Mechanistic insights for (alkyl)methacrylate-based cationic hybrid gels reinforced with sepiolite, montmorillonite, mica, bentonite and kaolin: A comparative thermodynamic study with all-in-one formulations

Abstract

A series of clay-based cationic hybrids from N,N-dimethylaminoethyl methacrylate and 2-acrylamido-2-methyl-1-propanesulfonic acid in the presence of various clays; montmorillonite (Mt), bentonite (Bnt), sepiolite (Sep), kaolin and mica was prepared by in-situ free-radical polymerization. The primary objective was to understand the macroscale properties of ionically modified (alkyl)methacrylate-based polymers reinforced with nanometer-sized clays. Water absorption, repeated on-off switching in salt solutions, elasticity and efficiency in removal of anionic indigo carmine (IC) dye, widely used in textile industry for denim production, were comparatively analyzed depending on the clay type. Compared to Sep and Mt-integrated hybrids, the mechanical property was weaker due to the inhomogeneous distribution in the structure, while the mica and kaolin-integrated gels were resistant to high compression. Mt- and Sep-addition increased the swelling by four-fold, while the swelling in the presence of mica was limited. Hybrids exhibited good salt resistance and pH-sensitive swelling, with largest change in the volume being observed in Mt-doped gels, while least change was in Kaolin-doped ones. Based on the effect of salinity, the swelling of hybrids in aqueous solutions of monovalent and divalent metal nitrates followed the decreasing order as KNO₃ > NaNO₃ > Ba(NO₃)₂. Regardless of clay type, hybrid cryogels prepared at low polymerization temperature followed Fickian diffusion, while at higher temperatures, the clay type altered the mechanism and non-Fickian diffusion was observed in hybrid hydrogels doped with Mt, kaolin and mica. Anionic IC adsorption, which reached equilibrium in 30 min, varied between 93.4 % and 97.6 %. The presence of Kaolin and Bnt caused a significant increase in adsorption, while a decrease was observed in presence of Sep. Adsorption isotherms determined by three parameter models were compared with two-parameter models in linear and non-linear fitting of equilibrium data; Freundlich isotherm model and pseudo-first order kinetic model had the best fitness. Comparative results demonstrated that clay-integrated (alkyl)methacrylate-based cationic hybrids can be employed as effective and economical adsorbents for removal of anionic dyes from aqueous environments.