Selective gold extraction from e-waste leachate via sulfur-redox mechanisms using sulfhydryl-functionalized MOFs.

Facile recovery of gold from e-waste by integrating chlorate leaching and selective adsorption using chitosan-based bioadsorbent

The hydrometallurgical recovery of gold from electronic waste and gold slag is a hot research topic. To develop a cost-effective and environmentally friendly adsorbent for gold recovery, four types of amino-acid (arginine, histidine, methionine, and cysteine)-functionalized cellulose microspheres were prepared via a radiation technique. The adsorption performance of the amino acid resins toward Au(III) ions was systematically investigated by batch experiments. The amino acid resins could absorb Au(III) ions at a wide pH range. The adsorption process was followed by the pseudo-second-order model and Langmuir model. The theoretical maximum adsorption capacity was calculated as 396.83 mg/g, 769.23 mg/g, 549.45 mg/g, and 636.94 mg/g for ArgR, HisR, MetR, and CysR, respectively. The amino acid resins could effectively and selectively recover trace Au(III) ions from the leaching solutions of printed circuit board and gold slag waste. Lastly, the mechanism underlying amino acid resin’s Au(III) ion recovery capability was investigated by FTIR, XRD, and XPS analyses. This work describes a series of cost-effective gold adsorbents with excellent selectivity and adsorption capacity to boost their practical application.

A structure‐oriented design of zirconium‐based metal‐organic frameworks (Zr‐MOFs) for long‐chain per‐ and polyfluoroalkyl substances (PFAS) is critically needed, as current efficacy faces limitations, including restricted micropore functionalization, limited coordination sites, and weak host–guest interactions. Here, we present a surface‐localized oligomeric grafting strategy to functionalize UiO‐66‐NH 2 with proximally arranged oligoalkyl‐quaternary ammonium (PAOQ) motifs, yielding UiO‐66‐ L 3 sorbent. Unlike conventional grafting approaches, the PAOQ motifs containing repetitive functional groups predominantly extend beyond surface pore layers, circumventing intrinsic grafting limitations of the MOF backbone and achieving a 2.2‐fold increase in group loading. This tailored architecture enables strong synergistic interactions through precise spatial alignment of quaternary ammonium moieties and conformationally flexible oligoalkyl chains, promoting dual‐mode coordination with long‐chain PFAS. Consequently, UiO‐66‐ L 3 exhibits rapid kinetics (equilibrium within 5 min) and high sorption capacities (403–1872 mg g −1 ) for long‐chain PFAS and derivatives, outperforming alkylated/quaternized analogs by up to 13.3‐fold. Density functional theory (DFT) calculations reveal that electronic redistribution induced by oligoalkyl segments enhances the electrostatic potential at adjacent ammonium centers, enabling chain‐length‐dependent selectivity with sorption coefficients ranging from 2.3 to 4.9. Our work highlights the critical role of surface chemistry modulation in Zr‐MOFs and advances structure‐oriented design of high‐performance sorbents for long‐chain PFAS and derivatives.

Covalent organic frameworks constructed by flexible alkyl amines for efficient gold recovery from leaching solution of e-waste

Electronic waste is a dominant global issue with over 50 million tons generated annually. Still, as an amalgamation of precious and rare raw materials, electronic waste is a considerable economic resource with the most valuable components located on the printed circuit boards. Gold is widely used in electronics in numerous applications, although principally for contact points and external connectors. The recovery of gold, due to its high value, is one of the main motivations for recycling e-waste. Although pyrometallurgy and hydrometallurgy processing are still the preferred modes of recovery for gold, the use of high-energy consuming pyro-methods, and the use of gold cyanidation that uses harmful lixiviants are increasingly discouraged. Thiourea has received attention as an alternative lixiviant for gold leaching due to its fast reaction kinetics and less harmful nature. This review aims to provide an up-to-date evaluation of thiourea-gold leaching studies from electronic waste, with emphasis on the recent progression from the classic chemical method to a more sustainable hybrid bioleaching-based system, while its challenges are highlighted. The complementary methods applied for gold retrieval from the pregnant solution are also described with a focus on sustainable methods that have the potential to provide a closed-loop system, the key objective for material recovery in a circular economy.Graphical

The extraction of gold (Au) from electronic waste (e-waste) has both environmental impact and inherent value. Improper e-waste disposal poses environmental and health risks, entailing substantial remediation and healthcare costs. Large efforts are applied for the recovery of Au from e-waste using complex processes which include the dissolution of Au, its adsorption in an ionic state and succeeding reduction to metallic Au. These processes themselves being complex and utilizing harsh chemicals contribute to the environmental impact of e-waste. Here, we present an approach for the simultaneous recovery and reduction of Au3+ and Au+ ions from e-waste to produce solid Au0 forms, thus skipping several technological steps. We develop a nanoscale cross-dimensional composite material via self-assembly of two-dimensional graphene oxide and one-dimensional chitosan macromolecules, capable of acting simultaneously as a scavenger of gold ions and as a reducing agent. Such multidimensional architecture doesn't require to apply any voltage for Au adsorption and reduction and solely relies on the chemisorption kinetics of Au ions in the heterogeneous GO/CS nanoconfinements and their chemical reduction on multiple binding sites. The cooperative phenomena in ionic absorption are responsible for the extremely high efficiency of gold extraction. The extraction capacity reaches 16.8 g/g for Au3+ and 6.2 g/g for Au+, which is ten times larger than any existing gold adsorbents can propose. The efficiency is above 99.5 wt.% (current limit is 75 wt.%) and extraction ability is down to very low concentrations of 3 ppm.

In this study, metallic molybdenum nanoparticles confined in the nanopores of a zirconium-based MOF (Zr-MOF), MOF-808, are prepared by a self-limiting decoration of spatially isolated Mo­(VI) sites on the hexa-zirconium nodes of MOF-808, followed by the electrochemical reduction of Mo­(VI) to metallic Mo. The obtained pore-confined Mo exhibits reversible redox activity in a neutral aqueous electrolyte and serves as the pseudocapacitive material for negative electrodes. By introducing another MOF-based pseudocapacitive material that can be used for positive electrodes, a manganese-decorated Zr-MOF-carbon nanotube nanocomposite, as a demonstration, all-Zr-MOF-based asymmetric pseudocapacitors with an aqueous electrolyte are fabricated.

The development of high-performance adsorbents with exceptional adsorption capacity, superior selectivity, and excellent regenerability is crucial for sustainable precious metal recovery. Functionalized metal–organic frameworks (MOFs) show particular promise in this regard, where the strategic incorporation of functional groups can significantly enhance adsorption performance. In this study, we developed a series of MIL-101(Fe) with tunable compositions by varying the ratio of the two organic ligands. The amino-functionalized NH2-MIL-101(Fe) demonstrated outstanding gold (Au(III)) recovery performance from electronic waste wastewater, achieving an experimentally measured adsorption capacity of 4077.47 mg·g–1 at 308 K while maintaining excellent regenerability over multiple cycles without significant capacity loss. Comprehensive investigation of adsorption kinetics and thermodynamics, along with systematic evaluation of pH effects and competitive ions, revealed the material’s robust performance under various conditions. Remarkably, NH2-MIL-101(Fe) exhibited exceptional selectivity for Au(III) recovery, even in complex real leaching solutions from central processing units (CPUs) and random access memory (RAM) containing high concentrations of interfering ions. To facilitate practical application, we successfully immobilized NH2-MIL-101(Fe) on expanded perlite, creating a composite material that maintained a high gold recovery efficiency during prolonged operation in simulated wastewater. Mechanistic studies demonstrated that the exceptional performance stemmed from synergistic effects between amino group coordination and Fe-node-mediated reduction, where introduced –NH2 groups played a pivotal role in both Au(III) adsorption and reduction to Au(0). This work not only presents a highly efficient and selective adsorbent for gold recovery but also provides valuable insights into designing functionalized MOF materials for sustainable resource recovery from electronic waste.

The recovery of gold from electronic waste using locally available and environmentally friendly materials were investigated. Bio-adsorbent for gold recovery was prepared from Citrus sinensis peels through mechanical processing and chemical modification. Some ceramic computer processor units were leached using aqua-regia as lixiviant. Gold was recovered from the solution using the prepared bio-adsorbent and Sodium Meta Bisulphite (SMB). The recovery efficiency of the bio-adsorbent was determined and results compared to that of SMB. The adsorption of gold at different contact times was determined and the concentration of gold in solution was found using Agilent 4100 Micro Plasma Atomic Emission Spectroscopy (MP-AES). The results obtained showed that the bio-adsorbent has an adsorption capacity of 98.4% for gold and an adsorption kinetics correlation coefficient of 0.99989. This is comparable with SMB which has 95.4%. adsorption capacity and an adsorption kinetics correlation coefficient of 0.98597. The higher adsorption capacity of the bio-adsorbent is attributed to the binding sites and the chemical modification which increases the reducing properties of the functional groups present in the Citrussinensis peels.

This dissertation investigates the electrochemical deposition (ECD) and electrophoretic deposition (EPD) processes involving citrate (cit)-stabilized Au nanoparticles (NPs) and calcium alginate (Ca-Alg-Au NP) hydrogel films. The first part of the dissertation investigates the influence of NP size, surface area (SA) coverage, and ligand stabilizers on the electrochemical growth of Au on Au NPs, aiming to enhance our understanding of the growth process during the nucleation and growth of metal NPs. The growth kinetics depend on Au NP size (4 nm > 15 nm > 50 nm) for a constant SA coverage, as determined by the overpotentials for ECD. The growth rate increases with increasing SA, showing reversible kinetics for all NP sizes above a threshold SA. In addition, alkanethiol self-assembled monolayers (SAM)-coated Au NPs show a decrease in growth kinetics as the SAM chain length increases while showing the same NP size dependence as cit Au NPs. Faster growth kinetics on Au NPs relative to glass/ITO allows for a two-step EPD/ECD amplification for electrochemical detection of fM solution concentrations of Au NPs by anodic stripping voltammetry (ASV). This approach is sensitive, simple, fast, low cost, and has the potential for automation and portability, making it promising for Au detection for mining, electronic waste, and bioassays. The second part of this dissertation explores the EPD of Ca-Alg and hybrid Ca-Alg-Au NP hydrogel films utilizing Au NP-catalyzed electrooxidation of hydroquinone (HQ). This approach induces a localized pH decrease at the electrode surface, facilitating Alg and Au NP deposition followed by Ca2+ gelation. The catalytic action of Au NPs reduces the EPD potential, presenting promising applications in sensors, biological studies, electrocatalysis, and energy-related research. Electrochemical understanding in Ca-Alg-Au NP films involves investigating the behavior of Fe(CN)63-/4- on Pt, Au, and glassy carbon electrodes coated with EPD-deposited Ca-Alg and Ca-Alg-Au NP hydrogel films. Reversible electrochemistry with decreased Fe(CN)63- diffusion follows an initial equilibration time. The amide-functionalized hydrogel impeded electrochemistry, likely due to hindered mass and ion transfer. Incorporating Au NPs initially enhances Fe(CN)63- redox activity but hampers it at higher loads, indicating a significant change in mass and ion transfer, likely altering hydrogel viscoelastic and fluid properties. Finally, EPD of Ca-Alg and Ca-Alg-Au NPs in pipette tips and Pt/Ir scanning tunneling microscopy (STM) tips enables electrochemical measurements (Fe(CN)63-, Au ECD, Au ASV, and Au NP EPD) in microscale hydrogel environments as small as 10 microns. Microscale electrochemistry differs significantly from bulk hydrogels, exhibiting reversible

Uptake and recovery of gold ions from electroplating wastes using eggshell membrane

The reducing property of activated carbon fibre and its application in the recovery of gold

In this work, biochar (BC), activated carbon (AC), and graphene oxide (GO) were thiol-functionalized using 3-mercaptopropyltrimethoxysilane (3-MPTS) (named as BCS, ACS, and GOS, respectively). BCS, ACS, and GOS were synthesized mainly via the interaction between hydrolyzed 3-MPTS and surface oxygen-containing functional groups (e.g., -OH, O-C=O, and C=O) and π-π interaction. The materials before and after modification were characterized and tested for mercury removal, including sorption kinetics and isotherms, the effects of adsorbent dosage, initial pH, and ionic strength. Pseudo-second-order sorption kinetic model (R2= 0.992~1.000) and Langmuir sorption isotherm model (R2 = 0.964~0.998) fitted well with the sorption data of mercury. GOS had the most -SH groups with the largest adsorption capacity for Hg2+ and CH3Hg+ (449.6 and 127.5mg/g), followed by ACS (235.7 and 86.7mg/g) and BCS (175.6 and 30.3mg/g), which were much larger than GO (96.7 and 4.9mg/g), AC (81.1 and 24.6mg/g), and BC (95.6 and 9.4mg/g). GOS and ACS showed stable mercury adsorption properties at a wide pH range (2~9) and ionic strength (0.01~0.1mol/L). Mercury maybe removed by ligand exchange, surface complexation, and electrostatic attraction.

Copper leaching from electronic waste for the improvement of gold recycling

Gold recovery from electronic waste (e-waste) presents a significant materials challenge, as conventional systems lack structural tunability, exhibit poor metal selectivity, and fail to integrate adsorption and photoreduction functionalities effectively. Herein, we present a topology-guided strategy based on two isomeric π-conjugated covalent organic frameworks (COFs) (v-2D-COF-Dz and v-2D-COF-Pz), incorporating diazine (ortho-N) and pyrazine (para-N) units, respectively, for integrated gold adsorption and photoreduction. The nitrogen topology not only stabilizes the π-conjugated backbone but also significantly modulates the frameworks' optoelectronic and redox properties. Notably, v-2D-COF-Dz exhibits a markedly higher Au3⁺ adsorption capacity (2465versus 1854mgg-1), attributed to enhanced interfacial electron localization and directional charge transfer to Au3⁺ centers. Both frameworks achieve over 99% gold extraction from real e-waste leachates under visible-light irradiation. Spectroscopic characterization and DFT analysis reveal that nitrogen topology governs metal coordination strength and electronic coupling, enabling precise Au-Cl bond activation. This work establishes a nitrogen-topology-directed design paradigm for light-driven, COF-based precious metal recovery systems.