Physico-chemical Speciation of Lead in South San Francisco Bay

Abstract

The speciation of lead at a site in the South San Francisco Bay was determined using a combination of physical size fractionation and electrochemical analyses. The ‘total dissolvable’ Pb was 8·1nM from analysis of an acidified unfiltered sample. The ‘dissolved’ Pb was equal to 0·20nM (41ng l−1), only 2·5% of the ‘total dissolvable’ Pb. The difference yielded the ‘particulate’ Pb equal to 7·9nM (1·6μg l−1). Results from crossflow ultrafiltration indicated that almost all (0·19nM) of the dissolved Pb was ‘in solution’ [<10K nominal molecular weight (MW)] and that colloidal Pb (10K MW to 0·2μm)accounted for onlyc.1% of the dissolved Pb at this station. This small concentration (0·01nM) of colloidal Pb may be attributed to the low amount of organic carbon associated with colloid size fraction as determined by dissolved organic carbon analyses.The chemical speciation of lead was determined in the dissolved sample and ultrafiltered sub-sample. Differential pulse anodic stripping voltammetry (DPASV) on a thin mercury film (TMF) rotating glassy carbon disk electrode (RGCDE) was used to distinguish the kinetically labile inorganic species (Pb′) from the Pb-chelated by organic ligands (PbLi). Lead titration results were similar for both samples revealing that Pb′, PbLiand 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 influenced by two classes of Pb-binding ligands. In the dissolved sample, the concentration of the stronger class of ligands was [L1]=0·89±0·35nM, with a conditional stability constant ofKcondL1,Pb′=3±1×1010M−1. The weaker class was [L2]=12·8±1·9nM, withKcondL1,Pb′=4±1×108 M−1. The presence of these ligands, in excess of the dissolved Pb, resulted in [Pb′]=7±2pM and [Pb2+]=0·3pM (62pg 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]=0·15nM and [PbL2]=0·03nM. More significantly, there were large concentrations of unchelated Pb-binding ligands, (Li′), available to buffer the free Pb2+concentration in the event of perturbations in dissolved Pb.