FeCl3-Activated Agro-Waste Biochars for Enhanced Dye Adsorption: Unveiling the Role of Iron Oxide Active Sites

In this study, activated biochars derived from spent coffee grounds (CAC-600) and lemon pomace (LAC-600) were prepared through pyrolysis with FeCl3 activation and evaluated for the selective adsorption of Acid Blue 74 (AB74), a dye widely used in the denim textile industry. FeCl3 activation significantly increased the surface area and pore development relative to the pristine biochars, while also promoting the formation of Fe2O3 phases on the activated biochars surfaces. The activated biochars exhibited comparable adsorption capacities of 39.44 and 37.16 mg·g−1 for CAC-600 and LAC-600, respectively, indicating that adsorption performance was governed mainly by the activation process rather than by the precursor biomass. Isotherm and kinetic models revealed heterogeneous adsorption behavior involving surface interactions combined with internal diffusion. The materials showed stable adsorption performance within a pH range of 4–10. Competitive adsorption experiments demonstrated preferential adsorption of AB74 over Acid Red 1 (AR1), confirming the selectivity of LAC-600 and CAC-600. Density Functional Theory (DFT) calculations revealed a cooperative adsorption mechanism combining π-surface interactions with localized Fe-oxide anchoring sites on the graphene-based model, increasing the adsorption energy by approximately 24 kcal·mol−1 relative to carbon-only systems. These findings demonstrate the potential of Fe-activated agro-industrial biochars as adsorbents for dye removal from aqueous media.