Ecotoxicity of binary mixtures of cholinium-based ionic liquids and salts to microalgae

Abstract

Due to the widespread use of ionic liquids (ILs) in various applications and their frequent combination with salts in processes or products, mixtures of ILs and salts are likely to be present in aquatic environments. The present study aimed to assess the combined ecotoxicity of mixtures of cholinium-based ILs (cholinium bicarbonate, benzyldimethyl(2-hydroxyethyl)ammonium chloride, cholinium bitartrate and cholinium dihydrogencitrate) and salts (potassium phosphate tribasic and sodium citrate dihydrate) to the microalga Raphidocelis subcapitata, a highly sensitive microalgal species. The 96 h-EC₅₀ values for the endpoint yield increased as follows: [Chol][DHCit] (EC₅₀ = 85.2 mg L⁻¹) < [Chol][Bit] (EC₅₀ = 110.8 mg L⁻¹) < [Chol][Bic] (EC₅₀ = 310.5 mg L⁻¹) < [BzChol]Cl (EC₅₀ = 766.3 mg L⁻¹), generally following the expected increase of ecotoxicity with increased hydrophobicity. Both CA and IA models could describe the observed ecotoxicity, but a better fit was achieved with the CA model, with antagonistic interactions observed in 5 of the 8 tested mixtures. The salt K₃PO₄ was found to be less ecotoxic than NaCit·2H₂O and, simultaneously, to promote stronger antagonism, and is thus recommended to be used in future processes or product design with cholinium-based ILs, hence supporting the advancement of green chemistry. Synergism was not significant in any mixture, despite being observed under specific conditions, particularly when the IL was dominant in the mixture and above 1 TU. The antagonism observed for most of the mixtures, associated with the hormesis observed for all mixtures, suggests that mixtures of ILs and salts will likely be less environmentally hazardous than predicted based on their individual toxicities. Since ILs commonly present high water solubility and good stability, further studies addressing the effects of mixtures with ILs should be performed, contributing to improving the risk assessment of ILs.