The impact of carbon felt thickness and de-bundled felt fibers on dye adsorption: The external surface matters

Adsorption onto carbon-based materials has emerged as a prominent method for removing contaminants from aqueous solutions, gaining notable attention recently. In this study, two activated carbon (AC) felt materials, with very similar Brunauer-Emmet-Teller (BET) surface areas (∼1850 m²/g) but different thickness (2 mm and 3 mm), were oxidized using concentrated nitric acid to modify their surface properties and thus further improve their adsorption capacity towards crystal violet (CV) dye. The oxidation process caused a notable decline in BET surface area (∼550 m²/g) and pore volume, coupled with a modest rise in average pore size. Batch adsorption tests were conducted to assess the CV adsorption efficiency of oxidized AC felt materials, both in their original form and as de-bundled fibers. Evaluation of the adsorption performance in groundwater and seawater solutions was also carried out. Thermodynamic analysis revealed that the adsorption process is spontaneous, endothermic, and primarily driven by entropy. Results from kinetic experiments revealed that the thinner felt shows better performance compared to the thicker counterpart, removing ∼70 % and ∼20 % of the CV dye after 10 min, respectively. However, the de-bundled fibers from the thinner and thicker felt materials display much higher uptakes, specifically ∼95 % and ∼45 % after 10 min, respectively, confirming that the external available surface area matters most in kinetic studies.