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The rapidly accumulating amounts of waste electrical and electronic equipment (WEEE) is one of the biggest environmental concerns in modern societies, and this problem will be further accelerated in the future. The use of supercritical CO2 (scCO2) mixed with acids has been proposed as a greener solvent system compared to conventional cyanide and aqua regia solvents, however, the mechanisms of scCO2 in metal extraction from WEEE are still poorly understood. Thus, this study focused on the physical, structural, and chemical interactions between scCO2/acid solvents and complex layered components in waste printed circuit boards (WPCBs), one of the common WEEEs. Our study showed that the use of scCO2-based pretreatment allows faster leaching of metals including copper (Cu) in the subsequent hydrometallurgical process using H2SO4 and H2O2, while allowing gold (Au) recovery as hydrometallurgically delaminated solids. This enhancement is due to the selective leaching of Ni and unique inner porous structures created by ScCO2/acid treatment via dissolving the Ca-silicate-bearing fiberglass within the WPCB. Thus, the scCO2-based pretreatment of WPCBs shows a multifaceted green chemistry potential relating to the reduction in solvent usage and targeted recovery of Au prior to shredding or grinding that would reduce any loss or dilution of Au in the subsequent waste stream. 

The amount of electronic waste (e-waste) globally has doubled in just five years, from approximately 20 million tons to 40 million tons of e-waste generated per year. In 2016, the global amount of e-waste reached an all-time high of 44.7 million tons. E-waste is an invaluable unconventional resource due to its high metal content, as nearly 40% of e-waste is comprised of metals. Unfortunately, the rapid growth of e-waste is alarming due to severe environmental impacts and challenges associated with complex resource recovery that has led to the use of toxic chemicals. Furthermore, there is a very unfortunate ethical issue related to the flow of e-wastes from developed countries to developing countries. At this time, e-waste is often open pit burned and toxic chemicals are used without adequate regulations to recover metals such as copper. The recovered metals are eventually exported back to the developed countries. Thus, the current global circular economy of e-waste is not sustainable in terms of environmental impact as well as creation of ethical dilemmas. Although traditional metallurgical processes can be extended to e-waste treatment technologies, that is not enough. The complexity of e-waste requires the development of a new generation of metallurgical processes that can separate and extract metals from unconventional components such as polymers and a wide range of metals. This review focuses on the science and engineering of both conventional and innovative separation and recovery technologies for e-wastes with special attention being given to the overall sustainability. Physical separation processes, including disassembly, density separation, and magnetic separation, as well as thermal treatment of the polymeric component, such as pyrolysis, are discussed for the separation of metals and non-metals from e-wastes. The subsequent metal recovery processes through pyrometallurgy, hydrometallurgy, and biometallurgy are also discussed in depth. Finally, insights on future research towards sustainable treatment and recovery of e-waste are presented including the use of supercritical CO 2 .