Fabrication of hierarchically porous monolithic layered double hydroxide composites with tunable microcages for effective oxyanion adsorption

Abstract

Removal of toxic substances from industrial wastes is an urgent issue for realizing a sustainable society. Layered double hydroxides (LDHs) are expected to be an effective adsorbent for toxic anions, especially oxyanions, because of their high anion adsorption capacity and reusability. Monolithic LDH materials with rational meso- and macropores are expected to show high adsorption capacity/rate towards targeted toxic substances owing to their large specific surface area and liquid transport property. Besides fabricating hierarchical pores, size control of microcages in LDH crystals is required to achieve selective removal of oxyanions. Herein, we prepared hierarchically porous monolithic LDH composites with tunable microcages by changing the combination of cationic species in the LDH crystal. Monolithic Mg–, Mn–, Fe–, Co– and Ni–Al type LDH composites with hierarchical pores were successfully prepared via an epoxide-mediated sol–gel reaction accompanied with phase separation. The monolithic Co–Al type LDH composite with hierarchical pores exhibited the highest CrO₄²⁻ adsorption capacity because its microcage size easily fits the CrO₄²⁻ size. Also Co–Al type LDH composites adsorbed different oxyanions, depending on their affinity with the LDH, in a SO₄²⁻, CrO₄²⁻, MoO₄²⁻ and HVO₄²⁻ co-existing solution. The pore size controllability in discrete length-scales of micrometers, nanometers, and picometers offers LDHs with tailored surface chemistries and physical properties desirable for effective and selective oxyanion adsorption.