Seasonal changes of lead mobility in sediments in algae- and macrophyte-dominated zones of the lake

Abstract

This study examined lead (Pb) pollution in algae- and macrophytes-dominated sediments, using diffusive gradient in thin films (DGT) and dialysis (HR-Peeper) techniques. Lead pollution varied by season in the two different ecotype sediments. In the algae-dominated zone, the highest concentrations of DGT-labile Pb and dissolved Pb occurred in April and July, respectively. The reductive dissolution of Fe/Mn oxides was identified as an important driver for Pb releases in April and July. This was supported by the decrease of the reducible fraction of Pb in sediments during those sampling periods. Furthermore, dissolved organic matter (DOM) complexation with Pb in sediments also significantly increased the dissolved Pb concentrations in July. The Pb-DOM complexes accounted for 95% of the total chemical species of Pb in pore water, calculated by Visual MINTEQ 3.1 model. Low concentrations of labile and dissolved Pb were observed in October and January; these resulted from the formation of Pb-sulfide precipitates and adsorption by Fe/Mn oxides. It was supported by the high rate of Pb(HS)2 precipitation (saturation index > 0), at 36%, in October samples and the high reducible fraction of Pb in sediments in January samples. In the macrophytes-dominated region, there was a decrease of labile and dissolved Pb concentrations in April and July. It is likely because of the uptake of Pb by submerged macrophyte roots and the Fe/Mn plaques in the root surface. High concentrations of labile and dissolved Pb were observed in October and January, likely resulting from the DOM complexation with Pb in sediments. This was supported by the fact that the Pb-DOM complexes accounted for 90% and 87% of the total chemical species of Pb in October and January, respectively.