Surface-Localized ZIF-8 on Polymer Foams for Efficient Pb(II) Adsorption Prepared by a Pickering High Internal Phase Emulsion Templating Method

Abstract

Metal–organic framework (MOF)/polymer composites are promising adsorbents for heavy metal ion removal, but the effective utilization of MOF particles is often limited by particle aggregation, polymer encapsulation, and poor processability of powdered adsorbents. Herein, a ZIF-8-functionalized porous foam was fabricated using a Pickering high internal phase emulsion (HIPE) template, enabling ZIF-8 to be incorporated into an interconnected porous polystyrene framework while retaining accessible MOF-related domains. During emulsification, ZIF-8 nanoparticles served as interfacially active particles that may assist emulsion stabilization together with Span-80. After polymerization, washing, and drying, the emulsion template was transformed into an open-cell porous polymer structure, with ZIF-8 retained on the pore walls and pore-edge regions. Increasing the nominal ZIF-8 loading from 2.5 to 5 wt % improved the compressive response, with the stress at 60% strain increasing from 318.09 ± 32.18 to 559.16 ± 50.69 kPa. The best Pb(II) adsorption performance was obtained at 3.75 wt % ZIF-8, corresponding to the highest apparent MOF accessibility of 94.34%, while excessive loading reduced site accessibility. The adsorption kinetics followed a pseudo-second-order model with a fitted equilibrium capacity of 273.97 mg g–1, normalized to the experimentally determined ZIF-8 mass contained in the composite. Equilibrium adsorption over an initial Pb(II) concentration range of 10–1000 mg L–1 was reasonably described by both Langmuir and Freundlich models, with a slightly better fit to the Freundlich model. Compared with powdered ZIF-8, the composite foam offers improved shape adaptability, easier handling, and collection. Furthermore, the Pickering HIPE strategy maintained relatively high apparent MOF accessibility after ZIF-8 immobilization, despite slower adsorption kinetics than powdered ZIF-8. This work provides an interfacial assembly strategy with accessible MOF domains for heavy metal ion removal.