Oxidation kinetics of Fe(II) in a eutrophic Swiss lake

The rate of oxidation of ferrous iron was measured in samples from Lake Greifen, a eutrophic lake in Switzerland. Fe(ll) concentrations were followed using an automated flow injection analysis system employing luminol-based chemiluminescence detection of Fe(ll). For kinetic studies at pH > 7.8, the system was modified to allow a time resolution of less than 1 s. Oxidation rates were measured in unfiltered samples at 2-2000 nM Fe(ll). The pH was varied between 6.8 and 8.3 by bubbling with CO2 and synthetic air. Above pH 7.4, rates were consistent with the rate law determined in pure carbonate systems. Between pH 6.8 and pH 7.3, however, the apparent rate was independent of pH. This surprising finding may be explained by some naturally occurring (organic, colloidal, or surface) ligand(s) that accelerate the oxidation of Fe(II). The relative importance and pH dependence of the direct reaction with O-2 in comparison to that with H2O2 was determined, and the enhancement of the overall rate was attributed to the reaction of Fe(II) with oxygen.