Propeline: a green alternative to Ethaline for electrochemical recovery of precious metals

Ionometallurgy can be considered as an alternative to conventional hydrometallurgy for precious metals recovery, reducing process hazards and toxic waste production. More specifically, ionic solvents are of particular interest for electrometallurgy processes, since being intrinsically conductive and stable in a broad range of potentials.At the edge of ionic solvents, Deep Eutectic Solvents (DES) are better candidates than regular ionic liquids, being much less expensive and greener, since they are often partially bio-sourced and they exhibit a higher biodegradability, as for betaine, choline-based DES. We recently demonstrated that palladium (Pd) and gold (Au) can be quantitatively leached and recovered in a single electrochemical reactor using Ethaline, a DES composed of choline chloride (ChCl) as hydrogen bond acceptor and ethylene glycol (EG) as hydrogen bond donor, typically in a 1:2 molar ratio. However, if ChCl can be considered as a “green” reactant, this is not the case for EG. Comparable DES with a lower toxic nature than Ethaline can be obtained by replacing EG by other glycols e.g. propylene glycol (PG), widely used in cosmetics and pharmacology: the resulting DES with PG is Propeline. The present paper deals with the potential of this less known DES in the recovery of precious metals.Because the change in the hydrogen bond donor with PG leads to a modification of the DES bulk properties, the first part of this work deals with the determination of Propeline density, viscosity, conductivity and electrochemical stability, which are properties of interest for electrochemical processes. The influence of water content on these properties was thoroughly investigated. Values of the above property parameters are compared to those obtained with Ethaline as a reference DES. In a second part, we present the performances of Propeline for the selective electrochemical leaching of Ag, Pd and Au. The performance of Propeline in leaching was evaluated in ambient atmosphere, i.e. in the presence of water at percent levels. Leaching efficiencies could be studied after thorough development of analytical procedures dedicated to elemental analysis e.g. ICP-EOS in DES. The speciation of leached metals was determined by use of cross-linked analysis, namely UV-vis, and EXAFS/XANES spectroscopic techniques, in both cases with comparison with those in Ethaline. Systems (leached metal species-DES) were then thoroughly studied by electrochemical methods. In particular, diffusion coefficients of the solvated metal species were determined by electrochemical transient and stationary techniques, in the aim of leached metal recovery by electrochemical deposition.