Pseudovoltammetry was used to evaluate the actual Pb complexation occurring in natural water samples of varying oxygen and sulfide concentration. In pseudovoltammetry, the potential at which metal−ligand complexes are broken up to form the metal amalgam is used to determine the complexes' thermodynamic stability constants (KTHERM; corrected for metal and ligand side reaction coefficients) via the Nernst expression. This methodology removes the need for any metal additions and for subsequent modeling using fitting criteria, which provide only conditional stability constant data (KCOND). Using known organic ligands, a chelate scale ranging from log KTHERM = 4 to log KTHERM = 20 was developed as a template for comparison with samples collected from two stations of different salinities and at several depths in the Chesapeake Bay. These samples were observed to contain up to five different ligand compounds of unknown structural composition (log KTHERM greater than 8) with the strongest ligand fraction exceeding log KTHERM greater than 39 (the maximum observable thermodynamic stability constant due to the reduction of Na+). One possible explanation for the observed complexation is the existence of lead sulfide clusters. This was supported by laboratory experiments using electrochemistry and ICRminusFTMS, which confirmed the formation of electrochemically inert multinuclear clusters with high stability constants (e.g., M3S3, log KTHERM = 62.9). However, in all field samples, (sub)nanomolar levels of acid-leachable sulfide were recovered at pH 5.0 to 6.2, which could be attributed to dissociation of lead sulfide complexes with moderate acidity. Recovery of sulfide increased from less than 10% of the total dissolved Pb concentration (Pbdiss) in surface waters to 100% of the Pbdiss in the sulfide-rich bottom waters at the higher salinity location.
