Trace metal complexation in natural waters

1
Trace metal complexation innatural waters

• - Pseudopolarography
• - Metal complexing capacity (MCC)

• Dario Omanovic, Petra Cmuk, Ivanka Pižeta
• Center for Marine and Environmental Research,
• Ruder Boškovic 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

2
Distribution of trace metals

• Operationally defined
• Particulate - gt 0.45 µm
• Dissolved - lt 0.45 µm
• Colloidal - between 1 kD and 0.45 µm
• Truly dissolved - lt 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 0.97010-7 M log Kapp
8.37 Theoretical CdCC 110-7 M log Kapp
8.44
9
Model titrations two ligands
Stability constants _at_ µ 0.1 M Log K CdNTA
9.76 Log K CdEDTA 16.4
Theoretically ?Erev 0.059log K / n Exp. for
CdNTA ?E 0.320 V log K 10.8
10
Model titrations two ligands
Eacc -0.75 V CdCC 0.97310-7 M
Eacc -1.15 V CdCC 0.50310-7 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 4.210-8 M Log Kapp 9.2
14
Seawater sample clean (Zlarin)
Copper
Two well separated waves - labile copper
complexes _at_ E -0.34 V - inert copper
complexes _at_ 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