The U.S. Army, U.S. Navy, U.S. Department of Energy, and commercial producers of explosives, propellants and pyrotechnics are required by government regulation to monitor areas where ordnance-related compounds (ORCs) are disposed. Groundwater contamination is a public health concern because of the potential toxicity of ordnance-related chemical residue.
For these highly reactive species, the Environmental Protection Agency (EPA) recommends laboratory analysis by high performance liquid chromatography for ORC identification and quantitation in micrograms per liter or parts per billion levels. Liter volumes of groundwater must be collected from monitoring wells, shipped to the laboratory, and subjected to lengthy preparation steps prior to use with this method.
Advances in the field of electrochemistry, microfluidics and protein engineering, specifically in electrochemical reduction of ORCs in microfluidic flows and in evolution of peptides that recognize ORCs, may provide a means to develop an in-situ method of detection. This may replace the need for sample transportation and laboratory analysis as well as collection and preparation of large volumes of groundwater.
Setting the Stage
Based on the Resource Recovery and Conservation Act, the EPA regulates the disposal of solid hazardous waste and monitors disposal sites. Groundwater in the vicinity of munitions disposal sites and training areas must be monitored for potential groundwater contamination by ORCs. The nature of live fire training, military operational tempo and reduction in number of training sites due to base realignment, has lead to increased levels of ORCs in high-use areas. This amplifies environmental concerns for the local water supply.
The long-term monitoring (LTM) of groundwater for ORCs at both contaminated and remediated sites, which can be required for as many as 30 years, involves collection, transport and analysis of chemical, physical and possibly microbiological samples. Significant portions, 50 percent to 70 percent, of the total costs associated with LTM projects are associated with these activities. The ability to analyze samples directly in the field could significantly reduce LTM costs; however, many of the current field analytical methods produce screening data rather than definitive data that can be used for regulatory decision making.
The Engineer Research and Development Center (ERDC) of the U.S. Army Corps of Engineers (USACE) is currently engaged in a research program to develop field analytical technologies for more effective monitoring that will be acceptable to the regulatory community and meet the compliance and fiscal needs of the Army. Some of the requirements that are being addressed in the program include:
- quick turnaround time of less than 4 hours;
- cost reduction of 25 percent to 50 percent, compared to traditional laboratory analysis;
- detection of ORCs at levels of concern;
- portability, remote operation and in situ operation; and
- data generation that is acceptable to federal, state and local regulatory agencies.
The objective of the research program is development and deployment of emerging new technologies for a real-time in-situ monitoring system for detection of munitions-related compounds and chemicals associated with military training activities.
A New Approach
A collaborative research initiative with ERDC’s Environmental Laboratory combines electrochemistry, protein engineering and microfluidics to study fundamental aspects of on-site detection of parts per billion concentration levels of TNT, DNT and RDX, the most common explosive ingredient of munitions. Electrochemical detection is a cost effective option for on-site monitoring of ORCs in groundwater. The required hardware is portable and robust. Further, the equipment can integrate with a computer-controlled microfluidic processing station. These features enable on-site sample handling and quick turn-around times within a single, self-contained device.
The goal of the current research effort is to develop and evaluate a design for such a device, by optimizing electrochemical detection, designing and developing a microfluidic device, and integrating possible “leap ahead” technology through the addition of a protein affinity collection and concentration stage. Detection is divided into three stages; sample collection, separation and concentration, and electrochemical reduction. The most recent focus has been on electrochemical reduction and implementation in a microfluidic device.
Early observations using an unmodified gold rotating disc electrode showed that detection of TNT and DNT at 100-ppb or 100-mg/L is readily possible by electrochemical means. Progress in the area of detection of TNT using square wave voltammetry, an electrochemical analytical technique, inspired efforts with a functionalized gold electrode. Using a gold electrode treated with a self-assembled monolayer (SAM) and square wave voltammetry, TNT and DNT may be simultaneously detected from a mixture with resolution in the parts-per-billion level.
The ability to detect both of these contaminants simultaneously is believed to be due in part from the chemistry of SAM. The compound 11-mercapto 1-undecanol (MCU) is an alkanethiol used to modify the gold surface with a layer of carbon chains on the order of nanometers. The end terminus of the carbon chain is a hydroxyl group that possesses hydrogen bonding properties. The slightly negatively-charged nitro groups on the TNT and DNT are attracted to the slightly positively charged hydrogens on the hydroxyl groups. TNT and DNT possess three and two nitro groups respectively; thus, it is expected that TNT is more readily attracted to the SAM surface and therefore reacts first.
To test this hypothesis, a SAM made from a different carbon chain, 1-dodecanethiol, with a methyl end group was attached to the gold electrode surface. The methyl group does not demonstrate hydrogen bonding properties like the hydroxyl group. The change in surface condition from hydroxyl group to methyl group changed the specificity of the signal. When the gold electrode was treated with MCU, the current signal from TNT and DNT displays two clear peaks and the 1-dodecanthiol surface treatment shows a slight increase in current without peak specificity. The peak positions for TNT and DNT were confirmed through experiments with one ORC analyte.
The next phase of research focused on design and fabrication of a microfluidic device. The device consists of a single flow channel formed in a polymer known as polydimethylsiloxane resting on a glass slide. A gold wire working electrode and platinum counter electrode are positioned below a silver-silver chloride reference electrode. To date, parts-per-billion concentration levels of TNT and DNT were detected without surface modification of the gold using milliliters of solution. Peptide-based binding methods also are being developed and will be integrated into the device that can exploit separation and concentration. Future experiments with SAMs and mixtures as well as real-world samples from groundwater taken from the vicinity of munitions plants are planned.