Quaternary Ammonium-Modified Chitosan Aerogels for Adsorbing Caramel Pigments: Mass Transfer and Density Functional Theory Analysis

In this study, a quaternary ammonium-modified chitosan aerogel (QCSA) was developed for the decolorization of remelt syrup. Caramel pigments (a representative pigment from remelt syrup) were used as the adsorption model substrate to study the adsorption performance of QCSA. A novel Wen Li-Wei Wei adsorption mass transfer mixed (LWAM) phenomenological mathematical model was used to analyze the mass transfer mechanism of QCSA adsorption of caramel pigments. Density functional theory (DFT) was used to investigate the microscopic interaction mechanism of QCSA adsorption of caramel pigments. The results showed that the equilibrium adsorption capacity of QCSA for caramel pigments at initial concentrations of 60, 80, and 100 mg/L were 198, 263, and 308 mg/g and the corresponding decolorization rates were 99.8%, 98.2%, and 92.4%, respectively. Analysis using the LWAM phenomenological mathematical model showed that the adsorption rate-limiting steps were jointly determined by external diffusion, internal diffusion, and site binding. The DFT analysis showed that the adsorption mechanism of caramels by QCSA was dominated by electrostatic interactions. Weak interactions, such as hydrogen bonds, occurred between the oxygen atoms of –COO^–/–COOH (caramel pigment) and the hydrogen atoms of –OH/–NH₃⁺ (protonated QCSA), with caramel pigment molecules serving as hydrogen bond acceptors. Among them, O···H H-bonds played a significant role, accounting for 37.88% of the total H-bond superficial area. In summary, the LWAM phenomenological mathematical model can accurately calculate the capture amounts of liquid films, pore channels, and sites at any time during the adsorption process, thereby providing a new perspective for elucidating the underlying mass transfer mechanism. DFT analysis facilitates the understanding of intermolecular interactions in adsorption systems at the atomic level, offering theoretical support for optimizing adsorbent design.