An in situ study of the effect of nitrate on the reduction of trichloroethylene by granular iron
Introduction
Nitrate falls into two areas of research interest related to the use of granular iron for groundwater remediation. Permeable walls of granular iron were developed as a passive, in situ treatment method for dissolved chlorinated solvents (Gilham and O'Hannesin, 1994). Both the application of iron to the treatment of nitrate contamination Cheng et al., 1997a, Cheng et al., 1997b, Rahman and Agrawal, 1997, Huang et al., 1998, Zawaideh and Zhang, 1998, Jin and Chiu, 1999, and the effect of the presence of nitrate on the remediation of chlorinated solvents Siantar et al., 1995, Siantar et al., 1996, Wust et al., 1998, Schlicker et al., 2000, Farrell et al., 2000a have been subjects of recent studies. This paper focuses on the latter topic.
There is growing evidence that nitrate may have a detrimental effect on the ability of granular iron to treat chlorinated solvents. A number of studies have observed shifting TCE concentration profiles in column experiments in the presence of nitrate. It has been suggested that the presence of nitrate results in the formation of a passive surface film on the iron that interferes with TCE reduction Siantar et al., 1995, Siantar et al., 1996, Wust et al., 1998, Schlicker et al., 2000, Farrell et al., 2000a. These findings are consistent with the classification of nitrate and nitrite as oxidizing inhibitors. Oxidizers cause passivation of metals by initially reacting with the metal, causing an increase in pH and a positive shift in potential. As a result, high valency oxide species form and remain stable at the metal surface, creating a film which acts as a barrier to further reaction with the metal (Cohen, 1978 and references therein).
A possible weakness of previous work on the effect of nitrate on chlorinated solvent reduction by granular iron is that the proposed surface passivation by nitrate was not substantiated by analysis of the iron surface film Siantar et al., 1995, Siantar et al., 1996, Wust et al., 1998, Schlicker et al., 2000, or the film was analyzed using ex situ methods like X-ray diffraction (Farrell et al., 2000a). Permeable walls of granular iron operate under anaerobic conditions, and surface films on iron formed anaerobically can be altered by exposure to oxygen and/or vacuum (Melendres, 1996). In order to accurately characterize conditions at the iron surface, in situ, nondestructive analysis techniques are preferred.
The current work was undertaken with a focus on using in situ analysis techniques in order to characterize the influence of nitrate on the iron surface film. Experiments were conducted with columns designed to allow for the in situ monitoring of iron surface films with Raman spectroscopic and open circuit potential (iron corrosion potential) measurements.
A further point to make is that previous studies have been performed with relatively pure iron materials that were initially free of thick oxide films. In this study, Connelly iron is used, a commercial iron product typical of those used in barriers. This material is impure, consisting of cast iron and low alloy steels. Furthermore, it is covered with a passive high-temperature oxidation film, which forms during the production of the granular iron material. The film consists of an inner layer of Fe3O4 and an outer passive layer of Fe2O3 (Odziemkowski et al., 2000). This passive layer should prevent mechanisms involved in contaminant treatment, including direct electron transfer and catalytic hydrogenation, from occurring. However, contaminant reduction does occur using this material, and in previous work, it was shown that the passive layer was removed in a manner consistent with an autoreduction reaction, upon contact with water and with a solution containing 1.5 mg/l TCE (Ritter et al., 2002). The autoreduction process involves electrons from the iron metal core reducing the hematite layer and releasing Fe2+upon contact with solution (Pryors and Evans, 1950), or reducing the hematite to magnetite at the iron surface (Haruyama et al., 1969). The exposed layer of magnetite would then permit contaminant reduction, as there is evidence that magnetite allows both electron transfer (Schultze, 1978) and catalytic hydrogenation King et al., 2000, Marcus and Protopopoff, 1997, Flis and Zakroczymski, 1992 to occur; two of the mechanisms that have been proposed for the reduction of chlorinated solvents by iron. Hence, this work examines the interaction of nitrate with the pre-existing oxide film, and the influence of this interaction on TCE reduction by granular iron.
Section snippets
Experimental approach
Groundwater flow through permeable granular iron walls was simulated in experiments using laboratory columns filled with granular iron. The effect of nitrate on TCE reduction by iron was investigated in an experiment with an influent solution of 1.5 mg/l (0.011 mM) TCE+100 mg/l (1.6 mM) nitrate. Two control columns were also run, one saturated directly with a solution of 100 mg/l nitrate, and a second control in which the column was pre-treated with Millipore water prior to introduction of the
Migrating TCE concentration profiles: TCE adsorption
The concentration of TCE in solution initially decreased along the length of the column (Fig. 1). However, the profile migrated with time, as observed by others in the presence of nitrate Wust et al., 1998, Schlicker et al., 2000, Farrell et al., 2000a, until the effluent concentration was almost equal to that of the influent solution. Schlicker et al. (2000) suggested that the main effect of nitrate was to delay the onset of TCE reduction, and divided their TCE concentration profiles into an
Acknowledgements
The authors wish to acknowledge the invaluable lab assistance of Greg Friday and Wayne Noble. We would also like to acknowledge the assistance of Dr. L. Gui. Funding for this study was provided through the NSERC, Motorola, EnviroM et al. Industrial Research Chair held by Dr. R.W. Gillham, and by an NSERC Postgraduate Scholarship held by K. Ritter.
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2020, Water ResearchCitation Excerpt :In addition, NO3− affects the passivation of the oxide film on the iron surface. A high concentration of NO3− may stabilize the original passivation film (e.g., hematite) (Lu et al., 2017; Ritter et al., 2003) or form a new passivating iron oxide layer (e.g., FeOOH or a nonporous maghemite) (Mishra and Farrell, 2005; Schlicker et al., 2000) on the surface of ZVI by increasing the pH and redox potential of the system, which hinders the electron transfer. The reaction of Fe0 with H2O, which generates H2 and iron oxide (Eq. (8)) (Qin et al., 2018), is among the most fundamental processes that compete for electrons in aqueous solutions containing ZVI.3Fe0 + 4H2O → Fe3O4 + 4H2