Project 6 Update Archive

2014

Our goal is to model and extend the approaches we’ve developed in our previous research on oxidant production by zero-valent iron nanoparticles and employing state-of-the-art surface science technology to further understand the reactions of persulfate in soil and groundwater that can be used to optimize treatment, predict the rate of contaminant transformation and prevent the release of toxic products of incomplete oxidation of hazardous waste contaminants.  We have studied various remediating concepts and our findings from in situ chemical oxidation (ISCO) treatments have been widely disseminated and application of our approach is being replicated.  Research in Project 6 resulted in the development of a compact and inexpensive treatment system that has the potential to be employed for wellhead treatment or point-of-use treatment in situations where water is contaminated with organic chemicals.  The system has numerous advantages over existing approaches because it can be operated without the need to replenish reagents or replace materials.  We filed a provisional patent for this electrochemical water treatment system.

 

2013

During the current project period, we focused our efforts on understanding the mechanisms through which activated persulfate transforms organic contaminants and the role of minerals and aquifer solids in the activation process.  We also investigated the development of catalysts that could be used to increase the effectiveness of bioremediation by persulfate.  We conducted experiments to quantify bioremediation efficiency, benzene oxidation using persulfate, in the presence of representative redox-active minerals collected from real aquifers in five sites in California and Arizona.  We repeated the experiments using aliquots of aquifer material that were subjected to a selective extraction procedure intended to remove surface deposits of iron and manganese oxide.  The resulting efficiency values were similar to those obtained using unmodified aquifer materials, indicating that the reaction mechanism of persulfate with the surface of metal oxides is likely different than the reaction of hydrogen peroxide with surface metal oxides.  Experiments to assess the rate of persulfate activation in the heterogeneous system provided additional insight into the mechanisms of the process.  In the presence of 1 mM benzene, the rate at which persulfate was activated increased significantly relative to experiments conducted in the absence of benzene or at lower concentrations of the compound.  However, the observed increase in activation rate was less in experiments with aquifer materials, suggesting that the initial activation step initiates a chain reaction.  In the presence of benzene, and presumably other organic contaminants, the initial product of the one-electron oxidation of the organic compound results in consumption of additional persulfate ions and propagation of the chain.  At lower concentrations of benzene, or in the absence of an organic compound, the rate of persulfate loss decreases and the efficiency of the reactions decreases as the persulfate is converted to sulfate through the oxidation of water to oxygen.

Experiments also were conducted to identify the unknown intermediate produced when persulfate was used to oxidize benzene.  Previous analyses by HPLC/MS indicated that the presence of a parent ion with a molecular weight of 126 Da, which was consistent with a formula of C6H6O3.  Analyses were conducted by adding dinitrophenylhydrazine (DNPH) prior to analysis.  The formation of an adduct upon addition of DNPH is further evidence for the presence of an aldehyde. As far as we know, there are no published reports on the toxicity of hydroxymucondialdehyde.  However, it is a structurally related benzene metabolite that is known to play a role in benzene toxicity.  Further work is underway to isolate and confirm the identity of this unknown intermediate.

Experiments were also conducted as part of our effort to develop a catalyst for oxidation of persulfate as an ex situ treatment.  Initial experiments suggested that many of the catalysts used to activate hydrogen peroxide, such as clay-containing minerals and silica-containing solids, would not result in increases in rates necessary to make the process attractive relative to existing treatment methods.  During the previous progress period, we identified graphene oxide as a catalyst that had a high potential for activating persulfate in an ex situ treatment system.  Experiments conducted during this project period indicated that graphene oxide activates persulfate at rates that are approximately four times faster than those observed for the most reactive of the naturally occurring minerals (i.e., goethite) on a surface-area normalized basis.  For a 100 mg/L graphene oxide solution, approximately 40% of the persulfate will be activated after 7 days at 25˚C.  Initial results suggest that this approach may be particularly well suited to regeneration of graphene oxide or activated carbon that has been used to remove PFOA from contaminated water.

Significance

We determined that the rate at which persulfate is converted into oxidants increases in the presence of benzene, providing the possibility for relying upon the slow activation of persulfate as a means of allowing the oxidant to move further away from the injection point before it reacts.  The finding that the rate of activation increases in the presence of minerals commonly found in aquifers suggests that in situ applications are the most fruitful avenue for further studies. The slow rate of persulfate activation by minerals means that the treatment might have advantages over peroxide based approaches because fewer injection wells would be needed. The research conducted as part of this project will be useful in predicting the rates at which persulfate is activated under different conditions encountered during remediation.  We also identified an unknown intermediate from benzene oxidation that we now suspect may be an aldehyde.  Because persulfate is used for remediation, the presence of toxic byproducts merits additional study prior to widespread adoption of the practice.

Future Plans

We will continue to develop the models of persulfate activation and benzene oxidation described above.  We will also initiate experiments on the use of activated persulfate to transform perfluorinated organic compounds.  We also will continue our efforts to identify the unknown compound produced when benzene is oxidized and will assess its toxicity using in vitro techniques, such as high content screening.  Finally, we will continue our efforts to develop catalysts for persulfate activation as well as the use of Fe[III] citrate complexes and a graphite cathode for treatment of organic contaminants, such as benzene and phenol.  The Project 6 team has been meeting with, and presenting new methods to, remedial engineers and will soon be looking for opportunities to test methods at a field site.

 

2012

Currently under review.

 

2011

Currently under review.

 

2010

Overall goals
The investigators aim to develop and test new approaches for oxidizing contaminants that are difficult to treat with existing technologies (e.g., PCBs, 1,4-dioxane and perfluorinated compounds) and apply these approaches to make treatment systems more robust and efficient. Successful completion of the proposed research will result in new oxidative treatment systems that will substantially reduce the costs of remediating contaminants that are difficult to clean-up.

What we have done so far
The investigators facilitate the reaction of iron with oxygen or hydrogen peroxide so that it produces large amounts of powerful oxidants, such as hydroxyl radicals, 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 first phase of the research, the investigators demonstrated that, under conditions normally encountered in contaminated soil and groundwater, only a small fraction of the oxygen or hydrogen peroxide reacts with the iron-containing particles to produce hydroxyl radicals.  The reaction pathway responsible for much of the loss of oxidant appeared to produce reactive species that were incapable of oxidizing most important organic contaminants encountered at Superfund sites.

To make more efficient use of the potential for iron-containing particles to facilitate oxidation of recalcitrant contaminants, their subsequent research has focused on ways of increasing the yield of hydroxyl radicals when oxygen or hydrogen peroxide reacts with iron.  Research showed that using a heterogeneous catalyst in which the iron was associated with silica or aluminum oxide increased oxidant yields from hydrogen peroxide by almost two orders of magnitude at near-neutral pH values relative to hydrogen peroxide decomposition on iron oxides.

During the past year they have shown that when iron is associated with aluminosilicate clay minerals it exhibits high yields for oxidant production.  They also observed that dissolved silica present in groundwater interacts with the surfaces of iron oxides to decrease their catalytic activity, prolonging the lifetime of hydrogen peroxide in the subsurface.

Important discovery
The results are significant to Superfund site chemical oxidation applications in which hydrogen peroxide is added to soil or groundwater to remediate recalcitrant organic contaminants.  The research suggests that hydrogen peroxide addition is much more likely to succeed when the soil or aquifer contains iron in association with aluminosilicate clay minerals.  Furthermore, it may be possible to manipulate the subsurface to remove iron oxides or increase the amount of iron associated with aluminosilicate clays as a means of improving the performance of these remediation systems.

 

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.