Speciation and quantification of Hg in sediments contaminated by artisanal gold mining in the Gualaxo do Norte River, Minas Gerais, SE, Brazil

Abstract

The Iron Quadrangle in SE Brazil was, in the eighteenth century, one of the most important Au producing regions of Brazil. In this region, gold is produced, even today, by artisanal methods that use Hg to increase the extraction efficiency with no control of Hg release to water systems and the atmosphere. In this context, the Gualaxo do Norte River is of particular interest; its springs are located in the Doce River basin, an important Brazilian basin that supplies water for 3.5 million people. The main goal of this work was to quantify and speciate the Hg in the sediments of the Gualaxo do Norte River using a direct mercury analyzer and gas chromatography-pyrolysis-atomic fluorescence detection system. Statistical analyses consisted of principal component analysis, aiming to assess interactions among elements and species and to group the variables in factors affecting the properties of sediment. The results show that total Hg (THg) and methylmercury (CHHg) concentrations in samples ranged from 209 to 1207 μg kg and from 0.07 to 1.00 μg kg, respectively (methylation percentages from 0.01 to 0.27%). Thermal desorption analysis showed that mercury is mainly present in the oxidized form, and correlation analyses pointed to a relationship between THg and MnO, indicating that manganese can oxidize and/or adsorb Hg. Together, MO and CHHg are important parameters in the third principal component, indicating the influence of OM on the methylation process. This first investigation on Hg methylation in this small-scale gold mining area points to the possibility of Hg bioaccumulation and to the need of better understanding the biogeochemical cycle of Hg in this area. Samples were collected in 2012, prior to the 2015 Fundão Dam disaster. The results are also a record of the characteristics of the sediment prior to that event.

Dissolution Potential of Elemental Mercury in the Presence of Bisulfide and Implications for Mobilization

“…While this study used aqueous bisulfide as a sulfide source, sulfidation of Hg 0 L could also occur with other pools of reduced sulfur in the environment, such as polysulfides, , inorganic sulfides (e.g., FeS), and organic thiols found within natural organic matter (NOM). , Limited research suggests that these different types of sulfur may also impact sulfidation dynamics and long-term stability. For example, the reaction of Ag NPs with metal sulfides (e.g., ZnS and CuS) resulted in sulfidation structures that differed from the products of Ag NPs reacted with aqueous bisulfide .…”

“…While this study used aqueous bisulfide as a sulfide source, sulfidation of Hg 0 L could also occur with other pools of reduced sulfur in the environment, such as polysulfides, 36,56 inorganic sulfides (e.g., FeS), 57 and organic thiols found within natural organic matter (NOM). 58,59 Limited research suggests that these different types of sulfur may also impact sulfidation dynamics and long-term stability.…”