Removal of methylene blue and basic blue 41 from aqueous solutions using silica fume: A central composite design approach

This study evaluates the effectiveness of silica fume (SF) as a sustainable and economical adsorbent for eliminating the cationic dyes Basic Blue-41 (BB-41) and Methylene Blue (MB) from aqueous solutions. Unlike conventional adsorbents, SF offers a low-cost valorization pathway for an industrial by-product, while demonstrating superior dye uptake performance. SF's morphology and surface characteristics were examined using FESEM and FTIR analyses. Batch adsorption experiments were carried out under different operational parameters, including pH, adsorbent dosage, contact time, and initial dye concentration. Equilibrium studies revealed that adsorption followed the Freundlich isotherm, indicating a heterogeneous surface with multilayer adsorption, while Langmuir analysis confirmed high maximum monolayer capacities of 108.95 mg/g for BB-41 and 189.31 mg/g for MB. Kinetic modeling showed that both dyes followed the pseudo-second-order model (R 2 = 0.9977 for BB-41 and R 2 = 0.9991 for MB), suggesting chemisorption as the dominant mechanism, involving stronger dye–surface interactions beyond physical adsorption. The optimal removal efficiency for BB-41 reached 97.12 % at pH 10, using 0.25 g of SF for 20 min with a 20 mg/L dye concentration. For MB, the maximum predicted efficiency was 98.94 % under similar conditions at pH 9.81, a dosage of 0.14 g, and 31 min contact time. These results highlight the novelty of employing SF as a high-capacity, heterogeneous, and chemisorption-driven adsorbent, underscoring its promise for practical dye-laden wastewater treatment.