Sustainable adsorption of reactive dyes onto a carbon-silica hybrid derived from biogenic silica waste: Thermodynamic, isotherm, and DFT insights

The removal of synthetic dyes from wastewater remains a major environmental challenge owing to their toxicity and persistence. This work investigates the adsorption of two anionic azo dyes Reactive Black 5 (RB5) and Reactive Red 141 (RR141) onto carbon-silica hybrid (CAM) obtained as a purified residue from biogenic silica production, aiming to elucidate the underlying mechanisms and assess their potential as efficient, low-cost adsorbents for dye-contaminated water treatment. Batch adsorption experiments were conducted to examine kinetic, equilibrium, and thermodynamic behaviors. Density Functional Theory (DFT) simulations, combined with non-covalent interaction (NCI) and quantum theory of atoms in molecules (QTAIM) analyses, were performed to explore the nature and strength of dye-surface interactions. The CAM surface was modeled with silanol (-Si-OH) and siloxane (-Si-O-Si-) groups to reflect its reactive sites. The optimal pH for adsorption was 3 for both dyes, with removal percentages exceeding 92% within 240 min. The Sips model gave the best statistical fit to the equilibrium data of both dyes; the fitted Sips heterogeneity exponent revealed a markedly heterogeneous surface for RB5 (exponent well below unity, consistent with a strong Freundlich fit) and near-monolayer behavior for RR141 (exponent close to unity at the lower temperatures). CAM exhibited maximum experimental adsorption capacities of 124.8 mg g −1 for RB5 and 109.9 mg g −1 for RR141, in the range reported for hybrid silica-carbon and activated-carbon adsorbents under comparable conditions. Negative ΔG 0 and ΔH 0 values confirmed spontaneous and exothermic adsorption, while the positive ΔS° values, attributed to the release of ordered hydration water and counter-ions upon dye binding, indicate that the process is also entropically favorable; the DFT analyses revealed hydrogen bonding, electrostatic attraction, and π-π stacking as the primary stabilizing forces. The synergistic integration of experimental and theoretical insights provides a robust molecular-level understanding of dye-adsorbent interactions, indicating CAM as a promising low-cost adsorbent for anionic-dye removal, whose performance under realistic wastewater conditions warrants further evaluation.