Strategic Environmental Research And Development Program Research Articles

Per- and polyfluoroalkyl substances (PFAS)-free aqueous film forming foam formulations: Chemical composition and biodegradation in an aerobic environment

Per- and poly-fluoroalkyl substances (PFAS) have been historically used in fire-suppression activities, including aqueous film-forming foam (AFFF), given their unique structures and characteristics. However, the same properties that make them durable, resistant, and valuable also make them persistent, bioaccumulative, and toxic. This has led to recent investigations on alternative PFAS-free AFFF formulations for suppression of hydrocarbon-based fires. Such foams should contain certain classes of chemicals to remain effective, while being more environmentally friendly; part of the latter involves thoroughly characterizing the constituents and performing necessary biodegradability and toxicity studies. As part of the Department of Defense’s Strategic Environmental Research and Development Program (SERDP), we performed comprehensive constituents’ characterization and assessed the biodegradability of seven PFAS-free AFFF and a reference PFAS-based AFFF. We discovered that most of PFAS-free formulations share similar constituents with similar biodegradability through a 28-day reaction period. In PFAS-based AFFF, 4:2 and 6:2 fluorotelomer sulfonates (6:2 FTS) were identified with 6:2 FTS concentration at ∼ 9000 ng/mL. In biodegradation studies, the concentrations of 4:2 FTS and 6:2 FTS decreased while generating some perfluoroalkyl acids (PFCAs). These results increase the current knowledge on the biodegradability of AFFF alternatives but also highlight the need for future efforts on development of novel PFAS-free AFFF formulations that are more biodegradable than PFAS-based AFFF.

Assessing Electrolyte Transport through an Aircraft Coating System Using EIS

Electrochemical impedance spectroscopy (EIS) is frequently employed to nondestructively evaluate coating systems. However, there are several challenges associated with this approach. One of these challenges is associated with isolating the physical processes that are contributing to the measured impedance. We attempted to address this concern by measuring several of the coating system components before starting the exposures and attempted in-situ measurements of some properties during the course of the exposures.In this work, we analyze impedance data obtained from a two-layer coating system that was applied to AA 2024 substrates that were subjected to either immersion and simulated outdoor exposure conditions. The impedance data from the full coating system is compared with data obtained from coating systems in which gold electrodes were inserted between the layers of the coating system that could be used to monitor changes in each coating layer. Several equivalent circuit models were evaluated before settling on a modified coatings equivalent model. Using this approach, we were able to estimate several coating system parameters, including electrolyte penetration depth as a function of temperature, and solution concentration from the values obtained from the equivalent circuit model. Acknowledgments The Department of Defense Strategic Environmental Research and Development Program (SERDP) sponsored this project under work unit 5399. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Office of Naval Research, the U.S. Navy, the Department of Defense, or the U.S. government.

Acute Toxicity of Eight Aqueous Film-Forming Foams to 14 Aquatic Species.

Researchers have developed numerous per- and polyfluoroalkyl substances (PFAS)-free aqueous film-forming foam (AFFF) formulations to replace PFAS-containing AFFF used for fire suppression. As part of the Department of Defense's Strategic Environmental Research and Development Program (SERDP), we examined the direct lethal effects of seven PFAS-free AFFF and a PFAS-containing AFFF on 14 aquatic species using a series of lethal concentration (LC50) tests. We assessed the LC10, LC50, and LC90 values using log-logistic and logit analyses. Across all aquatic species tested, we discovered that exposure to at least one PFAS-free AFFF was more or as toxic as exposure to the PFAS-containing AFFF. For most cases, National Foam Avio F3 Green KHC 3% and Buckeye Platinum Plus C6MILSPEC 3% were the most and least toxic formulations, respectively. Moreover, we found consistency among results from multiple experiments using the same minnow species (Pimephales promelas) and among closely related taxa (e.g., daphnids, amphibians). Lastly, the LC50 values for AFFF formulations trended lower for tested marine species as compared to those of freshwater species. These results dramatically increase the current knowledge on the potentially toxic effects of AFFF but also highlight the need for additional research and the development of new PFAS-free AFFF that are more "ecologically friendly" than those containing persistent PFAS.

Convergence of Levenberg-Marquardt Estimates of Nonlinear Parameters in EIS Analysis of Coated Aluminum Panels

Equivalent circuit models are frequently employed to analyze electrochemical impedance spectroscopy (EIS) data. However, aside from some simple cases in which the equivalent circuit element parameters can be estimated directly from a plot of the data, the parameters for each of the equivalent circuit elements must often be estimated through the implementation of a nonlinear least-squares regression method. One frequently-used approach is the powerful Levenberg-Marquardt algorithm. This algorithm functions as a damped least-squares method that performs an interpolation between the steepest-descent and the Gauss-Newton methods for estimating nonlinear parameters. However, there are several challenges to using a Levenberg-Marquardt algorithm to model EIS data. The nonlinear nature of the impedance equations along with the difference in the magnitude of the parameter values can result in the algorithm failing to converge to a local minimum in the parameter space. This convergence challenge can be seen in determining equivalent circuit values for undamaged coating systems that have a high pore resistance, a small capacitance, as well as contributions from the underlying metal substrate. A Levenberg-Marquardt algorithm, along with a heuristic approach to use the raw data to improve the initial parameter estimates, is implemented that provides an improved approach for nonlinear regression fits of equivalent circuit models. A comparison of three equivalent circuit models fits, with residuals, for a two-layer coating system on an aluminum substrate is shown in the attached figure.Figure caption: Plot of EIS data (+ symbols) from a cured coating system prior to any environmental exposures compared with 3 equivalent circuit model fits (solid lines). The model fits were obtained using an NRL-developed Levenberg-Marquardt nonlinear regression fitting routine.The Department of Defense Strategic Environmental Research and Development Program (SERDP) sponsored this project under work unit 5399. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Office of Naval Research, the U.S. Navy, the Department of Defense, or the U.S. government. Figure 1

Monitoring Coating System Changes Using Equivalent Circuit Fit Parameters

Organic coating systems are used to manage the corrosion of metal surfaces subjected to atmospheric degradation. These coating systems are frequently complex and multilayered and capable of providing several functions, including: substrate adhesion, corrosion inhibition and electrolytic barriers. In addition, the service environment can be equally complex with exposure to UV radiation, wet/dry cycling, and application of thermal and physical stresses on the substrate and coating system alike.Refining our ability to predict service life and explain how these coatings evolve over the course of their lifetimes provides an opportunity to decrease maintenance costs and aid in future coating development. These objectives can be achieved by the development of predictive models that incorporate the fundamental effects of relevant service environment parameters. This would provide reliable damage predictions based upon the measured environmental data thereby decreasing our dependence on proxy corrosion sensors.To that end, the first phase of this project has focused on testing and monitoring an aircraft coating system that is comprised of a surface pretreatment, an epoxy primer with inhibitors, and a polyurethane topcoat exposed to a variety of degradation conditions that simulate the extreme ranges of environmental stresses expected to be seen in-service. Coating condition was monitored using electrochemical impedance spectroscopy (EIS) and characterized with equivalent-circuit models so that changes to the coating system properties could be quantified and tracked over time.Regression models for the changes in the coating system properties are being developed and will be validated by follow-on outdoor exposure testing in the next phase of the program. Acknowledgments The Department of Defense Strategic Environmental Research and Development Program (SERDP) sponsored this work under program WP19-1017. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Office of Naval Research, the U.S. Navy, or the U.S. government.

Strategic resources for assessing PFAS ecological risks at AFFF sites.

The use of aqueous film forming foam (AFFF) in the United States has caused concern about the potential effects of per- and polyfluoroalkyl substances (PFAS) on ecological resources. Moreover, the limited availability of scientific information and a lack of guidance have collectively resulted in significant challenges for ecological risk assessors supporting site-specific investigations and management decisions at PFAS-impacted sites. To address these needs, the environmental science and technology program of the Department of Defense (DoD), the Strategic Environmental Research and Development Program (SERDP), began funding four desktop review projects in 2018. Project teams were tasked with developing strategic guidance for assessing ecological risk at AFFF-impacted DoD sites. The projects focused specifically on ecological risk assessment practices for threatened and endangered (T&E) species; however, the recommendations and approaches presented in the projects are also applicable and adaptable to common species. The present review provides risk assessors and site managers an overview of how the resources provided in these projects can support ecological-risk-based management decisions at AFFF and other PFAS-impacted sites. Additionally, we synthesize perspectives offered by the four projects on a path forward for PFAS ecological risk assessment, including research needs that we recommend should be prioritized by the scientific community. Integr Environ Assess Manag 2021;17:746-752. © 2021 SETAC.

Identifying and Managing Aqueous Film-Forming Foam-Derived Per- and Polyfluoroalkyl Substances in the Environment.

The use of aqueous film‐forming foam (AFFF) has resulted in the widespread occurrence of per‐ and polyfluoroalkyl substances (PFAS) in groundwater, drinking water, soils, sediments, and receiving waters throughout the United States and other countries. We present the research and development efforts to date by the Strategic Environmental Research and Development Program (SERDP) and the Environmental Security Technology Certification Program (ESTCP) to measure PFAS in the environment, characterize AFFF‐associated sources of PFAS, understand PFAS fate and behavior in the environment, assess the risk to ecological receptors, develop in situ and ex situ treatment technologies for groundwater, treat soils and investigation‐derived wastes, and examine the ecotoxicity of PFAS‐free fire suppression formulations. Environ Toxicol Chem 2021;40:24–36. © 2020 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC. This article has been contributed to by US Government employees and their work is in the public domain in the USA.

US Department of Defense-Funded Fate and Transport Research on Per- and Polyfluoroalkyl Substances at Aqueous Film-Forming Foam-Impacted Sites.

The U.S. Department of Defense (DoD) has invested heavily in research intended to improve our understanding of the composition, fate, and transport behavior of per- and polyfluoroalkyl substances (PFAS) mixtures at sites where aqueous film-forming foam (AFFF) was previously used. The vast majority of this research has been funded through the Strategic Environmental Research and Development Program (SERDP) and the Environmental Security Technology Certification Program (ESTCP). While much uncertainty remains, the PFAS fate and transport research funded by SERDP and ESTCP has collectively contributed to a growing understanding of the complexity and importance of vadose zone processes and resulting soil retention. This article summarizes much of this work and the overarching conclusions to date. This article is protected by copyright. All rights reserved.

Simulation and testing results for a sub-bottom imaging sonar

The problem of detecting buried unexploded ordnance (UXO) is addressed with a sensor deployed from a shallow-draft surface vessel. This sonar system produces three-dimensional synthetic aperture sonar (SAS) imagery of both surficial and buried UXO across a range of environments. The sensor’s hardware design was based in part upon data created using a hybrid modeling approach that combined results from separate environmental scattering and target scattering models. This hybrid model produced synthetic sensor data where the sensor/environment/target space could be modified to explore the expected operating conditions. The simulated data were also used to adapt a set of existing signal processing algorithms for formation of three-dimensional acoustic imagery.Recently, the sonar system has been integrated to a test platform, and experiments have been conducted at a trial site in the Foster Joseph Sayers Reservoir near Howard, PA. This test site has been prepared with several buried man-made objects. Initial results show that fully buried targets can be detected. This research was supported in part by the U.S. Department of Defense, through the Strategic Environmental Research and Development Program (SERDP).

Spatial interpolation of noise monitor levels

Continuously recording noise monitoring stations provide feedback of the noise environment at monitor locations. While this feedback is useful, it only provides information at a few point locations, and in many cases it is of interest to know the noise level(s) at the locations between and beyond noise monitoring locations. In this study, we test the accuracy of several spatial interpolation models with experimental data collected during the Strategic Environmental Research and Development Program (SERDP) Community Attitudes Towards Military Blast Noise study. These datasets include 9 months of blast noise events captured at two different study locations. In both cases, a small number of monitors (e.g., 3–9) were located over a large region of interest (e.g., 1–8 km2), thus providing realistic operational conditions. The utility of deterministic (e.g., nearest neighbor, Delaunay triangulation, thin plate splines, etc.) and stochastic (e.g., geostistical or kriging) interpolation models for estimating single-event and cumulative noise levels is examined using leave-one-out cross validation. The accuracy of each approach is assessed with the root-mean-square-error (RMSE), and we discuss the practical implications of implementing such approaches in real-time systems.

Effects of seabed curvature on the scattered acoustic field

Knowledge of the acoustic properties of the sea bottom provides valuable input into the development and performance of information processing algorithms designed to detect and identify, e.g., man-made objects near to, or embedded in, the seabed. This information is also useful for improving modeling and simulation results in reverberation-limited environments. A critical challenge in this area is in allowing for range dependent bathymetry as it can affect the propagating acoustic field in ways that are difficult to predict. Typical approaches to modeling variable bathymetry assume either piecewise flat or tilted (linear) geometry; however, this only serves as a discrete approximation to reality. This study investigates the effects of mild curvature on the 3D acoustic field in frequency and angle space via finite difference time domain simulations. We present results and discuss the implications on forward modeling of the acoustic interaction with the seabed. [Work supported by the Strategic Environmental Research and Development Program (SERDP).]

Copper Dissolution in Chloroaluminate Ionic Liquids

The anodic dissolution of metal substrates under convective mass-transport conditions can often lead to considerable improvement of the surface roughness of the substrate through a process known as electropolishing. Electropolishing processes, which are a cornerstone of metal surface finishing, are most commonly carried out in aqueous electrolyte solutions, especially mixtures of concentrated sulfuric and/or phosphoric acids, or in the single acids. However, depending on the metal and conditions used, anodic dissolution in aqueous solutions sometimes results in the formation of unwanted passive films of insoluble salts and/or metal oxides, which limit the efficacy of the electropolishing process. In some circumstances, ionic liquids or room-temperature molten salts may be suitable replacement electrolytes for the aqueous acids commonly used for electropolishing. Among these ionic liquids are the organic salt-based chloroaluminates, such as AlCl3-1-ethyl-3-methylimidazolium chloride (EtMeImCl). Because of their adjustable Lewis acidity, chloroaluminate ionic liquids offer a unique solvation environment for many metals. The Lewis basic AlCl3-EtMeImCl ionic liquids, which contain unbound chloride ion, provide a solvation environment that is not unlike aqueous NaCl, except without the H2O. On the other hand, the AlCl3-rich Lewis acidic ionic liquids are a very different story because there are no “hard” ligands available to form isolable anionic complexes. Nevertheless, many metal ions are very soluble in these ionic liquids. In this investigation, we have focused on the anodic dissolution of copper, not only to assess the utility of acidic and basic chloroaluminates as electrolytes for electropolishing metals, but to understand the kinetics of the electrodissolution of metals in these versatile solvents. As is well known, copper is vitally important in the electronics industry, especially with regard to the preparation of interconnects for computer chips by electrodeposition. As with all electrochemical deposition processes, anode dissolution must be considered in the overall cell reaction. There is also an extensive body of information about the kinetics and mechanism of the anodic dissolution of copper and its alloys under simulated marine conditions, i.e., in aqueous NaCl solutions. Thus, another goal of this investigation is to compare the dissolution of copper in the chloride-rich ionic liquids with dissolution in aqueous chloride solutions. In this presentation, we will describe a kinetic analysis of the anodic dissolution of copper in the Lewis acidic and basic compositions of the AlCl3-EtMeImCl ionic liquid. An example of our research results is given in the figures below. Figure 1 shows current density-time transits recorded at a copper RDE in the Lewis acidic ionic liquid at different potentials and at a fixed rotation rate. Clearly, the current density, j, is potential dependent, but independent of time. Plots of j versus the square root of electrode rotation rate, w1/2, which were constructed from data resulting from a series of experiments similar to those used to produce Fig. 1, are shown in Fig. 2. According to the Levich equation, for a mass transport-limited process, j should show a linear dependence on w1/2 with the line passing through the origin of the plot. This is obviously not the case here, indicating that the dissolution process probably proceeds under mixed control. Figure 3 shows plots of 1/j versus 1/w1/2 based on the Koutecky-Levich equation for a one-electron reaction, 1/j=1/j k + kc /k a [1.61(FD 2/3)-1 ν1/6 ] ω-1/2. These plots clearly show a potential-dependent intercept, indicating a kinetic step in the dissolution process. The interpretation of this data, as well as that resulting from similar experiments conducted in Lewis basic ionic liquids, will be discussed in this presentation. This work was funded by the Strategic Environmental Research and Development Program (SERDP) through contract DE-AC05-00OR22725 to ORNL with subcontract to the University of Mississippi. Figure 1

Portable Aluminum Deposition System: An Alternative Plating Process for Aluminum Electrodeposition Based on Ionic Liquids

Electrodeposited aluminum and aluminum-transition metal alloys are promising materials for use as corrosion-protective coatings for steel. In fact, it is thought that electrodeposited aluminum might be a replacement for cadmium in some applications. This substitution is important because the process used for plating Cd is associated with significant health and environmental concerns. From health and environmental perspectives, the chromium plating process is also problematic, and substitutes for chromium are eagerly sought. Among the possible alternatives are aluminum-transition metal alloys such as Al-Mn and Al-W. These alloys are sufficiently durable to replace electroplated chromium in at least some applications. Aluminum plating is not feasible from aqueous solutions because water is reduced and hydrogen is evolved prior to aluminum deposition. Thus, the plating process must be carried out in a nonaqueous environment. Most commercial aluminum plating processes utilize mixtures containing pyrophoric alkylaluminum fluorides dissolved in toluene, e.g., the Siemens SIGAL® process. This very effective technology has been purchased and improved by AlumiPlate, Inc., but the newer process is still based on similar pyrophoric plating solutions and continues to invoke significant health and safety concerns. Aluminum electrodeposition from ionic liquid electrolytes, particularly organic salt-based chloroaluminates, such as aluminum chloride-1-ethyl-3-methylimidazolium chloride (AlCl3-EtMeImCl), is an attractive alternative to the classical methods for preparing aluminum coatings described above. These well known ionic liquids have low vapor pressures, low viscosities, high electrical conductivities and thermal stabilities, and so far as known, relatively low toxicities. Commercially marketed plating solutions based on these ionic liquids with the addition of proprietary additives are now marketed under the BASIONICS trademark by BASF. In our work, we have developed a Portable Aluminum Deposition System (PADS) for plating aluminum and its alloys from Lewis acidic AlCl3-EtMeImCl and related chloroaluminate ionic liquids. This technology was developed as an alternative to typical open bath electrodeposition processes and is suitable for field applications. The utility of the PADS device was demonstrated by electrodepositing Al and Al-Mn films on copper and carbon steel. These electrodeposition experiments were performed using constant potential and constant current plating methods. The deposits were evaluated based on alloy composition, compactness, surface morphology, durability, and corrosion resistance. The Al-Mn films produced with the PADS device were specular, dense, and adherent, and they rivaled the quality of similar deposits prepared in an open bath (see figure below). As expected, the Al-Mn films demonstrated significantly better resistance to chloride-pitting corrosion and much greater hardness compared to pure Al films. In this presentation, we will describe the relationship between the operational parameters of the PADS device and the composition and overall quality of the resulting Al-Mn deposits. This work was funded by the Strategic Environmental Research and Development Program (SERDP) through contract DE-AC05-00OR22725 to Oak Ridge National Laboratory with subcontract to the University of Mississippi Figure 1

Polymer Gel Membranes for Aluminum Deposition and Rechargeable Aluminum Ion Batteries

Aluminum and its many versatile alloys are routinely used as surface coatings for the corrosion protection of many metals, offering both barrier and sacrificial protection. Among various methods for aluminum coating, electrodeposition is the most attractive one because it leads to thin, economical coatings that usually are adherent and do not affect the structural and mechanical properties of the substrate. Unfortunately, neither aluminum nor its alloys can be electrodeposited from aqueous solutions because hydrogen is evolved before aluminum is plated. Thus, it is necessary to employ nonaqueous solvents (both molecular and ionic) for this purpose. Room-temperature ionic liquids (low-temperature molten salts) possess a unique set of physical properties such as nonflammability, negligible vapor pressure, high ionic conductivity, and high thermal, chemical and electrochemical stability. Therefore, they open the door for the electroplating of reactive elements, which is impossible using aqueous or other organic solvents. Thus far, most research about the use of ionic liquids for electrodeposition of aluminum and its alloys has focused on chloroaluminate anions, which were obtained by mixing anhydrous AlCl3 with organic chloride salt such as 1-ethyl-3-methyl imidazolium chloride (EMImCl) and 1-(1-butyl)pyridinium chloride (N-BPCl) etc. Unfortunately, because of the hygroscopic nature of AlCl3 and the chloroaluminate, the electroplating has to be performed in an inert-gas atmosphere. 1 One way to reduce the moisture sensitivity of the chloroaluminate based ionic liquids for practical application is to form polymer gel electrolytes, either by impregnating liquid electrolytes into the preformed membranes or co-cast polymer and liquid electrolytes, or by copolymerization of monomers in the presence of plasticizers. In this paper, polymer gel membranes were synthesized using the latter approach, that is, copolymerization of monomers in the presence of plasticizers. The gel membranes are transparent with good mechanical properties, which can be used for practical applications. Fig. 1 shows the ionic conductivities of the gel membranes containing different amount of eutectic mixture of AlCl3 and 1-ethyl-3-methylimidazolium chloride (EMIm.Cl) in a molar ratio of 1.5:1. At 20oC the ionic conductivities of the gel membranes containing 50, 60, 70 and 80 wt% of AlCl3-EMIm.Cl are 5.3 x 10-5, 2.0 x 10-4, 8.7 x 10-4 and 1.7 x 10-3 S cm-1, respectively. Fig. 2 shows a typical cyclic voltammetry of the membrane containing 60 wt% of AlCl3-EMIC (1.5:1) with copper plate as the working electrode and aluminum plate as the counter electrode. Reversible aluminum deposition and stripping is clearly observed. It is noticed that the current densities increase with increasing the scan cycles, indicating an activation process, probably due to the residual surface oxide on the Al plate. For rechargeable aluminum ion battery application, reversible aluminum deposition/ stripping with high coulombic efficiency is a necessary step to realize stable cycling performance. Therefore, this gel membrane can also be used for rechargeable aluminum ion batteries. 2More details about the gel membrane will be presented at the meeting. Acknowledgements:This work was funded by the Strategic Environmental research and Development Program (SERDP) (WP2316). 1. Liao, Q.; Pitner, W. R.; Stewart, G.; Hussey, C. L.; Stafford, G. R., J. Electrochem. Soc. 1997, 144 (3), 936. 2. Lin, M. C., Gong, M., Lu, B. , Wu, Y., Wang, D. Y. , Guan, M. Angell, M. , Chen, C. , Yang, J. , Hwang, B. J., Dai, H., Nature, 2015, 520, 324–328 Figure 1

Aluminum Dissolution in the Aluminum Chloride 1-Ethyl-3-Methylimidazolium Chloride (AlCl3-EtMeImCl) Ionic Liquid

There is increasing interest in the use of chloroaluminate ionic liquids (ILs) as solvents for the electrodeposition of aluminum. This effort is to replace the electrochemical solvent currently used for the commercial plating of aluminum in the Siemens Sigal process, which is based on pyrophoric alkylaluminum fluoride compounds dissolved in toluene. Low melting chloroaluminate ILs such as AlCl3-1-ethyl-3-methylimidazolium chloride (AlCl3-EtMeImCl)1are popular choices for this application. Although reactive with moisture, these non-flammable ILs are otherwise relatively benign. BASF SE recently marketed a successful aluminum plating technology based on this ionic liquid. The plating of Al from organic salt based chloroaluminates must be carried out under Lewis acidic conditions (> 50 m/o AlCl3) so that the reducible species, Al2Cl7 -, is present in the IL 4Al2Cl7 - + 3e- ↔ Al + 7AlCl4 - (1) This requirement stems from the fact that the reduction potential of the coordinately saturated AlCl4 - ion is more negative than that for the organic cation. This factor prohibits Al plating in basic and neutral (£ 50 m/o AlCl3) ILs. However, Al2Cl7 - (AlCl3) can be introduced into these ILs by the anodization of Al in order to change the composition to > 50 m/o AlCl3. Furthermore, during Al plating from this IL, an aluminum anode must be present in an undivided cell to resupply Al2Cl7 -that is lost from the ionic solvent during the plating process. Although the electrodeposition of Al from organic salt-based chloroaluminates has been thoroughly investigated for more than 50 years, virtually nothing is known about this anode reaction, i.e., the reverse of Eq. 1. Recent work in our laboratory involving Al anodization in the related inorganic molten salt, LiAlBr4-NaAlCl4-KAlCl4 (3:5:2), determined that under certain conditions this process is complicated by passivation of the Al electrode with AlCl3(s). In the absence of passivation, this multistep reaction is itself relatively slow, proceeding under mixed kinetic/mass transfer control.2 In this presentation, we describe an investigation of the anodic dissolution of Al in AlCl3-EtMeImCl (> 50 m/o AlCl3) carried out with rotating disk electrode (RDE) voltammetry. Figure 1 shows current-time plots recorded during potential step experiments at an Al RDE. The anodization process becomes limited by passivation of the Al electrode as the rate of the anodization process is increased and/or the temperature is decreased. As expected, this phenomenon occurs more readily in ILs with greater Lewis acidity. At smaller overpotentials, where passivation can be avoided, Levich plots recorded at the Al RDE (Fig. 2) were non-linear, indicating that Al anodization proceeds under mixed control in this ionic solvent as well. The exchange current density, j 0, for the anodization reaction was determined as a function of the IL composition by using classical Tafel analysis (Fig. 3). The mechanism of the anode reaction, and its implication for high rate Al plating will be discussed. ACKNOWLEDGEMENT :This work was funded by the Strategic Environmental Research and Development Program (SERDP) through contract DE-AC05-00OR22725. 1. J. S. Wilkes, J. A. Levisky, R. A. Wilson and C. L. Hussey, Inorg. Chem., 21, 1263 (1982). 2. C. Wang and C. L. Hussey, ECS Trans., 64, 257 (2014). Figure 1

Aluminum Anodization in the Low-Melting LiAlBr4-NaAlCl4-KAlCl4 Molten Salt

Inorganic chloroaluminate molten salts are of interest as electrolytes for the high-rate plating of aluminum because of their high conductivity and low cost. Unfortunately, low-melting Lewis acidic inorganic chloroaluminate molten salts such as AlCl3-rich AlCl3-NaCl are problematic because of the loss of volatile AlCl3, which restricts their use to sealed systems. Most common Lewis neutral chloroaluminate salts that are based on a single alkali chloride salt, e.g., equimolar AlCl3-NaCl or NaAlCl4, are problematic as well because of their high melting points. However, Hjuler et al.1 described a complex mixture of inorganic haloaluminate salts, LiAlCl4-NaAlCl4-NaAlBr4-KAlCl4 (3:2:3:2) [or LiAlBr4-NaAlCl4-KAlCl4 (3:5:2)] with a melting point of only 86 oC. This Lewis neutral salt was originally formulated for use as an electrolyte for rechargeable Al/molten salt/Ni3S2batteries. The facile electrodeposition of aluminum from this salt was also demonstrated. Any electrolyte solution that is used for the continuous electrodeposition or plating of aluminum normally requires an aluminum anode to resupply the aluminum species that is reduced at the cathode and lost from the solution during the plating process. Much research has been directed at the mechanism of the plating reaction at the cathode in haloaluminate melts, but little attention has been directed to the reactions taking place at the anode. As described in a now classical paper by Holleck and Giner,2passivation phenomena at the aluminum anode may ultimately limit the rate of the plating process in these molten salts as well as their use as battery electrolytes. In this presentation, we describe the behavior of the aluminum anode in the LiAlCl4-NaAlCl4-NaAlBr4-KAlCl4 molten salt at 130 oC. The anodization of Al in this melt is a complex process. Figure 1 shows current - time plots recorded at an Al rotating disk electrode at different applied potentials, E app, versus the equilibrium potential, E eq. (0 V versus Al). As shown in the Levich plots in Fig. 2, for E appup to 0.60 V, the anodization of Al is a mass transport-limited process according to the well-known reversible reaction:Al + 7AlX4 - ↔ 4Al2X7 - + 3e- (1)where X = Cl + Br. However, as E app is increased above this value, the current decreases significantly over a short time period, indicating that the amount of AlX4 - supplied by mass transport is not sufficient to support the reaction in Eq. 1. This results in passivation of the Al anode with AlX3(s) according to the reaction Al + 3AlX4 - ↔ 4AlX3(s) + 3e- (2) Once the Al electrode develops a passive layer, the current reaches a steady-state value determined by the rate of AlX3(s) formation (Eq. 2) and dissolution according to the reaction AlX3(s) + AlX4 - ↔ Al2X7 - (3)The Levich plot in Figure 2 suggests that this dissolution process is also mass transport limited. References 1H. A. Hjuler, S. von Winbush, R. W. Berg, and N. J. Bjerrum, J. Electrochem. Soc., 136, 901-906 (1989). 2G. L. Holleck and J. Giner, J. Electrochem. Soc., 119, 1161-1166 (1972). Acknowledgement This work was funded by the Strategic Environmental Research and Development Program (SERDP) through contract DE-AC05-00OR22725 to Oak Ridge National Laboratory with subcontract to the University of Mississippi.

Corrosion Resistance of Aluminum Films Electroplated on Steel from Chloroaluminate Ionic Liquids Containing Toluene as a Co-Solvent

Aluminum is a widely used material for corrosion resistant coatings. From a thermodynamic perspective, it is a very active metal. However, its excellent corrosion resistance stems from the formation of a robust, self-healing ~50 nm thick barrier film of Al2O3, which is formed spontaneously upon exposure to air at ambient temperature. Unfortunately, it is established that aluminum cannot be plated from aqueous solutions under normal conditions because hydrogen is evolved before most Al(III) species are reduced to Al. A number of commercial Al plating processes have been developed. Typically, these processes are based on pyrophoric mixtures of AlCl3 or AlBr3 with LiAlH4 dissolved in suitable organic solvents, such as ethers or mixtures of substituted benzenes.The first report in which a room-temperature haloaluminate ionic liquid was used to plate Al appeared in 1948.1 Although moisture-sensitive themselves, such ionic liquids, particularly those based on mixtures of AlCl3 and quaternary ammonium chloride salts, have many safety and convenience advantages over the currently used commercial Al plating baths. In this work, we report the pulsed-current plating of Al on steel substrates from the Lewis acidic AlCl3-1-ethyl-3-methylimidazolium chloride (EtMeImCl) ionic liquid with and without organic co-solvents. Compared to DC methods, pulsed-current techniques can produce electrodeposited metals with smaller grain size and greater brightness, as well as less internal stress and correspondingly greater corrosion resistance.Some preliminary results for Al films electrodeposited on steel from this ionic liquid are shown in Figs. 1 and 2. The surface morphology and corrosion resistance of these films were interrogated with optical microscopy and electrochemical impedance spectroscopy (EIS) in aqueous NaCl, respectively. Micrographs of typical Al electrodeposits (Fig. 1) indicate that their surface morphologies changed significantly with changes in t on and t off. For example, the grain size of the Al film prepared with t on = 0.25 s is much smaller than that for deposits prepared with t on =1 s. Figure 2 shows complex plane impedance plots resulting from EIS experiments. These plots show at least one time constant. A circuit model for an electrode coated with an inert porous layer2 was fitted to the data in Fig. 2 to extract information about the pore resistance (R p) of the coating. The pore resistance reflects the corrosion resistance of the electrodeposited films. Figure 2 shows that the best results were obtained for t on = 0.25 s and t off = 0.50 s in agreement with the results obtained by microscopy. It was generally found that films with the best corrosion resistance were obtained with smaller values of t on. Optimization of the parameters for the pulse plating of Al will be discussed with respect to the surface structure, grain size, corrosion resistance, and brightness of the deposits. References 1F. H. Hurley, U.S. Pat. 2,446,331 (1948). 2M. E. Orazem and B. Tribollet, Electrochemical Impedance Spectroscopy, (New Jersey: John Wiley and Sons, 2008). Acknowledgement This work was funded by the Strategic Environmental research and Development Program (SERDP) through contract DE-AC05-00OR22725 to Oak Ridge National Laboratory.

Long‐Term Performance of Chemically and Physically Modified Activated Carbons in Air Cathodes of Microbial Fuel Cells

AbstractActivated carbon (AC) is a low‐cost and effective catalyst for oxygen reduction in air cathodes of microbial fuel cells (MFCs), but its performance must be maintained over time. AC was modified by three methods: 1) pyrolysis with iron ethylenediaminetetraacetic acid (AC‐Fe), 2) heat treatment (AC‐heat), and 3) mixing with carbon black (AC‐CB). The maximum power densities after one month with these AC cathodes were 35 % higher with AC‐Fe (1410±50 mW m−2) and AC‐heat (1400±20 mW m−2), and 16 % higher with AC‐CB (1210±30 mW m−2) than for plain AC (1040±20 mW m−2), versus 1270±50 mW m−2 for a Pt control. After 16 months, the Pt cathodes produced only 250±10 mW m−2. However, the AC‐heat and AC‐CB cathodes still produced 960–970 mW m−2, whereas plain AC produced 860±60 mW m−2. The performance of the AC cathodes was restored to >85 % of the initial maximum power densities by cleaning with a weak acid solution. Based on cost considerations among the AC materials, AC‐CB appears to be the best choice for long‐term performance.

Extractive Sampling and Optical Remote Sensing of F100 Aircraft Engine Emissions

The Strategic Environmental Research and Development Program (SERDP) has initiated several programs to develop and evaluate techniques to characterize emissions from military aircraft to meet increasingly stringent regulatory requirements. This paper describes the results of a recent field study using extractive and optical remote sensing (ORS) techniques to measure emissions from six F-15 fighter aircraft. Testing was performed between November 14 and 16, 2006 on the trim-pad facility at Tyndall Air Force Base in Panama City, FL. Measurements were made on eight different F100 engines, and the engines were tested on-wing of in-use aircraft. A total of 39 test runs were performed at engine power levels that ranged from idle to military power. The approach adopted for these tests involved extractive sampling with collocated ORS measurements at a distance of approximately 20–25 nozzle diameters downstream of the engine exit plane. The emission indices calculated for carbon dioxide, carbon monoxide, nitric oxide, and several volatile organic compounds showed very good agreement when comparing the extractive and ORS sampling methods.

Bistatic scattering from underwater unexploded ordinance and the impact of burial.

Interest in exploring various sonar approaches for detecting underwater unexploded ordnance (UXO) has been growing in large part because of the strong support of the Strategic Environmental Research and Development Program (SERDP). Among the many issues now being explored are the following two fundamental questions: what are the broadband acoustic scattering characteristics associated with typical submerged UXO, and how are these impacted by the bottom sediment? Recently, the authors reported laboratory grade, monostatic acoustic scattering measurements on UXO targets in the free field. In the present study, further echo measurements are obtained on UXO both to infer what merits may exist for exploiting bistatic echo responses as well as to address the effect of sediment on the target echo characteristics. The measurements, which were carried out in the NRL sediment pool laboratory and free-field facilities, include bistatic free-field scattering for three source directions viz. 0°, 45°, and 90°, and bistatic measurements in the sediment facility with the target proud, half-buried, and fully buried. The data suggest that access to bistatic echo information in operations aimed at detecting submerged UXO-like targets could provide an important capability, especially for buried targets. [Work supported by SERDP and ONR.]