Adsorption of Ag/Au nanoparticles by ordered macro-microporous ZIF-67, and their synergistic catalysis application

Abstract

Silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs) are universally used in diverse fields, but the use and treatment of various commodities will adversely affect the human health. Therefore, the recovery and immobilization of AgNPs and AuNPs from wastewater has important economic and social benefits. The ordered macro-microporous ZIF-67 single crystal (SOM-ZIF-67) was prepared by double solvent-assisted strategy. It was used as an adsorbent to remove precious metal pollutants, AuNPs and AgNPs. Under the optimal experimental conditions, the adsorption process of nanoparticles on SOM-ZIF-67 conformed to the pseudo-second-order kinetics. The theoretical maximum adsorption capacity of SOM-ZIF-67 towards AgNPs and AuNPs was 1250 mg g−1 and 2000 mg g−1, respectively. Through some characterization methods, it can be inferred that the adsorption mechanism might include ion-dipole interaction, metal-ligand interaction, and hydrogen bond, chelation and ion exchange between AgNPs/AuNPs and nitrogen and oxygen atom, and reduction etc. Furthermore, the interpenetrating macropores and micropores of SOM-ZIF-67 further promoting the adsorption capacity. Meanwhile, after the adsorption, the composite material (SOM-ZIF-67@AgNPs or SOM-ZIF-67@AuNPs) was developed as a new kind of heterogeneous catalyst that had good catalytic activity and cycle catalytic performance for 4-nitrophenol in the presence of NaBH4. At last, the adsorbent and catalyst could be regenerated. Simultaneously recycling of precious noble metal was also possible. Thus, the prepared SOM-ZIF-67 was a potential green adsorbent for the removal and recovery of precious metal nanoparticles. The secondary utilization as an effective catalyst would elaborate the sustainability of the method.