Novel cyclic silsesquioxane-based polymers as high-performance adsorbents for dye removal

Abstract

In this study, three cyclic silsesquioxane-based polymers were synthesized via Heck reactions of octavinyl-substituted all-cis-T₄ siloxane with three distinct dibromo aromatic compounds. Their properties were systematically characterized, revealing excellent thermal stability and high porosity, along with fluorescent properties under λ = 365 nm UV irradiation. Furthermore, the adsorption performance of these polymers toward crystal violet (CV), Congo red (CR), methyl orange (MO), and malachite green (MG) was evaluated. All polymers exhibited higher adsorption capacities for cationic dyes (CV and MG). Adsorption kinetics followed pseudo-second-order models, and equilibrium data fit the Freundlich isotherm, indicating multilayer adsorption on heterogeneous surfaces. Mechanistic analysis using the intraparticle diffusion, Boyd, and Bangham models suggested a synergistic diffusion-controlled process, with film diffusion as the rate-limiting step, while intraparticle diffusion and pore diffusion contributed to overall kinetics. Adsorption efficiency for cationic dyes improved under alkaline conditions, and all polymers retained over 95% dye removal after five adsorption–desorption cycles, demonstrating excellent reusability.