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 CO2 uptake, moisture uptake, and methylene blue (MB) adsorption experiments. The results demonstrated that CBB exhibited the highest CO2 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 CO2 uptake, providing strong supporting evidence that naturally occurring mineral species play an important role in the CO2 adsorption mechanism. The quantity and type of naturally occurring potassium-containing oxides and salts strongly influenced CO2 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 CO2 capture and dye removal.