Metals: From Speciation in the Aqueous Phases to the Liquid–Liquid Extraction Mechanism

International audience ; Since the Manhattan Project in the early 1940s, the separation and recovery of metals from solution by solvent extraction has gained a lot of attention and it is nowadays, together with precipitation and ion exchange, one of the three main separation techniques for metals recovery, largely used in mining, nuclear and metallurgical industries as well as in waste water treatment. For years, in the solvent extraction field, the primary objective was to find the "good" solvent-extractant couple in order to develop an efficient separation system for a given metal. This was mainly driven by the fact that the metal resources were rather stable (mainly the richer ores of more or less defined composition). Once this couple was identified , the optimum extraction parameters were searched for. Tremendous efforts have been successfully applied in the adjustment of the extracting phase characteristics, mainly based on the extracting agent/diluent couple, possibly complemented by a phase modifier to avoid third phase formation, a synergist, etc. Considering the rather limited number of suitable organic solvents, most of the work has dealt with fine-tuning of the extracting agent's chemical structure. Based on the latter consideration, solvent extraction systems are traditionally divided into cation exchangers, anion exchangers and neutral extractants. This is related to the extraction mechanism that thus seems mainly, if not entirely, governed by the extracting phase composition. Our world is a changing one and the increase in demand for metals has induced interest in a broader variety of starting materials, where waste electronic equipment (WEE) or ores of poorer grade are now considered as possible starting materials for metal extraction , separation and recovery. Furthermore, environmental regulations have strengthened