Covalent organic frameworks for critical metal recycling from waste

Critical metals play a pivotal role in driving the emerging era of renewable technologies, including wind turbines, solar panels, and water electrolysis. However, their importance has surged over the years, primarily due to diminishing primary sources and a lack of viable alternatives. This escalating disparity between demand and supply necessitates a proactive approach to explore supplementary resources. Among the potential avenues, secondary sources such as e-waste, nuclear waste, and seawater offer promising alternatives for critical metal production through the concept of urban mining. In this context, the hydrometallurgical route has emerged as a notably efficient methodology. Notably, solid-phase extraction (SPE) within hydrometallurgy presents an environmentally friendly avenue, yielding reduced hydrocarbon waste. This method showcases superior efficacy, particularly in scenarios where target metals exist in low concentrations, surpassing the conventional liquid–liquid extraction techniques. Central to the successful extraction of critical metals from waste leachate solutions, primarily in robust acidic environments, lies the development of stable and high-performing sorbents. The recent emergence of covalent organic frameworks (COFs) holds considerable promise due to their intricate architecture, porous nature, and versatile design capabilities. This comprehensive review delves into the myriad applications of COF materials as effective sorbents within the realm of solid-phase extraction, facilitating the recovery of metals from secondary sources. The discussion encompasses various manifestations of COFs, including their employment as powders, composites, and components of mixed matrix membranes. Additionally, the review elucidates the methodologies employed for desorbing the extracted metals and subsequently regenerating COFs, enabling multiple extraction cycles. By offering a holistic insight into the structural nuances of COFs and their pivotal role in recycling critical metals from waste, this review not only advances our understanding but also serves as a guidepost for the future design and development of novel COFs for this crucial endeavor.