Long-term effect of nitrate on Cr(VI) removal by Fe(0): column studies

Lab-scale parallel continuous-flow column experiments were performed to assess the long-term effect of nitrate (NO3 (-)) on hexavalent chromium (Cr(VI)) removal by scrap iron (Fe(0)). The first column (L1) was fed with the Cr(VI) solution and the second column (L2) was loaded with the Cr(VI) + NO3 (-) solution. Raman spectroscopy and scanning electron microscopy energy-dispersive X-ray analyses (SEM-EDS) were conducted to investigate the changes of the iron oxides on Fe(0). The results showed that the process of Cr(VI) removal by Fe(0) was divided into three different stages in the presence of NO3 (-): inhibition period (<198 pore volumes (PVs)); promotion period (198∼1025 PVs); and complete passivation period (1025∼1300 PVs). During the 462∼1025 PVs, Cr(VI) removal capacity in L2 was about 2.5 times higher than that in L1, and the longevity of L2 than L1 was 275PVs longer. NO3 (-) exhibited the most dominant effect on the Cr(VI) removal by Fe(0) in the last two stages. New magnetite (Fe3O4) produced by the redox reaction of NO3 (-) and Fe(0) was discovered on the surface of the Fe(0) obtained from L2. The new generated Fe3O4 could directly reduce the Cr(VI) and could also act as an inhibitor for the formation of passive film on the Fe(0) surface as well as an electron mediator that facilitated electron transport from Fe(0) to adsorbed Cr(VI).