Selenium oxyanions: Highly selective uptake by a novel anion exchanger
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Toshihiko Ohnuki Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki, 319-1195 Japan
Sridhar Komarneni Department of Crop and Soil Sciences and Materials Research Institute, 205 Material Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 23 May 2002; accepted 16 September 2002)
We report the extremely high and selective uptake of selenium oxyanions by a novel anion exchanger, Ni1−x Zn2x(OH)2(OCOCH3)2 x ⭈ nH2O (0.15 < x < 0.25). The tested Ni–Zn basic salt (x ⳱ 0.24) exhibited very high selectivity for Se(IV) [Kd ⳱ 9.0 × 104 cm3/g with an initial Se(IV) concentration of 1 × 10−4 M] in the presence of 0.1 M Cl− solution. The uptake of Se(IV) on the Ni–Zn basic salt was irreversible when treated with solutions containing 1 N Cl−, 1 N NO3−, or 1 N PO43−. This novel exchanger also showed high Kd (2.6 × 103 cm3/g) for Se(VI), and therefore it is expected to be useful for decontamination and removal of selenium oxyanions from contaminated water.
Selenium is used extensively in the manufacture and production of photoelectric cells, rubber, pigment, etc.1,2 Selenium solubilized and released into wastewater exists in +4 and +6 oxidation states under oxidizing conditions. 3,4 Chemical species of selenium with these oxidation states are oxyanions [Se(IV)O32− and Se(VI)O42−] with high solubility and mobility in both aquatic and soil environment under oxidizing conditions.3,4 Although selenium is an essential nutrient at low concentrations, Se at high concentrations is toxic to living things including humans,5 so the concentration of selenium in drinking water has been regulated at low levels (e.g., 0.05 ppm in the United States and 0.01 ppm in Japan). Recovery of selenium from wastewater is, therefore, needed to prevent release of selenium from industrial facilities into the environment. Many efforts have been made for developing techniques such as ion exchange6 and other methods such as co-precipitation, bioremediation, chelation, and adsorption to effectively decontaminate and remove selenium from wastewater.7–11 Although many types of natural, as well as synthetic, selective cation exchangers are available for separating
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 17, No. 12, Dec 2002
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hazardous cations,12–16 very few selective anion exchangers are available for the ion exchange separation of hazardous oxyanions of Se, As, Tc, etc. In this study, we discovered that Ni–Zn basic salt of nominal composition Ni1−x Zn2x(OH)2(OCOCH3)2x ⭈ nH2O (0.15 < x < 0.25) very selectively removed selenium(IV) and selenium(VI) oxyanions from solution by anion exchange process. The exchange of Se(IV) on the Ni–Zn basic salt was found to be irreversible, which was revealed by desorption experiments. The Ni–Zn basic salt is analogous to the well-known anionic clays. Anionic clays are natural or synthetic layered double hydroxides with a pos
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