Sulfidation mitigates the passivation of zero valent iron at alkaline pHs: Experimental evidences and mechanism.

Groundwater pH is one of the most important geochemical parameters in controlling the interfacial reactions of zero-valent iron (ZVI) with water and contaminants. Ball milled, microscale ZVI (mZVIbm) efficiently dechlorinated TCE at initial stage (<24 h) at pH 6-7 but got passivated at later stage due to pH rise caused by iron corrosion. At pH > 9, mZVIbm almost completely lost its reactivity. In contrast, ball milled, sulfidated microscale ZVI (S-mZVIbm) didn't experience any reactivity loss during the whole reaction stage across pH 6-10 and could efficiently dechlorinate TCE at pH 10 with a reaction rate of 0.03 h-1. Increasing pH from 6 to 9 also enhanced electron utilization efficiency from 0.95% to 5.3%, and from 3.2% to 22%, for mZVIbm and S-mZVIbm, respectively. SEM images of the reacted particles showed that the corrosion product layer on S-mZVIbm had a puffy/porous structure while that on mZVIbm was dense, which may account for the mitigated passivation of S-mZVIbm under alkaline pHs. Density functional theory calculations show that covered S atoms on the Fe(100) surface weaken the interactions of H2O molecules with Fe surfaces, which renders the sulfidated Fe surface inefficient for H2O dissociation and resistant to surface passivation. The observation from this study provides important implication that natural sulfidation of ZVI may largely contribute to the long-term (>10 years) efficiency of TCE decontamination by permeable reactive barriers with pore water pH above 9.