Arginine and lysine-functionalized layered double hydroxides as efficient sorbents for radioactive Co²⁺ removal by chelate-facilitated immobilization

An increase in heavy metal contamination in aquatic environments require an efficient sorbent for the removal and reuse of toxic elements. We attempted to synthesize arginine/lysine-functionalized MgAl LDHs in one-pot without using any hazardous alkaline reagents. The LDHs produced at lower temperatures showed larger numbers of amino acids on their surfaces, while these are exchanged with CO₃²⁻ at higher temperatures. The arginine/lysine present on the surface of LDH enhanced the adsorption of Co²⁺ and showed the highest adsorption capacity of 1.159 and 1.170 mmol/g for the LDHs functionalized with lysine and arginine, respectively. Kinetics studies indicated that the adsorption of Co²⁺ occurred by multiple mechanisms. The Co²⁺ adsorption on these amino acid functionalized LDHs occurs by the formation chelation complex with amino acid, which provide better vicinity of Co²⁺ to basic LDH that facilitating the enhanced immobilization. The sorption of other divalent metal ions on these arginine/lysine functionalized LDHs followed the order of Co²⁺ > Ni²⁺ > Mn²⁺ > Fe³⁺. The Co²⁺ forms diamine-like coordination that is stable on the surface of LDH and causes higher sorption densities, while other metals form partial glycine-like coordination that detaches the amino acid from the surface of LDH, thereby leading to lesser sorption capacity. The conversion of anionic LDH in to a cationic sorbent was successfully fabricated by these arginine/lysine-assisted methods and explored for the remediation of Co²⁺ from aqueous solution.