Removal of Selenium Oxyanions from Aqueous Solutions by Ion Exchange: Equilibrium, Kinetics, and Mechanistic Modeling

Selenium (Se) is an essential micronutrient but toxic at high concentrations, posing challenges for water treatment. This study investigated the removal of selenate (SeO₄ ^2-) and selenite (SeO₃ ^2-) using the strong-base anion-exchange resin IRA-900, particularly in the presence of competing sulfate (SO₄ ^2-). The performance of the commercially available resin IRA-900 was systematically investigated. The batch equilibrium behavior was studied in both single- and binary-component systems, and the kinetic behavior was investigated in single-component systems. Results confirmed a selectivity order of SeO₄ ^2- > SO₄ ^2- > SeO₃ ^2-, indicating preferential SeO₄ ^2- removal over competing SO₄ ^2- but lower affinity for SeO₃ ^2-. The maximum total exchange capacity was determined to be 2.04 mequiv/g. Furthermore, SeO₃ ^2- uptake was found to be pH-dependent, whereas SeO₄ ^2- uptake remained stable across a broad pH range. From a modeling perspective, the Law of Mass Action model effectively described equilibrium data, and a transport-reaction modeling framework captured removal kinetics of oxyanions including film and intraparticle diffusion. Finally, X-ray photoelectron spectroscopy confirmed ion exchange between chloride and Se oxyanions as the primary removal mechanism. These findings provide fundamental insights into the removal of Se oxyanions from aqueous solutions by ion exchange.