Regulating the interlayer SO42--induced rebound of SeO42- in green rust coupled with iron nanoparticles for groundwater remediation

Green rust (GR) is an interlayer anion-containing Fe(II)/Fe(III) mineral material that is versatile in removing a series of ionic contaminants in water. Taking SeO₄^2- (Se(VI)) as the target contaminant, this study identified that the removal processes of Se(VI) by GR could be divided into three stages: initial rapid interlayer exchange, followed by a rebound, and finally slow removal. In addition, as the percentage of SO₄^2- in GR interlayer increased, the Se(VI) removal by GR gradually decreased. To mediate the SO₄^2--induced rebound of Se(VI), the coupling of GR with iron nanoparticles (nFe⁰@GR) was proposed in this study and it was found that the removal efficiency of Se(VI) by nFe⁰@GR was 3.53 folds greater than that of GR. This study further revealed that the enhanced reactivity of nFe⁰@GR with Se(VI) could be attributed to the re-equilibration of SO₄^2- driven by the formed GR in situ. Since it had a weaker electrostatic repulsion with interlayer SeO₄^2- than pristine GR, the Se(VI) could be quickly removed by nFe⁰@GR without the rebound. Moreover, the nFe⁰@GR was demonstrated to be effective in immobilizing Se(VI) from simulated groundwater and has a great potential to reduce the risk of Se(VI) re-release into the environment.