(Contact: M. Tercier-Waeber, E. Bakker and V. Slaveykova)

Background

Measurements of relevant fractions of trace metals in natural waters are essential to improve our understanding of their behavior and long-term impact [1,2]. The individual physico-chemical forms of the metal, in natural waters, may include particulate, colloidal and dissolved species such as simple inorganic and organic complexes [3]. For many aquatic organisms, the free hydrated ions of Cd(II), Cu(II) and Pb(II) and their mobile and labile complexes are the bioavailable and toxic species [4]. The determination of free metal ions is regarded as crucial to assess the capability or limitation of the free-ion activity model (FIAM) [4] and the biotic ligand model (BLM) [4-6] conventionally used to predict toxicological impact of trace metals in environmental systems. To achieve this, the traditional monitoring approaches, based upon discrete sampling followed by laboratory separation and analysis, are inadequate due to the numerous possible artifacts that may occur during sampling and sample handling. Sample perturbations include contaminations of trace metals or their losses by adsorption onto the walls of containers, but also, for samples collected from depths, speciation changes due to variations in temperature, pressure, CO2 , O2, and/or H2S content, and consequently in pH, redox potential, and solubility of solids known to adsorb trace metals. The use of expensive clean procedure and facilities (such as clean rooms) minimize the problem of contaminations but are ineffective to solve the problem of chemical species instability. There is thus an urgent need of field-deployable tools allowing reliable and sensitive (down to the pM level) in situ detection of the free metal ions.

Goal

The goal of the project is to further characterize, first in well controlled synthetic solution then in natural samples, two analytical devices developed in our laboratory to allow in situ monitoring of the free metal ion concentrations:

• a Complexing Gel Integrated Microelectrode (CGIME)
• a Hollow Fiber Permeation Liquid Membrane Device (HFPLM).

The CGIME is prepared by the successive deposition of micro-layers of a chelating resin, an antifouling agarose gel and Hg on an 100-interconnected Ir-based microelectrode array. The measurement principle of this sensor is based on: accumulation of trace metals on the chelating resin in proportion to their free ion concentration in solution during equilibration of the sensor with the natural samples; release of the chemically pre-concentrated metal in acidic solution and simultaneous detection using Square Wave Anodic Stripping Voltammetry (SWASV).

HFPLM is a separation and pre-concentration technique. It is based on a water immiscible organic liquid containing a carrier selective for the species of interest immobilized on a microporous support separating two aqueous solutions, viz. the source (sample) solution containing the metal ions to be transported and the strip solution (into which the transported metals are trapped and pre-concentrated). Me ions accumulated in the strip solution are analyzed either in-situ by electrochemical techniques or ex-situ by atomic spectrometry or ion chromatography.

Project activities

The student will choose model metal-ligand chelating systems that are suitable for the present study. Target metal ions include copper(II) and lead(II) because of their environmental relevance and the possibility of extracting these ions selectively into an organic membrane phase, which is important for their recognition with potentiometric sensors and the HFPLM technique. The aqueous metal ligand binding systems should be well understood in terms of their binding equilibrium and kinetic (speed of complexation/decomplexation) characteristics.

Potentiometric membrane electrodes for the chosen target metal ion will be fabricated and characterised, and subsequently used to directly detect the uncomplexed fraction of the metal ion in solutions containing environmentally relevant ligand species. This data will be used to correlate to the performance of the CGIME and/or HFPLM techniques in controlled laboratory experiments. If interested, the student will have an opportunity to describe the fundamental working properties of these two techniques by finite element numerical modeling and to evaluate the working limits of these techniques. Once the working characteristics are sufficiently characterized in synthetic solutions, the student will applied the CGIME and/or HFPLM techniques in freshwater sample analysis performed:

• first in laboratory in microcosms under various controlled conditions; spiking of the samples with known concentrations of ligands and metal ions will be performed to strengthen the data set in order to fully establish the methodology
• than on board and/or in situ during field campaign(s) in Lake Leman, with potentiometric and in-situ GIME stripping voltammetric sensor probe [1] as the reference methods.

The goal of this evaluation is to confirm that the CGIME and HFPLM techniques accurately trace the free metal ion concentrations in the natural sample.

Number of students and formation

• 1 or 2 students characterizing one technique or both in parallel
• Chemist, biochemist, environmental scientist

Supervisors

Marylou Tercier-Waeber and Eric Bakker. Chemical sensor group, Département de Chimie Minérale, Analytique et Appliquée Vera Slaveykova. Aquatic Biogeochemistry and Ecotoxicology Group, Insitute Forel

Place of work

Département de Chimie Minérale, Analytique et Appliquée, Sciences II, 30 Quai E.-Ansermet, Genève. Institut F.-A. Forel, Versoix.

References

1. M.-L. Tercier-Waeber, M. Taillefert. Remote in situ voltammetric techniques to characterize the biogeochemical cycling of trace metals in aquatic systems. (Invited review).Journal of Environmental Monitoring, 10 (2008) 30-54.
2. K. Wilkinson and J. Buffle, in Physicochemical Kinetics and Transport at Biointerfaces, (Eds.: H.P. van Leeuwen, W. Köster), IUPAC Series on Analytical and Physical Chemistry of Environmental Systems, John Wiley, Chichester 2004, pp. 445-533.
3. P.R. Paquin, J.W. Gorsuch, S. Apte, G.E. Batley, K.C. Bowles, P.G.C. Campbell, C.G. Delos, D.M. Di Toro, R.L. Dwyer, F. Galvez, R.W. Gensemer, G.G. Goss, C. Hogstrand, C.R. Janssen, J.C. McGeer, R.B. Naddy, R.C. Playle, R.C. Santore, U. Schneider, W.A. Stubblefield, C.M. Wood, K.B. Wu, Comp. Biochem. Physiol. C 2002, 133, 3-35.
4. S. Noël, M.-L. Tercier-Waeber, L. Lin, J. Buffle, O. Guenat, M. Koudelka-Hep. Integrated micro-analytical system for simultaneous voltammetric measurements of free metal ion concentrations in natural waters. Electroanalysis, 18 (2006) 2061-2069.
5. M.-L. Tercier-Waeber, F. Confalonieri, G. Riccardi, A. Sina, S. Noel, J. Buffle, F. Graziottin. Multi Physical-Chemical Profiler for real-time in situ monitoring of trace metal speciation and master variables: development, validation and field application. Mar. Chem., 97 (2005) 216-235.
6. N. Parthasarathy, M. Pelletier, J. Buffle, Hollow fiber based supported liquid membrane: a novel analytical system for trace metal analysis. Anal. Chim. Acta 1997, 350, 183-195.
7. E. Bakker, E. Pretsch. Potentiometry at trace levels. TrAC 20 (2001) 11-19.