Mobility and bioavailability of mercury in sediments of the southern Baltic sea in relation to the chemical fractions of iron: Spatial and temporal patterns.

Marine sediments play a significant role as reservoirs for mercury (Hg), a bioaccumulative toxic pollutant that poses risks to human and ecosystem health. Iron (Fe) has been recognized as an influential factor in the complexation and bioavailability of Hg in sediments. However, limited studies have investigated the interactions between the chemical fractions of these elements in natural settings. This study aims to examine the fractions of Hg and Fe in sediments of the Baltic Sea, a region historically impacted by Hg pollution. The Hg fractions were determined using the thermodesorption technique, while sequential extraction was employed to identify the Fe fractions. The findings confirm the crucial role of Fe in the formation, as well as the horizontal and vertical distribution of labile and stable Hg in marine sediments. Factors such as the contribution of organic matter, the presence of reactive Fe, and Fe associated with sheet silicates emerged as significant drivers that positively influenced the content of the most labile Hg fractions, potentially affecting the mobility and bioavailability of Hg in the marine environment. • Fe fractions and sediment features explained 76% of Hg level and fraction variability. • Content of labile Hg fractions (Hg ADS1 , Hg ABS) and reactive Fe (Fe R) was proportional. • Share of labile Hg increased with the content of clay minerals and organic matter. • Organic matter inhibited precipitation of HgS and Fe sorption on sediment particles. • Hg and Fe indicated peaked sediment pollution in the second half of the 20th century. [ABSTRACT FROM AUTHOR]