Effect of acidification of mercury methylation and volatilization in an oligotrophic Northern Wisconsin lake
Steffan, Robert J.
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The effects of experimental acidification on mercury methylation and volatilization were measured in sediment, water, and attached microbial communities (on artificial substrates) from an acid-susceptible Wisconsin lake. This research was initiated to determine potential mechanisms causing elevated mercury concentrations in biota of low pH waters. Methylation and volatilization rates were quantified by incubating samples with 203Hg(NO3) and measuring the formation of CH3 203HGplus and volatile 203Hg species, respectively. The effects of acidification on these processes were assessed by experimental acidification of samples with either HCl or H2S04. Methylation was greatest in surficial sediments (0-10 cm) from the profundal zone of the lake, and significantly less (p<0.01) in littoral and deeper sediments. The amounts of methylmercury produced in water and artificial substrate samples were very small, near the detection limit. Methylation in surficial sediments was significantly (p<0.01) decreased by lowering the pH with either HCl or H2S04 in the pH range 6.4-3.0. Methylation in sediments acidified to pH 5.0 did not increase during a 74 d incubation. The addition of sulfate (as Na2S04) further decreased methylation in sediments acidified to pH 4.5, but did not affect methylation in non-acidified sediments (pH 6.4). Thus, the elevated mercury concentrations observed in biota of low pH waters are not due to enhanced methylation activity. Mercury volatilization, which was almost entirely abiotic in the present study, was greatest in water. Mercury volatilization in sediments was initially faster than methylation, but decreased sharply 20 min after addition of 203Hg2plus, presumably due to the binding of mercury to particulates. Mercury volatilization in water was decreased by acidification, but the effect of acidification on volatilization in sediments was highly variable. The abiotic volatilization of mercury may therefore by inhibited in low pH waters, resulting in increased retention of atmospherically deposited mercury. Decreased volatilization may explain increased concentrations of mercury in biota of low pH lakes.
Mercury -- Environmental aspects
Acid pollution of rivers, lakes, etc