On the fate of deep eutectic solvents after their use as reaction media: the CO2 production during downstream and ultimate disposal

Abstract

Deep Eutectic Solvents (DES) have emerged as an alternative for many applications in different chemical sectors (to be used during the upstream and downstream processing, or as performance additives). While traditionally coined as green solvents, the petrochemical and energy-demanding origin of some DES components, together with some reported toxicological data, have been often overlooked. This perspective discusses the possible fate of DES once they have been used as synthetic reaction media, particularly related to the downstream unit to recover the product from the reaction mixture, and to the final DES disposal. The Total Carbon Dioxide Release (TCR) (measured as kg CO₂ per kg product) is used to compare different options. After a downstream processing to recover the product often involving an organic solvent which will be incinerated (producing CO₂), the used DES media can either be incinerated or diluted to some degree to be divested to a Wastewater Treatment Plant (WWTP). A mild wastewater treatment – involving state-of-the-art microbial processing steps, appears more promising than the incineration option, both in terms of CO₂ production, as well as to avoid the potential formation of halide compounds (e.g. from chloride) during the incineration. However, to reach the WWTP, a key factor is the dilution degree of the DES, and the biodegradability that DES (components) may display by wastewater microorganisms. At a range of 1 : 20 dilution, a production of ∼16 kg CO₂ per kg product may be expected in the WWTP, for a synthesis in DES containing 100 g substrate loading per L. Research is urgently needed to assess whether the treatment of DES as diluted wastewater may be a sustainable (and economic) solution for the DES fate.