Performance evaluation of a zerovalent iron reactive barrier: Mineralogical characteristics

There is a limited amount of information about the effects of mineral preci pitates 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 an d (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 co ntaminated 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, si derite, and FeS occurred predominantly in the shallow portion. Additionally , aragonite and FeS were present in upgradient deeper zone where groundwate r first enters the Fe-o section of the barrier. After 15 months in the barr ier, 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, lar ger amounts of corrosion (similar to 10-150 mu m thick corrosion rinds) occ urred on cemented iron particles where groundwater first enters the barrier . Bicarbonate/ carbonate concentrations were high in this section of the ba rrier. Byproducts of this corrosion, iron oxyhydroxides, were the primary b inding material in the cementation. Also, aragonite acted as a binding mate rial to a lesser extent, while amorphous FeS occurred as coatings and infil ings. Thin corrosion rinds (2-50 mu m thick) were also found on the uncemen ted individual Fe-o filings in the same area of the cementation. If corrosi on continues, the estimated lifespan of Fe-o filings in the more corroded s ections 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 F e-o filing surfaces may hinder this corrosion but they may also decrease re active surfaces. This research shows that precipitation will vary across a single reactive barrier and that greater corrosion and subsequent cementati on of the filings may occur where groundwater first enters the Fe-o section of the barrier.