Determination of Pb complexation in oxic and sulfidic waters using pseudovoltammetry

Tim F Rozan¹, George W Luther, Doug Ridge

¹College of Marine Studies, University of Delaware, 700 Pilottown Road, Lewes, Delaware 19958, USA.

|September 12, 2003

Summary

Pseudovoltammetry revealed strong lead (Pb) complexation in Chesapeake Bay waters, identifying unknown ligands and lead sulfide clusters. Sulfide recovery increased with salinity, indicating Pb-sulfide complex dissociation in acidic conditions.

Area of Science:

Environmental Chemistry
Analytical Chemistry
Geochemistry

Background:

Lead (Pb) complexation in natural waters influences its bioavailability and toxicity.
Traditional methods often yield conditional stability constants (KCOND), limiting understanding of true thermodynamic stability (KTHERM).
Pseudovoltammetry offers a direct method to determine KTHERM without metal additions or complex modeling.

Purpose of the Study:

To evaluate actual Pb complexation in natural waters using pseudovoltammetry.
To determine the thermodynamic stability constants (KTHERM) of Pb complexes.
To investigate the role of oxygen and sulfide concentrations on Pb complexation.

Main Methods:

Pseudovoltammetry was employed to measure Pb complexation.
A chelate scale using known organic ligands was developed for comparison.
Samples from the Chesapeake Bay at varying salinities and depths were analyzed.
Laboratory experiments with electrochemistry and ICR-FTMS confirmed lead sulfide cluster formation.

Main Results:

Up to five unknown Pb-ligand compounds with log KTHERM > 8 were detected.
The strongest ligand fraction exceeded log KTHERM > 39.
Lead sulfide clusters (e.g., M3S3, log KTHERM = 62.9) were confirmed as electrochemically inert.
(Sub)nanomolar levels of acid-leachable sulfide were recovered, increasing with salinity.

Conclusions:

Pseudovoltammetry successfully determined high KTHERM values for Pb complexes in natural waters.
Lead sulfide clusters are a plausible explanation for observed strong complexation.
Sulfide recovery indicates dissociation of Pb-sulfide complexes under acidic conditions, with increased dissociation at higher salinities.

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Determination of Pb complexation in oxic and sulfidic waters using pseudovoltammetry

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Determination of Pb complexation in oxic and sulfidic waters using pseudovoltammetry

Area of Science:

Background:

Purpose of the Study:

Main Methods:

Main Results:

Conclusions:

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