Dissolved Selenium(VI) Removal by Zero-Valent Iron under Oxic Conditions: Influence of Sulfate and Nitrate

Abstract

Dissolved Se(VI) removal by three commercially available zero-valent irons (ZVIs) was examined in oxic batch experiments under circumneutral pH conditions in the presence and absence of NO₃ ^- and SO₄ ^2-. Environmentally relevant Se(VI) (1 mg L^-1), NO₃ ^- ([NO₃-N] = 15 mg L^-1), and SO₄ ^2- (1800 mg L^-1) were employed to simulate mining-impacted waters. Ninety percent of Se(VI) removal was achieved within 4-8 h in the absence of SO₄ ^2- and NO₃ ^-. A similar Se(VI) removal rate was observed after 10-32 h in the presence of NO₃ ^-. Dissolved Se(VI) removal rates exhibited the highest decrease in the presence of SO₄ ^2-; 90% of Se(VI) removal was measured after 50-191 h for SO₄ ^2- and after 150-194 h for SO₄ ^2- plus NO₃ ^- depending on the ZVI tested. Despite differences in removal rates among batches and ZVI materials, Se(VI) removal consistently followed first-order reaction kinetics. Scanning electron microscopy, Raman spectroscopy, and X-ray diffraction analyses of reacted solids showed that Fe(0) present in ZVI undergoes oxidation to magnetite [Fe₃O₄], wüstite [FeO], lepidocrocite [γ-FeOOH], and goethite [α-FeOOH] over time. X-ray absorption near-edge structure spectroscopy indicated that Se(VI) was reduced to Se(IV) and Se(0) during removal. These results demonstrate that ZVI can be effectively used to control Se(VI) concentrations in mining-impacted waters.

Figure 1

Scanning electron microscopy (SEM) images of unreacted ZVI: (a) CGPM, (b) PM, and (c) QMP. Scale bars are 20 μm.

Figure 2

Dissolved Se concentrations with time (data points) and best-fit first-order reaction (lines) for Se removal by ZVI for B1, B2, B3, and B4 batches. The dotted horizontal line represents 90% removal.

Figure 3

Measured pH and Eh values for B1, B2, B3, and B4 batches.

Figure 4

Nitrate (as N) and ammonium (as N) concentrations with time for B3 and B4 batches.

Figure 5

Measured Se K edge XANES spectra of reacted ZVI in (a) CGPM, (b) PM, and (c) QMP forms of reference Se compounds with different oxidation states (selenate, selenite, elemental selenium, and selenide). The dashed vertical lines represent K edge energies for the different oxidation states of Se. The energy values of the first inflection point (E₀) and the white line (most intense peak) for the reference standards are presented in Table S1.