Selenium Sequestration in a Cationic Layered Rare Earth Hydroxide: A Combined Batch Experiments and EXAFS...

Selenium is of great concern owing to its acutely toxic characteristic at elevated dosage and the long-term radiotoxicity of ⁷⁹Se. The contents of selenium in industrial wastewater, agricultural runoff, and drinking water have to be constrained to a value of 50 μg/L as the maximum concentration limit. We reported here the selenium uptake using a structurally well-defined cationic layered rare earth hydroxide, Y₂(OH)₅Cl·1.5H₂O. The sorption kinetics, isotherms, selectivity, and desorption of selenite and selenate on Y₂(OH)₅Cl·1.5H₂O at pH 7 and 8.5 were systematically investigated using a batch method. The maximum sorption capacities of selenite and selenate are 207 and 124 mg/g, respectively, both representing the new records among those of inorganic sorbents. In the low concentration region, Y₂(OH)₅Cl·1.5H₂O is able to almost completely remove selenium from aqueous solution even in the presence of competitive anions such as NO₃^-, Cl^-, CO₃^2-, SO₄^2-, and HPO₄^2-. The resulting concentration of selenium is below 10 μg/L, well meeting the strictest criterion for the drinking water. The selenate on loaded samples could be desorbed by rinsing with concentrated noncomplexing NaCl solutions whereas complexing ligands have to be employed to elute selenite for the material regeneration. After desorption, Y₂(OH)₅Cl·1.5H₂O could be reused to remove selenate and selenite. In addition, the sorption mechanism was unraveled by the combination of EDS, FT-IR, Raman, PXRD, and EXAFS techniques. Specifically, the selenate ions were exchanged with chloride ions in the interlayer space, forming outer-sphere complexes. In comparison, besides anion exchange mechanism, the selenite ions were directly bound to the Y³⁺ center in the positively charged layer of [Y₂(OH)₅(H₂O)]⁺ through strong bidentate binuclear inner-sphere complexation, consistent with the observation of the higher uptake of selenite over selenate. The results presented in this work confirm that the cationic layered rare earth hydroxide is an emerging and promising material for efficient removal of selenite and selenate as well as other anionic environmental pollutants.