Project Update Archive
Project 6: Contaminant oxidation using nanoparticulate and granular
zero-valent iron
2009
Zero-valent iron (i.e., Fe0) is unstable in water and is readily oxidized to ferrous (Fe[II]) and ferric (Fe[III]) iron. When Fe[II] on the surface of zero-valent iron reacts with oxygen or hydrogen peroxide it produces oxidants such as hydroxyl radical (OHl) that are capable of degrading chemical contaminants. Due to their high surface area and reactivity, these reactions are especially fast on iron nanoparticles, raising the possibility of using iron nanoparticles for oxidative remediation of contaminants.
In the initial phase of our research, we demonstrated that, under conditions normally encountered in contaminated soil and groundwater, most of the Fe0 and Fe[II] associated with iron nanoparticles is oxidized through pathways that do not produce OHl. Most of the recalcitrant organic contaminants encountered at Superfund sites react with OHl but not the other oxidizing species produced by the nanoparticles. As a result, large amount of iron nanoparticles are needed for remediation.
To enhance the efficiency of OHl production, we altered the coordination environment of iron. Addition of organic compounds, such as oxalate or EDTA, enhances the production of OHl by approximately an order of magnitude. Therefore, addition of small amounts of relatively benign compounds (e.g., oxalate and EDTA are both used as food additives) along with iron nanoparticles can be produce high yields of oxidants needed for contaminant remediation. Addition of polyoxotungstate also enhances the yield of OHl by over an order of magnitude. Polyoxotungstate may be useful in ex situ treatment systems because it can be attached onto glass or silica surfaces where it can act as a catalyst to convert oxygen or hydrogen peroxide into OHl.
To achieve the efficient production of OHl that we have observed in the presence of iron-complexing organic compounds and polyoxotungstate in a more cost-effective manner, we developed an iron-containing catalyst in which the iron is associated with aluminum or silica. The efficient conversion of hydrogen peroxide to OHl that we have observed on the catalyst may provide a cost effective means of treating contaminated groundwater. It also may provide insight into ways of improving the efficiency of in situ remediation techniques that rely upon the conversion of hydrogen peroxide into OHl on mineral surfaces encountered in soils.
2008
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.
2007
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.
2006
The Contaminant Oxiadation Using Nanoparticulate and Granular Zero-Valent Iron project 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 researchers’ objective is to assess the potential for using oxidants to remediate contaminated groundwater and soil. The oxidants project researchers are studying are produced during the corrosion of granular and nanoparticulate zero-valent iron (ZVI) by oxygen. The overall hypothesis that the researchers are to test 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.
During this period project researchers focused on their objective to determine the effects of solution composition on the rates of oxidant production, iron corrosion and contaminant transformation. The researchers’ plan is to determine the rate of oxidant production and overall yield in the presence of different types of nanoparticulate and granular ZVI in solutions of different pH.
In this initial phase of the research, project researchers successfully developed an experimental system for assessing the efficiency of the ZVI oxidation reaction under controlled conditions. This system allowed the researchers to quantify a mass balance for all of the reactive species in their experimental system. Project researchers found that contrary to previous studies, the data indicate that the efficiency of the reaction increases between pH 3 and 6. This suggests that the ZVI oxidative process may be more efficient than previously believed at pH ranges encountered in the environment.