Mercury dynamics in a small Northern Minnesota lake: water to air exchange and photoreactions of mercury

摘要

Mercury speciation at a small seepage lake in Northern Minnesota was shown to be influenced by photoreduction and photooxidation. Fluxes of Hg~0 from water to air were greatest in the warmer, sunnier months in 2001 to 2002; however, correlation with solar radiation was weak. The daytime evasional loss was generally from water to air and was estimated at 5.3 pmol m~(-2) h~(-1) for 2001 and 6.2 pmol m~(-2)h~(-1) for 2002 using a two-layer gas transfer model. Losses of Hg_((aq))~0 in the dark over 10 days were observed in lake water (0.02 h~(-1)), Milli-Q~(TM) water, and HPLC grade water (0.002 h~(-1)) and agreed with reported pseudo-first-order loss rates in the dark in other freshwaters. Using a mercury arc lamp, the pseudo-first-order loss rate of Hg~0 in water from Spring Lake was found to range from 0.39 to 0.76 h~(-1). Other sinks for Hg~0 exist through reaction with ozone, hydroxyl radical, and possibly singlet oxygen. A second-order reaction rate constant for Hg_((aq))~0 and ~·OH of 1.0 x 10~9 M~(-1) s~(-1) was estimated based on data from experiments and the literature. Although less reactive, there is a higher steady state concentration of ozone in lake water compared to hydroxyl radical. Consequently, loss of Hg_((aq))~0 by ozone may predominate. Potential oxidation of Hg_((aq))~0 by singlet oxygen using rose bengal as a sensitizer could not distinguish between oxidation by rose bengal and by singlet oxygen itself. Chloride enhanced the oxidation of Hg_((aq))~0 and should be considered in the mercury cycle in the ocean.