reactions, which are mediated by microbial processes. MLL has been shown capable of sequentially reducing CrVI over three consecutive spiking cycles.Compared with CrVI and iAsV, monomethylarsonic acid (MMAV) and dimethylarsinic acid (DMAV) transformation is more complex. Besides microbial reduction of MMAV to monomethylarsonous acid (MMAIII) and methylation of MMAV to DMAV, a number of sulfur–containing organoarsenic species, including dimethyldithioarsinic acid, dimethylmonothioarsinic acid and monomethyldithioarsonic acid, were found in the MMAV- and DMAV-spiked MLL. Bacteria were shown to play a crucial role in arsenic transformation, which related to the microbial reduction of sulfate to sulfide and subsequent thiolation of the organoarsenic compounds in the MLL. The arsenic transformation in MLL demonstrated the extreme complexity of arsenic speciation. It highlighted the potential risk of human exposure of highly toxic thio-organoarsenicals and MMAIII formed in a landfill environment.The preliminary study of mercury speciation in MLL demonstrated that soluble inorganic mercury (Hg2+) and organic methylmercury (HgCH3+) were transformed to insoluble species. Different concentration profiles of dissolved Hg2+ and HgCH3+ in sterilised and non-sterilised MLL demonstrated that pH, bacteria and MLL characteristics all have contributed to the mercury transformation through chemical, physical and microbial processes.