Lead and copper speciation in remote mountain lakes
Limnol. Oceanogr., 50(3), 2005, 995-1010 | DOI: 10.4319/lo.2005.50.3.0995
ABSTRACT: We determined the chemical speciation of lead and copper in remote mountain and high-latitude lakes at different times of the year, providing background data for regions in Europe least affected by anthropogenic effects. The lakes are characterized by low ionic strength, clear waters, and oligotrophic conditions; are fed predominantly by atmospheric precipitation; and are ice-covered during a large part of the year. Lead- and copper-complexing ligands were determined by cathodic stripping voltammetry with ligand competition using Calcein blue and salicylaldoxime, respectively. Water column averages of the dissolved lead concentrations ranged from 0.2 to 0.9 nmol L-1, with generally lowest levels during the prolonged winter ice cover, whereas dissolved copper concentrations, varying from 1 to 1.7 (±0.1) nmol L-1, showed no significant seasonal variations. The lead speciation was controlled by complexation with ligands at concentrations mostly below 2 nmol L-1, with values for the stability constant (log K'PbL) of 12.5-13.7; calculated 9PbL ionic lead concentrations (-log[Pb2+] values) were 11.9-14.5. Copper complexation was controlled by ligands at concentrations of 12- 21 nmol L-1, with values for log K'CuL of 13.5-14.0 and -log[Cu2+] values of 14.4-15.1. The concentration of the copper-binding ligands, but not those for lead, varied seasonally, with about 50% higher concentrations during open water conditions compared to periods of ice cover. The data were consistent with the presence of only one class of ligands for copper and lead. The stability constant of the ligands is similar to that of fulvic acid; however, evidence regarding the actual nature of the ligands is still lacking. The lake data show that (1) competition between calcium and lead causes a reduction in the stability constant of approximately one log-unit for each order of magnitude of Ca2+, and (2) lead scavenging in the lakes is moderated by complexation.