Insights into the liquid extraction mechanism of actual high-strength phenolic wastewater by hydrophobic deep eutectic solvents

Abstract

Liquid-liquid extraction using recently developed hydrophobic deep eutectic solvents (HDESs) shows great potential for the removal of phenol from wastewater. But most of the research limited themselves by being focused on the wastewater with phenol concentration lower than 1000 mg·L−1 and most importantly, the extraction mechanism is still lacking in deep understanding. We pilot to remove phenol from actual industrial wastewater containing ultra-high-concentration phenol (∼54,000 mg·L−1) using HDESs, and also carried out a systematic investigation on their extraction mechanism. Eight HDESs composed of dl-menthol (hydrogen bond acceptor, HBA) and various fatty acids (hydrogen bond donor, HBD) were synthesized and the effects of various important conditions, such as HBA/HBD molar ratio, HDES/wastewater mass ratio, and initial concentration of phenol in wastewater, on their extraction efficiencies were evaluated. The experimental results showed that all the prepared HDESs possess excellent extraction capacity for phenol with the best one being obtained from dl-menthol/nonanoic acid (C9) based HDES (molar ratio 2:1) within 5 min at room temperature (97 %). Successive extraction was performed to minimize the amount of solvents used for economic consideration. Regeneration and reuse of HDESs after phenol extraction was achieved by activated carbon recovery. The extraction mechanism was investigated experimentally by Fourier transform infrared (FTIR) and 1H nuclear magnetic resonance (NMR) characterization, together with the predicted activity coefficients of phenol in solvents by COSMO-RS model and experimentally determined distribution coefficient (D) between HDES and aqueous phase. The hydrogen bond formed between HDES and phenol during extraction was experimentally identified. Moreover, quantum computation by DFT method was applied to model the extraction process, and the extraction mechanism was investigated more in-depth by theoretical computation. The results of DFT calculation provided good agreement with the experimental data and a new six-membered ring-like structure formed during the extraction process between HDES and phenol molecule was suggested for the first time to the best of our knowledge. We hope this work will provide guidance on the study of liquid extraction process/mechanism using HDESs and promote their practical application in the future.