High-efficiency gold recovery by additive-induced supramolecular polymerization of β-cyclodextrin

Bifunctional chitosan/tannin aerogel for gold recovery via electrostatic attraction and in-situ reduction

Enhancing gold recovery from wastewater significantly boosts resource efficiency and safeguards the environment. Addressing this need, we have proposed a straightforward and efficient one-step approach utilizing a multifunctional cellulose/chitosan/tannin aerogel (CCTG) to recover Au (III) through a combined adsorption-reduction mechanism. Under optimal conditions of pH 3 and 35 °C, our CCTG demonstrates a remarkable maximum adsorption capacity of 1761.6 mg·g-¹ , swiftly reaching equilibrium within 80 min. The adsorption kinetics indicates a pseudo-second-order model, suggesting a rapid and efficient interaction. Further investigation into adsorption isotherms reveals that both higher initial Au concentrations and elevated temperatures facilitate the adsorption capacity, with the Langmuir model offering a superior fit, indicative of monolayer chemisorption verifying a testament to the selective and specific nature of the process. Remarkably, CCTG maintains its exceptional adsorptive capability across varying ionic strengths. In binary metal systems of Au(III) with Cu (II), Cd (II), Co (II), Ni (II), and Cr (III) respectively, CCTG displays a preferential affinity for Au (III). The underlying mechanism of this performance is a dual-action process: electrostatic attraction facilitated by chitosan and reductive capacity endowed by tannic acid. Our work presents a compelling strategy for the practical application of gold recovery from wastewater in an efficient and environmentally friendly manner.

Materials utilizing supramolecular host-guest binding for gold extraction

The dicyanoaurate anion plays a central role in the gold mining industry. Its recovery from an aqueous solution is dominantly achieved using activated carbon, which, however, suffers from several drawbacks. Herein, we report a simple preparation of a new material containing the bambusuril macrocycle physically sorbed on the surface of silica gel particles. We utilized the ability of bambusuril to form a unique material that extracts dicyanoaurate from an aqueous solution via supramolecular host-guest interactions. Notably, the material selectively removed dicyanoaurate over dicyanoargentate. Equilibrium sorption and desorption of the anion were achieved within minutes at ambient temperature, significantly outperforming activated carbon.

Decatungstate-Driven Photocatalytic Pathways for Sustainable and Cleaner Recovery of Precious Metals

The recovery of precious metals from waste streams is crucial for sustainable resource utilization but remains hindered by traditional methods involving high toxicity, energy consumption, and environmental pollution. Here, we present a photocatalytic strategy employing hydrothermally synthesized decatungstate ([W10O32]4-) homogeneous ion catalysts to achieve simultaneous oxidation and reduction of precious metals under ambient conditions. This innovative approach integrates solvent-controlled reaction pathways, enabling efficient dissolution and recovery of precious metals from diverse waste sources, including electronic waste (e-waste), platinum membrane electrodes, and platinum-containing catalysts. The decatungstate catalyst exhibits exceptional performance, with an apparent quantum yield of 0.027%-nearly double that of commercial TiO2 (0.014%)-and achieves recovery efficiency of 80%-100% for platinum, surpassing 21 tested photocatalysts. The process adheres to a solid-phase dissolution model and remains against ionic interference. Time-dependent density functional theory (TD-DFT) calculations corroborate experimental UV-vis spectra, while electron-hole pair analyses elucidate atomic and molecular contributions to photocatalytic activity. Density functional theory (DFT) further validates the thermodynamic feasibility of the reaction pathways. By combining high efficiency, ambient operational conditions, and scalability, this work establishes decatungstates as a sustainable benchmark for green precious metal recovery, addressing the limitations of traditional methods and advancing innovation in resource circularity.

Ultra-Efficient and Selective Gold Separation via Second-Sphere Coordination of Aurous Dihalide Using a Nonporous Amorphous Superadsorbent

The escalating demand for gold, coupled with dwindling terrestrial reserves, underscores the urgent need for innovative separation strategies, including e-waste recycling and seawater extraction. However, the development of ultra-efficient, highly selective adsorbents capable of recovering trace amounts of gold from complex aquatic matrices remains a formidable challenge. Herein, a covalent organic superphane cage is reported as a nonporous amorphous superadsorbent (NAS) for selective and efficient gold recovery via intermolecular second-sphere coordination of AuBr₂⁻ (or AuCl₂⁻) ions, subsequently converted to metallic gold through disproportionation. NAS demonstrates outstanding performance, including an exceptional gold uptake capacity of 2750 mg g⁻¹, ultrafast adsorption kinetics (40 s), broad pH tolerance (1-11, up to 6 M acids), and remarkable gold uptake even in 36 wt.% HCl solution (821 mg g⁻¹). NAS achieves over 99% selective gold recovery, even amidst excess competing ions, retaining efficacy across 30 regeneration cycles. Its versatile and scalable design enables applications in gold separation from gold-bearing e-waste, catalytic residues, gold ores, and seawater. A large-scale trial recovered 23.8 Karat gold from printed circuit board leachates, positioning NAS as a sustainable and eco-friendly solution for industrial and environmental gold recovery.

Carbon Defects as Highly Active Sites for Gold Detection and Recovery

The use of precious metals (PMs) in many areas, such as printed circuit boards, catalysts, and target drugs, is increasing due to their unique physical and chemical properties, but their recovery remains a great challenge in terms of zero-valent PMs as the final product. We report a highly hydrophilic carbon dot (CD) as a reductant (electron donor), in which the defects in CD served as efficient active sites for zero-valent PMs recovery with an electron-donating capacity of ~1.7 mmol g-1. The reduction of gold follows a two-step dynamic model characterized by the formation of nano-gold nuclei (initial rapid electron transfer process) followed by an Ostwald ripening process (subsequent slow process). Finite element method (FEM) simulation shows that the reaction efficiency and confinement effect of AuCl4 - ions are positively correlated with defect density, indicating that the quantitative control of carbon defect density is the key to enhancing reduction activity. Combining density functional theory (DFT) with XPS and FTIR technology, we found that the electron is transferred from CD to Au(III) via hydrogen bonding. This nano carbon material can be exploited to recover gold from e-waste water directly, with the characteristics of reducing energy consumption and avoiding environmental pollution.

A Gold Rush: Designing Hydrogels for Selective Recovery in Wastewater Containing Mixed Metal Ions

The use of synthetic hydrogels in wastewater treatment represents a promising and scalable approach to achieving clean water. By modulation of their chemical structure, hydrogels can effectively remove a wide range of toxic compounds, including emerging organic pollutants and heavy metals. For the latter, recovery is essential for both environmental protection and metal recycling. The increasing demand for gold, a nonrenewable metal widely used in many technologies, calls for methods for its selective recovery from complex metal cation solutions. This study explores easy-to-make poly(acrylamide-co-acrylic acid) hydrogels as adsorbents for gold recovery from industrial wastewater containing other precious metals. Such material can reduce gold cations into elemental nanoparticles and microparticles in acid environments at room temperature. This process offers a potential route for metal recovery that is not based on weak interaction or complex formation. Batch tests demonstrate a good adsorption capacity (up to 124 mg/g) and efficient separation from other precious metal ions (Ru, Ir, Pd, Pt, and Rh) in a solution that closely mimics realistic industrial waste conditions. These hydrogels would enable gold recovery also from other complex metal solutions, including those derived from the dissolution of electronic wastes.

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.

Challenges and Applications of Supramolecular Metalate Chemistry

While the supramolecular chemistry of simple anions is ubiquitous, the targeting and exploitation of their metal-containing relatives, the metalates, is less well understood. This mini review highlights the latest advances in this emergent area by discussing the supramolecular chemistry of metalates thematically, with a focus on the exploitation of metalates in a diversity of applications, including medical imaging and therapy, environmental remediation, molecular magnetism, catalysis, perovskite materials, and metal separations. The unifying features of these systems are identified with a view to allow the supramolecular chemist to target the unique material properties of the metalates, even in areas that are currently relatively immature.

Porous Polypyrrolidines for Highly Efficient Recovery of Precious Metals through Reductive Adsorption Mechanism

The recycling and utilization of precious metals have emerged as a critical research focus in advancing the development of the circular economy. Among numerous methods for recovering precious metals such as gold, adsorbents with both high adsorption selectivity and capacity have become key technologies. This article incorporated the N-phenylpyrrolidine into a flexible porous polynorbornene backbone to create a class of distinctive porous organic polymers, named BIT-POP-14-BIT-POP-17. Through a reductive capture mechanism, the reductive adsorption sites of N-phenylpyrrolidine coordinate selectively with precious metals, the reduced metal is captured by the hierarchically porous polymers with flexible backbone. This approach leads to remarkable gold recovery efficiency, achieving a record of 2321 mg g-1 at ambient conditions, and 3020 mg g-1 under UV light, surpassing the theoretical limit. The porous polymers are filled in a column for a continuous uptake of gold from waste printed circuit boards (PCBs), showing recovery efficiency toward gold as high as 95% after 84 h. Overall, this work offers a new perspective on designing novel adsorbents for precious metal recovery, providing inspiration for researchers to explore novel adsorption modes and contribute to the advancement of the circular economy.

Multicomponent Polymerizations of Elemental Sulfur, CH2Cl2, and Aromatic Amines toward Chemically Recyclable Functional Aromatic Polythioureas

The exploration of new polymer materials required the development of efficient, economic, robust, and scalable synthetic routes, taking energy consumption, environmental benefit, and sustainability into overall consideration. Herein, through retro-polymerization analysis of functional aromatic polythioureas, a multicomponent reaction of elemental sulfur, CH2Cl2, and aromatic amines was designed with the assistance of fluoride, and efficient, economic, and robust multicomponent polymerizations (MCPs) of these three abundantly available cheap monomers, elemental sulfur, CH2Cl2, and aromatic diamines, were developed to realize scalable conversion directly from sulfur to a series of functional aromatic polythioureas with high molecular weights (Mn up to 50,800 g/mol) in excellent yields (up to 98%). The synergistic cooperation of the strong and selective coordination of thiourea with gold ions and the redox property of aromatic polythiourea enable in situ reduction of Au3+ to elemental gold under a normal bench condition. Furthermore, the functional aromatic polythiourea could be chemically recycled through aminolysis with NH3·H2O to afford a diamine monomer in 83% isolated yield. The development of elemental sulfur-based MCP has brought the opportunity to access cost-effective and sustainable sulfur-containing functional polymer materials, which is anticipated to provide a solution for the utilization of sulfur waste and making profitable polymer materials.

Selective Gold Precipitation by a Tertiary Diamide Driven by Thermodynamic Control

The simple diamide ligand L was previously shown to selectively precipitate gold from acidic solutions typical of e-waste leach streams, with precipitation of gallium, iron, tin, and platinum possible under more forcing conditions. Herein, we report direct competition experiments to afford the order of selectivity. Thermal analysis indicates that the gold-, gallium-, and iron-containing precipitates present as the most thermodynamically stable structures at room temperature, while the tin-containing structure does not. Computational modeling established that the precipitation process is thermodynamically driven, with ion exchange calculations matching the observed experimental selectivity ordering. Calculations also show that the stretched ligand conformation seen in the X-ray crystal structure of the gold-containing precipitate is more strained than in the structures of the other metal precipitates, indicating that intermolecular interactions likely dictate the selectivity ordering. This was confirmed through a combination of Hirshfeld, noncovalent interaction (NCI), and quantum theory of atoms in molecules (QTAIM) analyses, which highlight favorable halogen···halogen contacts between metalates and pseudo-anagostic C-H···metal interactions in the crystal structure of the gold-containing precipitate.

Advances in natural polysaccharides for gold recovery from e-waste: Recent developments in preparation with structural features

The increasing demand for gold because of its high market price and its wide use in the electronic industry has attracted interest in gold recovery from electronic waste (e-waste). Gold is being dumped as solid e-waste which contains gold concentrations ten times higher than gold ores. Adsorption is a widely used approach for extracting gold from e-waste due to its simplicity, low cost, high efficiency, and reusability of adsorbent material. Natural polysaccharides received increased attention due to their natural abundance, multi-functionality, biodegradability, and nontoxicity. In this review, a brief history, and advancements in this technology were evaluated with recent developments in the preparation and mechanism advancements of natural polysaccharides for efficient gold recovery. Moreover, we have discussed some bifunctional modified polysaccharides with detailed gold adsorption mechanisms. The modified adsorbent materials developed from polysaccharides coupled with inorganic/organic functional groups would demonstrate an efficient technology for the development of new bio-based materials for efficient gold recovery from e-waste. Also, future views are recommended for highlighting the direction to achieve fast and effective gold recovery from e-waste in a friendly and sustainable manner.

Tuning CH Hydrogen Bond-Based Receptors toward Picomolar Anion Affinity via the Inductive Effect of Distant Substituents

Inspired by nature, artificial hydrogen bond-based anion receptors have been developed to achieve high anion selectivity; however, their binding affinity is usually low. The potency of these receptors is usually increased by the introduction of aryl substituents, which withdraw electrons from their binding site through the resonance effect. Here, we show that the polarization of the C(sp3 )-H binding site of bambusuril receptors, and thus their potency to bind anions, can be modulated by the inductive effect. The presence of electron-withdrawing groups on benzyl substituents of bambusurils significantly increases their binding affinities to halides, resulting in the strongest iodide receptor reported to date with an association constant greater than 1013  M-1 in acetonitrile. A Hammett plot showed that while the bambusuril affinity toward halides linearly increases with the electron-withdrawing power of their substituents, their binding selectivity remains essentially unchanged.

Thermally Reduced Graphene Oxide Membranes Revealed Selective Adsorption of Gold Ions from Mixed Ionic Solutions

The recovery of gold from water is an important research area. Recent reports have highlighted the ultrahigh capacity and selective extraction of gold from electronic waste using reduced graphene oxide (rGO). Here, we made a further attempt with the thermal rGO membranes and found that the thermal rGO membranes also had a similarly high adsorption efficiency (1.79 g gold per gram of rGO membranes at 1000 ppm). Furthermore, we paid special attention to the detailed selectivity between Au3+ and other ions by rGO membranes. The maximum adsorption capacity for Au3+ ions was about 16 times that of Cu2+ ions and 10 times that of Fe3+ ions in a mixture solution with equal proportions of Au3+/Cu2+ and Au3+/Fe3+. In a mixed-ion solution containing Au3+:Cu2+:Na+:Fe3+:Mg2+ of printed circuit board (PCB), the mass of Au3+:Cu2+:Na+:Fe3+:Mg2+ in rGO membranes is four orders of magnitude higher than the initial mass ratio. A theoretical analysis indicates that this selectivity may be attributed to the difference in the adsorption energy between the metal ions and the rGO membrane. The results are conducive to the usage of rGO membranes as adsorbents for Au capture from secondary metal resources in the industrial sector.