Speciation of dissolved copper and nickel in South San Francisco Bay: a multi-method approach

Abstract

A multi-method approach was used to investigate the chemical speciation of dissolved copper and nickel in South San Francisco Bay. Dissolved copper speciation was determined by four different analytical approaches: competitive ligand equilibration-cathodic stripping voltammetry [CLE-CSV], differential pulse anodic stripping voltammetry using a thin mercury film rotating glassy carbon disk electrode [DPASV(TMF-RGCDE)], DPASV using a hanging mercury drop electrode [DPASV(HMDE)], and chelating resin column partitioning-graphite furnace atomic absorption spectrometry [CRCP-GFAAS]. Dissolved nickel speciation was determined by CLE-CSV and CRCP-GFAAS. Each of the methods employed provided useful insight into the chemical speciation of dissolved copper and nickel. CLE-CSV provided the best characterization of the stronger (but less predominant) of the two organic copper-complexing ligands detected, while DPASV(TMF-RGCDE) provided the truest measurement of inorganic copper, and therefore, the best characterization of the weaker organic copper-complexing ligand which exerted the strongest influence on dissolved copper speciation. These methods provide complementary results which can be combined to provide a more complete understanding of the chemical speciation of copper. DPASV(HMDE) and CRCP-GFAAS both provide an operational measurement of labile copper which includes some fraction of labile organic complexes in addition to inorganic copper species. CLE-CSV and CRCP-GFAAS both yielded similar results for nickel speciation indicating the presence of a single class of extremely strong nickel-complexing ligands. The dissolved copper in South San Francisco Bay is found to exist predominantly as organic complexes (80–92%). About 27% of dissolved copper was complexed with the stronger (L1; [CuL1 ≈ 13 nM, log K′CuL1 > 13.5) of the copper-complexing ligands detected, while copper complexed with the weaker ligand L2 comprised the greatest fraction of total dissolved copper species (52–65%; [CuL2] = 20–30 nM; log K′CuL2 = 9.0–9.6). A third to a half of total dissolved nickel was complexed by one, extremely strong class of organic ligands ([NiL] = 17–28 nM; log K′NiL > 17); the remaining total dissolved nickel existed as inorganic or labile forms.