Fixed-bed column performance of amino-functionalized MCM-41 mesoporous silica for Cr(VI) removal: Optimization of operating conditions and regeneration studies

Abstract

Hexavalent chromium (Cr(VI)) is a highly toxic and priority hazardous contaminant in aqueous systems, posing serious risks to human health and environmental safety. The development of efficient, regenerable materials capable of operating under continuous conditions remains a key challenge for sustainable water treatment. In this study, the removal of Cr(VI) from aqueous solutions was investigated using aminopropyl-functionalized ordered mesoporous silica (MCM-41-NH2) in fixed-bed columns. Through an in-depth physicochemical characterization of the solid phase (FTIR, XPS, TGA, and N2 sorption), the presence of mesoporous ordering, a monomodal pore size distribution, high specific surface area and surface amino functional groups was established. Column experiments systematically evaluated the effects of inlet Cr(VI) concentration (10-40 mg/L), flow rate (2-4 mL/min), and bed height (0.6-1.2 cm). Factorial design analysis identified inlet concentration as the dominant operational parameter. Under optimized conditions (40 mg/L, 2 mL/min, 1.2 cm), a maximum uptake capacity of 110.5 mg/g was achieved, while a series column configuration enhanced removal efficiency by approximately 15% compared to a single bed. Independent and continuous monitoring of total chromium and Cr(VI) in the column effluent provided evidence of simultaneous adsorption and partial reduction to Cr(III). The sorbent also demonstrated a gradual decline in performance (≈30%) over multiple NaOH regeneration cycles. Overall, these results highlight the potential of MCM-41-NH₂ as an efficient material for Cr(VI) removal under continuos operations conditions.