INHIBITION OF MERCURY METHYLATION IN ANOXIC FRESH-WATER SEDIMENT BY GROUP-VI ANIONS

The addition of group VI anions to sediment slurries resulted in the i nhibition of the rate of mercury (Hg) methylation. The ranking of inhi bition is as follows: tellurate (TeO42-) > selenate (SeO42-) > molybda te (MoO42-) > tungstate (WO42-). In sediment slurries treated with TeO 42- and SeO42-, methylmercury (MeHg) formation was significantly inhib ited (p < 0.05) at the concentrations > 50 nM of TeO42- and > 270 nM o f SeO42-, while the significant inhibition (p < 0.05) of Hg-methylatio n by MoO42- and WO42- was observed in slurries spiked at final concent rations greater than or equal to 100 mu M and greater than or equal to 700 mu M, respectively. Increasing the sulfate (SO42-) concentration while using fixed concentrations of inhibitors led to the partial rees tablishment of some MeHg production in WO42--treated slurries, whereas , no such significant change was noticed in sediment slurries treated with MoO42- and TeO42-. These observations suggested that WO42- inhibi ts Hg methylation by a competitive mechanism, while MoO42- and TeO42- are noncompetitive inhibitors. Selenate and SO42- showed a qualitative ly similar effect on Hg methylation at concentrations tested, in that each showed stimulation at low concentrations and inhibition at high c oncentrations. The depression of MeHg formation by group VI anions was not accompanied by an inhibition of general microbial activity, sugge sting that only particular microorganisms, such as sulfate-reducing ba cteria, are responsible for Hg methylation. Finally, in the concentrat ion ranges encountered in most natural aquatic environments, the inhib ition of MeHg production by group VI anions is unlikely, except in sys tems where those elements are found in anomalously high concentrations .