HR: 1340h
AN: H33E-1694    [Abstracts]
TI: Comparison of Iron Sulfide and Zero-Valent Iron as Reactive Materials for the Removal of Arsenic From Groundwater
AU: * Henderson, A D
EM: henderad@umich.edu
AF: University of Michigan Department of Civil and Environmental Engineering, EWRE Bldg. 1351 Beal Ave., Ann Arbor, MI 48109, United States
AU: Demond, A H
EM: averyd@umich.edu
AF: University of Michigan Department of Civil and Environmental Engineering, EWRE Bldg. 1351 Beal Ave., Ann Arbor, MI 48109, United States
AB: Zero-valent iron (ZVI) installed in permeable reactive barriers (PRBs) has been shown to be an effective remediation agent for several contaminants, including arsenic (As), a redox-active oxyanion present in reduced form as arsenite, AsO3(3-), and in oxidized form as arsenate, AsO4(3-). Work performed has shown greater removal of arsenic by iron sulfide (FeS), as mackinawite, than by ZVI under anaerobic conditions, recommending the use of FeS in PRB systems. For both ZVI and FeS PRB systems, the interaction of the reactive porous media with groundwater species, and calcium and carbonate in particular, is key to maintaining the permeability and reactivity of the PRB, both of which are necessary for continued treatment. If geochemical conditions are favorable, CaCO3(s) may precipitate, thus reducing permeability and passivating the reactive surface, preventing further remediation. In a statistical review of field PRB performance (Henderson and Demond, Env. Eng. Sci., 2007), it was found that alkalinity, as an indicator of the potential for precipitation of carbonate solids, was correlated to increased risk of PRB failure. A combination of experimental and geochemical modeling approaches is being used to investigate the quantity of calcium carbonate formation in anaerobic FeS and ZVI systems. Column tests with FeS to date have resulted in behavior unlike that observed with ZVI. In the ZVI columns, a pH increase has allowed the precipitation of CaCO3(s), which led to a reduction in permeability. In the FeS columns, the effluent pH and aqueous calcium concentrations were essentially the same as the influent, suggesting that the buffer capacity of carbonate prevented a pH increase, thus precluding the precipitation of CaCO3(s). Geochemical modeling suggests that the interaction of carbonate and FeS may self-regulate in PRB systems: at high carbonate concentrations, when the precipitation of CaCO3(s) could reduce permeability, the buffer capacity provided by the carbonate precludes the pH rise necessary for precipitation. Based on this work, it appears that FeS has additional attributes that recommend it as a reactive medium for in situ removal of arsenic from groundwater.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1042 Mineral and crystal chemistry (3620)
DE: 1828 Groundwater hydraulics
DE: 1831 Groundwater quality
SC: Hydrology [H]
MN: 2007 Fall Meeting