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

## TL;DR

This study explores how a resin can remove harmful selenium compounds from water, even when sulfate is present.

## Contribution

The study provides new insights into the selective removal of selenate over sulfate using ion exchange and confirms the mechanism via X-ray analysis.

## Key findings

- The resin IRA-900 shows higher selectivity for selenate (SeO4^2–) over sulfate (SO4^2–) and selenite (SeO3^2–).
- The maximum exchange capacity of the resin was found to be 2.04 mequiv/g.
- X-ray photoelectron spectroscopy confirmed ion exchange as the primary removal mechanism.

## Abstract

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.