Life cycle assessment of olive stone-derived activated carbon

Activated carbon (AC) derived from biowastes is widely considered a sustainable alternative to fossil-based adsorbents; however, its environmental performance remains constrained by energy-intensive production processes. This study investigates olive stone (OS)-derived AC, produced via pyrolysis followed by chemical activation using NaOH and KOH, as a model system to reveal the dominant role of pyrolysis and activation in environmental impacts and the influence of functional unit selection on sustainability interpretation. Environmental impacts were evaluated using two functional units: per kg of AC produced and per kg of dye adsorbed, to reflect its application in adsorption-based water treatment systems. Contribution analysis identifies pyrolysis as the dominant contributor to climate change (CC) and net energy demand (EN), primarily due to high electricity consumption under inert conditions. Sensitivity analysis further confirms that pyrolysis energy demand strongly governs CC and EN outcomes. On a mass-based basis, NaOH-activated AC shows lower impacts (2.9 vs. 3.1 kg CO₂ eq kg⁻¹ AC); however, the higher adsorption capacity of KOH-activated AC (882 vs. 815 mg g⁻¹) offsets this advantage under a performance-based functional unit. Overall, the results demonstrate that both process-level hotspots and functional unit selection are critical in determining the true sustainability of OS-derived AC, and conclusions based solely on mass-based metrics may be misleading.