| Summary: | The research on selenium presence in water resources has revealed the need to determine the respective aquatic species. As selenium oxyanions SeO<sub>3</sub><sup>2−</sup> (Se<sup>IV</sup>) and SeO<sub>4</sub><sup>2−</sup> (Se<sup>VI</sup>) predominate in natural waters, their determination is essential, mainly due to different ecotoxicity properties, as well as to different removal options from relevant-polluted waters. This study focuses on the SeO<sub>3</sub><sup>2−</sup>/SeO<sub>4</sub><sup>2−</sup> speciation/separation and determination through the selective adsorption of SeO<sub>3</sub><sup>2−</sup> only onto specific iron oxy-hydroxides (FeOOHs). For this purpose, the laboratory prepared FeOOHs examined along with the commercially available relevant material (Bayoxide), which was found to present optimum results for the speciation of selenium oxyanions, at the low concentration range 10–100 μg/L, using a dose of 0.5 g/L of adsorbent and gently stirring for 30 min at the usually encountered pH value of 7.3 ± 0.2. Moreover, the relevant experiments showed that the other major ions Cl<sup>−</sup>, HCO<sub>3</sub><sup>−</sup>, NO<sub>3</sub><sup>−</sup>, SO<sub>4</sub><sup>2−</sup>, Ca<sup>2+</sup>, Mg<sup>2+</sup>, Na<sup>+</sup>, possibly found in most natural waters at the concentration range 0–200 mg/L, as well as silicon, total organic carbon (TOC) of natural organic matter (NOM) and iron at the concentration range 0–50 mg/L, 0–5 mg/L and 0–1 mg/L, respectively, did not interfere with the selective adsorption of Se(IV). Furthermore, the most important advantage of this selective speciation method is its implementation/combination with all commonly applied analytical methods for the determination of total selenium.
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