Pseudopolarographic determination of Cd2+ complexation in freshwater.

Pseudopolarography was used to detect Cd2+ complexes in samples collected at several locations along the Potomac River in June and September, 2004. Irrespective of site and sampling time, no weak inorganic Cd2+ species were present. However, up to two stable Cd(2+)-organic complexes were detected at each site. These unknown Cd2+ complexes were characterized by their half-wave potential (E1/2). The E1/2 values indicated certain Cd2+ complexes were common at different sites during each sampling but different complexes were observed in June and September. A Cd2+ chelate scale, generated from model ligands, was used to estimate the thermodynamic stability constants (K(THERM)) of the unknown complexes, which ranged from log K(THERM) = 21.5-32.0. Pseudopolarography did not recover all Cd2+ in the samples. This was partly attributed to highly stable Cd-sulfide species; owing to the presence of acid volatile sulfide at concentrations greater than total dissolved Cd2+. These electrochemically inert species may be multinuclear Cd-sulfide clusters and/ or nanoparticles with K(THERM) values that exceed the detection window of pseudopolarography (log K(THERM) > 34.4).

Determination of Pb complexation in oxic and sulfidic waters using pseudovoltammetry.

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 > 8) with the strongest ligand fraction exceeding log KTHERM > 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 ICR-FTMS, 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 5.0-6.2, which could be attributed to dissociation of lead sulfide complexes with moderate acidity. Recovery of sulfide increased from < 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.

Aqueous copper sulfide clusters as intermediates during copper sulfide formation.

Using a combination of experimental techniques, we show that Cu(II) reduction by sulfide to Cu(I) occurs in solution prior to precipitation. EPR and 63Cu NMR data show that reduction to Cu(l) occurs during the reaction of equimolar amounts of Cu(II) with sulfide. 63Cu solution NMR data show that Cu(I) is soluble when bound to sulfide and is in a site of high symmetry. EPR data confirm that Cu(I) forms in solution and that the mineral covellite, CuS, contains only Cu(I). Mass spectrometry data from covellite as well as laboratory prepared solid and solution CuS materials indicate that Cu3S3 six-membered rings form in solution. These trinuclear Cu rings are the basic building blocks for aqueous CuS molecular clusters, which lead to CuS precipitation. In controlled titration experiments where sulfide is slowly added to Cu(II), Cu3S3 rings and tetranuclear Cu molecular clusters (Cu4S5, and Cu4S6) form; the rings are composed primarily of Cu(II). During cluster formation from Cu3S3 condensation, some Cu(II) is released back into solution, indicating that Cu(II) reduction does not occur until after Cu-S bond and higher order cluster formation. Analysis of the frontier molecular orbitals for Cu(II) and sulfide indicate that an outer-sphere electron transfer is symmetry forbidden. These results are consistent with the formation of CuS bonds prior to electron transfer, which occurs via an inner-sphere process.