Enhancing Arsenate Adsorption Kinetics in Iron(III)-Crosslinked Chitosan Beads

Iron(III)-crosslinked chitosan (Fe-Ch) offers great potential for the sustainable removal of arsenic from contaminated water sources due to its high adsorption capacity and selectivity for As(V) over background competitors such as phosphate (P(V)). However, this material can have low surface area and high activation energy for chemisorption, which can contribute to a low adsorption rate limiting its scalability. This study investigated parameters impacting the adsorption kinetics of Fe-Ch beads and assessed methods to improve their performance. Batch adsorption experiments and molecular dynamics simulations demonstrated that the identity of anions present during synthesis plays a significant role in the final structure and degree of crosslinking of the beads. Acetate in the synthesis was found to lead to a lower degree of crosslinking and higher surface area, enhancing adsorption kinetics. Moreover, glycine in the synthesis was found to lead to the highest adsorption rates, as its interaction with iron increases the hydrophilicity of the beads and may provide a coordination environment that has higher affinity for arsenate than crosslinked chitosan evidenced by density function theory calculations. We also compared the impact of drying techniques on adsorption performance. Freeze-dried beads were found to be rate-limited by chemisorption while air-dried beads were found to be rate-limited by intraparticle diffusion until the beads reached a swollen state. With the approaches evaluated in this study, the pseudo-second-order rate constant of Fe-Ch beads was increased by 24-fold compared to literature studies without altering selectivity, enhancing their suitability for As(V) remediation.