| Popis: |
Adsorption of Cd, Pb and Cu (pH 7) has been studied as a function of time during the slow oxidation (24 hr.) of ferrous iron naturally present in samples of anoxic lake water. A simple surface complexation model fits the Cd and Pb behaviour. Initial uptake of these metals is fairly evenly distributed across all size fractions of the precipitate. There appears, though, to be a transfer of Pb, and to a lesser extent Cd, from the smaller to the larger-size fractions during the course of the precipitation. This is explained in terms of slow desorption kinetics coupled with the slow aggregation of the precipitate. Cu behaves unexpectedly when spiked at a low concentration at the beginning of the experiment. An initial high uptake is focussed on the small-size fraction (0.1–0.4 μm). This is followed by a steady desorption from this size fraction. Three hypotheses to account for this behaviour are explored, but none provides a wholly satisfactory answer. |
