Highly Selective Recovery of Gold by In Situ Magnetic Field-Assisted Fe/Co-MOF@PDA/NdFeB Double Network Gel.

There are enormous economic benefits to conveniently increasing the selective recovery capacity of gold. Fe/Co-MOF@PDA/NdFeB double-network organogel (Fe/Co-MOF@PDA NH) is synthesized by aggregation assembly strategy. The package of PDA provides a large number of nitrogen-containing functional groups that can serve as adsorption sites for gold ions, resulting in a 21.8% increase in the ability of the material to recover gold. Fe/Co-MOF@PDA NH possesses high gold recovery capacity (1478.87 mg g-1) and excellent gold selectivity (Kd = 5.71 mL g-1). With the assistance of an in situ magnetic field, the gold recovery capacity of Fe/Co-MOF@PDA NH is increased from 1217.93 to 1478.87 mg g-1, and the recovery rate increased by 24.7%. The above excellent performance is attributed to the efficient reduction of gold by FDC/FC+, Co2+/Co3+ double reducing couple, and the optimization of the reduction reaction by the magnetic field. After the samples are calcined, high-purity gold (95.6%, 22K gold) is recovered by magnetic separation. This study proposes a forward-looking in situ energy field-assisted strategy to enhance precious metal recovery, which has a guiding role in the development of low-carbon industries.

Development of complete hydrometallurgical processes for gold recovery from ICs and CPUs using ionic liquids.

Integrated circuits (ICs) and central processing units (CPUs), essential components of electrical and electronic equipment (EEE), are complex composite materials rich in recyclable high-value strategic and critical metals, with many in concentrations higher than in their natural ores. With gold the most valuable metal present, increase in demand for gold for EEE and its limited availability have led to a steep rise in the market price of gold, making gold recycling a high priority to meet demand. To overcome the limitations associated with conventional technologies for recycling e-waste, the use of greener technologies (ionic liquids (ILs) as leaching agents), offers greater potential for the recovery of gold from e-waste components. While previous studies have demonstrated the efficiency and feasibility of using ILs for gold recovery, these works predominantly concentrate on the extraction stage and often utilise simulated solutions, lacking the implementation of a complete process validated with real samples to effectively assess its overall effectiveness. In this work, a simulated Model Test System was used to determine the optimal leaching and extraction conditions before application to real samples. With copper being the most abundant metal in the e-waste fractions, to access the gold necessitated a two-stage pre-treatment (nitric acid leaching followed by aqua regia leaching) to ensure complete removal of copper and deliver a gold-enriched leach liquor. Gold extraction from the leach liquor was achieved by liquid-liquid extraction using Cyphos 101 (0.1 M in toluene with an O:A = 1:1, 20 °C, 150 rpm, and 15 min) and as a second process by sorption extraction with loaded resins (Amberlite XAD-7 with 300 mg of Cyphos 101/g of resins at 20 °C, 150 rpm and 3 h). In both processes, complete stripping and desorption of gold was achieved (0.5 M thiourea in 0.5 M HCl) and gold recovered, as nanoparticles of purity ≥95%, via a reduction step using a sodium borohydride solution (0.1 M NaBH4 in 0.1 M NaOH). These two hydrometallurgical processes developed can achieve overall efficiencies of ≥95% for gold recovery from real e-waste components, permit the reuse of the IL and resins up to five consecutive times, and offer a promising approach for recovery from any e-waste stream rich in gold.

Highly Efficient and Selective Extraction of Gold from Thiosulfate Leaching Solution Using Functionalized Dicationic Ionic Liquids.

Thiosulfate leaching has been regarded as a promising alternative to cyanidation, but it still faces the challenge of the recovery of low content of gold from high concentrations of thiosulfate solutions. Liquid-liquid extraction is a method to address this issue but is still limited by the use of volatile and toxic organic solvents. To overcome this limitation, this work synthesized some functionalized dicationic ionic liquids (DILs) to serve as extraction solvents for the recovery of the gold-thiosulfate complex, [Au(S2O3)2]3-, from thiosulfate solutions. Experimental results indicated that the DILs showed higher extraction rates toward [Au(S2O3)2]3- compared with their monocationic-based counterparts, likely due to the stronger electrostatic interaction between the dications of the ILs and [Au(S2O3)2]3-. The transfer of [Au(S2O3)2]3- from the water phase to the IL phase was identified as an anion exchange and endothermic process. The rate of extraction was limited by the anion exchange process occurring at the IL-water interface. The extraction ability of ILs highly depended on the type of anion; specifically, the ILs with anions that had strong hydrogen-bonding ability exhibited high extraction ability toward [Au(S2O3)2]3-. Finally, DILs proved effective in the recovery of [Au(S2O3)2]3- from an actual gold leaching solution and exhibited high selectivity toward coexisting ions, indicating their potential as environmentally friendly solvents for gold recovery.

Anion-Regulated Ionic Covalent Organic Frameworks for Highly Selective Recovery of Gold from E-Waste.

The existing electronic waste (e-waste) and leaching solutions generated by industries accumulate significant amounts of gold (Au), even in excess of those in natural minerals. Therefore, the recycling of Au is extremely significant for the potential sustainability of chemical industry. By designing ionic covalent organic frameworks (COFs), here we synthesize a series of Ionic-COF-X (X=Cl-, Br-, AcO-, and SO42-) by anion regulation strategy. All these ionic COFs exhibit ultrahigh gold adsorption efficiency and excellent regeneration. Moreover, anion regulation could indeed affect the Au capture performance. In particular, when Cl- ions serve as counter ions, the Au capacity of Ionic-COF-Cl could reach 1270.8 mg g-1. Moreover, in the actual CPU leaching solution test, the selectivity of Ionic-COF-Cl towards Au3+ ion hits 39000 and 4600 times higher than that of Cu2+ and Ni2+ ions, respectively, suggesting that the Ionic-COF-Cl is a promising material for highly selective recovering gold from actual e-waste. DFT calculations further reveal that counter ions can regulate the adsorption affinity of ionic COF framework toward Au. In short, this work provides a useful anion regulation strategy to design ionic COFs as a promising platform for gold selective recovery from actual e-waste.

Extraction of Gold Based on Ionic Liquid Immobilized in UiO-66: An Efficient and Reusable Way to Avoid IL Loss Caused by Ion Exchange in Solvent Extraction.

Ionic liquids (ILs) have received considerable attention as a promising green solvent for extracting metal ions from aqueous solutions. However, the recycling of ILs remains difficult and challenging because of the leaching of ILs, which is caused by the ion exchange extraction mechanism and hydrolysis of ILs in acidic aqueous conditions. In this study, a series of imidazolium-based ILs were confined in a metal-organic framework (MOF) material (UiO-66) to overcome the limitations when used in solvent extraction. The effect of the various anions and cations of the ILs on the adsorption ability of AuCl4- was studied, and 1-hexyl-3-methylimidazole tetrafluoroborate ([HMIm]+[BF4]-@UiO-66) was used for the construction of a stable composite. The adsorption properties and mechanism of [HMIm]+[BF4]-@UiO-66 for Au(III) adsorption were also studied. The concentrations of tetrafluoroborate ([BF4]-) in the aqueous phase after Au(III) adsorption by [HMIm]+[BF4]-@UiO-66 and liquid-liquid extraction by [HMIm]+[BF4]- IL were 0.122 mg/L and 18040 mg/L, respectively. The results reveal that Au(III) coordinated with the N-containing functional groups, while [BF4]- was effectively confined in UiO-66, instead of undergoing anion exchange in liquid-liquid extraction. Electrostatic interactions and the reduction of Au(III) to Au(0) were also important factors determining the adsorption ability of Au(III). [HMIm]+[BF4]-@UiO-66 could be easily regenerated and reused for three cycles without any significant drop in the adsorption capacity.

Porous Covalent Organic Framework Based Hydrogen-Bond Nanotrap for the Precise Recognition and Separation of Gold.

Utilizing weak interactions to effectively recover and separate precious metals in solution is of great importance but the practice remains a challenge. Herein, we report a novel strategy to achieve precise recognition and separation of gold by regulating the hydrogen-bond (H-bond) nanotrap within the pore of covalent organic frameworks (COFs). It is found that both COF-HNU25 and COF-HNU26 can efficiently capture AuIII with fast kinetics, high selectivity, and uptake capacity. In particular, the COF-HNU25 with the high density of H-bond nanotraps exhibits an excellent gold uptake capacity of 1725 mg g-1 , which is significantly higher than that (219 mg g-1 ) of its isostructural COF (COF-42) without H-bond nanostrap in the pores. Importantly, the uptake capacity is strongly correlated to the number of H-bonds between phenolic OH in the COF and [AuCl4 ]- in water, and multiple H-bond interactions are the key driving force for the excellent gold recovery and reusability of the adsorbent.

Tailoring the Asymmetric Structure of NH2 -UiO-66 Metal-Organic Frameworks for Light-promoted Selective and Efficient Gold Extraction and Separation.

Designing adsorption materials with high adsorption capacities and selectivities is highly desirable for precious metal recovery. Desorption performance is also particularly crucial for subsequent precious metal recovery and adsorbent regeneration. Herein, a metal-organic framework (MOF) material (NH2 -UiO-66) with an asymmetric electronic structure of the central zirconium oxygen cluster has an exceptional gold extraction capacity of 2.04 g g-1 under light irradiation. The selectivity of NH2 -UiO-66 for gold ions is up to 98.8 % in the presence of interfering ions. Interestingly, the gold ions adsorbed on the surface of NH2 -UiO-66 spontaneously reduce in situ, undergo nucleation and growth and finally achieve the phase separation of high-purity gold particles from NH2 -UiO-66. The desorption and separation efficiency of gold particles from the adsorbent surface reaches 89 %. Theoretical calculations indicate that -NH2 functions as a dual donor of electrons and protons, and the asymmetric structure of NH2 -UiO-66 leads to energetically advantageous multinuclear gold capture and desorption. This adsorption material can greatly facilitate the recovery of gold from wastewater and can easily realize the recycling of the adsorbent.

Recovery of gold from e-waste via food waste byproducts.

Global materials' and energy constraints and environmental issues call for a holistic approach to waste upcycling. We propose a chemically rational, cost-effective and environmentally friendly recovery of non-leaching gold from e-waste using aqueous chemistry with hydrogen peroxide, an environmentally benign oxidant, and lactic acid, a food chain byproduct. The oxidation of the base metals enables the release of gold in its metallic state in the form of flakes subsequently separated via filtration. Our main byproduct is a precursor of Cu2O, a relevant metal oxide for solar energy conversion applications. The recovered gold was characterized by scanning electron microscopy, energy dispersive spectroscopy and x-ray photoelectron spectroscopy to gain insight into the morphology of the flakes and their chemical composition. Furthermore, recovered gold was used to successfully fabricate the source and drain electrodes in organic field-effect transistors.

Irreversible Amide-Linked Covalent Organic Framework for Selective and Ultrafast Gold Recovery.

Design of stable adsorbents for selective gold recovery with large capacity and fast adsorption kinetics is of great challenge, but significant for the economy and the environment. Herein, we show the design and preparation of an irreversible amide-linked covalent organic framework (COF) JNU-1 via a building block exchange strategy for efficient recovery of gold. JNU-1 was synthesized through the exchange of 4,4'-biphenyldicarboxaldehyde (BA) in mother COF TzBA consisting of 4,4',4''-(1,3,5-triazine-2,4,6-triyl)trianiline (Tz) and BA with terephthaloyl chloride. The irreversible amide linked JNU-1 gave good stability, unprecedented fast kinetics, excellent selectivity and outstanding adsorption capacity for gold recovery. X-ray photoelectron spectroscopy along with thermodynamic study and quantum mechanics calculation reveals that the excellent performance of JNU-1 for gold recovery results from the formation of hydrogen bonds C(N)-H⋅⋅⋅Cl and coordinate interaction of O and Au. The rational design of irreversible bonds as both inherent linkage and functional groups in COFs is a promising way to prepare stable COFs for diverse applications.

Environmentally Benign, Rapid, and Selective Extraction of Gold from Ores and Waste Electronic Materials.

The extraction of gold from ores and electronic waste is an important topic worldwide, as this precious metal has immense value in a variety of fields. However, serious environmental pollution and high energy consumption due to the use of toxic oxidation reagents and harsh reaction conditions is a well-known problem in the gold industry. Herein, we report a new chemical method based on the combined use of N-bromosuccinimide (NBS) and pyridine (Py), which has a greatly decreased environmental impact and reagent cost, as well as mild reaction requirements. This method can directly leach Au0 from gold ore and electronic waste to form AuIII in water. The process is achieved in a yield of approximately 90 % at room temperature and a nearly neutral pH. The minimum dose of NBS/Py is as low as 10 mm, which exhibits low toxicity towards mammalian cells and animals as well as aquatic creatures. The high leaching selectivity of Au over other metals during gold leaching is demonstrated, showing that this method has great potential for practical industrial application towards the sustainable refining of gold from ores and electronic waste.