Speciation, reactivity, and cycling of Fe and Pb in a meromictic lake

A suite of analytical techniques were combined to study the chemical specia tion of Fe and Pb in the water column of a lake characterized by a biogenic meromixis (Paul Lake, MI). Depth profiles of Fe2+ and "dissolved" Pb displ ay significant concentration gradients below the chemocline, i.e., they inc rease from below detection limit to ca. 100 mu M for Fe2+ and 2 nM for Pb,. Significant correlations between particulate organic matter, hydrous iron oxides, and particulate Pb suggest that Pb is scavenged by Fe-rich particle s formed at the oxic-anoxic transition. Transmission electron microscopy sh ows that particles of hydrous iron oxides form complex aggregates with natu ral organic matter at and below the oxic-anoxic transition. Experiments wit h batch reactors show that these organo-mineral moieties remove Pb rapidly during their formation. Thermodynamic calculations predict that FeS and PbS are respectively saturated and oversaturated in the monimolimnion, althoug h the presence of neither FeS nor PbS was observed. This suggests that the solubilities of Fe and Pb are influenced by complexation. Voltammetric expe riments on filtered samples show that Pb is weakly complexed in the mixolim nion and strongly complexed in the monimolimnion. A conditional stability c onstant for Pb complexation is determined using metal titration curves assu ming a simple 1:1 stoichiometry and gives logK(cond) = 9.4 +/- 0.8 M-1 in t he monimolimnion. These speciation results are confirmed by ion exchange ch romatography: which demonstrates that more than 98% of Pb is complexed by n atural organic matter. Copyright (C) 2000 Elsevier Science Ltd.