Performance Evaluation of a Zerovalent Iron Reactive Barrier: Mineralogical Characteristics

There is a limited amount of information about the effects of mineral precipitates and corrosion on the lifespan and long-term performance of in situ Fe 0 reactive barriers. The objectives of this paper are (1) to investigate mineral precipitates through an in situ permeable Fe 0 reactive barrier and (2) to examine the cementation and corrosion of Fe 0 filings in order to estimate the lifespan of this barrier. This field scale barrier (225‘ long × 2‘ wide × 31‘ deep) has been installed in order to remove uranium from contaminated groundwater at the Y-12 plant site, Oak Ridge, TN. According to XRD and SEM-EDX analysis of core samples recovered from the Fe 0 portion of the barrier, iron oxyhydroxides were found throughout, while aragonite, siderite, and FeS occurred predominantly in the shallow portion. Additionally, aragonite and FeS were present in up-gradient deeper zone where groundwater first enters the Fe 0 section of the barrier. After 15 months in the barrier, most of the Fe 0 filings in the core samples were loose, and a little corrosion of Fe 0 filings was observed in most of the barrier. However, larger amounts of corrosion (∼10−150 μm thick corrosion rinds) occurred on cemented iron particles where groundwater first enters the barrier. Bicarbonate/carbonate concentrations were high in this section of the barrier. Byproducts of this corrosion, iron oxyhydroxides, were the primary binding material in the cementation. Also, aragonite acted as a binding material to a lesser extent, while amorphous FeS occurred as coatings and infilings. Thin corrosion rinds (2−50 μm thick) were also found on the uncemented individual Fe 0 filings in the same area of the cementation. If corrosion continues, the estimated lifespan of Fe 0 filings in the more corroded sections is 5 to 10 years, while the Fe 0 filings in the rest of the barrier perhaps would last longer than 15 years. The mineral precipitates on the Fe 0 filing surfaces may hinder this corrosion but they may also decrease reactive surfaces. This research shows that precipitation will vary across a single reactive barrier and that greater corrosion and subsequent cementation of the filings may occur where groundwater first enters the Fe 0 section of the barrier.