In situ synthesized Ag nanoparticles encapsulated inside carbonized ZIF-8 for efficient Cr(VI) reduction

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Effective reduction of carcinogenic contaminant Cr(VI) to low-toxic Cr(III) is of global importance for environmental and water source protection. Herein, a stable metal-organic framework (MOF), ZIF-8, was carbonized as a template to synthesize a novel nanocomposite involving Ag nanoparticles (NPs) inside (Ag@C-ZIF-8). Surprisingly, the Ag NPs synthesized in situ in the Ag+@C-ZIF-8/AB system exhibited outstanding reduction ability toward Cr(VI) and eliminated 100 % of Cr(VI) (20 mg/L) within 10 min in an initial pH range of 3–5. The Cr(VI) removal efficiency was much higher compared with these systems of AB (25.5 %), C-ZIF-8/AB (44.6 %), Ag+/AB (66.8 %), and Ag@C-ZIF-8/AB (39.3 %). Increasing the initial pH of the solution and the amount of C-ZIF-8 template accelerated Cr(VI) reduction. In four cycles of experiments, over 92 % of Cr(VI) was reduced without any treatment, demonstrating the excellent stability and reusability of Ag@C-ZIF-8. X-ray photoelectron spectroscopy (XPS) characterization and mechanism exploration experiments indicated that Ag NPs catalyzed AB dehydrogenation and efficiently reduced Cr(VI) together with the generated hydrogen. This is the first report on carbonized MOF-based Ag NPs synthesized in situ for Cr(VI) reduction by combining catalytic AB dehydrogenation with metal nanoparticle reducibility. This work provides new avenues for the utilization of heavy metal ions in water environments and the development of novel and efficient reductants/catalysts of Cr(VI) removal.