Core-shell alginate-sulfonic acid networks crosslinked by multivalent cations (Ca²⁺/Al³⁺) for atmospheric water...

A ternary core-shell hydrogel composite (Alg/CaAl@PSS) was synthesized via a two-step ionotropic gelation and interfacial polymerization strategy, combining a Ca²⁺/Al³⁺ cross-linked alginate core with a polystyrene sulfonic acid (PSS) shell mediated by an intermediate polyaniline layer, for application in atmospheric water harvesting. The effect of gelation pH (3-8) on core morphology and elemental composition was systematically investigated using SEM and EDS, revealing that pH = 7 produced the most homogeneous cross-linked network, with Al/Ca ratios of 2.90, 2.55, and 8.08 measured at pH 5, 7, and 8, respectively, reflecting a transition from Al³⁺-dominated to Ca²⁺-dominated coordination and subsequent Al(OH)₃ deposition at alkaline pH. FE-SEM-EDS mapping and high-resolution XPS indicates successful core-shell architecture formation, resolving carboxylate, sulfonate, and three distinct nitrogen states (imine, amine, protonated nitrogen) characteristic of polyaniline, alongside Al 2p, Ca 2p, and Li 1s signatures verifying multivalent-cation coordination. FT-IR and TGA/DTG analyses further validated the crosslinked framework and demonstrated a 4.5-fold increase in mass loss upon water adsorption (14.28% to 64.30%), while BET/BJH analysis revealed a hierarchical mesoporous-macroporous structure (29.82 m² g^-1) arising from combined alginate cross-linking and PSS shell porosity. Kinetic analysis across six humidity/temperature conditions showed consistently superior pseudo-second-order (PSO) fits (R ² = 0.97-0.99), attributed to multi-site water interactions among carboxylate, sulfonate, and imine/amine groups. Notably, water uptake capacity increased progressively across regeneration cycles rather than declining, a phenomenon linked to thermally induced ionic cross-link redistribution between Ca-carboxylate and Ca-sulfonate coordination states. These findings establish Alg/CaAl@PSS as a promising, regeneration-stable sorbent for sustainable atmospheric water harvesting applications.