Thermodynamics of Co(II) Complexation with Nitrate and Chloride in the [C4mim][Tf2N] Ionic Liquid

Room temperature ionic liquids (RTILs) emerged as “green” solvents for numerous applications in the last two decades. RTILs are salts generally composed by an organic cation and an inorganic anion in the liquid state at 25 °C. They present high thermal and chemical stability, non-flammability, wide electrochemical window, low volatility and low toxicity [1]. Moreover, these properties can be finely tuned by systematically altering the structure of cations and anions. Due to these attractive features, RTILs have been used as "green" substitutes of volatile organic solvents in a number of applications related to the energy and environmental fields (e.g. separations, extractions, electrochemistry and catalysis). Among these, the use of RTILs for the separation and recycling of “critical” metals deriving from mining or high-tech waste was proposed in the last decade [2]. The recycling of cobalt assumed a great importance, due to the growing demand related to the use in key technologies, such as Li-ion batteries or motors for electric mobility [3]. The current processes for cobalt recovery in the hydrometallurgical route from aqueous solutions have some advantages such as method flexibility, high purity and low energy consumption [4] and some works on the application of RTILs in such process have appeared in the last years [4,5]. Among the available RTILs, those based on the alkylimidazolium (Cnmim+) cation and bis(trifluoromethylsulfonyl)imide (Tf2N-) anion have been studied for metal extractions in recent years [6]. However, only few works were focused on the nature of the dissolved metals and their speciation in RTILs [7], despite these are fundamental data to understand the separation processes. In this communication, the results on Co(II) complex formation with nitrate and chloride in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][Tf2N]) are reported. The interest in the Co2+ speciation with nitrate and chloride is due to the fact that the liquid samples containing the metal to be recovered usually contain high concentrations of these anions. The nature of the species formed in dry and water-saturated [C4mim][Tf2N] is determined by means of spectrophotometric, calorimetric and theoretical methods. The results evidence the great effect of water on the nature and stability of the species formed. References: [1] a) K. M. Docherty, C. F. Kulpa, Jr., Green Chem. 2005, 7 (4), 185-189; b) M. Armand, F. Endres, D. R. MacFarlane, H. Ohno, B. Scrosati, Nat. Mater. 2009, 8, 621-629. [2] a) A. P. Abbott , G. Frisch, J. Hartley, K. S. Ryder Green Chem. 2011, 13 (3), 471-481; b) A.P. Paiva, C.A. Nogueira, Waste Biom.Valoriz. 2021, 12, 1725-1747. [3] J. Piatek., S. Afyon, T. M. Budnyak, S. Budnyk, M. H Sipponen, A. Slabon, Adv. Energy Mater. 2020, 11, 2003456. [4] E. A. Othman, A. G. J van der Ham, H. Miedema, S. R. A Kersten, Sep. Purif. Technol. 2020, 252, 117435. [5] L. Xu, C. Chen, M.-L. Fu, Hydrometallurgy 2020, 197, 105439. [6] M. Gras, N. Papaiconomou, E. Chaînet, I. Billard, Solvent Extr. Ion Exch. 2018, 36 (6), 583-601. [7] M. Busato, A. Lapi, P. D’Angelo, A. Melchior, J. Phys. Chem. B 2021, 125 (24), 6639.