Hierarchically Porous Melamine-Formaldehyde Resin Microspheres for the Removal of Nanoparticles and Simultaneously As the Nanoparticle Immobilized Carrier for Catalysis

Hierarchically porous melamine-formaldehyde resin microspheres (MFRM) with mesopores/macrospores (10-90 nm) and flow-through pores (1000 nm) were first prepared by sacrificing template approach. MFRM had satisfactory theoretically maximum adsorption capacity based on Langmuir model for gold (179.2 mg g(-1)) and silver (132.5 mg g(-1)) nanoparticles (AuNPs and AgNPs), which were one kind of high concern emerging environmental contaminants. Based on molecular docking study, the interaction mechanism was ascribed as electrostatic interaction, metal ligand and regium-pi bonds. In addition, the hierarchically porous structure of MFRM was beneficial to the adsorption. The abundant mesopores provided large surface area and thus high adsorption capacity; the flow-through pores and macropores ensured full access of the relatively large adsorbate, herein nanoparticles, to the adsorbent, and facilitated fast mass transfer during adsorption. After adsorption, the hybrid materials, that is, MFRM adsorbing AuNPs or AgNPs, succeeded as heterogeneous catalysts in reducing 4-nitrophenol to 4-aminophenol with reasonable catalytic activity and recyclability, which was the second usage of noble metallic nanoparticles. Finally, MFRM were regenerated by acidic thiourea, and meanwhile leaching of precious noble metal was possible. The prepared hierarchically porous MFRM was a potential green adsorbent for the removal of AuNPs/AgNPs with the second usage of the nanoparticles as catalysts, recycling of precious noble metal and reuse of the adsorbent, suggesting the sustainability of the present approach.