Project 6: Contaminant oxidation using nanoparticulate and granular zero-valent iron
Summary
Presentation: Overview of Project 6
The main objective of this project is to assess the potential for using oxidants produced during the corrosion of granular and nanoparticulate zero-valent iron (ZVI) by oxygen to remediate contaminated groundwater and soil. These objectives are being realized by studying the reaction mechanisms involved in oxidant production and contaminant transformation and the efficiency of potential treatment methods under conditions similar to those that are likely to be employed in treatment systems. The overall hypothesis is that the oxidative ZVI system offers a practical, cost-effective means of remediating contaminants that have the greatest impact on human health at Superfund sites. This investigation of the reaction mechanisms focuses on the role of solution chemistry and surface structure on the rate of contaminant transformation. To gain insight into the processes occurring on or near ZVI surfaces, chemical processes occurring in the solution phase are being measured in conjunction with studies conducted using techniques designed to probe the surface, such as potentiometry, surface enhanced Raman spectroscopy and electrochemical quartz microbalance methods. This investigation of the potential applications of the oxidative ZVI system to contaminant remediation focuses on permeable reactive barriers and water infiltration systems used to treat organic contaminants and drinking water treatment systems used to remove arsenic. These studies extend the research in oxidant formation mechanisms to account for the effect of oxide coatings on the ZVI surfaces on contaminant oxidation rates and transport of contaminants to and from the corroding iron surfaces. This research has the potential to provide innovative and cost-effective ways of removing contaminants from groundwater and drinking water that are difficult or expensive to treat by conventional methods. The development of these technologies could reduce human exposure to organic and inorganic contaminants of concern.
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Project Update
Zero-valent iron (i.e., Fe0) is unstable in water and is readily oxidized to ferrous (Fe[II]) and ferric (Fe[III]) iron. When zero-valent iron is oxidized by oxygen, reactive intermediate species are formed that are capable of oxidizing chemical contaminants. Previous research has suggested that it might be possible to exploit these reactions to remediate chemicals that are frequently detected at Superfund sites. Due to their high surface area and reactivity, these reactions are especially fast on iron nanoparticles, raising the possibility that oxidants could be delivered to contaminated soil and groundwater on nanoparticles.
The formation of oxidants on iron nanoparticles depends strongly on solution conditions. As a first step in identifying the optimal solution conditions, experiments were conducted over a wide range of pH values (i.e., pH 2-9). Results from these and related experiments indicated that two types of oxidants were being produced. At low pH values, the reactions produced hydroxyl radical, whereas reactions at higher pH values ferrate ion (i.e., Fe[IV]) served as the main oxidant. This finding is significant because ferrate is a weaker, less selective oxidant than hydroxyl radical. Ultimately, it may be possible to exploit Fe[IV] in remediation of sites that are contaminated with arsenite (i.e., As[III]) or organic compounds that contain functional groups that react with Fe[IV].
Another aspect of solution chemistry that affects oxidant production in the zero-valent iron system is coordination of Fe[II] and Fe[III] by dissolved ligands. To identify ligands that increase the yields of oxidants, side-by-side experiments were performed in the presence and absence of ligands. The results suggest that EDTA increased the yield of Fe[IV] but did not change the oxidant or reaction mechanism. In contrast, complexation of iron by polyoxometalate (POM) increased the yield of oxidants and shifted the mechanism from Fe[IV] production to hydroxyl radical production. These results suggest that POM might provide a means of producing a high yield of hydroxyl radical at circumneutral pH values.
Publications
- Keenan, C.R. and David L. Sedlak. 2008. Factors affecting the yield of oxidants from the reaction of nanoparticulate zero-valent iron and oxygen. Environmental Science & Technology. (http://pubs.acs.org/journals/esthag/)
42:1262-1267. (in press) - Keenan, C.R. and David L. Sedlak. 2008. Ligand-enhanced reactive oxidant generation by nanoparticulate zerovalent Iron and oxygen. Environmental Science & Technology. (http://pubs.acs.org/journals/esthag/) 42(18):69366941. doi:10.1021/es801438f (http://dx.doi.org/10.1021/es801438f)
- Lee, Changha and David L. Sedlak. 2008. Enhanced formation of oxidants from bimetallic nickel-iron nanoparticles in the presence of oxygen. Environmental Science & Technology. (http://pubs.acs.org/journals/esthag/) 42(22):8528-8533.
- Lee, Changha, C.R. Keenan, and David L. Sedlak. 2008. Polyoxometalate-enhanced oxidation of organic compounds by nanoparticulate zero-valent iron and ferrous ion in the presence of oxygen. Environmental Science & Technology. (http://pubs.acs.org/journals/esthag/) 42(13):4921-6. doi:10.1021/es800317j (http://dx.doi.org/10.1021/es800317j)