Notable increases in metal consumption and declining ore grades in recent decades have stressed the significance of dilute solutions as secondary sources of valuable metals. Moreover, environmental considerations and the imperative of sustainable development have further emphasized their treatment. Therefore, finding an efficient solution for separating metals from dilute solutions has attracted the attention of numerous researchers. This paper reviews the purification processes of dilute solutions and highlights key achievements of published research works. Although this study focuses on evaluating the efficiency of recently developed aqueous-phase purification methods, such as immobilized ligands, ionic liquids, and air-assisted solvent extraction, the application of conventional processes to treat these solutions, such as solvent extraction, ion exchange, membranes, chemical precipitation, and adsorption are also briefly outlined. To provide a comprehensive assessment, more than 200 research articles were reviewed, and their key findings are stated in this study. This research contributes to the advancement of knowledge of metal recovery from dilute solutions and sheds light on the dynamic evolution of this field.

The high demand for rare earth permanent magnets (REPMs) for use in advanced electrical equipment has led to the classification of some rare earth elements (REEs), such as neodymium, praseodymium, and dysprosium, as strategic raw materials by the European Union. Finding an efficient method with economic benefits and minimal environmental impact is one of the challenges of recycling REPMs. A novel method to recover REPM metals by dissolving them with citric acid and separating the rare earth elements using the solvent extraction method as the next step has been introduced. Therefore, this research investigates the efficiency and kinetics of leaching REPM swarf by citric acid (0.5 to 2.5 M). The results showed that increasing the citric acid concentration from 0.5 M to 1.45 M significantly enhanced the dissolving rate of all studied elements. Although the results showed that the optimal concentration of citric acid in the leaching reagent was between 0.9 and 1.45 M, the REE dissolving reaction with a leaching reagent containing 1.45 M and 0.9 M citric acid reached equilibrium conditions at about 120 and 180 min, respectively. It could be concluded that using citric acid as an organic acid can dissolve the metals in REPM swarf with high efficiency and a fast dissolving rate. This issue can be considered key for recycling REPMs and the future REE supply challenge.

In recent years, the increasing demand and rising prices of rare earth elements (REEs), along with their attendant supply risk (about 95% of these elements are supplied by China), have led the European Commission to consider REEs as critical raw materials. Developing and optimizing processes for recovering REEs from secondary sources such as NdFeB magnets is fundamental in this context. A novel method to recover REEs by leaching with citric acid and subsequently separating these elements using the solvent extraction method has been introduced. Therefore, this research investigates the leaching efficiency of REEs, Fe, and B from NdFeB magnets. A full factorial design, with 18 experimental setups, was conducted to optimize the citric acid concentration (1–3 mol/L), leaching time (1–3 h), and solid–liquid ratio (5–10%wt./vol.). All tests were carried out at room temperature and 150 rpm. Different optimizations (response surface methodology (RSM) and artificial neural network (ANN) analysis) are used to maximize the REEs’ leaching efficiency. RSM resulted in a maximum extraction yield of total rare earth elements (TREEs) of about 89% in the investigated experimental plan. This result is similar to that for ANN analysis (about 86%), but more accurate than that for RSM. In fact, for the ANN, an overall R-value higher than 0.99 was obtained. This result indicates that the developed ANN can be used as an accurate model for estimating the leaching efficiencies of REEs from NdFeB magnets.

The growing demand for ultra-low sulfur fuels has intensified interest in recovering strategic metals from the large volumes of hazardous hydrodesulfurization catalysts that are discarded yearly. This work evaluates a task-specific ionic liquid, tri-n-octylammonium bis(2-,4-,4-trimethylpentyl)-phosphinate [TOA][Cy272], synthesized by the acid–base neutralization of tri-n-octylamine and Cyanex 272. FT-IR spectroscopy confirmed complete proton transfer and the formation of a stable ion pair. Liquid–liquid extraction tests were conducted with synthetic Co–Ni–Mo solutions (0.1–2.5 g/L each), a varying ionic liquid concentration (10–50 vol%), pH (1.5–12.5), and organic/aqueous ratio (1:1). At 35 vol% of ionic liquid and pH 2, the extraction efficiency for Mo reached 94%, with separation factors βMo/Ni = 12 and βMo/Co = 7.5; Co and Ni uptake remained ≤15%. Selectivity decreased at higher metal loadings because of ionic liquid saturation, and an excessive ionic liquid amount (>35%) offered no benefit, owing to viscosity-limited mass transfer. Stripping studies showed that 1 M NH₄OH stripped about 95% Mo, while leaving Co and Ni in the organic phase; conversely, 2 M HCl removed 92–98% of Co and Ni, but <5% Mo. Overall Mo recovery of about 95% was obtained by a two-step extraction/stripping scheme. The results demonstrate that [TOA][Cy272] combines the cation exchange capability of quaternary ammonium ILs with the strong chelating affinity of organophosphinic acids, delivering rapid, selective, and regenerable separation of Mo from mixed-metal leachates and wastewater streams.

The increasing demand for molybdenum, nickel, and cobalt in high-tech industries has led to a need for efficient and sustainable metal recovery from secondary sources. Conventional solvent extraction methods face environmental concerns, high reagent consumption, and limited selectivity. Ionic liquids (ILs) have emerged as promising alternatives due to their low volatility, high thermal stability, and exceptional metal ion selectivity. In this study, the bifunctional ionic liquids [TOA][CY272] and [A336][P204] were investigated for selective extraction of molybdenum, nickel, and cobalt from synthetic solutions simulating industrial waste leachates. The ILs were dissolved in kerosene, and their extraction efficiencies were compared with raw components to evaluate improvements in selectivity and efficiency. Key operational parameters, such as metal ion concentration, extractant concentration, and pH, were optimized for maximum recovery. Two stripping agents, acidic and alkaline, were tested to enhance selective metal recovery. Results showed [A336][P204] achieved over 95% extraction efficiency for molybdenum with less than 4% co-extraction of nickel and cobalt, while [TOA][CY272] exhibited a maximum efficiency of less than 75% and lower selectivity. The appropriate stripping agent facilitated efficient metal separation. These findings highlight the potential of bifunctional ionic liquids as sustainable and environmentally friendly extractants for recovering strategic metals from secondary sources.