Valorization of Aromatic Coconut Wastes into Biochars for Carbon Dioxide Uptake and Dye Adsorption: Adsorption Behavior and Economic Feasibility

The purpose of this research is to investigate the potential of aromatic coconut waste-derived biochars, namely coconut husk biochar (CHB) and coconut empty fruit bunch biochar (CBB), as low-cost, sustainable, and locally available adsorbents. Biochars were characterized using SEM, XRD, XPS, and XRF to evaluate their physical and chemical properties, followed by CO 2 uptake, moisture uptake, and methylene blue (MB) adsorption experiments. The results demonstrated that CBB exhibited the highest CO 2 uptake of 4.44 mmol g −1 , outperforming CHB (2.39 mmol g −1 ) under temperature-programmed desorption. Notably, the water-washed biochar (CBB-w) exhibited a marked decrease in CO 2 uptake, providing strong supporting evidence that naturally occurring mineral species play an important role in the CO 2 adsorption mechanism. The quantity and type of naturally occurring potassium-containing oxides and salts strongly influenced CO 2 and moisture uptake. In contrast, isotherm analyses using the Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich models indicated that CHB exhibited a superior MB adsorption capacity (30 mg g −1 ), reflecting the different adsorption mechanisms governing gas- and liquid-phase adsorption. The estimated production cost of aromatic coconut waste-derived biochar ranged from approximately US$0.83–1.11 kg −1 , depending on production scale. These results demonstrate that aromatic coconut waste-derived biochar represents a promising low-cost and sustainable adsorbent for environmental applications, particularly CO 2 capture and dye removal.