Choice of Solvent

Dissolution of noble metals in organic solvents is a concept of high interest and exhibits inherent opportunity for improved selectivity. The choice of a solvent is paramount when evaluating the dissolution procedures. Biomass-derived solvents are often advantageous, compared to ones from petroleum-based sources. This includes properties, such as low toxicity, high biodegradability, lower VOCs release, and reduced amount of (less or non-toxic) pollutants that are generated during their manufacture. Consequently, processes based on biomass-based solvents, in many cases, have advantages such as reduced disposal costs and improved worker safety. The utilization of biomass-based solvents is, therefore, expected to lower the environmental impact and play an important role in enhancing the sustainability of the processes.^{50, 51, 67, 68}

Chloride anions can coordinate to the surface of an elemental metal like Cu and Au, thus reducing their oxidation potential and after oxidation stabilize the metal cations in the solution.^{2, 16, 25, 27} Following the reasoning above, we focused herein on inexpensive, biodegradable, and easy-to-prepare DES made of toxicologically well-characterized components, choline chloride (ChCl), a bulk chemical and additive for chicken food and naturally occurring urea (U).^{69, 70} This combination (reline) offers both high ionic strength as well as high chloride concentrations.^{50, 51} The DES was prepared beforehand by mixing 1 equivalent of ChCl and 2 equivalents of U at 80 °C to afford liquid, which remains in the liquid state even at room temperature.^{69, 70} For the following dissolution experiments metal wires were chosen as substrates. They were submerged into different dissolution media and weighed after specific times to quantify the amount of dissolved metal.

Dissolution of Cu

Using H₂O₂ (30 % aq.) as oxidant, Cu dissolves slowly in ChCl/U DES at 60 °C, and wire mass loss is plotted as a function of time in Figure 2. After 48 h the medium, consisting of 3714 mg of DES and 0.5 mL H₂O₂ (30 % aq), absorbs 2.7 wt % of Cu with an initial dissolution rate of 0.48 mol/m²h (See SI, section 2.). With increasing concentrations of dissolved Cu, the medium turns from green to blue, and finally, a blue precipitate of Cu₂Cl(OH)₃ starts to form (confirmed by EDS and XRD, Figures S2–S3). This sets the upper Cu concentration (3 wt %), where the system can work. It is, however, possible to modify the system to absorb any desirable amount of Cu since the amount of dissolved metal depends on the quantity of solvent and oxidant as well as other reaction parameters. Overall, the dissolution of Cu under benign conditions using green oxidant and safe DES gives clear advantages over treatment in noxious mineral acids like HNO₃.

Selectivity

While the ChCl/U/H₂O₂ medium was successful in dissolving Cu, it was unable to etch either Ag or Au. The computational assessment of thermodynamics and plausibility of dissolution in ChCl/U for all three metals was conducted. This assessment was based on a thermodynamic cycle parametrized using DFT calculations with a hybrid cluster/continuum solvent model (Figure 3) employing the ωB97X-D4⁸⁹ functional and def2-TZVPPD^{91, 92} basis set as implemented in Orca. The thermodynamic cycle computes the free energy for oxidative dissolution of the metal in ChCl/U, by separating the chemical process into the free energy of dissolution in water and the solubility free energy when the metal ions are transferred from water to ChCl/U (For more information on the computational approach^73-87 and details,^88-105 see Supporting Information, section 5.). The computational results show that ChCl/U significantly stabilizes Cu²⁺, Ag⁺, and Au³⁺ cations and that oxidative dissolution is exergonic for all three metals with values of −78.4, −28.6, and −44.3 kcal/mol, respectively. These values are calculated with respect to the reduction of H₂O₂ in water. This finding suggests that oxidative dissolution of Ag and Au in ChCl/U should be thermodynamically possible by tuning the reaction conditions. Oxidation of the Ag wire surface was in fact observed and EDS measurements (Figure S9) confirmed the formation of an AgCl passivation layer—a more stable phase that was excluded from DFT calculations. Metallic gold remains in the ChCl/U solution, even in the presence of I₂ or KBr,²⁸ without mass loss or visible changes. A different oxidant is needed to dissolve Au.

Dissolution of Au

The changing of H₂O₂ to Oxone, a sustainable inorganic oxidant, dissolution of both Cu and Au takes place in ChCl/U (Figure 2), while for Ag, passivation and deterioration of the wire surface was observed. The ChCl/U/Oxone suspension (3714 mg of DES and 0.5 g of Oxone) at 60 °C was able to dissolve 1.1 wt % of Au with an initial dissolution rate of 0.11 mol/m²h in the form of Au³⁺ as proven by UV/Vis spectroscopy and comparison to a solution of KAuCl₄ (Figure S6). The formation of Cl₂ was detected with the DPD indicator, but UV/Vis spectroscopy studies did not confirm its presence. However, we cannot exclude chlorine as a low-concentration, transient oxidant that is immediately consumed by metals in ChCl/U/Oxone. In the less reactive ChCl/U/H₂O₂ system, chlorine, if generated, likely reacts with the water present. This leaves peroxide as an active oxidant in this system and raises the desired selectivity in the dissolution (See SI, section 6.). As a result, the robust ChCl/U/Oxone system based on an inexpensive and green oxidant provides efficient dissolution of Au under mild reaction conditions. Importantly, the developed method has a clear advantage over the utilization of dangerous acids like aqua regia, expensive processes like iodine-iodide leaching, or dissolutions in various other organic solvents where large amounts of (expensive) organic ligands must be used.

Dissolution of Ag

Since Cl⁻ passivates an Ag surface, a method for Ag dissolution should be based on non-chloride media. Different DES based on non-toxic and non-volatile weak acid that is produced naturally by fermentation, lactic acid (LA), and compounds containing amino groups like alanine, glycine, and NH₄OAc were explored.^{67, 71, 72} It was finally realized that pure LA could dissolve Ag without any additives using just 30 % aqueous H₂O₂ as an oxidant. What is more, a high amount of dissolved Ag, 7.0 wt % was reached in this system (3714 mg of LA and 0.5 mL of 30 % aq. H₂O₂) after 24 h at 60 °C (Figure 2) with a high initial dissolution rate of 3.32 mol/m²h. H₂O₂ was in this case also a probable active oxidant species as the formation of peroxolactic acid was not observed by NMR (See SI, section 6.). Utilization of safe natural acids like LA for the dissolution of Ag opens a fascinating possibility for sustainable noble metal recycling.

Selectivity

DFT calculations were applied to determine the free energy of oxidative dissolution in LA, yielding values of −34.1, −9.2, and 49.5 kcal/mol for Cu²⁺, Ag⁺, and Au³⁺ ions, respectively, with respect to the reduction of H₂O₂ in water (Figure 3, right). The large positive value for Au implies that it is least likely to dissolve in LA, whereas the negative values for Cu and Ag suggest that the dissolution in the real systems might be thermodynamically allowed. In fact, no dissolution of Au was experimentally observed in LA/H₂O₂, while Cu dissolved readily, but reached only around a quarter of the amount of dissolved Ag in a given system which highlights the importance of specific metal-ion interactions.

Coinage Metals in Recyclable Ionic Media

When retrieving the dissolved metals from the solution and recycling of the dissolution media, metal species can be precipitated in antisolvents that would dissolve the ionic media.¹⁰⁶ Here, we studied various antisolvents for the ChCl/U/H₂O₂ system containing copper and found that EtOH dissolves DES and precipitates Cu²⁺ as Cu₂Cl(OH)₃. It was later recognized that (NH₄)₂CO₃ can be added to retrieve Cu²⁺ quantitatively as a mixed Cu salt (Figure 4, Figures S12–S13). In the next step, EtOH from the filtrate was removed, and the remaining ChCl/U was dried and recycled in 83 % in the average of three runs (See SI, section 3.1.). The observed slightly blueish color of regenerated DES indicated that traces of Cu²⁺ remain dissolved, yet not influencing the dissolution capability of reused DES. The possibility to close the recycling loop exhibits clear advantages over the treatment of metals in water-based media where precipitation with antisolvent is rarely viable.¹⁰⁶

Ag dissolved in LA/H₂O₂ can be treated with organic antisolvents like acetone or EtOAc to dissolve LA while Ag lactate is precipitated quantitatively as a gelatin-like material (Figure 4). Its identity was confirmed after drying by IR, UV/Vis, NMR, and EDS (Figures S16–S22) and comparing it to commercial Ag lactate. After the removal of water and EtOAc, LA was recovered at 85 % as an average of three runs and reused as dissolution media (See SI, section 3.2.). This example shows that benign organic media can be used to directly recover pure silver compounds and that provides a promising selective and straightforward approach for recycling of Ag in real applications.

When ChCl/U/Oxone system containing Au³⁺ was dissolved in EtOH, the precipitation of residual Oxone and its byproducts took place (Figure 4). As a showcase, the reduction of EtOH-soluble Au³⁺ ions could be achieved directly with Mg (Figure S23). The DES was recycled in 87 % on average of three runs and reused as dissolution media (See SI, section 3.3.). The treatment of the system with dissolved metal using antisolvents proved as an advantageous approach for closing the recycling loop by application of organo-soluble media for leaching.

Sequential Selective Dissolution of Coinage Metals

The results above provided us the opportunity to develop a sequential selective dissolution concept for coinage metals and multi-metal substrates like WEEE. The logical path for sequential treatment is to first dissolve Cu selectively in ChCl/U/H₂O₂ where Ag and Au are not soluble, then Ag in LA/H₂O₂, where Au is not soluble, followed by dissolving Au in ChCl/U/Oxone system (Table 1). It is worth mentioning that Ag starts to partially deteriorate (formation of AgCl, Figures S10–S11) after 32 h when Cu²⁺ ions are present in the ChCl/U/H₂O₂ system. This indicates that reaction times and ion concentrations are crucial parameters for optimizing selective dissolution steps for multi-metal substrates.

Waste printed circuit boards (WPCBs) were selected as real-life WEEE substrates (Figure S25). First, 15 g of gold fingers with Au plating were soaked in DCM and acetone to remove silicones, brominated, and other possible coatings followed by aqueous HCl to dissolve the base metals that compose the solder. WPCB pieces were then treated twice in the ChCl/U/H₂O₂ system to dissolve all Cu, followed by the aforementioned recycling procedure. The precipitated Cu₂Cl(OH)₃ was cemented to obtain 111 mg of a powder where 98 % of the metal content was Cu (Table S6).

Due to the lack of Ag in the WPCB substrates, we focused directly on Au dissolution. The Au from detached connection plates (Figure S26) was subsequently leached in the ChCl/U/Oxone. After closing the recycling loop, 6 mg of a powder was obtained after cementation where 97 % of the metal content was Au demonstrating the feasibility of the applied approach in the recycling of real-life WEEE substrates (Table S7).

A different secondary source material was chosen to be leached in the LA/H₂O₂ system. Despite the prevalence of solar energy and the high silver consumption within this industry, recycling methods for Ag from solar panels are widely unexplored due to their complex composition. A waste solar panel consisting of a plastic motherboard, protection glass, ceramics, and Al and Ag electrodes held together with different adhesives and polymers was chosen as a suitable substrate.

We crushed and dismantled the selected solar panel by soaking the cut pieces in organic solvents and manually removing the adhesive layer between ceramics with Ag electrodes and protection glass (Figure S27). The Al electrode was first removed by soaking the crushed solar panel pieces (7 g) in 10 % aqueous HCl followed by extraction of Ag into the LA/H₂O₂ system. As the low silver concentration in LA prevents its precipitation as Ag lactate using EtOAc, Ag⁺ was precipitated as AgCl (35 mg) from an aqueous solution of NaCl where 99.9 % of metal content was Ag (Figure S28, Table S8), showing the potential of the system even for recycling solar panels.

The one metal – one method approach for sequential dissolution can also be applied when all three metals are present in WEEE. This was simulated by combining the WPCB square with Cu grid, Ag wire, and part of connection fingers with Au plating (Figure 5). After removing polymer coatings, plastics, and base metals, Cu, Ag, and Au were sequentially leached in ChCl/U/H₂O₂, LA/H₂O₂, and ChCl/U/Oxone systems, respectively. Due to the slow deterioration of Ag in the ChCl/U/H₂O₂ system when Cu²⁺ is present, it is essential to fine-tune the parameters like ion concentrations and leaching time when quantitative retrieval of metals is an objective.

Comparative Analysis of the Methods

Compared to traditional recycling technologies that include dissolution in mineral acids and toxic lixiviants like cyanide as well as modern hydrometallurgical approaches like glycine, thiosulphate, cyanide, and iodine-iodide-based methods, the approaches presented herein are relatively inexpensive, environmentally sound, and moderately efficient (Table 2). An additional benefit of the current approach that helps to further reduce the overall environmental impact as well as cost is the recyclability of the dissolution media and the retrieval of pure metal species with a simple antisolvent approach. The methods presented herein exhibited lower starting dissolution rates (0.11–3.32 mol/m² h) compared to dissolution in mineral acids and some other water-based methods including iodine-iodide leching (0.9 mol/m² h),¹⁸ but were comparable to other methods using organic solvents, like leaching in organic aqua regia (0.3 mol/m² h)⁵ or ligand-assisted leaching with I₂ in acetone (0.18 mol/m² h).³⁶ However, the dissolution proceeded faster than in ligand-assisted dissolution in EtOH using O₂ as an oxidant (0.06⋅10–3 mol/m² h)³⁸ and faster than in some traditional water-based methods like glycine leaching (0.001–0.04 mol/m² h)¹⁷ and hydrometallurgical cyanidation (0.004 mol/m² h).³⁵ Additional exploration into life-cycle assessment is imperative to thoroughly examine the economic and environmental sustainability of the traditional and recent methods.