Salicylic acid-modified defective NH2-MIL-125/sodium alginate composite aerogel beads for enhanced Pb(II) and Cd(II) removal

The threat of Pb(II) and Cd(II) in water demands advanced adsorbents. While metal-organic frameworks (MOFs) have demonstrated great potential for heavy metal ions adsorption, their microporous nature and powdery form limit practicality. Thus, we engineered a sodium alginate composite aerogel bead loaded with salicylic acid-modified defective NH 2 -MIL-125, for the efficient removal of Pb(II) and Cd(II) from aqueous solutions. By adjusting the solvent volume ratio to regulate the crystal morphology and introducing salicylic acid as a fragmented ligand to compete with NH 2 -BDC to coordinate with Ti-oxo clusters, a defective Ti-MOFs with mesoporous structure (pore size increased from 2.30 to 8.93 nm) and abundant active sites was successfully developed. Further integration with sodium alginate formed millimeter-scale aerogel beads, improving the reusability and enhancing adsorption performance: the maximum adsorption capacities of 428.01 mg g −1 for Pb(II) and 179.67 mg g −1 for Cd(II), respectively. The adsorption process, described by the pseudo-second-order kinetic and Sips isotherm models, is driven by multilayer chemical adsorption via electrostatic attraction, ion exchange and coordination reactions with −COOH, −NH 2 and −OH groups. The beads also exhibit excellent selectivity and reusability, maintaining high efficiency of ∼97 % for Pb(II) over 5 cycles. This work provides a promising strategy for designing high-performance and recyclable adsorbents for adsorbing heavy metal ions from water.