Mechanistic role of nitrate anion in TCE dechlorination by ball milled ZVI and sulfidated ZVI: Experimental investigation and theoretical analysis

摘要

Mechanistic role of NO3- in trichloroethylene (TCE) dechlorination by ball milled, micro-scale sulfidated and unsulfidated ZVI (e.g., S-mZVI(bm) and mZVI(bm)) was explored through experiments and density functional theory (DFT) calculations. Sulfidation inhibited NO3- reduction by mZVI(bm) as S weakened its interaction with NO3-. mZVI(bm) reduced NO3- within 2 h. This just resulted in a short-term electron competition during the dechlorination process by mZVI(bm) and hardly affected its sluggish dechlorination kinetics (complete TCE dechlorination in 11 d). On the contrary, NO3- suppressed TCE dechlorination by S-mZVI(bm). This was attributed to that inhibited NO3- reduction by S-mZVI(bm) (40 % reduction in 6 h) induced continuous electron competition with TCE during the time span of its dechlorination by S-mZVI(bm). NO3- reduction was also observed to facilitate formation/ crystallization of Fe3O4 on both ZVI particles, promoting dechlorination by mZVI(bm) after 4 d while not taking effect to the S-mZVI(bm)/TCE system, as its dechlorination time was too short for the surface of S-mZVI(bm) to transform. This observation has important implication on groundwater remediation by ZVI or sulfidated ZVI PRBs under a scenario of upgradient anthropogenic release of NO3-.