In this work, the important role played by metal ions such as Fe(II/III), Cr(III) and Ni(II) in the formation and binodal behaviour of aqueous biphasic systems (ABS) composed of HCl and the ionic liquid, [P44414]Cl, or the polymer, PEG-600, is investigated. The concentration of metal ions used in this work exceeds several g/L for an industrial foreseen application. Experiments have also been carried out by varying the concentration of metal ions at different temperatures. Fe exhibits a totally different behaviour compared to Ni and Cr. In particular, the binodal curves in presence of the ionic liquid are far from the classical curves found in the literature, displaying an onion-shape form, while for Ni and Cr, the curves follow the classical trend. When any of the three metal ions is mixed with the polymer and HCl medium, only Fe(III) induces a biphasic system. Insights into the chemical driving forces at work are discussed.Introduction In recent years, neutral Aqueous Biphasic Systems (ABS), composed of different polymer-polymer, polymer-salt or salt-salt combinations, plus water as the main protagonist, have been taken into consideration to selectively extract metal ions. In particular, the use of the [P44414]Cl (tri-butyl tetradecyl phosphonium chloride) ionic liquid (IL) or of the polyethene glycol (PEG) polymer for metal extraction has already been the subject of several papers. 1-6 The use of these compounds, more appropriately called biphasic inducers in these classical ABS, helps to make the process eco-sustainable. The [P44414]Cl hydrophilic ionic liquid (i) is less toxic than the respective hydrophobic IL 7 , (ii) can be used in much lower quantities than organic volatile solvents and (iii) the extraction kinetics is favored by its low viscosity. In regards to the PEG, this is an inexpensive, nontoxic, non-flammable compound which is commercially available in very large quantities. These systems are intrinsically much eco-friendlier than the classical liquid-liquid systems where organic solvents are used as main components and could thus be favorably considered by the industry. However, in order to comply with any industrial demand, extraction should proceed from highly loaded wastes, which is currently very rarely the case in the literature. Furthermore, most industrial wastes are highly acidic, which is not compatible with the most classical ABSs. Therefore, in the first step, we took advantage of the recently described Acidic Aqueous Biphasic Systems (AcABS) 8 loaded with very high concentrations of either Cr, Ni or Fe to perform fundamental studies in view of potential industrial applications. In this work, aqueous biphasic systems (ABS) are formed from solutions containing a single metal element, HCl as the acid, and using either [P44414]Cl or PEG as biphasic inducer. We focused on the influence of the metal, in particular on the contribution coming from the type of metal used and on its concentration. Furthermore, the influence of temperature has been taken into consideration for industrial needs. Finally, we reviewed past literature which proposed some possible explanations and we discussed these under the light of our experimental results. Experimental Section Chemicals Polyethene glycol with an average molecular weight of 600 (abbreviated as PEG-600) was acquired from Acros Organics. The ionic liquid tributyl tetradecyl phosphonium chloride, [P44414]Cl, was acquired from Interchim (Montluçon, France). The declared purity was > 95%. Some basic physicochemical properties of PEG-600 are reported in the supplementary materials. The hydrochloric acid, HCl, (37 wt% = 12 mol/L, in water) was acquired from Sigma-Aldrich. The following hydrated salts have been used: NiCl2.6H2O pure at 98% acquired from Rectapur, CrCl3.6H2O pure at 98% acquired from Fluka, FeCl2.4H2O pure at 98% and FeCl3.6H2O pure at 99% were acquired from Merck. All the chemicals were used without further purification. The water used in all experiments was passed through a Milli-Q apparatus purification system commercialized by Merck (18MΩ). AAS (Atomic Absorption Spectroscopy) single element standard solutions (1000 mg/L) were acquired from Perkin Elmer for Cr and from Chem-Lab for Fe and Ni. Sample preparation