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Article

Pseudopolarographic Determination of Cd²⁺ Complexation in Freshwater

Supporting Info

Jeffrey J. Tsang,* Tim F. Rozan, Heileen Hsu-Kim,^† Katherine M. Mullaugh, and George W. Luther, III*

University of Delaware, College of Marine Studies, 700 Pilottown Road, Lewes, Delaware 19958, U.S.

Environ. Sci. Technol., 2006, 40 (17), pp 5388–5394

DOI: 10.1021/es0525509

Publication Date (Web): August 5, 2006

Address correspondence to either author. Phone: +1 (302) 645-4208 (G.W.L.); +1 (302) 645-4257 (J.J.T.). Fax: +1 (302) 645-4007. E-mail: luther@udel.edu (G.W.L.); jtsang@udel.edu (J.J.T.).

†

Current address: Duke University, Department of Civil and Environmental Engineering, Box 90287, Durham, NC 27708.

Abstract

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 Cd²⁺-organic complexes were detected at each site. These unknown Cd²⁺ complexes were characterized by their half-wave potential (E_1/2). The E_1/2 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).