Adsorptive removal of Congo red by surfactant modified cellulose nanocrystals: a kinetic, equilibrium, and mechanistic investigation

Abstract

A cellulose nanocrystal (CNC) based adsorbent was synthesized by modifying pristine CNC with various amounts of a positively-charged surfactant (CTAB) and was used to study the adsorption behavior of Congo red (CR) in aqueous medium. The interaction of CTAB with CNCs, and potential alterations on the chemical and physical structure of CNCs are studied, and the synthesized adsorbent, modified cellulose nanocrystal (MCNC) was characterized using Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, elemental and zeta potential analysis. The amount of surfactant used for modification was optimized to maximize the adsorption capacity of the adsorbent. Furthermore, it was found that the amount of surfactant affects the CR-MCNC interactions and determines the mechanism of adsorption. The kinetics followed a pseudo-second order and intra-particle diffusion model implying that the rate-controlling step of the adsorption process was first dominated by film-diffusion, and consequently by intra-particle diffusion. Thermodynamic studies on the system suggested that the adsorption process is spontaneous and exothermic. Characterization of the adsorbent, before and after adsorption, coupled with the kinetic and isotherm studies indicated that electrostatic attraction, hydrogen bonding, and hydrophobic attraction are the main mechanisms/interactions of adsorption. The adsorbent is highly stable in water and retains its original adsorption capacity after successive dialysis cycles.