A reliable in-situ probe for dynamic metal speciation in natural waters has been a dream for all scientists that study trace metals in the environment. The objective of this project is to advance toward that goal by developing a better in-situ measurement probe based in the recent advances of electroanalytical techniques.
The difficulties connected with development of these probes arise first from the very low concentrations of the trace metals in natural waters (typically below 10-8M); second from the need of bringing the equipment to the field (autonomy problems), third the problems posed by the myriad ligands existing in the natural waters that are able to bind metals (speciation problems) and last but by no means the least the fact that natural waters are generally subject to changing conditions and are practically never at chemical equilibrium (dynamic problems).
In natural waters only a small portion of the total dissolved metal exists as free hydrated cations because metal ions form stable complexes with a large variety of dissolved inorganic and organic ligands and also adsorbs onto colloids and suspended matter and can interact with micro-organisms. Most of these show a polyfunctional and a polyelectrolytic character and, thus, have a broad, net range of free energies of complex formation, formation/dissociation rate constants and sizes, hence controlling the bioavailability, toxicity and mobility of the metal ions.
As of recently it also become important to take into account the impact of anthropogenic stabilized nanoparticles. These are unusually stable as compared with natural particles of similar size thus adding a new level of complexity to this already complex mixture.
A fundamental aspect that arise from the system not being in chemical equilibrium is that a correct interpretation of the fate and environmental impact of metal complexes must consider the importance of the reactivity and fluxes of the metal compounds and the relative time scales of these processes (dynamic speciation).
Due to their environmental relevance lead, cadmium, copper, zinc and silver will be the main trace metals focused in our study. Our approach to develop a better in-situ dynamic sensor is to use the potential of recently developed stripping electroanalytical techniques, Scanned stripping chronopotentiometry (SSCP) and Absence of gradients and nernstian equilibrium stripping technique (AGNES).
Regarding autonomy problems to develop and in-situ probe electrochemical devices have the advantage of being easily available in small sizes and been battery operated. The drawback was the need to degas the sample (purging with nitrogen) to eliminate oxygen interferences. Nevertheless it has been recently shown by one our team members (Parat) that by using a thin mercury film the electrochemical experiments can be carried out without degassing thus making its in-situ deployment feasible.
To validate an analytical procedure it is necessary to compare its results with a well-established method. For this purpose we will include in our in-situ probe a Donnan Membrane device (DMT).
Bringing the probe to the sample is but the first step in a long way. As important as it is, it mean nothing if the signal obtained is too complex to be deciphered. It is necessary to understand the signal obtained and validate the analytical methods used. In complex matrices this implies a three way approach: first the analysis of increasingly more complex model systems in the laboratory, second the study of a pilot-scale environment spiked with the model systems and finally in-situ investigation of different field situations (usually different seasons).
One of the key aspects of this project is to assess if sample fractionation will be necessary to correctly interpret the sensor data. This aspect will be assessed in the initial pilot scale and field studies and if found necessary an in-situ fractionation device will be added to the probe.
To understand the results it is fundamental to apply models that can describe and predict the behavior of the system. Thermodynamic modeling will include the NICA-Donnan model (nica donnan) to account for metal ion organic ligand interactions and the CD-MUSIC model (cd music) to account for metal binding onto mineral surfaces. Dynamic modeling will also be applied to develop a flux transport model to discriminate which parameters control metal ion speciation in complex aquatic systems (fluxy or mhedin).