Does the recarbonization of artisanal gold mining soil increase mercury retention?

Abstract

High levels of mercury (Hg) in soils and humans are found in the Amazon region and other biomes in Brazil due to gold mining. Most of the gold in these regions is extracted through artisanal mining, which can worsen contamination of soils and the environment by Hg. Artisanal mining involves the use of inadequate equipment and procedures that result in low efficiency in extracting gold from the ground. Artisanal mining, in addition to being inefficient in extracting gold and causing health problems due to lack of personal protective equipment, has no legal responsibility for recovering the area and is generally associated with illegal activities. The mining activity also promotes the loss of carbon (C) from the soil, affects Hg retention, and favors CO2 emissions into the atmosphere. Faced with this problem, the C sequestration in the soil (recarbonization) can be essential not only for reducing climate change, but also reducing the accessibility to Hg, collaborating to achieve a balance between the resilience of the ecosystem and the C dynamics in the atmosphere. Dissolved organic matter (DOM) in the soil can strongly interact with Hg and affect its speciation, solubility, mobility, and toxicity, and the strong binding of Hg by DOM is attributed to the coordination of Hg at reduced sites of sulfur within the DOM. However, its magnification is limited to mineralogical conditions, quality of native organic matter, and climatic conditions, making the study necessary in different biomes. Due to the importance of the organic matter for Hg retention and organic carbon (OC) sequestration in the soil, we plan to evaluate in soils from different biomes i) the saturation capacity of organic carbon in mining soil; ii) Hg sequestration by Fe-OC-Hg ternary complexes; and iii) investigate the spatial distribution of C functional groups and their associations with soil mineral phases. To achieve these goals, we will combine DOM and Hg batch sorption studies along with scanning transmission X-ray microscopy (STXM), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and high-resolution X-ray absorption and fluorescence spectroscopy. We hope that the combination of these techniques will make it possible to analyze the distribution of organic functional groups in the soil as well as the possible preferentially sorbed species. Thus, recarbonization can be a viable alternative to increase Hg retention and store OC.