Sr(Ⅱ) adsorption using amidoxime-functionalized Calcium-Alginate gel beads derived from coral skeletons: kinetic, isotherm, thermodynamic, and mechanisms studies

In this study, Ca-alginate-Amidoxime (Ca-Alg-AO) beads synthesized from coral skeleton waste and functionalized with amidoxime groups were utilized for Sr(Ⅱ) adsorption in aqueous solutions. The physicochemical characterization was conducted using FTIR, TGA-DTA, SEM-EDX, and XRF. Characterization confirmed successful functionalization, as evidenced by the presence of ACN functional groups at 2251 cm-1, C = N stretching vibrations at 1651 cm-1, and N–O vibrations at 939 cm-1, as well as thermal stability up to 282.2–652.1 ℃ and enhanced structural density. XRF analysis revealed an increase in Ca content from 94.63% to 94.76%, while SEM showed improved surface structural density. In aqueous solutions, the beads achieved an adsorption capacity of 13.58 mg/g and an efficiency of 84.8%, following pseudo-second-order kinetics and the Langmuir isotherm model. Thermodynamic analysis indicated an exothermic and spontaneous adsorption process driven by entropy. In seawater, the adsorption capacity decreased to 4.02 mg/g due to ion competition and Sr(Ⅱ) speciation changes. Adsorption mechanisms involved ion exchange and metal-ligand complexation at lower pH levels, while higher pH promoted electrostatic interactions and complexation, with Sr(OH)2 precipitation at excessive pH. Reusability tests demonstrated that the beads retained 75.7% of their initial adsorption capacity after five cycles, highlighting excellent durability and potential for environmental applications.