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

Mercury speciation at a small seepage lake in Northern Minnesota was shown to be influenced by photoreduction and photooxidation. Fluxes of Hg0 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 Hg0(aq) in the dark over 10 days were observed in lake water (0.02 h−1), Milli-Q™ 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 Hg0 in water from Spring Lake was found to range from 0.39 to 0.76 h−1. Other sinks for Hg0 exist through reaction with ozone, hydroxyl radical, and possibly singlet oxygen. A second-order reaction rate constant for Hg0(aq) and OH of 1.0×109 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 Hg0(aq) by ozone may predominate. Potential oxidation of Hg0(aq) 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 Hg0(aq) and should be considered in the mercury cycle in the ocean.