Simple and modified chitosan gel beads from a natural source as a bio-sorbent for water defluoridation: Experimental and computational perspectives

Abstract

Water contaminated with fluoride (F -) is widely acknowledged as a major global issue that poses serious health and environmental risks. Adsorption is one of the many fluoride removal treatment methods that has been widely and successfully investigated. In this work, we describe a simple process for making both cross-linked and uncross-linked chitosan gel beads from shrimp waste. SEM, FTIR, XRD and TGA analysis confirmed the successful synthesis of the non-cross-linked chitosan gel beads (Cs beads) and cross-linked chitosan gel (Cs-Ech beads) whose swelling behavior, stability and fluoride removal efficiency were studied. The adsorption conditions were also optimised. An adsorption capacity of 16.59 mg/g was achieved after just 30 min and at pH = 6.8, 10 mg/L and T = 25 °C. It was shown that chitosan gel beads cross-linked with epichlorohydrin removed more fluoride than non-cross-linked chitosan gel beads. The rate of adsorption of fluoride ions is determined by the pseudo-first order kinetic equation. On the other hand, equilibrium data was best described by Langmuir adsorption isotherms. Density functional theory calculations (DFT) based on quantitative analyses were used to explore the interfacial interaction mechanisms, offering valuable perspectives on the inter- and intra-molecular interactions. Further studies on weak interaction were conducted, focusing on the binding contribution of electrostatic interaction and vdW potential. DFT results confirmed the strong adsorption capacity and coordination behavior of F - ions on the cross-linked Cs-Ech, suggesting that the Ech group offers preferred adsorption sites.