Study On Nanoscale Zero-Valent Iron And Its Modified Bimetallic Pd/Fe Materials In Nitrate Reduction

Nitrate pollution is becoming more and more serious in recent years.Nitrate （NO₃^-）is becoming a common pollution factor in surface and groundwater.How to effectively repair the nitrate pollution in water and develop a green and efficient nitrate reduction technology has been the focus of research in this field.Among them,nanoscale zero-valent iron has the advantages of good reduction performance,large specific surface area,strong permeability and environmental friendly features,which makes it show a good application prospect in this field.How to improve the reduction efficiency/electron utilization efficiency of iron-based nanomaterials in chemical reduction of NO₃^-and the selectivity of reduction products to N₂is the current difficulties of this technology.In this paper,nanoscale zero-valent iron material was prepared by liquid-phase reduction method and modified by surface replacement to obtain Pd/Fe bimetallic materials.The effects of material dosage,initial pH,initial NO₃^-concentration and Pd loading on the reduction efficiency and product selectivity were investigated.The morphology,size,surface structure and chemical composition of Fe based nanoscale materials were characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,X-ray photoelectron spectroscopy（XPS）,transmission electron microscopy（TEM）and infrared spectroscopy（FTIR）.The results of product analysis and first principles calculation（DFT）were used to explore the mechanism of nitrate pollution degradation by Fe based nanoscale materials.（1）The nanoscale zero-valent iron material synthesized in this experiment showed a significant effect on the reduction of nitrate pollution in water.Nitrate nitrogen（NO₃^--N）with a concentration of 20 ppm can be completely removed within 3hours.However,the selectivity of N₂is only 20.5%.DFT simulation of the evolution path of NO*（an important intermediate species in the reaction）on the Fe surface confirmed that there may be three reaction paths for the formation of N₂.Compared with the formation of NH₃,there are higher reaction barriers.In the environment with abundant H*,NH₃is the main reduction product of nanoscale zero-valent iron materials.This experiment also proved that the nanoscale zero-valent iron material can keep a good removal effect of NO₃^-in the polluted water at a certain pH,and the selectivity of product N₂presents a volcanic trend with the increase of pH.When pH is 8.0,the selectivity of N₂is up to 25.4%.When pH is 5.0 and 9.0,the selectivity decreases to9.1%and 16.7%.The effects of different initial concentrations of nitrate and different dosage of nanoscale zero-valent iron on nitrate removal are basically consistent.Removal rate increases significantly with the increase of Fe/N ratio,while the selectivity of N₂decreases with the increase of Fe/N ratio.In general,it is difficult to achieve efficient and one-way regulation of nitrate reduction products of nanoscale zero-valent iron materials by controlling environmental conditions due to complex process and many influencing factors.（2）After surface modification with Pd component,Pd/Fe bimetallic nanomaterials not only ensure high nitrate removal efficiency,but also significantly improve the conversion of products to N₂.Through the study of Pd loading on the surface,it is found that the optimal mass fraction ratio of Pd to Fe is between 12-18%,which can ensure more than 90%nitrate removal rate in 3 hours reaction time and improve N₂selectivity to 44%.In contrast,the Cu/Fe system formed by using Cu as the second component is difficult to improve the selectivity.DFT calculation compared the adsorption energies of NO*,N*,N₂O*,NH₂*and active H*on Cu,Pd and Fe surfaces.It was found that the order of adsorption intensity was Fe>Pd>Cu.According to the calculation and experimental results,it is speculated that the second component mainly regulates the adsorption intensity of nitrogen species and H*on the surface of Fe.Too strong adsorption of nitrogen species and too weak adsorption of H*will restrict the conversion of products to N₂.Therefore,the moderate adsorption intensity on Pd surface is conducive to improving the selectivity of N₂.The simulation results of the evolution paths of NO*on the surface of Pd further confirmed that Pd could reduce the energy barrier of the reduction reaction to N₂,so as to obtain a better N₂ selectivity.