Core-shell Ca-alginate beads formed via inverse-gelation technique for the enhanced removal of lead(II) from aqueous solution.

Calcium-alginate (Ca-alginate) beads immobilizing various functional materials have demonstrated excellent Pb(II) adsorption in wastewater systems. Nonetheless, the immobilization mostly involves an external gelation technique, limiting the diffusion of targeted pollutants to the beads' inner region. This study prepared Ca-alginate beads with a distinctive core-shell, immobilizing different functional materials via an inverse gelation technique, to remove lead(II) ions from aqueous solutions. Fe2O3 nanoparticles were immobilized within the core and carbonaceous materials were immobilized in the shell of the beads. Core-shell beads with shell thickness of 0.39 mm was formed using concentration of alginate, activated carbon (AC), iron (III) oxide (Fe2O3) of 5 g/L, 20 g/L and 5 g/L respectively, under 20 minutes gelation time. Morphological analyses confirmed the successful formation of a well-defined core-shell structure in the beads (AC@Fe2O3). AC@Fe2O3 beads showed improved physical properties such as thermal and mechanical stability and exhibited improved specific surface area. Furthermore, kinetic studies indicated that Pb(II) adsorption on the beads followed a pseudo-second-order model. Core-shell beads exhibited faster adsorption rates compared to mixed-material beads formed using external gelation, with GO@Fe2O3 showing the highest rate constant (7.314 × 10-5 g/mg·min). Additionally, isotherm analysis revealed that GO@Fe2O3 beads also had the highest maximum adsorption capacity (qₘₐₓ = 192.308 mg/g), surpassing both AC-based and mixed-material beads. Lastly, the core-shell beads retained high Pb(II) removal over five cycles.