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.

Automated summary

The immobilization of powdery metal-organic-frameworks (MOFs) in a porous matrix is a promising approach for practical applications. In this study, porous alginate/ZIF-8 composite beads were synthesized using a green method, with ZIF-8 grown in alginate hydrogel beads and subsequently freeze-dried using only water as the reaction solvent. The hierarchically porous structure of the composite beads, consisting of macro- and meso-pores in the alginate matrix and micro-pores in the immobilized ZIF-8, enables enhanced adsorption properties. The adsorption performance of the composite beads towards water contaminants was evaluated using methylene blue as a probe molecule. The growth time of ZIF-8 was found to significantly influence the adsorption performance, with a maximum calculated adsorption capacity of 628.93 mg g(-1) at 298 K for ZIF-8@Alg30. The adsorption process followed the pseudo-second-order kinetic model and Langmuir isotherm model, and thermodynamic studies revealed that the process was spontaneous, endothermic, and irreversible.