Effective elimination of multiple heavy metal ions from wastewater using circular liquid-liquid extraction based on trioctylammonium carboxylate ionic liquids

The widespread discharge of effluents containing heavy metal ions (HMIs) from various industries into natural water bodies has become an alarming concern due to their harmful effects. Recently, ionic liquid (IL)-assisted liquid–liquid extraction (LLE) has emerged as a rapid and facile method for treating industrial effluents, effectively reducing the risk of secondary pollution. The primary objective of this study was to design novel hydrophobic ILs for the extractive removal of toxic HMIs from aqueous media. These hydrophobic ILs were designed by incorporating cyclic carboxylic acids (thiobenzoic acid, pyridine dicarboxylic acid, and pyrimidine dicarboxylic acid) with trioctylamine. The aim was to evaluate the effect of different coordinating atoms in the cyclic carboxylic acids on HMI extraction, while the trioctylamine moiety was chosen to provide the necessary hydrophobicity. The synthesized ILs were evaluated for their efficiency in the extractive removal of toxic HMIs, including Cu(II), Cd(II), Ni(II), and Pb(II), both individually and from their mixtures, under simulated industrial conditions. Thorough optimization was performed to identify the key parameters affecting extraction efficiency. The HMI concentrations were adjusted to their reported maximum contamination limits to ensure real-time applicability, and the ILs demonstrated successful extraction under these conditions. Experimental studies showed that the ILs were able to effectively remove HMIs even from mixtures, confirming their practical relevance. A detailed comparison with literature data revealed that the extraction mechanism primarily relied on the preferential coordination of metals, following the HSAB principle. Furthermore, metal ions were stripped back using disodium EDTA solution, facilitating the regeneration of the ILs for multiple reuse cycles and supporting a sustainable, circular process.