Highly efficient remediation of groundwater co-contaminated with Cr(VI) and nitrate by using nano-Fe/Pd bimetal-loaded zeolite: Process product and interaction mechanism

Hexavalent chromium and nitrate co-contaminated groundwater remediation are attracting extensive attention worldwide. However, the transformation pathways of chromium and nitrate and the interplay mechanism between them remain unclear. In this work, zeolite-supported nanoscale zero-valent iron/palladium (Z-Fe/Pd) was synthesized and used for the first time to simultaneously remediate Cr(VI) and nitrate. Transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy analyses confirmed that nanoscale zero-valent iron/palladium was successfully loaded onto zeolite and it exhibited good dispersibility and oxidation resistance. Results of batch experiments showed that the Cr(VI) and nitrate removal efficiencies decreased from 95.5% to 91.5% to 45% and 73%, respectively, with the initial solution pH increasing from 3.0 to 8.0. The removal rates and efficiencies of Cr(VI) and nitrate under anoxic conditions were higher than those under open atmosphere because the dissolved oxygen diminished the electron selectivity toward the target pollutants. Moreover, the presence of Cr(VI) inhibited nitrate reduction by forming Fe(III)-Cr(III) hydroxide to impede electron transfer. Cr(VI) removal was promoted by nitrate, within limits, by balancing the consumption and generation rate of Fe3O4, which enhanced electron migration from the Fe(0) core to the external surface. The removal capacities of Cr(VI) and nitrate reached 121 and 95.5 mg g−1, respectively, which were superior to the removal capacities of similar materials. Results of product identification, XRD, and XPS analyses of spent Z-Fe/Pd indicated that the reduction of Cr(VI) was accompanied by adsorption and co-precipitation, whereas the reduction of nitrate was catalyzed by the synergism of Fe(0) and Pd(0). An alternative to the simultaneous remediation of Cr(VI) and nitrate from groundwater under anoxic conditions is provided. [Display omitted] •Z-Fe/Pd was used for simultaneous remediation of Cr(VI) and nitrate.•Cr(VI) and nitrate removal capacity reached 121 and 99.5 mg g−1, respectively.•The process products of Cr(VI) and nitrate reduction were identified.•Nitrate enhanced Cr(VI) removal within limits.•Fe(II) and Pd(0) played significant roles in Cr(VI) and nitrate reduction. The main finding of the work: In-depth information on the transformation pathway of Cr(VI) and nitrate by Z-Fe/Pd and the interplay mechanism between them was obtained by macroscopic and microscopic techniques.