Methyl mercury in dental wastewater.

摘要

Dental wastewater is a major source of mercury (Hg) release to publicly operated treatment works; with reported annually discharge inventory of 9%-78% of total Hg loading. Hg in DWW has a potential to convert to a much more toxic methyl Hg, although mechanistic studies are few. In the present study, total and methyl Hg levels in DWW are significant, with total Hg spanning 5 orders of magnitude (0.02-5000 muM) and methyl Hg was found at levels (2-270 nM) comprising up to 0.1% or more of the total Hg in DWW. Based on these results and other published data, the annual total and methyl Hg loading to wastewater collection systems and receiving water systems were estimated to be 3488 kg, 2.02 kg, and 680 kg, 374 g respectively. These levels are on the order of discharge limits and provide conclusive proof that DWW is a major source of methyl Hg to POTWs.;DNA extracts indicate that the total eubacterial DNA was composed primarily of sulfate-reducing bacteria (SRB), which is significant because SRB are implicated as being Hg methylators. Highly significant correlations were found between methyl Hg and both Desulfobacteraceae (DSBAC) and Desulfovibrionacaea (DSVIB) DNA, both are known Hg methylators. These results suggest that SRB are implicated in DWW Hg methylation, and these results were confirmed by further incubation studies with various inhibitors added to DWW supernatant produced from raw DWW following 24 hours settling. The results indicate that some abiotic Hg methylation does occur, but the majority is biotic and due to SRB and other Hg methylating bacteria. Further, the results demonstrate that Hg bioavailability is not a limiting factor for biotic Hg methylation in DWW given the generally high levels of Hg. The inhibitor and bioaugmentation studies indicate that both non acetyl-CoA pathway utilizing SRB like Desulfobacter postgatei and acetyl-CoA SRB like Desulfovibrio desulfuricans are active methylators in DWW, with first order methylation rates of 0.003-0.005 hr-1. Finally, the significant production of methyl Hg in molybdate inhibited DWW leads to the conclusion that other non-SRB, such as Geobacter spp. are possibly methylating Hg in DWW.