Trichloroethene hydrodechlorination in water by highly disordered monometallic nanoiron

The small size and high surface-to-volume ratio makes nanoiron attractive for in situ remediation of groundwater contaminants that are susceptible to reductive transformation, e.g. trichloroethylene (TCE). Nanoiron synthesized from borohydride reduction of dissolved iron is the most widely studied. Its reactivity with chlorinated organics such as trichloroethylene (TCE) is unique compared to other nanoiron and to iron filings that are typically used for in situ groundwater remediation, e.g. (1) higher surface-area normalized TCE dechlorination reaction rate constants, (2) the formation of saturated reaction products, and (3) higher reaction rates in the presence of H-2. The objectives of this study were to confirm the ability of monometallic Fe(B) to activate and use H-2 for TCE hydrodechlorination and to determine how the nanoiron chemical composition and the degree of crystallinity influence nanoiron reactivity with TCE. Fresh (Fe(B)), partially oxidized (Fe(B)(ox)), and annealed (Fe(B)(cr)) nanoiron samples made from borohydride reduction of dissolved Fe(II) in a water/methanol solution were characterized by HRTEM, XRD, XPS, and N-2-BET. The TCE dechlorination rate and products and the dissolved iron and boron released during reaction with TCE were measured. Fe(B) and Fe(B)(ox) were poorly ordered and could activate and use H-2 to reduce TCE to ethane. Fe(B)(cr) was crystalline and could not activate and use H-2 and reduced TCE to acetylene. The poorly ordered structure rather than the presence of boron (up to 5 wt %) provided the ability of Fe(B) and Fe(B)(ox) to activate and use H-2 for TCE dechlorination. Fe(B) and Fe(B)(ox) underwent oxidative dissolution during TCE dechlorination, and the Fe-0 in the particles was fully accessible. Particle dissolution suggests that normalizing the observed reaction rate constants with the measured specific surface area for comparison with other types of Fe-0 may be inappropriate.