Anodic oxidation of Zr-5Fe alloy in ethylene glycol/fluoride electrolyte for Cr(VI) removal under sunlight

Abstract

In this work, Fe-doped ZrO2 nanotubes (Fe-ZNTs) were synthesized by anodization of Zr-5 wt% Fe alloy in fluoride-containing ethylene glycol (EG)/8 vol% H2O electrolyte at varying ammonium fluoride (NH4F) concentrations (0.1, 0.3 and 0.5 wt%) at 40 V for 3 h. The morphology and crystal structure of as-anodized Fe-ZNTs were analyzed by Field-emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD), respectively. The as-anodized Fe-ZNTs were annealed at 500 °C for 3 h in a tube furnace for crystallization of the oxide film. The crystal structure, surface functional groups, and surface chemistry of annealed Fe-ZNTs were analyzed by XRD, Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS), respectively. From the FESEM results, large inner diameter (79 nm) with short nanotubes (1.9 µm) were grown in 0.5 wt% NH4F due to enhanced chemical dissolution of Fe-ZNTs in high fluoride content. XRD result of as-anodized Fe-ZNTs indicate the enhanced intensity of tetragonal-ZrO2 peaks with increasing fluoride content from 0.1 to 0.3 wt% NH4F. The cubic-ZrO2 started to emerge at 0.5 wt% NH4F. For annealed Fe-ZNTs, pure monoclinic-ZrO2 was observed in 0.1 wt% NH4F, while tetragonal-ZrO2 + monoclinic-ZrO2, and cubic-ZrO2 + monoclinic-ZrO2 were co-existed in 0.3 wt% NH4F and 0.5 wt% NH4F, respectively. Cr(VI) photoreduction under sunlight indicates 93 % Cr(VI) removal efficiency over annealed Fe-ZNTs synthesized in 0.3 wt% NH4F which ascribed to high Cr(VI) adsorption and high photocatalytic activity of tetragonal-ZrO2.