SORPTION OF TRACE-METALS (CU, PB, ZN) BY SUSPENDED LAKE PARTICLES IN ARTIFICIAL (0.005 M NANO3) AND NATURAL (ESTHWAITE WATER) FRESH-WATERS

The sorption of Cu, Pb, and Zn onto natural lake particles, suspended in 0.005 M NaNO3 solution and in a natural lake water (Esthwaite Water , Cumbria, UK), was studied as a function of pH and time in a series o f laboratory experiments, under environmentally realistic conditions. The sorption of all three metals increased with increasing pH and reac tion time (2 h and 7 days). In 0.005 M NaNO3 solution, the well-define d sorption edges spanned 2-2.5 pH units for Cu and Pb, and approximate to 4 pH units for Zn. In the natural lake water, the Cu sorption edge was broader and both Cu and Zn were less strongly sorbed. The binding stability decreased in the order Pb > Cu > Zn. Competitive adsorption onto surface sites appeared to be the main factor determining the obs erved sorption behaviour. Application of a macroscopic metal exchange model to the 7 day NaNO3 results enabled the surface site concentratio n to be estimated as 0.79 +/- 0.07 mmol g(-1). The modelling exercise suggested that an observed shift in the sorption edge of Zn, in the pr esence of Pb and Cu, was due to competition for surface sites. The exp erimental data are in good general agreement with field observations o f trace metal behaviour in Cumbrian lakes. The almost total sorption o f Pb by lake particles throughout the in-situ pH range is compatible w ith previous field measurements including trace metal budgets and resi dence times. Dissolved Zn concentrations in the lake are lower than pr edicted by the sorption experiments, but the lower lake concentrations are consistent with the previously observed scavenging of Zn by plank tonic algae. Both the decreased sorption of Cu in the experiments with natural lake water, compared to that in NaNO3 solution, and the relat ively small-scale removal of dissolved Cu by particles in the lake its elf can partially be explained by humic complexation.