Copper extraction using protic ionic liquids: Evidence of the Hofmeister effect Camiel H.C. Janssen a , Norma A. Macías-Ruvalcaba a , Martha Aguilar-Martínez a , Mark N. Kobrak b,c,⇑ a Facultad de Química, Universidad Nacional Autónoma de México, Ciudad Universitaria, C.P. 04510 México D.F., Mexico b Department of Chemistry, Brooklyn College of the City University of New York, 2900 Bedford Ave., Brooklyn, NY 11210, USA c Department of Chemistry, Graduate Center of the City University of New York, 365 Fifth Ave., New York, NY 10016, USA article info Article history: Received 12 January 2016 Received in revised form 23 May 2016 Accepted 26 May 2016 Available online 27 May 2016 Keywords: Ionic liquids Metal extraction Hofmeister effect abstract Protic ionic liquids (PILs) have been used extensively in extractions of organic compounds but have seen relatively little use in the extraction of metals from aqueous phase. In this work, PILs based on trialkylam- monium alkanoates are investigated for their capacity to extract copper from the aqueous phase. The results demonstrate a high degree of copper extraction, and the structures of the PILs make this possible even in the absence of chelating agents. Charge balance indicates that neutral extraction is the dominant mechanism in all cases, avoiding the degradation of the ionic liquid phase associated with ion exchange mechanisms. Further, we observe that the extent of copper extraction depends strongly on the identity of the anion of the copper salt, and that in the presence of multiple anions the PILs display a strong selec- tivity for particular species. The study presents strong evidence that this is a manifestation of the Hofmeister effect, and a review of literature studies indicates that this phenomenon may be quite com- mon in IL-based extractions. We also demonstrate that the PILs used in this research are highly selective for copper from a mixture of alkali and alkaline earth metals, and that copper may be stripped from the IL phase using standard chelating agents. Ó 2016 Elsevier B.V. All rights reserved. 1. Introduction The increasing global demand for metals requires the develop- ment of innovative technologies for extraction and recovery of metals from aqueous streams. Currently the most common tech- nique for the extraction of commercially valuable metals from the aqueous phase is liquid-liquid extraction with molecular organic solvents [1–3]. Ionic liquids (ILs), salts that are molten at or near room-temperature, have recently emerged as very promis- ing alternatives to conventional organic solvents. Since the discovery of air and moisture stable ILs in the 1990s [4], an enormous range of IL species have been synthesized [5–8] and many studies have explored their application as media for the extraction of metals from aqueous phases [9–13]. As a class, ILs are largely non-flammable and non-volatile, potentially simpli- fying their use in commercial environments [5–7]. A more funda- mental advantage is that while ionic metal complexes are generally insoluble in organic phases, ILs are capable of solvating ionic as well as neutral metal complexes [14,15]. This amounts to the removal of a constraint on the coordination chemistry underly- ing extraction, offering researchers greater flexibility in process design. ILs therefore hold the potential for highly novel approaches to metal extraction. Here, we consider protic ionic liquids (PILs) for the extraction of copper (II) from aqueous phase. PILs are a class of ionic liquids based on the mixture of a Brønsted acid and a Brønsted base, such that molecular and ionic forms exist in equilibrium. In the present study, we combine trialkylamines with linear alkanoic acids, so that the IL phase equilibrium can be written HNR þ 3 ðILÞþ R 0 COO  ðILÞ  NR 3 ðILÞþ R 0 COOHðILÞ: ð1Þ Here, ‘‘(IL)” denotes solvation in the ionic liquid phase, and R and R 0 represent saturated, straight-chain alkyl groups. The fraction of ionic vs. molecular species in PILs is not easily measured, and the question of how to measure and define the ‘‘ionicity” of a PIL remains an active area of discussion [16,17]. However, there is evi- dence that while absolute pKa values are different in PIL and aque- ous phases, the relative pKa values of the acid and base remain the same [18]. We are not aware of any literature reports of aqueous pKa values for the ammonium and carboxylic acids of interest in the present study, and the low aqueous solubility of these species would make their determination problematic. Generally, carboxylic http://dx.doi.org/10.1016/j.seppur.2016.05.031 1383-5866/Ó 2016 Elsevier B.V. All rights reserved. ⇑ Corresponding author at: Department of Chemistry, Brooklyn College of the City University of New York, 2900 Bedford Ave., Brooklyn, NY 11210, USA. E-mail address: mkobrak@brooklyn.cuny.edu (M.N. Kobrak). Separation and Purification Technology 168 (2016) 275–283 Contents lists available at ScienceDirect Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur