Selenite Uptake by Ca-Al LDH: A Description of Intercalated Anion Coordination Geometries

Layered double hydroxides (LDHs) are anion exchangers with a strong potential to scavenge anionic contaminants in aquatic environments. Here, the uptake of selenite (SeO₃^2-) by Ca-Al LDHs was investigated as a function of Se concentration. Thermodynamic modeling of batch sorption isotherms shows that the formation of SeO₃^2--intercalated AFm (hydrated calcium aluminate monosubstituent) phase, AFm-SeO₃, is the dominant mechanism controlling the retention of Se at medium loadings. AFm-Cl₂ shows much stronger affinity and larger distribution ratio (R_d ∼ 17800 L kg^-1) toward SeO₃^2- than AFm-SO₄ (R_d ∼ 705 L kg^-1). At stoichiometric SeO₃^2- loading for anion exchange, the newly formed AFm-SeO₃ phase results in two basal spacing, i.e., 9.93 ± 0.06 Å and ∼11.03 ± 0.03 Å. Extended X-ray absorption fine structure (EXAFS) spectra indicate that the intercalated SeO₃^2- forms inner-sphere complexes with the Ca-Al-O layers. In situ X-ray diffraction (XRD) shows that basal spacing of Ca-Al LDHs have a remarkable linear relationship with the size of hydrated intercalated anions (i.e., Cl^-, SO₄^2-, MoO₄^2-, and SeO₃^2-). Contrary to AFm-SeO₃ with inner-sphere SeO₃^2- complexes in the interlayer, the phase with hydrogen-bonded inner-sphere complexed SeO₃^2- is kinetically favored but thermodynamically unstable. This work offers new insights about the determination of intercalated anion coordination geometries via XRD analyses.