Challenges for recycling ionic liquids by using pressure driven membrane processes

Abstract

Although the extremely low vapour pressure of ionic liquids prevents their emission to the atmosphere, they are at least partly miscible with water and will inevitably end up in the aqueous environment. One example where this can be expected to occur is the application of ionic liquids for the electrodeposition of metals. During industrial use, ionic liquids will also get mixed with other product streams and will have to be separated and recycled. Economically and ecologically it is important to recycle and re-use ionic liquids as efficiently as possible. The use of pressure driven membrane processes, nanofiltration, reverse osmosis and pervaporation, as a possibility to recycle ionic liquids from water was investigated. Ethaline200 (a deep eutectic formed between choline chloride and ethylene glycol) was used to perform these tests and the results were compared with those found in the literature. The osmotic pressure was determined experimentally to explain the results. High ion retentions (up to 0.95) were obtained, but the retention of the non-charged molecules was too low. For concentrating ionic liquids the osmotic pressure was found to be the limiting factor when using nanofiltration or reverse osmosis. Only a five-fold concentration of the ionic liquid was possible, to a maximum concentration of 20–25 vol% of ionic liquid. Pervaporation was investigated as an alternative. It was found to have limited usability for this application as the water content is too high. For low water contents, pervaporation is applicable although the flux is very low due to the presence of the ionic liquid, which decreases the activity of the water and thus the flux through the membrane. The necessary membrane area would be very high and makes pervaporation rather impractical.