Evidence of Tunable On−Off Sorption Behaviors of Metal Oxide Nanoparticles: Role of Ion Exchanger Support

Abstract

Metal oxide particlesnamely, oxides of Fe(III), Zr(IV), Ti(IV), and Al(III)are environmentally benign and exhibit amphoteric sorption behaviors around neutral pH; i.e., they can selectively bind both transition-metal cations (e.g., Cu2+) and anionic ligands (e.g., arsenate or HAsO42-). Because sorption sites reside predominantly on the surface, the metal oxides offer very high sorption capacity at nanoscale sizes on a mass basis, because of the high surface area-to-volume ratio. However, these nanoparticles are almost impermeable in fixed-bed columnar configuration or any flow-through system. The primary objective of this short communication is to present convincing experimental evidence to demonstrate that, by appropriately dispersing hydrated iron oxide (HFO) nanoparticles within a polymeric cation or anion exchanger, its amphoteric sorption capacity can be tailored to remove either metal cations or anionic ligands. Such hybrid cation and anion exchangers are also amenable to efficient regeneration. Thus, toxic metals and ligands can be separated and recovered quantitatively from the same solution using HFO nanosorbent but with different ion exchanger support. The on−off sorption behaviors of HFO and zirconium oxide nanoparticles within an ion-exchanger host are explained using the Donnan membrane principle. The selective binding sites always reside with inorganic metal oxide nanoparticles, whereas the polymeric ion exchanger support exerts the Donnan effect.