Hierarchical Micro–Mesoporous ZnO–SiO2/Carbon Composites: Synthesis, Structural Characterisation, and High-Capacity Adsorption of Cationic Organic Pollutants from Water

Hierarchical ZnO–SiO2/carbon composites (C-Zn1, C-Zn2, C-Zn3) were synthesised via the carbonisation of resorcinol–formaldehyde gels in the presence of ZnO-modified fumed silica, and characterised by N2 adsorption–desorption, FTIR, XRD, SEM, and zeta potential analysis. The composites exhibited hierarchical micro–mesoporous structures with BET surface areas of 467–499 m2 g−1; the non-microporous volume fraction increased from 0.09 (reference carbon RFC, 545 m2 g−1) to 0.54–0.63 upon ZnO–SiO2 incorporation. Adsorption of methylene blue (MB), crystal violet (CV), and rhodamine 6G (R6G) followed the Marczewski–Jaroniec isotherm model. Maximum adsorption capacities for the best-performing composite (C-Zn1) reached 1.22 mmol g−1 for MB, 1.04 mmol g−1 for CV, and 0.63 mmol g−1 for R6G, compared to 1.32, 1.17, and 0.67 mmol g−1 for unmodified RFC. Kinetic analysis revealed up to 3.5-fold faster adsorption rates for C-Zn1 relative to RFC (for CV and R6G), attributed to enhanced diffusion through mesoporous channels while preserving the micropore-driven capacity. Agar well-diffusion assays against four bacterial strains showed no inhibition zones for any composite, indicating that no biologically active concentration of zinc species was released under the assay conditions. The proposed approach yields composites with enhanced adsorption kinetics, preserved capacity, and confirmed non-leaching character, positioning them as effective candidates for water purification.