Preparation of hybrid β-chitosan – squid pen protein hydrogel beads by ionic liquid regeneration for adsorption of copper(ii) and zinc(ii) from wastewater†

Abstract

This study explores the use of squid pen protein to enhance the chemical stability and heavy metal ion (Cu²⁺ and Zn²⁺) affinity of β-chitosan. Hydrogel beads with enhanced porosity and scalability were prepared using 1-butyl-3-methylimidazolium acetate, ([BMIM][OAc]), which simultaneously functionalized β-chitosan by decreasing its crystallinity and enhancing binding site access, as indicated by Fourier transform infrared (FT-IR) spectroscopy, which revealed intensification of functional group expression. Notably, this functionalization compensated for the effects of glutaraldehyde crosslinking. However, initial experiments noted a reduction in adsorption capacity as the squid pen protein content increased, with Cu²⁺ and Zn²⁺ adsorption being particularly inhibited at lower pH levels due to protonation. Subsequent batch adsorption studies identified optimal conditions for Cu²⁺ and Zn²⁺ uptake, with 24-hours being adequate to appraoch equilibrium, and revealed that adsorption followed pseudo-second-order kinetics, indicative of chemisorption. Furthermore, analysis of adsorption kinetics by intraparticle diffusion revealed that mass transfer was rate-limiting, with Cu²⁺ and Zn²⁺ transport being a multi-step process involving successive and slower phases controlled by external diffusion, intraparticle diffusion and equilibrium, respectively. Lastly, equilibrium studies revealed that the adsorption of Cu²⁺ and Zn²⁺ corresponded with the Langmuir model, suggesting monolayer coverage with maximum adsorption capacities of 67.4 mg g⁻¹ for Cu²⁺ and 24.1 mg g⁻¹ for Zn²⁺. Overall, the potential of squid pen protein as an economical filler for β-chitosan-based adsorbents was validated alongside the efficiency of using [BMIM][OAc] for the non-toxic functionalization of β-chitosan. Support of green chemistry principles was evidenced by a high atom economy and low environmental impact, indicating a sustainable method for preparing effective biosorbents.