Green Synthesis of Hierarchically Porous Alginate/ZIF-8 Composite Beads for Water Treatment

The immobilization of powdery metal-organic-frameworks (MOFs) into porous matrix has risen to be a promising approach for their practical applications. This work presents the green synthesis of porous alginate/ZIF-8 composite beads through the in situ growth of ZIF-8 in alginate hydrogel beads crosslinked by Zn²⁺ and subsequent freeze-drying process, during which only water is used as reaction solvent. The macro- and meso-pores in alginate matrix and micro-pores in immobilized ZIF-8 endow the porous composite beads with hierarchically porous structure, which benefits their adsorption properties. Methylene blue (MB) is selected as probe molecule to assess the adsorption performance of the porous composite beads toward water contaminants. It is found that the growth time of ZIF-8 has a significant influence on the adsorption performance, and the calculated maximum adsorption capacity of ZIF-8@Alg30 for MB reaches 628.93 mg g⁻¹ at 298 K. Additionally, the adsorption process fits well with the pseudo-second-order kinetic model and Langmuir isotherm model, and the adsorption process is found to be spontaneous, endothermic and irreversible based on thermodynamic studies.