Sustainable adsorption of organic dyes from water using brewery spent grain: kinetic, computational and equilibrium studies

The removal of synthetic dyes from water requires adsorbents whose performance is governed by well-understood surface interaction mechanisms rather than solely by adsorption capacity. In this study, brewery spent grain (BSG), an abundant lignocellulosic agro-industrial residue, was investigated as a sustainable biosorbent for the selective removal of cationic Safranin T (ST) and anionic Congo Red (CR). Surface characterization revealed a porous morphology and a negative surface charge (ζ = −17.9 mV at pH 6.8), indicating a surface environment capable of charge-driven selectivity. Adsorption equilibrium followed the Freundlich model, consistent with heterogeneous surface behavior. The maximum adsorption capacity was 30.62 mg g −1 for ST and 1.36 mg g −1 for CR, demonstrating strong selectivity toward the cationic dye. Thermodynamic analysis, performed using dimensionless equilibrium constants in accordance with IUPAC recommendations, confirmed spontaneous adsorption for both dyes, with distinct energetic behavior. Density functional theory (DFT) calculations and electrostatic potential mapping provided molecular-level insight, revealing that ST adsorption is predominantly governed by electrostatic attraction, whereas CR adsorption is limited by surface repulsion and mediated by localized π–π and hydrogen-bonding interactions. By integrating experimental adsorption studies with electronic structure analysis, this work advances the understanding of interfacial phenomena governing dye–biosorbent interactions and highlights the role of surface charge and molecular electronic properties in adsorption selectivity. Although regeneration and textural surface area analyses were not addressed, the results demonstrate the potential of minimally processed BSG as a low-cost and mechanistically understood adsorbent for wastewater treatment applications.