Selenium Enrichment in Pore Water of Estuarine Sediments Subject to Salt Marsh Vegetation Bioirrigation (Patos Estuary,
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Selenium Enrichment in Pore Water of Estuarine Sediments Subject to Salt Marsh Vegetation Bioirrigation (Patos Estuary, Southern Brazil) Larissa Costa1 · Nicolai Mirlean1 Received: 26 February 2020 / Accepted: 19 August 2020 / Published online: 4 September 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Selenium (Se), iron (Fe), and free sulfides contents in pore waters were measured to study the liberation of soluble Se in suboxic conditions. The sediment core was collected in a salt marsh in Patos Lagoon estuary (southern Brazil), and it was obtained during a brackish water period, in a low intertidal stand vegetated by Spartina alterniflora. The redox potential (Eh), pH, andacid volatile sulfides (AVS) content were also investigated. Pore water results sustained the idea that S. alterniflora roots promote oxygen penetration to depths of ca. 10 cm below the salt marsh surface, increasing Eh and lowering the pH in this interval. High Se concentrations (e.g., 16.9 µg L −1), that are above US. EPA environmental criteria, were observed in the pore water to depths between 10 and 20 cm and are associated to low AVS contents and high concentrations of free sulfides. In the first 10 cm the lowering of Se contents probably happens due the low pH and biological volatilization of the metalloid. Keywords Selenium · Free sulfides · Salt marsh · Pore water · Environmental criteria Selenium (Se) is a metalloid which exists in a variety of oxidation states including selenide (Se2−), elemental Se (Se0), selenite (Se4+), selenate (Se6+), and several organic (e.g., selenomethionine) and volatile compounds (e.g., dimethylselenide, DMSe) (Hansen et al. 1998; Ansede and Yoch 2006). Selenium occurs in the Earth’s crust in concentrations from 0.05 to 0.09 µg g−1 (Alexander, 2015), and it mainly occurs as selenites and selenides in association with sulfide minerals (Skinner 1999). Besides other variables, such as clay and organic matter contents, the major factors that control Se forms in sediments are redox potential (Eh) and pH (El-Ramady et al. 2014). Selenate predominates in alkaline environments and has higher mobility than selenite, which commonly occurs in neutral or acidic sediments, and is easily sorbed on oxyhydroxides (Kabata-Pendias 2010). Pore water Se reflects the diagenetic reactions of selenite and selenate (Se4+ + Se6+) in the sediments (Alexander 2015). Partial remobilization of sedimentary Se occurs when the upper sediments become oxidizing (Velinsky and Cutter 1991). Those highly soluble * Larissa Costa [email protected] 1
Oceanography Institute, Federal University of Rio Grande, Av. Itália km 08 Campus Carreiros, Rio Grande, RS 96201‑900, Brazil
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forms of Se ( Se4+ + S e6+) are, therefore, bioavailable and potentially toxic (Hansen et al. 1998). Selenides (Se2−) exist in reducing acidic environments are very insoluble, resist oxidation, and are not taken up by plant and animals (Fordyce 2007). The toxicity of Se depends on its b
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