###### Abstract  
In line with the principles of the circular economy, this study aimed to develop a pyrolysis-activation regeneration process capable of producing highly porous carbon materials from spent granular activated carbon (GAC), which was generated by a high-tech electronics manufacturing company in Taiwan. Thermogravimetric analysis (TGA) and other thermochemical analyses were first conducted to investigate the thermal decomposition behavior of the spent GAC. Subsequently, the thermal regeneration system was employed to perform the N\({}_{2}\) pyrolysis and CO\({}_{2}\) activation experiments under various process conditions (i.e., 800, 850, and 900 \({}^{\circ}\)C for holding 0, 30, and 60 min, respectively). Analytical instruments included a surface area and porosimeter for pore property analysis, scanning electron microscopy (SEM) for porous texture observation, and energy dispersive X-ray spectroscopy (EDS) for surface elemental distribution analysis. The results revealed that the pore properties of thermally regenerated GAC were significantly improved compared to the spent GAC, indicating the effective removal or decomposition of adsorbed organics and deposited substances under the process conditions. Additionally, thermal regeneration via physical activation with CO\({}_{2}\) led to enhanced pore properties compared to simple pyrolysis. The maximum BET surface area achieved exceeded 720 m\({}^{2}\)/g, which was greater than those of spent GAC (approximately 425 m\({}^{2}\)/g) and N\({}_{2}\)-pyrolyzed GAC (approximately 570 m\({}^{2}\)/g) under the same regeneration conditions (i.e., 900 \({}^{\circ}\)C with a 30 min holding time). spent granular activated carbon; thermal regeneration; nitrogen pyrolysis; carbon dioxide activation; pore property +
Footnote †: journal: Applied Sciences  
0000-0002-4000-0002-4000][PERSON]  
2,545222,5452  
challenge, and it is often disposed of without reuse. In contrast, GAC, either in granulated form or pelletized form, is widely applied in gas-phase adsorption, including organic vapor recovery, indoor air purification, gas separation, respiratory protection, and vent gas purification [2]. Given the high cost of AC and the ease of regeneration [3], spent GAC used in industrial applications can be regenerated or reactivated using various methods, which will be further addressed in the subsequent section. Regenerating or reactivating AC offers both economic and environmental advantages, as it reduces the need for treating or disposing of spent AC. Various regeneration methods for exhausted or saturated AC have been reviewed in the literature, including thermal, chemical, biological, electrochemical, microwave, and oxidation techniques [4; 5; 6; 7; 8]. Although microwave thermal regeneration is considered a novel technique [9; 10; 11; 12; 13], conventional thermal regeneration (desorption) via pyrolysis under an inert gas (i.e., nitrogen) remains the most commonly applied method. However, one drawback of thermal regeneration is that it often fails to fully restore the original pore properties of the AC [14; 15; 16; 17; 18], thereby limiting its reuse in adsorption applications. To overcome this limitation, researchers have explored combinations of thermal and chemical oxidation methods. For instance, [PERSON] et al. regenerated spent GAC at 850 \({}^{\circ}\)C using N\({}_{2}\) (pyrolysis alone) as well as N\({}_{2}\) and CO\({}_{2}\) combined (pyrolysis plus activation) [19]. According to their results, pyrolysis alone was not able to restore the pore properties due to residual charred products remaining on the carbon surface and pores; however, using an oxidant was capable of removing adsorbed organics completely. [PERSON] et al. also regenerated spent GAC by combining N\({}_{2}\) pyrolysis at 500-800 \({}^{\circ}\)C with air activation at 600-800 \({}^{\circ}\)C for holding times between 0 and 60 min [20].