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-o reactive barriers. The objectives of this paper are (1) to investigate mineral precipitates through an in situ permeable Fe-o reactive barrier and (2) to examine the cementation and corrosion of Fe-o filings in order to estimate the lifespan of this barrier. This field scale barrier (225' long x 2' wide x 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-o 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 upgradient deeper zone where groundwater first enters the Fe-o section of the barrier. After 15 months in the barrier, most of the Fe-o filings in the core samples were loose, and a little corrosion of Fe-o filings was observed in most of the barrier. However, larger amounts of corrosion (similar to 10-150 mu m 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 mu m thick) were also found on the uncemented individual Fe-o filings in the same area of the cementation. If corrosion continues, the estimated lifespan of Fe-o filings in the more corroded sections is 5 to 10 years, while the Fe-o filings in the rest of the barrier perhaps would last longer than 15 years. The mineral precipitates on the Fe-o 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-o section of the barrier.