Facile Preparation of Millimeter‐Sized Sodium Alginate‐Silica Composite Spheres for Highly Selective Adsorption of Heavy Metal Ions

Abstract Millimeter‐sized (∼3.5 mm) sodium alginate‐silica composite spheres with macroporous structure and functional groups of amino‐ (NH 2 ‐SiO 2 /SA) and sulfhydryl (SH‐SiO 2 /SA), respectively, were successfully prepared by a simple hydrothermal copolycondensation. The obtained materials were characterized by FT‐IR, SEM, XPS, N 2 adsorption‐desorption techniques, and their heavy metal ion adsorption performance was evaluated in detail under different conditions. Characterization results revealed that the functional group plays a key role in the selectivity of metal ions, that is, NH 2 ‐SiO 2 /SA preferentially adsorbs the relatively hard Pb(II) ions while SH‐SiO 2 /SA favors the softer Ag(I) ion, which is attributed to the fact that S atoms are softer donor than N atoms. The adsorption kinetics and the adsorption equilibrium fitted well with the pseudo‐second‐order model and Langmuir adsorption isotherms, respectively. The adsorption experiments showed the maximum adsorption capacities of Pb(II) for NH 2 ‐SiO 2 /SA and Ag(I) for SH‐SiO 2 /SA are 460 mg/g and 417 mg/g, respectively, giving good stability without a significant decrease in 5 recycles. Therefore, the synthesized materials display promising adsorbents for recycling heavy metal pollutants from wastewater due to their good selectivity, high adsorption capacity, stability and easy recovery due to their large size.