Confining nano zero-valent iron into zeolitic imidazolate framework-8 for efficient Cr(VI) removal

Confining nano zero-valent iron (nZVI) into porous materials could prevent its aggregation and passivation. In this study, we confined nZVI into the cavity of zeolitic imidazolate framework-8 (ZIF-8) via sequential pre-adsorption ferric ion (Fe(III)) and NaBH 4 reduction processes. The optimized Cr(VI) removal rate of ZIF-8 confined nZVI (nZVI@ZIF-8) reached 1.2 × 10 −1 min −1 , 9.2- and 37.5-fold that of nZVI and ZIF-8, respectively. Interestingly, the removal rate constants of nZVI@ZIF-8 toward Cr(VI) still reached 9.6 × 10 −2 min −1 and 6.7 × 10 −2 min −1 with initial pH of 7.0 and 9.0, respectively, 13.0 (7.4 ×10 −4 min −1 ) and 23.9 times that of nZVI. More importantly, channels confinement significantly improved the reactive lifetime of nZVI for Cr(VI) removal. The electro-donating -NH 2 provided positive charge sites for CrO 4 2- or HCrO 4 - adsorption. Subsequently, the difference of Fermi level between nZVI and ZIF-8 drove electrons transfer from nZVI to ZIF-8 for surface Cr(VI) reduction. This study provides an avenue to enhance the performance of nZVI toward heavy metal ions removal by coupling surface adsorption and nano confinement.