A comparative study of different modification methods on zero-valent iron performance toward nitrate selective reduction to nitrogen

Although nanoscale zero-valent iron (nZVI) has been widely used for nitrate reduction, its application is always limited by the undesired generation of toxic ammonium. In this study, by selecting four commonly used nZVI modification methods (i.e., introducing Fe(II), activated carbon, Cu, and Pd), the roles of direct electron transfer and atomic hydrogen (H*) mediated reaction on nitrate selective reduction were examined. Results revealed that all the modification methods could enhance the reduction rates of nitrate with a promotion factor of 0.94-8.62, while the N-2-selectivity could only be increased from similar to 4.5 % to less than 25.7 % (in carbon amended system). The enhancing effect of these additives is determined to be highly associated with their ability to promote the direct electron transfer from Fe-0 core to nitrate by forming galvanic couples or conductive magnetite layer. Unexpectedly, multiple lines of evidence suggested that H* was not the dominant reducing species responsible for nitrate reduction in Pd-modified nZVI system (and also the other tested systems), despite the presence of Pd could significantly accelerate H* generation. Correlation analysis further showed that there was no reasonable relationship between N-2-selectivity and Fe-0 corrosion tendency and hydrogen formation rates of different modified nZVI systems. All these finding could improve our understanding of nitrate selective reduction by Fe-0.