APTES–chitosan-modified rice husk mesoporous silica for the adsorption of anionic and cationic dyes

The development of adsorbents capable of selectively removing dyes with different charges remains a significant challenge. This limitation is further intensified in silica-based adsorbents, which inherently exhibit weak affinity toward dyes with diverse molecular structures. This study aims to develop a functionalized mesoporous silica with enhanced adsorption capacity. In this study, mesoporous silica (Meso-SiO₂) was synthesized from rice husk–derived sodium silicate (Na₂SiO₃) via a CTAB-templated sol-gel route, then functionalized with (3-aminopropyl)triethoxysilane (APTES) and coated with chitosan to introduce amine-rich adsorption sites. FTIR analysis confirmed the successful incorporation of amine functional groups, while TGA revealed characteristic organic decomposition at 200–350 °C, indicating effective surface modification. SEM-EDX analysis further verified the morphological structure and the presence of C and N elements, while zeta potential measurements showed a reduction in surface charge negativity after functionalization. Adsorption studies using Meso-SiO₂–APTES–showed clear pH-dependent selectivity for dyes with different charges. The adsorption capacity of Meso-SiO₂–APTES–Chitosan towards methylene blue increases up to 2.5x that of the unmodified adsorbent, meanwhile, for congo red showed no significant differences. The adsorption kinetics for both dyes were best described by the pseudo-second-order model, indicating that the overall rate is governed by adsorption-site interactions. Equilibrium isotherm analysis indicated that methylene blue adsorption predominantly followed the Langmuir model, whereas Congo red adsorption exhibited combined Langmuir–Freundlich characteristics after APTES–chitosan modification. These findings demonstrate that dual-functionalized mesoporous silica offers an effective platform for achieving selective adsorption of dyes with different charges in aqueous systems.