PHYSICOCHEMICAL SPECIATION OF LEAD IN SOUTH SAN-FRANCISCO BAY

The speciation of lead at a site in the South San Francisco Bay was de termined using a combination of physical size fractionation and electr ochemical analyses. The total dissolvable' Pb was 8.1 nM from analysis of an acidified unfiltered sample. The 'dissolved' Pb was equal to 0. 20 nM (41 ng l(-1)), only 2.5% of the 'total dissolvable' Pb. The diff erence yielded the 'particulate' Pb equal to 7.9 nM (1.6 mu g l(-1)). Results from crossflow ultrafiltration indicated that almost all (0.19 nM) of the dissolved Pb was 'in solution' [<10K nominal molecular wei ght (MW)] and that colloidal Pb (10K MW to 0.2 mu m) accounted for onl y c. 1% of the dissolved Pb at this station. This small concentration (0.01 nM) of colloidal Pb may be attributed to the low amount of organ ic carbon associated with colloid size fraction as determined by disso lved organic carbon analyses. The chemical speciation of lead was dete rmined in the dissolved sample and ultrafiltered sub-sample. Different ial pulse anodic stripping voltammetry (DPASV) on a thin mercury film (TMF) rotating glassy carbon disk electrode (RGCDE) was used to distin guish the kinetically labile inorganic species (Pb') from the Pb-chela ted by organic ligands (PbLi). Lead tetration results were similar for both samples revealing that Pb', PbLi and excess unbound ligands were present primarily in the ultrafiltrate, rather than in the colloidal phase. The titration data can be interpreted as dissolved Pb being inf luenced by two classes of Pb-binding ligands. In the dissolved sample, the concentration of the stronger class of ligands was [L-1] = 0.89 /- 0.35 nM, with a conditional stability constant K-L1,Pb'(cond) = 3 /- 1 x 10(10) M-1. The weaker class was [L-2] = 12.8 +/- 1.9 nM, with K-L1,Pb'(cond) = 4 +/- 1 x 10(8) M-1. The presence of these ligands, i n excess of the dissolved Pb, resulted in [Pb'] = 7 +/- 2 pM and [Pb2] = 0.3 pM (62 pg l(-1)). While less than 2.4% of the ambient Pb was ' in solution', it existed chiefly in the form of organic complexes with [PbL1] = 0.5 nM and [PbL2] = 0.03 nM. More significantly, there were large concentrations of unchelated Pb-binding ligands (L-i'), availabl e to buffer the free Pb2+ concentration in the event of perturbations in dissolved Pb. (C) 1997 Academic Press Limited.