Effects of non-reducible dissolved solutes on reductive dechlorination of trichloroethylene by ball milled zero valent irons

Abstract

Non-reducible solution anions have been well recognized to affect reactivity of ZVI in dechlorinating chlorinated hydrocarbons. However, their effects and corresponding functional mechanisms on electron efficiency (εe) of ZVI remain unclear. In this study, mechanochemically modified microscale sulfidated and unsulfidated ZVI particles (i.e., S-mZVIbm and mZVIbm) and trichloroethylene (TCE) were used as model particles and contaminant to explore such effects. PO43− as a corrosion promoter enhanced initial dechlorination rate by both particles. However, its passivating role as a surface complex agent became significant at the later stage of dechlorination by mZVIbm, while sulfidation alleviated this effect without inhibition of dechlorination. Compared with enhancing dechlorination, PO43− promoted hydrogen evolution reaction (HER) to a higher extent, decreasing εe for both particles by 17–73 %. HCO3− negligibly affected dechlorination by both particles, while elevated HER. Thus, HCO3− [5 mM] decreased εe for S-mZVIbm and mZVIbm by 1.9 % and 22 %. Different from PO43− and HCO3−, Cl− and SO42− showed no significant effects on dechlorination, HER, and therefore εe for both particles. These results imply that even though some co-existing anions (i.e., PO43− and HCO3−) acting as corrosion promoters could improve the dechlorination by ZVIs, they would lead to decreased εe and shortened particle reactive lifetime.