Trace metal complexation in natural waters

1. Trace metal complexation innatural waters • Dario Omanović, Petra Cmuk, Ivanka Pižeta • Center for Marine and Environmental Research, • Ruđer Bošković Institute,Croatia • Yoann Louis, Rudy Nicolau • Laboratoire PROTEE, Université de Toulon et du Var • BP 132, 83957 La Garde, France • Cedric Garnier • LPTC, Université Bordeaux I, 351 Crs. de la Libération, F-33405 Talence CEDEX, France - Pseudopolarography - Metal complexing capacity (MCC)

2. Distribution of trace metals Operationally defined: • Particulate - > 0.45 µm • Dissolved - < 0.45 µm • Colloidal - between 1 kD and 0.45 µm • Truly dissolved - < 1 kD

3. Distribution of trace metals Physico-chemical classification: • Inorganic complexes • Organic complexes • Associated to particles

4. Distribution of trace metals Methodological (electrochemical) classification: • Labile complexes • Mostly inorganic complexes (Cl-, OH-, SO42-, ...) • Fast dissociation rate • Mostly reducable • Inert complexes • Mostly organic complexes • Very stable – high stability constant • Only partly reducable

5. Electrochemical characteristics Inert complexes Labile complexes

6. Construction of pseudopolarogram Voltammograms Pseudopolarogram

7. Model titrations – one ligand

8. Model titrations – one ligand Experimental: CdCC = 0.970×10-7 M log Kapp = 8.37 Theoretical: CdCC = 1×10-7 M log Kapp = 8.44

9. Model titrations – two ligands Stability constants @ µ = 0.1 M: Log KCdNTA = 9.76 Log KCdEDTA = 16.4 Theoretically: ΔErev = 0.059*log K / n Exp. for CdNTA: ΔE = 0.320 V log K = 10.8

10. Model titrations – two ligands Eacc = -0.75 V CdCC = 0.973×10-7 M Eacc = -1.15 V CdCC = 0.503×10-7 M

11. Seawater sample – addition of NTA • CADMIUM • Fast complexation with NTA • Two separate peaks of labile Cd and CdNTA

12. Seawater sample – addition of EDTA • CADMIUM • Very slow complexation with EDTA (cca. 3 h) • Two separate peaks of labile Cd and CdEDTA

13. Seawater sample – “not clean” (Šibenik) Copper Eacc = -0.45 V No separated well defined waves CuCC = 4.2×10-8 M Log Kapp = 9.2

14. Seawater sample – “clean” (Zlarin) Copper Two well separated waves: - labile copper complexes @ E = -0.34 V - inert copper complexes@ E = -1.40 V

15. Experimental setup - parameters Copper - “clean“ seawater (Zlarin) Eacc = -1.6 V, tacc = 300 s Eacc = -0.45 V, tacc = 297 s and Eacc = -1.6 V, tacc = 3 s Eacc = -0.45 V, tacc = 300 s

16. Experimental setup - parameters Copper - “clean“ seawater (Zlarin)

17. Seawater sample – “clean” (Zlarin) Cadmium without well separated waves

18. Conclusion • Pseudopolarography is a tool for the characterisation of an interaction of trace metal ions in natural samples • It is the “fingerprint” of the sample • The position and the shape of the waves give us additional information about complexing ability of the particular natural sample • It is very useful in complexing capacity determination measurements • The composition of natural water samples is very complex and, unfortunatelly, it is very hard to obtain behaviours like in model solutions • Additional efforts should be done to resolve problems associated with the experimental setup as well as to interpret data regarding both pseudopolarograms and metal complexing capacity determination