The first instance of an acidic aqueous biphasic system (AcABS) based on tributyltetradecyl phosphonium chloride ([P ][Cl]) and an acid is here reported. This AcABS exhibits pronounced thermomorphic behavior and is shown to be applicable to the extraction of metal ions from concentrated acidic solutions. Metal ions such as cobalt(II), iron(III), platinum(IV) and nickel(II) are found to partition preferentially to one of the phases of the acidic aqueous biphasic system and it is here shown that it successfully allows the difficult separation of Co from Ni , here studied at 24 and 50 °C.

The physicochemical properties of the ionic liquid N-ethyl-2-(2-methoxyethoxy)-N,N-bis(2-(2-methoxyethoxy)ethyl)ethan-1-ammonium bis(trifluoromethylsulfonyl)imide (NTFSI) as well as its solutions with sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) and sodium bis(fluorosulfonyl)imide (NaFSI) are compared in order to study the effects of the anion. The NaFSI solutions show weaker interactions as suggested by lower glass transition temperatures, lower densities, lower viscosities and higher conductivities as compared to their more strongly coordinating NaTFSI analogues. The transport properties follow Vogel-Tamman-Fulcher behaviour suggesting that the mixtures are fragile glass formers. The addition of a higher concentration of Na salts results in decoupling of the translational motion from the viscosity, as observed from Walden plots, and Na NMR data suggests that the sodium speciation is independent of the nature of the Na salt or the temperature but is affected by the salt concentration.

Acidic aqueous biphasic system (AcABS), in which the inorganic salt component of traditional aqueous biphasic system (ABS) is replaced by the inorganic acid inherently present in typical hydrometallurgical leachate solution, is shown to selectively separate cobalt from nickel, a separation relevant to the recycling of NiMH batteries. To overcome the limitation of electrodeposition in the presence of high acid concentration, a mixed ABS-AcABS approach is developed in which HCl is partially substituted by addition of a predictable amount of NaCl. This synergistic ABS-AcABS system retains the metal extraction efficiency of AcABS whilst diminishing the acid concentration required to induce phase separation as well as its distribution to the ionic liquid-rich phase. Selective deposition of cobalt in the presence of co-extracted manganese impurities was achieved in AcABS, ABS and ABS-AcABS systems. The morphology and composition of the obtained deposits as well as the Faradic efficiency of the process can be altered by varying the NaCl to HCl ratio and water content, resulting in highly tailored cobalt deposits. These results highlight the potential of AcABS derived systems as a new extractionseparation platform for the integrated hydrometallurgical treatment of critical metals, from leaching to electrodeposition.

Ionic-liquid-based acidic aqueous biphasic systems (IL-based AcABS) represent a promising alternative to the solvent extraction process for the recovery of critical metals, in which the substitution of the inorganic salt by an acid allows for a 'one-pot' approach to the leaching and separation of metals. However, a more fundamental understanding of AcABS formation remains wanting. In this work, the formation mechanisms of AcABS are elucidated through a comparison with traditional aqueous biphasic systems (ABS). A large screening of AcABS formation with a wide range of IL identifies the charge shielding of the cation as the primary structural driver for the applicability of an IL in AcABS. Through a systematic study of tributyltetradecylphosphonium chloride ([P44414]Cl) with various chloride salts and acids, we observed the first significant deviation to the cationic Hofmeister series reported for IL-based ABS. Furthermore, the weaker than expected salting-out ability of H3O+ compared to Na+ is attributed to the greater interaction of H3O+ with the [P44414]+ micelle surface. Finally, the remarkable thermomorphic properties of [P44414]Cl based systems are investigated with a significant increase in the biphasic region induced by the increase in the temperature from 298 K to 323 K. These finding allows for the extension of ABS to new acidic systems and highlights their versatility and tunability.

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The first instance of an acidic aqueous biphasic system (AcABS) based on tributyltetradecyl phosphonium chloride ([P 44414 ] [Cl]) and an acid is here reported. This AcABS exhibits pronounced thermomorphic behavior and is shown to be applicable to the extraction of metal ions from concentrated acidic solutions.M etal ions such as cobalt(II), iron(III), platinum(IV) and nickel(II) are found to partition preferentially to one of the phases of the acidic aqueous biphasic system and it is here shown that it successfully allows the difficult separation of Co II from Ni II ,here studied at 24 and 50 8 8C.

The continued electrification of society and the related growing demand for rechargeable batteries require in turn the elaboration of efficient and sustainable recycling strategies for their recovery and valorization. An important separation relevant to nickel metal hydride (NiMH) and lithium-ion battery recycling is the intertransition element separation between Ni(II), Co(II), and Mn(II). In this work, a fully aqueous process for the recovery of Mn(II) and Co(II) from concentrated Ni(II) effluents typical of NiMH battery leachate is disclosed consuming only Na 2 CO 3 . In the first instance, Mn is selectively precipitated as Mn(IV) by oxidation using ozone as an oxidant, resulting in a significant enrichment of Mn in the precipitate relative to its original solution concentration. Second, a thermo-and acid-responsive aqueous biphasic system (ABS) based on the ionic liquid (IL) tributyltetradecylphosphonium chloride ([P 44414 ]Cl) and NiCl 2 was used to recover Co(II). By using the high NiCl 2 content found in NiMH leachates both as the ABS phase former and salting-out agent, no additional salt is required. Through careful manipulation of the Co(II) to Ni(II) and the IL to Co(II) molar ratios, an effective and selective separation of Co(II) from Ni(II) was achieved. Finally, Co(II) is precipitated from the IL-rich phase and the IL is regenerated in one step by the addition of Na 2 CO 3 to induce a new phase separation.

Starting from a real Proton Exchange Membrane Fuel Cell (PEMFC) leachate, separation of the strategic metals platinum(IV) and cobalt(II) was carried out using two distinct strategies applying ionic liquids (ILs), either via solvent extraction (SX) or acidic aqueous biphasic system (AcABS). The versatility of ILs and their ability to provide original solutions to common issues in the field of used device recycling is illustrated, with both approaches critically compared. The highly hydrophobic tetradecylpyridinium bis(trifluoromethanesulfonyl)imide ([C 14 pyr][NTf 2 ]) was synthesized and applied to the separation of Pt/Co. This solvent was able to selectively extract over 98% of Pt(IV) from the PEMFC leachate in one step while leaving Co(II) in the aqueous phase and exhibited the highest reported partition of Pt(IV) in a [NTf 2 ]-based IL. In a second approach, the AcABS based on the fully water-miscible tributyltetradecylphosphonium chloride ([P 44414 ]Cl), HCl and water, is compatible with the concentrated nature of the leachate and extracts both platinum(IV) and cobalt(II) quantitatively. In a second stage, a selective precipitation step enabled the recovery of platinum(IV) in the form of an organometallic complex whilst leaving the co-extracted cobalt(II) in solution. This work represents the 2 first utilization of an AcABS for the recycling of a real waste technological object. These two different types of ILs in both cases resulted in an efficient separation of Pt(IV) from Co(II), using two very different pathways. The merit and transferability of each approach is critically compared, and suggestions are made as to the suitable condition range for each technique.