Improved sensors developed for long-term groundwater monitoring at DOE sites
Developing sensors for long-term monitoring of contaminated sites is an ongoing challenge. Current procedures involve costly pumping and sampling followed by time-consuming laboratory analysis. Sensors that operate in the ground in real time would significantly reduce costs and enable more effective long-term monitoring of groundwater contaminated with radionuclides and heavy metals such as technetium and chromium. However, in situ sensors must be selective and sensitive enough to meet very low-level detection requirements without using consumable reagents.
In work sponsored by the U.S. Department of Energy's Environmental Remediation Sciences Division, scientists at Pacific Northwest National Laboratory (PNNL) originally developed a minicolumn sensor for technetium that uses an innovative combination of separation chemistry and radiometric detection, meeting the sensitivity requirements (see figure). However, reagents were needed to regenerate the sensor column, which is disadvantageous for long-term monitoring.
Recently, the research team developed a new modality for these sensors called equilibration-based sensing, which overcomes the need for reagents and takes advantage of the dynamic nature of the analyte interaction with the separation material. Under these conditions, the sensing material in the column equilibrates with the analyte concentration in solution, and the sensor is reversible; that is, the amount on the column can go up or down with changes in concentration. If a blank solution is run through the column, the sorbed analyte will be eluted from the column. This process enables ongoing, real-time measurement of contaminant concentrations.
Initial experiments in groundwater matrices demonstrated the detection of technetium-99 at drinking-water-level standards (activity of 0.033 Bq/mL). Using spectrophotometric detection instead of radiometric detection, hexavalent chromium could be detected to levels below drinking-water standards of 50 ppb. The research appeared in the online version of Analytical Chemistry, the highest impact factor journal for primary research papers in the field, as an ASAP Article (research posted as soon as publishable—see link below). The authors are Matthew O'Hara, Jay Grate, and former PNNL staff member Oleg Egorov.
Schematic diagram of the radiometric sensor column configuration where the transparent column is placed between two photomultiplier tubes (PMT) for light collection. (© 2006 American Chemical Society).
Egorov OB, MJ O'Hara, and JW Grate. 2006. "Equilibration-based preconcentrating minicolumn sensors for trace level monitoring of radionuclides and metal ions in water without consumable reagents." Analytical Chemistry