How Functional Group Chemistry Governs Cr(VI) Adsorption on MCM-41: Experimental and Molecular-Level Insights

In this report, functionalized MCM-41 adsorbents (MCM-41/IL and MCM-41/NH₂) were synthesized, and their performances were systematically compared in Cr(VI) removal. The adsorption capacities reached 110.2 mg/g for MCM-41/IL and 153.3 mg/g for MCM-41/NH₂ at pH 5.6 with an adsorbent dosage of 1 g/L. Notably, functionalization led to a reduction in the surface area and pore volume, which significantly influenced their adsorption performance. The bulky ionic liquid (IL) molecules hindered the diffusion of Cr(VI) to the surface of MCM-41/IL, resulting in markedly lower adsorption and diffusion rates. Computational analyses further revealed that protonated amine-functionalized MCM-41 exhibits superior thermodynamic affinity for Cr₂O₇^2– removal through synergistic electrostatic interactions and multidirectional hydrogen bonds. This performance significantly exceeds that of pristine MCM-41 and the bulkier, aprotic IL-functionalized system. In competitive adsorption studies, organic pollutants reduced the adsorption efficiencies of both functionalized adsorbents by approximately 12–20%, whereas competing cationic metal ions had no significant effect on Cr(VI) removal. Additionally, both functionalized adsorbents maintained stable adsorption performance over five successive adsorption–desorption cycles. Overall, these results demonstrate that the size and nature of the functional groups play a critical role in governing the physicochemical properties and adsorption performance of functionalized MCM-41 materials.