Mercury reduction by black carbon under dark conditions

An accurate depiction of mercury (Hg) reduction is important to predict Hg biogeochemistry in both aquatic and soil systems. Although the photoreduction of Hg is well documented, reduction in the dark is poorly known and is thus the focus of this work. Black carbon (BC), an important constituent of organic matter in environments, can reduce Hg²⁺ in dark and oxygen-deficient conditions. Fast removal of Hg²⁺ in BC/Hg²⁺ solution was observed, with 4.99-86.88 L mg^-1h^-1 of the reaction rate constant, which could be ascribed to the combined actions of adsorption and reduction. Meanwhile, slow Hg reduction was obtained, compared to Hg removal, with 0.06-2.16 L mg^-1h^-1 of the reaction rate constant. Thus, in the initial stage, Hg²⁺ removal was mainly triggered by adsorption, rather than reduction. Afterward, the adsorbed Hg²⁺ on black carbon was converted into Hg⁰. Dissolved black carbon and aromatic CH on particulate black carbon were dominant triggers of Hg reduction for black carbon. During Hg reduction, the intastable intermediate, formed in the complex between aromatic CH and Hg²⁺, behaved as persistent free radicals, which could be detected by in situ electron paramagnetic resonance. Subsequently, the intastable intermediate was mainly converted into CO on black carbon and Hg⁰. Corresponding results of the present study highlight the important role of black carbon in the Hg biogeochemical cycle.