The exponential growth of electronic waste (e-waste) presents a significant environmental challenge and a substantial untapped resource of valuable metals like gold. E-waste leachates, produced during the pretreatment of e-waste, contain high concentrations of gold, making them an attractive source for recovery. Traditional gold recovery methods, such as solvent extraction and electrochemical reduction, often involve complex procedures and expensive reagents, limiting their scalability and sustainability. Adsorption offers a promising alternative, characterized by its cost-effectiveness, ease of operation, and reduced secondary pollution. However, the complex hydrochemical nature of e-waste leachates necessitates the development of highly selective and robust sorbents capable of overcoming challenges such as poor selectivity, structural instability, and functional group detachment observed in conventional sorbents like silica, activated carbon, and resins. Recent advances have focused on innovative sorbents with tailored microstructures and reactivity, including porous polymer materials and metal-based nanocrystals. While these materials demonstrate enhanced gold adsorption, they often suffer from limitations in large-scale production, including low yields, expensive synthesis, and the generation of hazardous waste. This necessitates the development of a sustainable and economically viable sorbent for widespread application in gold recovery from e-waste. Pyrocarbon, derived from pyrolyzed biomass, emerges as a promising candidate due to its abundance, cost-effectiveness, and intrinsic self-functionalization. Previous studies have explored the use of various pyrocarbon-based materials for gold recovery, including activated carbon, carbon nanotubes, biochar, and graphene oxide. However, a comprehensive understanding of the structure-function relationship of these materials, particularly concerning electron sources and the gold reduction process, remains limited. This lack of mechanistic understanding has hampered the development of highly efficient and selective pyrocarbon-based sorbents for practical gold recovery applications. This research aims to address this gap by developing and characterizing a novel alginate-derived pyrocarbon sorbent tailored for efficient and selective gold recovery, while also elucidating the underlying mechanisms involved.

Existing literature highlights the urgent need for sustainable and efficient gold recovery from e-waste leachates due to increasing e-waste generation and the limited supply of gold. While conventional methods like solvent extraction and electrochemical reduction exist, they are often hampered by high costs and complex procedures. Adsorption-based methods offer a promising alternative, but existing sorbents struggle with selectivity and stability in the complex chemical environment of e-waste leachates. Research on innovative materials like porous polymers and metal-based nanocrystals shows promise, but scalability and cost remain significant challenges. Pyrocarbon, a cost-effective material derived from biomass, has shown potential, but its structure-function relationship in gold recovery needs further investigation. Studies have explored various pyrocarbon forms, such as activated carbon, nanotubes, and biochar, but a complete understanding of the electron transfer mechanisms and optimal pyrocarbon properties for efficient gold recovery is lacking. This gap in knowledge motivates the current research to develop a highly efficient and selective pyrocarbon-based sorbent while comprehensively elucidating the underlying mechanisms.

The study employed a straightforward and eco-friendly method to synthesize alginate-derived pyrocarbon sorbents. Sodium alginate and calcium chloride were used as precursors. The pyrolysis temperature was systematically varied (500, 600, 700, and 800 °C) to optimize the sorbent's properties. The resulting pyrocarbons (PyCs) were characterized using various techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) surface area analysis, high-resolution TEM (HRTEM), X-ray diffraction (XRD), attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrical conductivity measurements. The gold recovery performance of the PyCs was evaluated through adsorption isotherm and kinetic experiments, assessing the effects of gold concentration, pH, ionic strength, and the presence of competing ions. Real-world CPU leachates were used to validate the sorbent's efficacy in practical applications. To investigate the underlying mechanisms of gold recovery, operando spectroscopy (in situ FTIR and time-resolved XAS) and density functional theory (DFT) calculations were employed. In situ FTIR monitored interfacial changes during gold adsorption, while XAS provided insights into the gold reduction process at the atomic level. DFT calculations explored the adsorption energies and reaction pathways involved in the interaction between gold ions and the pyrocarbon surface, including stepwise dechlorination processes. Finally, techno-economic analysis (TEA) was conducted to assess the economic viability of the pyrocarbon-based gold recovery process, considering costs associated with CPU waste leaching, sorbent production, gold recovery, and purification. The purity of the recovered gold was determined using standard methods.

The study found that the optimal pyrolysis temperature for gold recovery was 700 °C (PyC700). PyC700 exhibited a remarkably high theoretical adsorption capacity (Qm) of 2829.7 mg g⁻¹ and a rapid adsorption rate (k = 1.73 × 10⁻⁶ m s⁻¹). It achieved near-complete gold recovery efficiency (>99.5%) across a wide range of gold concentrations (1 to 1000 mg L⁻¹), pH values (1.0 to 8.0), and ionic strengths, demonstrating robust performance under diverse environmentally relevant conditions. The sorbent also showed excellent selectivity for gold over other metal ions present in e-waste leachates, with a distribution coefficient (Kd) for Au(III) approximately 5-7 orders of magnitude higher than that of competing metal ions. Mechanistic studies revealed that the reduction of Au(III) to Au(0) was facilitated by the electron-donating capacity of the pyrocarbon. Specifically, hydroxylation of the aromatic structures in the pyrocarbon during the gold adsorption process provided a continuous source of electrons, converting sp²-hybridized carbon to sp³-hybridized carbon and producing phenolic hydroxyl groups. The gold reduction proceeds through a stepwise nucleation mechanism involving the formation of intermediate gold-chlorine clusters, lowering energy barriers and accelerating the reduction process. DFT calculations supported this mechanism, showing a significant decrease in energy barriers during stepwise dechlorination. The efficacy of PyC700 was demonstrated using real-world CPU leachates. While the highly acidic nature of some leachates initially affected recovery efficiency, dilution improved performance, resulting in almost complete gold recovery. The recovered gold exhibited high purity (>99.82%, 23.96 karats). Techno-economic analysis indicated a substantial economic viability, with an input-output ratio of 1370%, potentially exceeding 2000% with a reduction in CPU waste price.

The findings of this study address the pressing need for sustainable and economically viable gold recovery from e-waste. The high recovery capacity, efficiency, selectivity, and stability of the alginate-derived pyrocarbon sorbent (PyC700) demonstrate its potential for large-scale application in e-waste recycling. The elucidated mechanism, involving hydroxylation and stepwise gold nucleation, provides valuable insights into the interaction between gold ions and pyrocarbon, guiding future design and optimization of similar sorbents. The exceptional selectivity of PyC700 addresses a major limitation of existing sorbents, minimizing the need for complex separation processes and improving the overall efficiency and cost-effectiveness of gold recovery. The techno-economic analysis confirms the significant economic benefits of this method, surpassing other reported methods. This research contributes significantly to the circular economy by promoting the sustainable recovery of valuable resources from e-waste, mitigating environmental impacts, and generating economic value.

This research presents a highly efficient and selective approach for gold recovery from e-waste leachates using a cost-effective alginate-derived pyrocarbon sorbent. The superior performance of PyC700, its economic viability, and the detailed mechanistic understanding contribute significantly to sustainable urban mining. Future research could explore other biomass sources for pyrocarbon production, optimizing pyrolysis parameters to further enhance sorbent properties, and investigating the scalability and integration of this technology into existing e-waste recycling processes. The development of similar tailored pyrocarbon sorbents for other valuable metals in e-waste is also a promising avenue for future exploration.

While the study demonstrates excellent performance, several limitations should be noted. The techno-economic analysis is based on specific cost factors and market conditions, and these could vary depending on location and scale. The effect of long-term use on the sorbent’s performance and potential regeneration strategies requires further investigation. The study focused primarily on CPU leachates; further testing with diverse e-waste materials would strengthen the generalizability of the findings. Finally, while the stepwise nucleation mechanism is proposed, further studies using advanced characterization techniques could provide more comprehensive confirmation of this mechanism.