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 (SeO) by Ca-Al LDHs was investigated as a function of Se concentration. Thermodynamic modeling of batch sorption isotherms shows that the formation of SeO-intercalated AFm (hydrated calcium aluminate monosubstituent) phase, AFm-SeO, is the dominant mechanism controlling the retention of Se at medium loadings. AFm-Cl shows much stronger affinity and larger distribution ratio (R ∼ 17800 L kg) toward SeO than AFm-SO (R ∼ 705 L kg). At stoichiometric SeO loading for anion exchange, the newly formed AFm-SeO 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 SeO 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, SO, MoO, and SeO). Contrary to AFm-SeO with inner-sphere SeO complexes in the interlayer, the phase with hydrogen-bonded inner-sphere complexed SeO is kinetically favored but thermodynamically unstable. This work offers new insights about the determination of intercalated anion coordination geometries via XRD analyses.