Complexation of cadmium by natural organic ligands in the central North Pacific

The complexation of Cd by natural organic ligands in the upper 600 m of the central North Pacific was determined with differential pulse anodic stripping voltametry at a thin mercury film, rotating glassy-carbon disk electrode. About 70 % of the dissolved Cd in surface waters was bound in strong complexes (K’,,,,‘,, c.,2, = 10’2.0 M-‘) by relatively Cd-specific organic ligands existing at low concentrations (0.1 nM). This ligand class is only observed within the surface 175 m and exhibits a concentration maximum at depths between 40 and 100 m. Due to the complexation of dissolved Cd in surface waters by this class of organic ligands, together with the vertical distribution of total dissolved Cd (ranging from 2 to 800 PM), the concentration of inorganic forms of Cd varies from o-O.7 pM in surface waters to 800 pM at 600 m. The concentration of free Cd ion ranges from 20 fM (- lo- 13.7 M) in surface waters to 22 pM at 600 m-a 1,OOO-fold variation! Although complexation with inorganic chloride ligands dominates the speciation of Cd in inter-mediate and deep waters, these results demonstrate that complexation with natural organic ligands is important in influencing Cd speciation in oceanic surface waters. Dissolved Cd exhibits a pronounced, nu