Efficiency of Sequential Extraction Schemes in Partitioning Toxic Elements in Acid Sulfate Soils With a Thionic Horizon

ABSTRACT The use of sequential extraction methods is a common practice for analysing the availability of potentially toxic elements in soils and sediments. However, the simultaneous presence of oxidisable and reducible minerals in acid sulfate soils with a thionic horizon raises questions about the accuracy of these methods. This study focused on the comparative evaluation of two sequential extraction schemes—the modified BCR method (four steps) and the modified Tessier method (six steps) ‐ in acid sulfate soils with a thionic horizon in the Delta of the Doce River region, Espírito Santo, Brazil. With particular attention to the presence of jarosite, a mineral associated with the sulfurization process, specific tests were conducted to better understand the performance of reducible extractors on its synthetic phase. The results highlight the complex dynamics of acid sulfate soils with a thionic horizon concerning the distribution of iron and potentially toxic elements. A significant divergence was observed between the two sequential extraction methods in identifying the main reservoir of potentially toxic elements. In the modified BCR method, metals were primarily associated with oxidizable iron fractions, including organic matter. In contrast, the modified Tessier method indicated metals in the reducible iron fractions, suggesting a different interpretation of the mobility and bioavailability of the elements. The comparative analysis revealed that the modified Tessier procedure is more effective in characterising acid sulfate soils with a thionic horizon for the following reasons: (1) it provides greater precision in evaluating the soil's exchangeable fraction; (2) it avoids underestimating the reducible fraction, capturing the dissolution of sulfide oxidation products; (3) it more accurately identifies elements associated with sulfide minerals; and (4) it offers a more precise quantification of the residual fraction. However, a limitation of the modified Tessier method is the omission of the influence of organic matter, an important component in the dynamics of elements in hydromorphic environments, emphasising the need for complementary approaches for a comprehensive understanding of the availability of potentially toxic elements (trace metal) in such soils.