ARS | Publication request: SELECTIVITY OF LIQUID MEMBRANE CADMIUM MICROELECTRODES BASED ON THE IONOPHORE, N, N, N', N'-TETRABUTYL-3,6-DIOXAOCTANEDITHIOAMIDE

Submitted to: Electronanalysis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 29, 1999
Publication Date: N/A

Interpretive Summary: Heavy metal (Pb, Zn, Cd, Cu, Cr and Ni) contamination of soils poses serious problems to both human health and agriculture in the U.S. One of the primary sites of entry of toxic heavy metals such as cadmium (Cd) into the food chain is via uptake into plants, with deposition in the edible portions. Thus, it is important that we understand and characterize the absorption and accumulation of heavy metals in plants. Also, there is considerable current interest in using the ability of plants to absorb heavy metals from the soil as a way to clean up contaminated soils. Current engineering-based technologies used to remediate soils (e.g., removal of top soil for storage in landfills) are quite costly, and often dramatically disturb the landscape. Recently, there has been considerable interest focused on the use of terrestrial plants to absorb heavy metals from the soil and concentrate them in the easily harvestable shoot tissues as an alternative remediation technology. One limit to the development of this technology is our lack of understanding of heavy metal transport processes in plants. This lack of understanding, in turn, has been limited by a lack of research technologies to study these processes. In this paper, we report on the development of a new microelectrode that is very selective for the toxic metal Cd. It can measure Cd in solution to very low levels. We are using this microelectrode to study and map Cd uptake along plant roots. This microelectrode will allow us to conduct detailed investigations into plant Cd uptake that previously were not possible.

Technical Abstract: The selectivity of heavy metal ion-selective electrodes has often been inadequately characterized in the past. Only recently, alternative methods have become available to assess the response toward highly discriminated ion solutions. In this article, the selectivity of vibrating liquid membrane cadmium-selective microelectrodes was characterized using the separate solution method, the fixed interference method, the matched potential method, and Bakker's method which demands one to condition the electrode membrane in discriminated ion solutions prior to measurement. In this study, the matched potential method gave selectivity values for the two least discriminated ions, lead(II) and copper(II), that were comparable to the ones obtained with SSM. Bakker's method, on the other hand, was the only method that reveals the extremely high ion selectivity over all other discriminated ions. This study shows that carrier-based cadmium microelectrodes possess sufficient selectivity for many in-situ applications, and that experimental biases may sometimes lead to experimental selectivity coefficient variations of about 12 orders of magnitude depending on the chosen method.