Adsorption of methyl violet using pH- and temperature-sensitive cellulose filament/poly(NIPAM-co-AAc) hybrid hydrogels

Abstract

Cellulose filament (CF) was modified via hybridization with poly (N-isopropylacrylamide-co-acrylic acid), which was prepared by free radical copolymerization of two monomers: N-isopropylacrylamide (NIPAM) as the thermosensitive component and acrylic acid (AAc) as the pH sensitive component. The copolymer was characterized with scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), and colloid tritation measurements. SEM and AFM images indicated the formation of semi-interpenetrating network (S-IPN) structure between cellulose filaments and poly(NIPAM-co-AAc). FTIR spectra displayed the characteristic peaks for CF and poly(NIPAM-co-AAc). TGA suggested that there was no degradation occurred for cellulose, and physical crosslinks between cellulose and poly(NIPAM-co-AAc) might be formed in the S-IPN. The amount of negative charges of CF/poly(NIPAM-co-AAc) was about 14.6 times that of neat CF. The adsorption of MV on CF/poly(NIPAM-co-AAc) reflected pH-dependence and pH 7.0 was the optimum pH value for MV adsorption. Adsorption isotherms at 293, 313 and 333 K could be described by Langmuir, Freundlich, Temkin and Sips models, amongst which Langmuir model had the best R2 values. At 293 K, the maximum adsorption capacity of S-IPN hybrids was 226.02 mg/g, which was higher than that of neat CF (64.83 mg/g). The negative values of ΔG0 and positive values ΔH0 calculated through thermodynamics equation indicated that the adsorption towards MV was a spontaneous and endothermic process. The adsorption kinetics followed a pseudo-second-order model and exhibited a three-stage intra-particle diffusion mode. The S-IPN hydrogels showed an increased desorption rate and an accelerated desorption process when temperature was increased. Furthermore, the S-IPN hydrogels showed that the desorption of MV was promoted by increasing temperature. Finally, the regenerated hydrogels reserved an adsorption capacity superior to that of the regenerated CF.