Sustainable β-cyclodextrin/polyethylenimine-encapsulated activated algae hydrogel beads for high-capacity Cd(ii) removal: adsorption performance, mechanism, thermodynamics, and Box–Behnken optimization

Abstract

This work details the formation of a novel bio-adsorbent from algae functionalized with glutamic acid after being activated with hyaluronic acid. The functionalized algae were encapsulated in β-cyclodextrin as well as polyethylenimine and crosslinked by epichlorohydrin to form FAACP hydrogel beads that have been used for the removal of cadmium(II) ions from wastewater, proving their potential use in environmental remediation. Full characterization was done by analytical tools such as XRD, FTIR, XPS, BET, and SEM-EDX. The FAACP hydrogel system surface area of 128.734 m² g⁻¹ is quite high. This study examined the effects of temperature, starting concentration of Cd(II) ions, pH, and quantity of FAACP on adsorption. The equilibrium followed the pseudo-second-order kinetic model and Langmuir isotherm adsorption isotherm; chemisorption was the predominant mechanism of adsorption that required an activation energy of 30.18 kJ mol⁻¹. This indicated that the adsorption process occurs by an increase in temperature, which means it is endothermic and spontaneous in nature. The Box–Behnken design under the response surface methodology using Design-Expert software enhanced the adsorption efficiency to a maximum value under optimum conditions: 0.02 g of FAACP in 25 mL at pH 6 with an adsorption capacity value of 254.75 mg g⁻¹ for Cd(II) ion solutions. X-Ray diffraction studies verified the stability and efficacy of the adsorbent throughout the process, and stability testing verified constant impurity elimination after six cycles of adsorption and desorption while maintaining the original chemical structure unchanged.