Tailoring starch-alginate beads for efficient heavy metal removal: the role of dual covalent-ionic crosslinking and drying method.

Abstract

Sustainable adsorbents derived from renewable feedstocks offer a promising route for heavy metal remediation in water. This study reports the synthesis of hybrid starch-alginate beads dual-crosslinked by citric acid (6-15 mg mL-1) and calcium ions through external gelation. A systematic evaluation was conducted to elucidate the synergistic effects of crosslinking degree and oven- or freeze-drying methodologies on physicochemical properties and adsorption performance of the biopolymer matrices. FTIR confirmed ester bond formation, while TGA indicated that increased crosslinking degree enhances thermal robustness. Morphological characterization revealed that freeze-drying successfully yields a highly porous structure (F6), with a swelling up to 2000% and a specific surface area of 8.56 m2 g-1, over 25 times higher than oven-dried compact beads (SBET = 0.33 m2 g-1). Highly crosslinked formulations (F3 and F6 prepared with 15 mg mL-1 of citric acid) displayed a marked selectivity for cationic metals, following the trend: Pb(II) > Cu(II) > Cr(III) > Cd(II) > Ni(II) > Zn(II) > Mn(II) ≫ Cr(VI). Equilibrium data were best described by the Sips model, indicating heterogeneous binding surfaces and highlighting similar maximum capacities for both F3 and F6 (up to 2.0 mmol g-1 for Pb(II) removal). Kinetic analysis revealed a pronounced structural-functional contrast, as porous F6 reached equilibrium up to 12 times faster than F3, due to reduced intraparticle diffusion resistance. Multicomponent and regeneration assays confirmed excellent operation stability, underscoring that the strategic integration of optimized covalent crosslinking and freeze-dried methods produces high-performance, stable, regenerable, and sustainable bio-based adsorbents for heavy metal remediation.