Layered Double Hydroxide/Chitosan Nanocomposite Beads as Sorbents for Selenium Oxoanions

Layered double hydroxide (LDH) nanoparticles are effective sorbents for selenium oxoanions but must be fabricated in a suitable fashion for implementation in water treatment applications using packed columns. In this work, we demonstrate the preparation of nanocomposite beads prepared from Mg-Al-CO₃ LDH nanoparticles and chitosan, a sustainable and biodegradable biopolymer. The synthesis of the nanocomposite beads is achieved by direct mixing or in situ synthesis of the LDH nanoparticles into the chitosan matrix. The effect of the preparation route on the nanocomposite structure, maximum loading of LDH in the composite, removal kinetics, and the maximum sorption capabilities for selenate and selenite oxoanions are studied and compared to LDH nanopowders and granular media. The results indicate that the in situ synthesis of LDH inside the beads leads to several favorable characteristics such as a higher mass loading of LDH and better dispersion of the nanoparticles while displaying good selenium removal over a wide pH range, superior sorption capacities to the nanopowder, and sorption kinetics similar to those of the granulated media. The maximum adsorption capacities for the nanocomposite beads from Langmuir isotherms were 17 mg/g for Se(IV) and ∼12 mg/g for Se(VI) with respect to the mass of LDH, which is higher than reported capacities obtained in chitosan beads embedded with other nanocrystalline metal oxide fillers. These results show that the LDH/chitosan nanocomposite beads are promising alternatives to granulated media for selenium removal and sheds light on how best to design and fabricate high performance and sustainable nanoenabled sorbents for water treatment applications.