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

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.