Superfund Research Program
Investigating the Transport of Heavy Metals in a Polluted Aquifer
The transport of toxic heavy metals in aquifers is often controlled by complex geochemical interactions with the oxides and hydroxides of both manganese (Mn) and iron (Fe). These interactions include processes such as sorption, oxidation/reduction reactions, and co-precipitation and dissolution. The Pinal Creek Basin near Globe, AZ is an example of a system contaminated by wastes from historic metal mining practices. The resultant acidic mining wastes contain iron, and have come in contact with alluvial sediments containing manganese. Under these conditions, the manganese dissolves (suspending the manganese ions in aqueous solution) and iron sulfato-hydroxides precipitate. Though iron and manganese are inextricably linked through these redox couples, the specific interactions are poorly understood--reaction rates vary by orders of magnitude, and there is a non-stoichiometric release of manganese into solution.
Researchers at the University of Arizona have been developing procedures to better characterize the mechanisms that control the interactions between iron and manganese. The investigators are thereby gaining a better sense of the potential for the continued reduction of the manganese oxides in the sediments. The presence and concentration of manganese oxides directly impacts the biogeochemical cycling of a large number of toxic trace constituents common in mining-waste polluted aquifers. In particular, manganese(III,IV) oxides not only affect the transport of toxic heavy metals due to their high sorptive capacities for metal ions, but these compounds oxidize many toxic organic and inorganic contaminants including, Cr(III), Co(II), aromatic hydrocarbons, and hydrogen sulfide.
In a recently submitted paper, the researchers at the University of Arizona have described the design and use of a flow-through reaction cell that is used to gain in situ, real-time X-ray absorption spectroscopy (XAS) data during geochemical reactions. By coupling solution chemistry data with the spectroscopic measurements, the researchers were able to show that the rate of reductive dissolution of manganese oxide, MnO2, by ferrous iron, Fe(II), decreased during the course of the reaction. The production of ferric iron, Fe(III), followed by the precipitation of ferric hydroxides, was responsible for the reduced rate of the dissolution of MnO2 as time progressed. In addition, the formation of an intermediate product containing reduced manganese as well as ferric iron was identified. This partially explains the non-stoichiometric release of manganese to the aqueous phase. The identification of this intermediate phase, as well as the rate of its formation and destruction, will enable the development of more accurate predictive models of the transport of manganese in the alluvial basin.
The design and use of the flow-through reaction cell described enables the researcher to gain information regarding the complex geochemical reactions involving manganese which cannot be obtained using bench-scale laboratory experiments or the more conventional spectroscopies. This flow-through cell has already been used to gain a better understanding of the mechanisms of microbially mediated manganese oxidation (Barger, et al, 2000). Therefore, not only has a more complete picture of the controls on the fate and transport of metals in a metal-mining polluted aquifer been gained, but also a tool has been developed that will enable researchers to understand other complicated geochemical problems.
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To learn more about this research, please refer to the following sources:
- Villinski JE, O'Day PA, Corley TL, Conklin MH. 2001. In situ spectroscopic and solution analyses of the reductive dissolution of MnO2 by Fe(II). Environ Sci Technol 35(6):1157-1163. doi:10.1021/es001356d PMID:11347928
- Villinski JE, O'Day PA, Corley TL, Conklin MH. 1999. A flow-through cell for in situ, real time X-ray absorption spectroscopy studies of geochemical reactions. In: Proceedings of the U.S. Geological Survey Toxic Substances Hydrology Program Technical Meeting - Volume 2 - Contamination of Hydrologic Systems and Related Ecosystems. U.S. Geological Survey Water-Resources Investigations Report 99-4018B. pp.217-226.
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