The chemical activation of a carbon precursor with KOH generally results in an activated carbon (AC) with a high specific surface area. However, this process generates a large volume of wastewater that includes dissolved alkali metals, existing mainly as K₂CO₃. Thus, wastewaters with a high concentration of dissolved K₂CO₃ can potentially be used in place of KOH as a chemical agent. In the present study, to reduce the thermal stability of K₂CO₃, which decomposes at temperatures greater than 891 °C, K₂CO₃ was chemically impregnated into carbon precursors prior to activation of the precursors. The thermochemical properties and activation efficiency of the carbon precursors treated with K₂CO₃ were compared with those of carbon precursors treated with KOH. Analysis by XPS indicated that C–O–K complexes formed on the surface of the carbon precursors; in addition, their peak intensities were approximately the same irrespective of the chemical agent used. However, the specific surface area of the K₂CO₃-impregnated AC was 2162 m²/g, which was ~70% of that of the KOH-impregnated AC (3047 m²/g) prepared using the same K/C molar ratio of 0.5. XRD results confirmed that both K₂CO₃ and KOH transformed into KHCO₃ and K₄H₂(CO₃)₃·1.5H₂O during the impregnation. The peak intensities of these compounds in the XRD pattern of the K₂CO₃-impregnated carbon precursors were two times greater than those in the pattern of the KOH-impregnated carbon precursors. These compounds eventually transformed into K₂CO₃, which hardly participated as a chemical agent at the temperature used in the present study (850 °C). Therefore, recrystallisation of K₂CO₃, even during the impregnation, appeared to adversely affect the degree of activation. Nevertheless, the specific surface area of the K₂CO₃-activated AC was still ~1.6 times greater than that of the untreated carbon precursor (1378 m²/g), suggesting that the use of wastewater as a chemical agent is feasible for resource recycling.

In this study, spent activated carbons (ACs) were collected from a waste water treatment plant (WWTP) in Incheon, South Korea, and regenerated by heat treatment and KOH chemical activation. The specific surface area of spent AC was 680 m²/g, and increased up to 710 m²/g through heat treatment. When the spent AC was activated by the chemical agent potassium hydroxide (KOH), the surface area increased to 1380 m²/g. The chemically activated ACs were also washed with acetic acid (CH₃COOH) to compare the effect of ash removal during KOH activation. The low temperature N₂ adsorption was utilized to measure the specific surface areas and pore size distributions of regenerated ACs by heat treatment and chemical activation. The functional groups and adsorbed materials on ACs were also analyzed by X-ray photoelectron spectroscopy and X-ray fluorescence. The generated ash was confirmed by proximate analysis and elementary analysis. The regenerated ACs were tested for toluene adsorption, and their capacities were compared with commercial ACs. The toluene adsorption capacity of regenerated ACs was higher than commercial ACs. Therefore, it is a research to create high value-added products using the waste.

Spent activated carbon (SAC) usually exhibits a low specific surface area due to its high ash contents. In this study, pre-treatments, such as heat and acid treatments, were optimized to improve this feature. The heat pre-treatment did not reduce the ash content, nor did it increase the surface area. Because metallic ions adsorbed in SACs turn into ash upon the heat treatment. In the acid pre-treatment, the volatiles and fixed carbon were increased with decreasing ash contents. In this study, it was found that the surface area increase was correlated with the ratio between fixed carbon and ash. Among the pre-treatment methods, the combined heat and acid pre-treatment method highly increased the ratio, and therefore led to the surface area increase. Additionally, the acid pre-treatment was carried out using different types of acid (organic and inorganic acids) solutions to further improve the surface areas. The organic acid treatment caused a significant structural collapse compared to the inorganic acid treatment, decreasing the surface area. In particular, H₃PO₄ effectively removed ashes adsorbed on the activated carbon surface and regenerated the exhausted activated carbon. Both the heat and acid pre-treatments before chemical activation resulted in the positive effects such as strong desorption of pollutants and ashes within the internal structure of the activated carbon. Therefore, the regeneration introduced in this study is methodically the best method to regenerate SAC and maintain a stable structure.

In this study, a feasible experiment on adsorbed natural gas (ANG) was performed using activated carbons (ACs) with high surface areas. Upgraded ACs were prepared using chemical activation with potassium hydroxide, and were then applied as adsorbents for methane (CH₄) storage. This study had three principal objectives: (i) upgrade ACs with high surface areas; (ii) evaluate the factors regulating CH₄ adsorption capacity; and (iii) assess discharge conditions for the delivery of CH₄. The results showed that upgraded ACs with surface areas of 3052 m²/g had the highest CH₄ storage capacity (0.32 g-CH₄/g-ACs at 3.5 MPa), which was over two times higher than the surface area and storage capacity of low-grade ACs (surface area = 1152 m²/g, 0.10 g-CH₄/g-ACs). Among the factors such as surface area, packing density, and heat of adsorption in the ANG system, the heat of adsorption played an important role in controlling CH₄ adsorption. The released heat also affected the CH₄ storage and enhanced available applications. During the discharge of gas from the ANG system, the residual amount of CH₄ increased as the temperature decreased. The amount of delivered gas was confirmed using different evacuation flow rates at 0.4 MPa, and the highest efficiency of delivery was 98% at 0.1 L/min. The results of this research strongly suggested that the heat of adsorption should be controlled by both recharging and discharging processes to prevent rapid temperature change in the adsorbent bed.

This study was a preliminary investigation of solid recovered fuel production from spent coffee grounds using the hydrothermal carbonization (HTC) technique. The spent coffee grounds (SCGs) were subjected to HTC at 170 to 250 °C. The biochar was characterized by proximate analysis, ultimate analysis, capillary suction time, time to filter, suspended solids, and particle size distribution. The biochar yields decreased with increasing HTC temperature and time. However, the higher heating value (HHV) of biochar increased with the HTC temperature and time. The H/C slop relative to the O/C atomic rate of spent coffee grounds was 0.10 with low decarboxylation selectivity. Considering the HHV of biochar and dehydration capacity depend on ratio of H/C vs. O/C, the optimum reaction temperature of HTC was 200 °C, and the biochar from SCGs is an attractive biochar.

Five types of odor-emitting exhaust gases from medical waste were selected, and their adsorption capacity and desorption efficiency were investigated using activated carbon. The selected gases included polar gases (hydrogen sulfide (H₂S) and ammonia (NH₃)) and non-polar gases (acetaldehyde (AA), methyl mercaptan (MM), and trimethylamine (TMA))). Commercial activated carbon with a specific surface area of 2276 m²/g was used as the adsorbent. For the removal of odor from medical waste, we investigated: (1) the effective adsorption capacity of a single gas (<1 ppm), (2) the effect of the adsorbed NH₃ gas concentration and flow rate, and (3) the desorption rate using NH₃ gas. The values of the effective adsorption capacity of the single gas were in the following order: H₂S < NH₃ < AA < MM < TMA, at 0.2, 4.2, 6.3, 6.6, and 35.7 mg/g, respectively. The results indicate that polar gases have a lower effective adsorption capacity than that of non-polar gases, and that the size of the gas molecules and effective adsorption capacity exhibit a proportional relationship. The effective adsorption performance of NH₃ gas showed an increasing trend with NH₃ concentration. Therefore, securing optimal conditions for adsorption/desorption is imperative for the highly efficient removal of odor from medical waste.