Synthesis of thiourea-based ionic liquids for extracting Cu ions and their toxicity analysis

With the increasing pressure on environmental protection, it was important to achieve an environmentally friendly and efficient medium to separate Cu2+ from electroplating wastewater. Therefore, two thiourea based hydrophobic ILs were screened and synthesized, which could be used as efficient extractants to extract Cu2+ by introducing ethyl and carboxyl groups as functional groups. A three-step method was adopted, then its structure was characterized. Meanwhile, the extraction experiments were optimized, and the extraction mechanism was explored by electrostatic potential analysis and FT-IR characterization. The extraction efficiency (E) of the adopted ILs reached to 96% and 99%, respectively, under 298.15 K and the ratio of extractant to solution to be extracted 1:50. The circulation extraction was investigated under the optimal conditions, it was proved that the adopted ILs had good separation result. Meanwhile, Pseudokirchneriella subcapitata was selected as the object of ILs toxicity test. The values of the concentration for 50% of maximal effect (EC50) were 1-(3-isopropylthiourea propyl) imidazole hexafluorophosphate ([HIPSM][PF6]): 79.09 mg/L and 1-(3-isopropyl thiourea carboxyethyl) imidazole hexafluorophosphate ([ITPSCM][PF6]): 177.5 mg/L, respectively. σ-profile polarity analysis was adopted to judge the change of interaction between ILs and non-polar biofilm and deduced the principle of toxicity reduction. The results showed that the introduction of carboxyl group not only improved the E, but also reduced the ecological toxicity of the adopted ILs, making the adopted ILs more environmentally friendly.