Investigation of the Long-Term Performance of Zero-Valent Iron for Reductive Dechlorination of Trichloroethylene

Abstract

This research investigated the long-term performance of zero-valent iron for mediating the reductive dechlorination of trichloroethylene (TCE). Over a 2-year period, rates of TCE dechlorination in columns packed with iron filings were measured in simulated groundwaters containing either 3 mM CaSO4, 5 mM CaCl2, or 5 mM Ca(NO3)2. At early elapsed times, TCE reaction rates were pseudo-first-order in TCE concentration and were independent of the solution pH. With increasing elapsed time, reaction rates deviated from pseudo-first-order behavior due to reactive site saturation and increased iron surface passivation toward the influent end of each column. The extent of passivation was dependent on both the TCE concentration and the background electrolyte solution. For most of the investigation, TCE reaction rates in 3 mM CaSO4 and 5 mM CaCl2 solutions were statistically identical at the 0.05 confidence level. However, TCE reaction rates in 5 mM Ca(NO3)2 were slower. In columns operated using chloride- and sulfate-containing waters, the effective half-life for TCE dechlorination increased from approximately 400 min after 10 days elapsed to approximately 2500 min after 667 days. The effective TCE half-life in the nitrate-containing water increased from approximately 1500 min after 10 days to approximately 3500 min after 667 days. Measurements of iron corrosion rates in nitrate and chloride solutions showed that nitrate contributed to increased iron surface passivation and decreased rates of iron corrosion. Corrosion current measurements indicated that halocarbon reduction on fresh iron surfaces was cathodically controlled, whereas on aged iron surfaces, iron corrosion was anodically controlled. Anodic control of iron corrosion contributed to the development of reactive site saturation with time and to similar reaction rates for TCE and perchloroethylene. Passivation of the iron surfaces was found to be dependent on the adhering tendency of the corrosion products and not on the overall mass of corrosion products in the columns. The decrease in TCE reaction rates over time can be attributed to anodic control of iron corrosion and not to increasing reactant mass transfer limitations associated with diffusion through porous corrosion products.

Calcite precipitation dominates the electrical signatures of zero valent iron columns under simulated field conditions

“…However, this mineral phase was not identified in our experiment which could possibly be attributed to the differences in flow rates and solution complexity, both influencing reaction kinetics and mineralogy (Gui and Devine, 1995;Farrell et al, 2000).…”

Testing the Suitability of Zerovalent Iron Materials for Reactive Walls

“…[13] Available data focus solely on geochemical factors affecting iron surface passivation. [13,14,36,38] For example hydrocarbonate is corrosive to ZVI resulting in a disruption of the surface passivation and additional "consumption" of ZVI. [24] However, the increased corrosion of iron leads to a higher amount of iron precipitates and subsequently to mineral precipitation, both of them causing cementation and decreased permeability of the iron wall.…”

Remediation of Soil Contaminated with Pyrene Using Ground Nanoscale Zero-Valent Iron

“…The trichloroethylene (TCE) and atrazine were dechlorinated effectively by ZVI, 5,6 as well as by the synthesized nanoscale iron (Fe 0 ) packed in columns. 7 The dechlorination by ZVI was capable to stepwise reduce polychlorinated dibenzo-p-dioxins in contaminated soil into octachlorinated dibenzo-pdioxin. 8 Adding 5% (wt/wt) Fe 0 resulted in Ͼ60% destruction efficiency for the five kinds of pesticide-contaminated soil.…”