Molybdenum-Oxysulfide-Functionalized MgAl-Layered Double Hydroxides─A Sorbent for Selenium Oxoanions

Effective removal of chemically toxic selenium oxoanions at high-capacity and trace levels from contaminated water remains a challenge in current scientific pursuits. Here, we report the functionalization of the MgAl layered double hydroxide with molybdenum-oxysulfide (MoO₂S₂) anion, referred to as LDH-MoO₂S₂, and its potential to sequester Se^VIO₄^2- and Se^IVO₃^2- from aqueous solution. LDH-MoO₂S₂ nanosheets were synthesized by an ion exchange method in solution. Synchrotron X-ray pair distribution function (PDF) and extended X-ray absorption fine structure (EXAFS) revealed an unexpected transformation of the MoO₂S₂^2- to Mo₂O₂S₆^2- like species during the intercalation process. LDH-MoO₂S₂ is remarkably efficient in removing SeO₄^2- and SeO₃^2- ions from the ppm to trace level (≤10 ppb), with distribution constant (K_d) ranging from 10⁴ to 10⁵ mL/g. This material showed exceptionally high sorption capacities of 237 and 358 mg/g for SeO₄^2- and SeO₃^2-, respectively. Furthermore, LDH-MoO₂S₂ demonstrates substantial affinity and efficiency to remove SeO₃^2-/SeO₄^2- even in the presence of competitive ions from contaminated water. Hence, the removal of selenium (VI/IV) oxoanions collectively occurs through reductive precipitation and ion exchange mechanisms. This work provides significant insights into the chemical structure of the MoO₂S₂ anion into LDH and emphasizes its exceptional potential for high-capacity selenium removal and positioning it as a premier sorbent for selenium oxoanions.