Anion-directed mechanistic divergence in chromium(VI) uptake on acid-activated red mud: diffusion-coupled chemisorption versus reaction-limited physisorption

The valorization of bauxite residue (red mud) into high-performance adsorbents is critical for circular economy frameworks; however, the acid-specific mechanisms governing adsorption remain mechanistically elusive. This study elucidates a quantitative mechanistic dichotomy between hydrochloric- and nitric-acid-activated red mud (HCl-RM and HNO₃-RM) for hexavalent chromium [Cr(VI)] removal, integrating kinetic modeling, spectroscopy, and dimensionless transport analysis. Although both treatments effectively neutralized alkalinity, their adsorption behaviors diverged fundamentally. HCl-RM exhibited mixed kinetic control with significant internal diffusion limitation (Bi>10, Da≈4.8), driven by exceptionally rapid surface reaction kinetics (pseudo-second-order rate constant k2 = 0.310 g mg−1 min−1). It achieved 68% Cr(VI) removal within 40 min—a 3.4-fold higher initial uptake rate compared to HNO₃-RM, and 3.2 times faster removal than commercial activated carbon. Crucially, FTIR spectroscopy, isotherm modeling, and geometric site-density validation refuted strictly monolayer binding, confirming a multilayer pore-filling mechanism. For HCl-RM, partial dealumination created a silica-rich mesoporous scaffold facilitating strong inner-sphere complexation at ≡Fe-OH2+ sites. Conversely, in HNO₃-RM, tightly coordinated nitrate ligands hindered direct chemisorption, shifting the mechanism toward slower, reaction-limited outer-sphere complexation and pore confinement (Da ≪ 1). Thermodynamic analysis confirmed that adsorption on both materials is spontaneous and entropy-driven. By explicitly linking activation chemistry, molecular-scale interactions, and transport-limited kinetics, this work establishes a rigorous engineering framework for red mud-based water treatment, directly supporting UN Sustainable Development Goals 6 and 12.