Diversity in the species and fate of chlorine during TCE reduction by two nZVI with non-identical anaerobic corrosion mechanism

There have been many studies on TCE degradation by synthesized nanoscale zero-valent iron (nZVI(B)) and commercial nanoscale zero-valent iron (nZVI(H)), but the effect of anaerobic corrosion on the dechlorination pathways and speciation distribution of chlorine is still unclear. Compared with nZVI(H) , nZVI(B) has a faster degradation rate of TCE and formation rate of CI- (aq) (k(SA), (TCE) = 3.67 +/- 0.85 x 10(-4 )& 2.17 +/- 0.13 x 10(-4) L. h(-1). m(-2) and k(obs, cl) = 0.344 +/- 0.027 & 0.166 +/- 0.010 mu M . h(-1) for nZVI(B) & nZVI(H), respectively). Based on the characterization of XRD, XPS and TEM during the anaerobic corrosion, the corrosion of nZVI(B) was dramatic under the dissolution-reprecipitation mechanism; but that of nZVI(H) was moderate and inward by maintaining the core-shell structure and shaping slightly rough and lumpy surface. Due to the different corrosion products (FeOOH for nZVI(B) and Fe3O4/gamma-Fe2O3 for nZVI(H) ) and the catalysis of boron on the nZVI(B) surface, the preferential dechlorination pathway of TCE was not identical by hydrogenolysis (nZVI(B) ) vs. reductive beta-elimination (nZVI(H)). Meanwhile, the dechlorination pathway of nZVI(H) was similar to that of ZVI and the reductive pathway to acetylene bypassed the formation of more toxic VC. This study shows that the high reactivity of nZVI(B) results in rapid corrosion with the side effect of enhanced adsorption of VC while nZVI(H )has a stable core-shell structure and less sorbed chlorine, which provides a new sight to access the ecological risk of nZVI due to the overlooked effect of nonidentical corrosion. (C) 2019 Published by Elsevier Ltd.