N-functionalized calcium alginate encapsulated with microalgae for efficient Pb removal: Mechanisms and performance in batch and column operations

In this study, a novel adsorbent called Ca@SP was developed by immobilizing microalgae protein (Spirulina platensis, SP) in an alginate matrix for enhanced Pb²⁺ removal from aqueous solutions. Synthesized via in situ crosslinking, Ca@SP leverages the synergistic effects of alginate's gel-forming ability and SP's N-rich biomass. Characterization of Ca@SP revealed a green spherical hydrogel with a BET specific surface area of 159.5 m²/g. The composite exhibited pH-dependent and highly selective adsorption for Pb²⁺ over other divalent metal ions, which can be explained by the relatively lower hydrated Gibbs free energy of Pb²⁺. The main adsorption mechanisms were identified as physical adsorption, electrostatic interaction, complexation, and ion exchange. Column experiments demonstrated Ca@SP's effectiveness under various operational conditions, including flow rate, influent concentration, and column heights. RSM was employed to optimize the adsorption process, with initial pH emerging as a critical parameter influencing Pb²⁺ removal efficiency. Desorption experiments showed that acidic solvents had higher desorption efficiency than alkaline solvents, and the potential for reusability was confirmed. These results suggest that Ca@SP has potential as an effective and sustainable adsorbent for Pb²⁺ removal from aqueous environments.