Impact of temperature and humic acid-assisted synthesis on selenium sorption onto iron oxide nanoparticles

Selenium is a key element for biological systems, but at elevated concentrations it can pose a risk to both the environment and human health. Therefore, developing effective strategies to limit its mobility is crucial. Here, we investigated the immobilization of selenite and selenate using iron-based magnetic adsorbents synthesized at various temperatures in the presence of humic acids (HA). Mössbauer spectrometry confirmed the presence of both (minor) magnetite and (major) maghemite components in the adsorbents. HA decreased the overall surface charge and size of the nanoparticles. Kinetic analysis revealed rapid and efficient removal of both species by the synthesized adsorbents at pH 3. The process was best described by pseudo-nth order kinetic model for selenite, whereas the kinetic data for selenate were inconclusive. This suggests a complex interaction between selenite and the adsorbent surfaces that cannot be adequately described by first- or second-order reaction kinetics since the sorption likely involved reductive immobilization, as evidenced by the presence of elemental selenium on the adsorbent surfaces, as revealed by X-ray photoelectron spectroscopy. Selenite sorption showed maximum capacity for adsorbents synthesized at 60 °C, without HA at 539.5 μmol⋅g⁻¹ and with HA at 528.4 μmol⋅g⁻¹. The highest maximum sorption capacity of selenate was observed for materials synthesized without HA at 30 °C at 340.9 μmol⋅g⁻¹. Desorption experiments demonstrated high regeneration efficiency for sorbents synthesized at lower temperatures, with desorption rates reaching up to 92 %. However, the presence of HA significantly reduced desorption efficiency, though this effect diminished as the synthesis temperature increased.