EXAFS and FTIR studies of selenite and selenate sorption by alkoxide-free sol–gel generated Mg–Al–CO3 layered double hydroxide with very labile interlayer anions

Abstract

Current research on Layered Double Hydroxides (LDHs, also known as hydrotalcites, HTs) is predominantly focused on their intercalations, but the industrial application of LDHs for anion exchange adsorption has not yet been achieved. It was recently recognized that, to develop LDH applications, these materials should be produced using methods other than direct co-precipitation. Mg–Al–CO₃ LDH produced using an alkoxide-free sol–gel synthesis showed exceptional removal properties for aqueous selenium species. Se K-edge EXAFS/XANES and FTIR studies (supporting the data by XRD patterns) were performed to explain the unusual adsorptive performance of Mg–Al LDH by revealing the molecular-level mechanism of HSeO₃⁻, SeO₄²⁻ and {HSeO₃⁻ + SeO₄²⁻} uptake at pH 5, 7 and 8.5. The role of inner-sphere complexation (exhibited by inorganic adsorbents with good performance) in adsorption of both selenium aqueous species was not confirmed. However, Mg–Al LDH fully met the other expectations regarding the involvement of the interlayer anions. The interlayer carbonate (due to its favorable speciation and generous HT hydration) gave a “second breath” to selenite sorption and was the only mechanism that controlled the removal of Se(VI). Because inner sphere complexation was the leading mechanism for selenite removal, ion exchange via surface OH⁻ and interlayer CO₃²⁻ species was the only mechanism for selenate removal; both of these species were easily bound to Mg–Al LDH (on its surface and gently parked into the interlayer forming a multilayer without violation of the structure of Mg–Al–CO₃ LDH). This work provides the first theoretical explanation of why it is more difficult to sorb selenate than selenite and which material should be used for this purpose.