Removal of Hexavalent Chromium from Water Using Cork-Based Biochars Obtained via Slow and Microwave Pyrolysis

Cork-derived biochars produced by slow pyrolysis (C-SP) and microwave-assisted pyrolysis (C-MWP) were evaluated for the removal of hexavalent chromium [Cr(VI)] from aqueous solutions. The materials were characterized by proximate/elemental analysis, N₂ physisorption (BET), thermogravimetry and FTIR. C-SP exhibited a higher specific surface area (174.5 m²·g^-1), a larger micropore volume and stronger O-H/C-O surface signatures than C-MWP, which showed partial aromatization and less-developed texture. Batch equilibrium adsorption experiments at pH 2.0 with initial Cr(VI) concentrations of 5-150 mg·L^-1 and kinetic experiments (c₀ = 40 mg·L^-1) were analyzed by nonlinear fitting to standard isotherm and kinetic models. The experimentally observed maximum uptakes were qmax,obs = 32.7 mg·g^-1 (C-SP) and 35.9 mg·g^-1 (C-MWP) at pH 2.0. Under screening conditions (pH 1.1, 45 min), removals reached 72% (1.44 mg·g^-1) for C-SP and 87% (1.73 mg·g^-1) for C-MWP. The Freundlich model provided the best fit to the equilibrium data for C-SP, consistent with adsorption on an energetically heterogeneous surface, whereas for C-MWP the fitted isotherm models did not yield physically meaningful parameters. Kinetic modeling showed rapid initial uptake consistent with energetic heterogeneity of surface sites. However, kinetic fits alone do not determine whether uptake is dominated by physical adsorption, chemical binding or redox reactions; direct surface/speciation analyses would be required to substantiate mechanistic assignments.