Arsenic (As) is considered a carcinogenic toxin, and nonferrous metal smelting activities can release significant quantities of As, causing an increase in As levels in the environment. A total of 221 samples of moss belonging to 5 species in 4 families were collected from the Wanshan District, the world third largest mercury mining area, in Guizhou Province, southwestern China, for analysis of total As (tAs) concentrations and As speciation. The results indicated that the tAs concentrations in these mosses ranged from 0.67 to 9.6 mg kg −1 , with an average of 2.4 ± 1.7 mg kg −1 . The dominant species of As in these mosses were inorganic forms of arsenite (As(III)) and arsenate (As(V)) with a total percentage of 87%, whereas the concentrations of organic forms (monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA)) were relatively low, with percentages of 11.8% and 6.0%, respectively. A significant correlation (r = 0.766, p < 0.01) was observed between As(V) and tAs. In the spatial distribution pattern of As, high concentrations of As in mosses were centralized around historical Hg retorting sites in the town of Wanshan, the villages of Gouxi and Yanwuping and sites downwind of these locations, suggesting that historical intensive Hg mining activities might be the most possible source of atmospheric As.
Photo-, microbial, and abiotic dark reduction of soil mercury (Hg) may all lead to elemental mercury (Hg(0)) emissions. Utilizing lab incubations, isotope signatures of Hg(0) emitted from mining soils were characterized to quantify the interplay and contributions of various Hg reduction pathways, which have been scarcely studied. At 15 °C, microbial reduced Hg(0) showed a negative mass-dependent fractionation (MDF) (δ202Hg = −0.30 ± 0.08‰, 1SD) and near-zero mass-independent fractionation (MIF) (Δ199Hg = 0.01 ± 0.04‰, 1SD), closely resembling dark reduced Hg(0) (δ202Hg = −0.18 ± 0.05‰, Δ199Hg = −0.01 ± 0.03‰, 1SD). In comparison, photoreduced Hg(0) exhibited significant MDF and MIF (δ202Hg = −0.55 ± 0.05‰, Δ199Hg = −0.20 ± 0.07‰, 1SD). In the dark, Hg isotopic signatures remained constant over the temperature range of 15–35 °C. Nonetheless, light exposure and temperature changes together altered Hg(0) MIF signatures significantly. Isotope mixing models along with Hg(0) emission flux data highlighted photo- and microbial reduction contributing 79–88 and 12–21%, respectively, of the total Hg(0) emissions from mining soils, with negligible abiotic dark reduction. Microorganisms are the key driver of soil Hg(0) emissions by first dissolving HgS and then promoting ionic Hg formation, followed by facilitating the photo- and microbial reduction of organically bound Hg. These insights deepen our understanding of the biogeochemical processes that influence Hg(0) releases from surface soils.
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