A comparative study on simultaneous removal of trichloroethylene and hexavalent chromium in groundwater by different types of sulfidized nanoscale zero-valent iron

Sulfidized nanoscale zero-valent iron (S-nZVI) has gained considerable attention in groundwater remediation on account of its remarkable reactivity, but there is limited research on simultaneous removal of trichloroethylene (TCE) and hexavalent chromium (Cr(VI)) by S-nZVI, and the effects of different sulfidation methods (co-sulfidation and post-sulfidation method) and sulfur agents (i.e., Na 2 S 2 O 4 , Na 2 S) on its simultaneous removal performance remains unclear. Herein, four types of S-nZVI particles (S-nZVI co -Na 2 S 2 O 4 , S-nZVI post -Na 2 S 2 O 4 , S-nZVI co -Na 2 S, S-nZVI post -Na 2 S) were synthesized using different sulfidation methods and sulfur agents, and their performances in TCE-Cr(VI) co-removal were systematically compared. For S-nZVI co , the removal of TCE is almost higher than that of S-nZVI post due to higher hydrophobicity, regardless of the presence or absence of Cr(VI). However, in TCE-Cr(VI) co-contamination, Cr(VI) can significantly passivate S-nZVI co -Na 2 S 2 O 4 , as FeCr 2 S 4 with poor conductivity was generated on the surface of the material, reducing its electron transfer ability. The removal of TCE by S-nZVI co -Na 2 S 2 O 4 decreased from 100 % (without Cr(VI)) to 53 % (with 40 mg/L Cr(VI)). In addition to higher Fe 0 content and relatively high hydrophobicity, S-nZVI co -Na 2 S has a higher specific surface area and proportion of S 2− species compared to S-nZVI co -Na 2 S 2 O 4 , exhibiting a greater advantage for Cr(VI) removal. Besides, there were only reduced sulfur species in S-nZVI co -Na 2 S, improving its reducing capacity. Therefore, S-nZVI co -Na 2 S had the highest TCE removal and Cr(VI) removal in the co-contamination, reaching 85 % and 100 %, respectively. These results suggest that S-nZVI co -Na 2 S has the potential to achieve simultaneous removal of TCE-Cr(VI) and suitable materials can be designed for the specific types of groundwater contamination.