The objective of this project was to explore the applicability of nuclear magnetic resonance relaxation-based sensors to determine key chemical parameters related to the natural attenuation of chlorinated ethenes. Recent progress in the development of chemical sensors based on nuclear magnetic resonance relaxation in the biomedical field may present opportunities for the development of chemical sensors based on similar principles for environmental applications. Small, portable sensors capable of measuring changes in chemical parameters related to the degradation of chlorinated ethenes may improve the ability to monitor the progress of remedial strategies based on monitored natural attenuation. The objective of this proof-of-concept study was to explore the viability of a nuclear magnetic resonance relaxation approach to measure variation in key chemical parameters indicative of the attenuation of chlorinated ethenes in groundwater.
This project focused on three key research questions:
- Can nuclear magnetic resonance sensor designs previously developed to study changes in pH, dissolved oxygen, and oxidative-reductive potential in biological organisms be used to perform the same measurements in groundwater under environmental conditions?
- Can polymer materials be identified that have potential to exhibit measurable changes in relaxation properties upon exposure to chlorinated ethenes?
- Can relaxation contrast agents be activated by exposure to chlorinated ethenes and thus be used to induce observable changes in polymer sensor materials?
The scientific principle exploited by these sensors was the relaxation of the nuclear magnetic resonance signals from these sensor materials in response to changes in their physical or electronic structure resulting from interactions with the chemical parameters of interest. Nuclear magnetic resonance was a physical property intrinsic to many common stable isotopes, including hydrogen-1 nucleus. When placed into a strong external magnetic field, the individual nuclear magnetic moments of a sensor material align and can be manipulated using radiofrequency radiation. Once excited, a nuclear magnetic resonance signal was easily observed; the rate of decay of this signal was highly sensitive to variations in the structural mobility of the material being studied (e.g., binding of water to a polymer surface or rigidity of a polymer) and by the presence of magnetic materials, including ferromagnetic and paramagnetic species.
Here, a variety of potential sensor materials were screened for sensitivity to key chemical parameters, including pH, oxidation-reduction potential, dissolved oxygen, tetrachloroethene, trichloroethene, 1,1-dichloroethene, and ethene by placing the sensor material in contact with prepared standard solutions using deionized water and then measuring the nuclear magnetic relaxation response. The sensor sensitivity for “natural” waters (i.e., groundwater) was also tested in a similar manner to investigate potential interferences not found in ideal standard solutions.
The proof-of-concept experiments conducted achieved partial success in meeting the key research objectives. For example, sensor materials were identified that produced a measurable response in the Transverse Relaxation (T2) properties upon exposure to changes in pH and dissolved oxygen, and upon exposure to chlorinated ethenes. Similarly, polymer sensor materials embedded with relaxation contrast agents were found to be activated by exposure to chlorinated ethenes. No sensor materials were identified that interacted with oxidation-reduction potential changes. Several other limitations were identified that limit the utility of magnetic relaxation sensors in field settings, including limited range in pH sensitivity (limited to the region of the pKa of the sensor material), non-unique T2 response for changes in dissolved oxygen, similar responses in T2 to individual chlorinated ethenes (i.e., cannot differentiate between constituents), and interference from naturally-occurring paramagnetic constituents such as iron and manganese found in groundwater.
For most measurements with any sensor material, the presence of iron and manganese in native groundwater is likely to prevent a simple correlation between measurements made in controlled (laboratory) and uncontrolled (field) environments. Any magnetic relaxation sensor employed in the field would need to be calibrated against a non-contaminated water sample taken from the same source, and calibrations would have to somehow account for natural variability in the site groundwater. Due to their paramagnetic properties, which strongly influence the response of paramagnetic sensor materials, variations in iron and/or manganese concentrations over time would interfere with the reliability of any measurements of the targeted chemical parameters, including pH, dissolved oxygen, and the concentration of chlorinated ethenes.
While the applications of relaxation-based sensors may not be well suited for the characterization of chlorinated ethenes in groundwater, other environmental contaminants may prove more fruitful. For example, the original development of these sensors was aimed at detecting trace concentrations of specific biomarkers of disease in biological samples; similar approaches could be developed to detect specific biomarkers of pathogens in water systems, or to detect contaminants such as heavy metals (i.e. mercury) that are able to form unique and specific interactions with structural moieties bound to magnetic nanoparticles (e.g. mercury binding to sulfur) in ways that chlorinated ethenes can not.
This study demonstrated some of the unique considerations and limitations for use of magnetic relaxation sensors in environmental applications. If further research is considered, it should focus on compounds that form unique and specific interactions with sensor materials, unlike the interactions of chlorinated ethenes.