Gold's unique properties (high electrical conductivity, corrosion resistance, high melting point) make it crucial in various applications, leading to its presence in significant quantities in electronic waste (e-waste). The increasing demand for gold coupled with its limited supply necessitates efficient recycling methods. Current gold recovery methods, such as pyrometallurgy, suffer from low efficiency, high energy consumption, and environmental hazards. Hydrometallurgy offers a more environmentally friendly alternative, employing leaching and selective extraction. Chemical/physical adsorption is a particularly benign option within hydrometallurgy, utilizing various porous organic polymers (POPs) like covalent organic frameworks (COFs), polymers of intrinsic microporosities (PIMs), porous aromatic frameworks (PAFs), and conjugated microporous polymers (CMPs) for gold recovery. While these POPs have shown promise, their often-complex syntheses and high-cost precursors limit their widespread application. This study explores the potential of N-heterocyclic carbenes (NHCs), known for their reactive and binding properties in catalysis, as a novel material platform for gold extraction. Specifically, we introduce a porous organic polycarbene (POPcarbene) network, designed for efficient and selective gold extraction from complex e-waste solutions, addressing the limitations of existing methods through a simple and scalable synthesis. The goal is to develop a material that can capture gold efficiently and selectively, even in the presence of other metal ions, and to elucidate the mechanism behind this high performance. Furthermore, life cycle assessment (LCA) and cost analysis will assess the environmental and economic feasibility of the proposed method.

Existing gold recovery methods from e-waste, such as hydrometallurgy and biometallurgy, have been explored. Hydrometallurgy uses digestive solutions to precipitate gold ions followed by selective extraction. Common methods within hydrometallurgy include solvent extraction, ion exchange, electrochemical reduction, and chemical/physical adsorption. Chemical/physical adsorption offers environmental benefits. Recent research highlights several POPs employed for gold recovery, including COFs (Qian et al., exhibiting fast kinetics for Au³⁺ adsorption), porphyrin-phenazine-based polymers (Nguyen et al., achieving high uptake from authentic e-waste), and polyamine-functionalized porous organic polymers (Ding et al., demonstrating high gold adsorption capacity under light irradiation). However, challenges remain regarding complex synthesis procedures and high-cost precursors in these approaches. The potential of N-heterocyclic carbenes (NHCs) and their derivatives as material platforms has been investigated in applications such as organic light-emitting diodes and oxygen species-responsive reagents. The concept of "polycarbene," which involves local assemblies of individual carbene units, is also being explored in various applications. The combination of porous structures and carbene units to create functional materials is a growing area of research. This study builds upon existing research in porous carbene materials and their application for small molecule and ion capture to develop a novel approach to gold extraction from e-waste.

This study synthesized a stable, easily-constructed porous organic polycarbene (POPcarbene) network, named Ptriaz-CN-A, for gold extraction from e-waste. The synthesis involved an ammonia-catalyzed molecular crosslinking of poly(1,2,4-triazolium)s, resulting in a covalently locked porous polymer. The characterization of Ptriaz-CN-A included ¹H NMR spectroscopy, gel permeation chromatography (GPC), N₂ sorption (BET equation), and scanning electron microscopy (SEM). The BET surface area was determined, and pore size distribution was analyzed using the nonlocal density functional theory (NLDFT) method. The chemical structure of the crosslinked network was investigated using Fourier transform infrared (FT-IR) spectroscopy and ¹³C NMR spectroscopy. X-ray photoelectron spectroscopy (XPS) analysis determined the surface properties and crosslinking degrees. The adsorption efficiency of Ptriaz-CN-A towards gold ions was assessed using adsorption isotherms in solutions of varying Au³⁺ concentrations (100-5000 ppm). Kinetic efficiency was studied under various pH values. The selectivity of Ptriaz-CN-A for Au³⁺ in the presence of other metal ions (Pt²⁺, Cu²⁺, Mg²⁺, Cr³⁺, Zn²⁺, Co²⁺, Ni²⁺) was evaluated in simulated e-waste solutions. Further characterization of the adsorbent after Au³⁺ capture included XRD, TEM, XPS, and NMR analyses. The ability to extract gold from authentic e-waste (disused CPU) was also tested using a leaching solution of N-bromosuccinimide (NBS) and pyridine (Py) at neutral pH. Desorption of gold from spent Ptriaz-CN-A was accomplished using an acidic thiourea solution to determine reusability. Density functional theory (DFT) calculations were performed to simulate the binding interactions between Ptriaz-CN-A and AuCl₄⁻/PtCl₄²⁻, providing insights into the adsorption and reduction mechanisms. Finally, a life cycle assessment (LCA) and cost analysis were conducted to evaluate the environmental impact and economic feasibility of Ptriaz-CN-A production at different scales.

The synthesized POPcarbene adsorbent (Ptriaz-CN-A) demonstrated an exceptional gold recovery capacity of 2.09 g/g and high efficiency (99.8%) in simulated e-waste solutions containing various competing metal ions. The adsorption isotherms followed the Langmuir model. Kinetic studies showed that the adsorption rate slightly decreased with increasing pH values but remained efficient. Compared to similar adsorbents with imidazolium and pyridinium groups, Ptriaz-CN-A exhibited the highest gold adsorption capacity. XRD, TEM, and XPS analyses confirmed the reduction of Au³⁺ to Au⁰ nanoparticles within the Ptriaz-CN-A structure during the adsorption process. The high selectivity of Ptriaz-CN-A for Au³⁺ was demonstrated even in the presence of significantly higher concentrations of other metal ions in simulated and authentic e-waste solutions. The adsorbent also showed the ability to extract gold at trace concentrations (100 ppb). XPS and NMR analysis revealed that the high performance is due to strong metal-carbene binding affinity and the ability to reduce gold ions to nanoparticles. DFT calculations supported the experimental findings, showing that the adsorption and reduction of gold are thermodynamically favorable processes while that of platinum and copper are not. LCA analysis indicated a significant reduction in environmental impact when scaled up, and cost analysis suggested that production costs are significantly reduced in industrial scale production, rendering the method economically viable. The adsorbent showed excellent reusability with a recovery efficiency above 95% after six cycles of adsorption-desorption.

The findings address the need for efficient and environmentally benign gold recovery methods from e-waste. The high gold recovery capacity and selectivity of the Ptriaz-CN-A adsorbent demonstrate a significant improvement over existing technologies. The strong metal-carbene binding affinity, confirmed by experimental and computational methods, explains the adsorbent's superior performance. The ability to reduce Au³⁺ to Au⁰ nanoparticles contributes to the high selectivity and efficient recovery. The economic and environmental feasibility, as demonstrated by the LCA and cost analysis, indicates a potential for industrial application. This study expands the application of carbene chemistry to materials science and provides a promising new approach to resource recovery. The results highlight the advantages of using this novel material for gold extraction from complex mixtures, reducing the environmental impact of e-waste processing.

This research successfully developed a porous organic polycarbene adsorbent (Ptriaz-CN-A) for highly efficient and selective gold recovery from e-waste. The material's superior performance is attributed to its unique structure and the strong metal-carbene interactions. The method's scalability and economic viability are supported by LCA and cost analysis. Future research could explore the application of this material to other precious metal recoveries and further optimization of the synthesis process for even greater efficiency and cost reduction.

While the study demonstrates excellent performance, several limitations exist. The current LCA and cost analysis are based on laboratory-scale production and may not fully reflect the economic and environmental impacts of industrial-scale operations. Further investigation into long-term stability and potential degradation of the adsorbent during multiple cycles is needed for optimization. A more detailed comparison with other state-of-the-art technologies across a range of parameters is required to rigorously evaluate the method's overall advantages.