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

Abstract

A suite of analytical techniques were combined to study the chemical speciation of Fe and Pb in the water column of a lake characterized by a biogenic meromixis (Paul Lake, MI). Depth profiles of Fe 2+ and “dissolved” Pb display significant concentration gradients below the chemocline, i.e., they increase from below detection limit to ca. 100 μM for Fe 2+ and 2 nM for Pb d. Significant correlations between particulate organic matter, hydrous iron oxides, and particulate Pb suggest that Pb is scavenged by Fe-rich particles formed at the oxic-anoxic transition. Transmission electron microscopy shows that particles of hydrous iron oxides form complex aggregates with natural organic matter at and below the oxic-anoxic transition. Experiments with 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, although the presence of neither FeS nor PbS was observed. This suggests that the solubilities of Fe and Pb are influenced by complexation. Voltammetric experiments on filtered samples show that Pb is weakly complexed in the mixolimnion and strongly complexed in the monimolimnion. A conditional stability constant for Pb complexation is determined using metal titration curves assuming a simple 1:1 stoichiometry and gives logK cond = 9.4 ± 0.8 M −1 in the monimolimnion. These speciation results are confirmed by ion exchange chromatography, which demonstrates that more than 98% of Pb is complexed by natural organic matter.