In situ quantification of Cobalt and Nickel in natural waters using innovative chemical sensors
Assessing the impact of trace metals on aquatic ecosystems and ultimately human health is challenging. Trace metals are distributed in a variety of redox states and chemical species that may vary continuously in space and time [1,2]. Only some trace metal species are bioavailable. The development of robust and adaptive submersible sensitive trace metal bioavailability-assessment tools is therefore required to support the establishment of environmental quality standards and guidelines based on realistic risk assessment to protect aquatic life and biodiversity, and ultimately human health.
Toward this aim, we developed on-chip chemical sensors consisting of an array of interconnected iridium-based microdiscs that are electroplated with appropriate sensing elements and covered with a hydrogel as efficient antifouling membrane [2,3]. Incorporated in in-house submersible probes and interrogated by Square Wave Anodic Stripping Voltammetry (SWASV), these gel-integrated microelectrode arrays (GIME) allow for the direct in situ quantification of the dissolved metal species that are available for uptake by phytoplankton (first chain of the food-web) [2.3]. To date, only trace metals that can be electrochemically reduced and pre-concentrated at the surface of the electrode could be measured. A wider range of trace metals may become accessible by adsorptive cathodic stripping voltammetry (AdCSV) as this approach allows for preconcentration by electrochemical adsorption of trace metal complexes upon adding a selective complexing agent to the sample. This technique has been mainly applied with renewable mercury or bismuth film electrodes. However, the renewal of the sensing element after each measurement is difficult to adapt for in situmeasurements.
We report here on the optimization and evaluation of AdCSV on GIME for the direct quantification of the sufficiently labile Co and Ni dissolved species in aquatic systems mediated by a ligand on a time scale of less than 10 minutes. The methodology was first characterized and validated in laboratory. The optimized protocol was then applied on a GIME incorporated in a submersible probe to in situ determine the sufficiently labile Co and Ni dissolved species in both fresh and sea waters. In addition, samples were collected to analyze the total acid-leachable (unfiltered samples), the total dissolved (0.2 µm filtered samples) and the “truly” dissolved (0.02 µm filtered samples) fractions. The combination of these four fractions allowed to better assess the Co and Ni speciation in the environment, something never achieved before. Selected examples of analytical optimization results in the laboratory and of field outcome will be presented.