Liquid–Liquid Extraction of Chromium(III) with Trihexylammonium Octanoate Using Thiocyanate as Extractant

Thiocyanate ions (SCN−) play a critical role in the extraction of Cr(III) from aqueous solutions into the ionic liquid trihexylammonium octanoate ([T6AC7COO]). Extractions were carried out at room temperature without pH adjustment, as the process remained effective despite pH variations during extraction. The systems were characterized by advanced techniques including UV-Vis, FT-IR, 1H and 13C NMR spectroscopies, ICP-OES and ion chromatography. In the absence of SCN−, [T6AC7COO] enables extraction at low Cr(III) concentrations (5.0–10.0 mM) via a neutral mechanism involving co-extraction of Cr(III) with its counter-anions (NO3− and Cl−) to maintain electroneutrality; however, unfavorable precipitation occurs at higher concentrations (20.0–50.0 mM). The addition of SCN− (200.0 mM) to the aqueous phase prior to extraction suppresses precipitation and significantly enhances extraction performance. Spectroscopic analyses confirm that SCN− migrates into the organic phase and are consistent with its coordination to Cr(III) through the nitrogen atom, acting as the dominant extracting agent, whereas the ionic liquid alone only enables extraction under limited conditions. Overall, these findings support an SCN−-driven ion-pair extraction mechanism, providing a selective and highly efficient strategy for Cr(III) removal from aqueous media with promising potential applications in environmental remediation and heavy metal wastewater treatment.