Cobalt recovery from lithium battery leachate using hydrophobic deep eutectic solvents: Performance and mechanism

This work introduces a sustainable and environmentally friendly process for extracting cobalt from leach solution of waste lithium-ion batteries, which can effectively reduce environmental pollution and alleviate shortage in cobalt resources. Three kinds of hydrophobic deep eutectic solvents (HDESs) were synthesized using di-(2-ethylhexyl) phosphate (DEHPA) as a hydrogen bond acceptor (HBA) and 1-butanol, 1-hexol and 1-octanol as hydrogen bond donors (HBD), which were cheap and easy to obtain and had little pollution to water. Among them, 1-butanol had the best extraction efficiency. At an extraction time of 6 min, O/A of 2:1, and pH of 5, the single extraction efficiency of 1-butanol-DEHPA for cobalt reached 95.64 %, which was higher than that when DEHPA was used alone, and the required amount of DEHPA was reduced. Furthermore, HDES exhibited good recycling ability. After five cycles of extraction, the extraction efficiency of HDES for cobalt remained above 80 %. The possible extraction mechanism was explored using the slope method, Fourier transform infrared spectroscopy, and hydrogen nuclear magnetic resonance spectroscopy. Through quantum chemical calculation and optimization, it has been demonstrated that a cation exchange exists between HDESs and cobalt ions, and non-extractive components play an important role in the extraction process. These findings will contribute to the development of environmentally friendly HDESs and improvement of cobalt recovery technology. Additionally, they will contribute to environmental protection and reduction of environmental pollution.