Sources of element release from acid sulfate soils to the environment: Elemental and mineralogical evidence from Northern Sweden

Acid sulfate soils (AS-soils) are sulfide-bearing sediments that remain benign in waterlogged conditions but become environmentally harmful after exposure to oxygen. Sulfide oxidation generates acidity, element mobilization, compositional changes in soils and water bodies, and adverse effects on biota. This study enhances the geochemical and mineralogical understanding of the element source in oxidized and unoxidized AS-soils samples from Luleå, northern Sweden. An adapted sequential extraction scheme was conducted to determine the element distribution in primary and secondary phases. The extraction differentiates the H2O-soluble from the exchangeable fraction, labile and stable organic fractions, differentiated reducible from oxidisable phases, and residual fractions. Mineralogical changes were monitored at each step of the extraction sequence. Compositional maps obtained through microprobe quantified trace elements within the pyrite structure, which are susceptible to mobilization after weathering. The mineralogical and compositional findings of this study support strategies to mitigate environmental impacts from AS-soils oxidation. It is shown that framboidal pyrites are highly reactive to oxidation, contributing to acidification and releasing elements such as Cu, Mn, Mo, and Ni, highlighting sulfide exposure risk. In unoxidized samples, S is present in primary Fe sulfides and as organic S, while in oxidized samples, S occurs as secondary sulfate minerals. Organic matter strongly associates with Cu, Mo, and S, and is an important source of these elements in the sediments. From the most labile phases and pore water, Cd, Mn, Mo, and S were predominantly leached, indicating their high bioavailability. These findings demonstrate the importance of maintaining AS-soils in waterlogged conditions to prevent acidification and metal mobilization.