Influence of different co-contaminants on trichloroethylene removal by sulfide-modified nanoscale zero-valent iron

Sulfide-modified nanoscale zero-valent iron (S-nZVI) has been widely employed to degrade various chlorinated organics due to its excellent properties. However, the influence of co-contaminants on its reactivity toward reductive degradation of chlorinated organics needs to be further investigated. In this study, S-nZVI was applied to remove trichloroethylene (TCE) in the presence of different co-contaminants (Cr6+, Cd2+, and NO3-) under acidic and alkaline conditions. The removal of TCE by S-nZVI in TCE/Cr6+ and TCE/Cd2+ mixed solution decreased significantly with the increasing concentration of Cr6+ and Cd2+. In contrast, the presence of NO3- enhanced the TCE removal by S-nZVI. The increasing initial pH from 5.61 to 9 resulted in a slight decrease of TCE removal in all reaction systems, while the increasing pH exerted negligible influence on the dechlorination of TCE. This indicated that the increasing pH only inhibited the adsorption of TCE. Simultaneously, the removal of co-contaminants was examined. S-nZVI had a poor sequestration of Cr6+, compared with that of Cd2+ and NO3-, which was due to the rapid surface passivation of S-nZVI by the products of Cr6+ reduction. SEM-EDS analysis verified that much more precipitates formed on the S-nZVI surface in the systems with Cr6+ and Cd2+ than NO3-. XPS analyses demonstrated the presence of surface layer of FeSx on S-nZVI, and the reactivity loss toward TCE degradation in TCE/Cr6+ and TCE/Cd2+ mixed solution was associated closely with the formation of Fe/Cr and Fe/Cd precipitates on the surface of S-nZVI.