Stability of Organic Iron Complexes in Dajiuhu Peats and Its Ecological Significance

In order to study the complexing stability of organic iron complexes from peat, the stability of DOM-Fe²⁺, Fe³⁺ in different molecular weights of Dajiuhu peatland was determined by potentiometric titration and fluorescence quenching titration. There are obvious differences between the results of potentiometric titration (4.0-6.1) and fluorescence quenching titration (1.5-4.1), which can be explained by the deprotonation of OH in high pH value and the oxidation of Fe²⁺ during the experiments. Potentiometric titration is suitable for studying the complexing stability of different molecular weights of DOM-Fe; the pH would not change during the experiment using fluorescence quenching titration method. Thus, the fluorescence method can be well applied to the element of variable valency. The results show that the complexing stability constants of DOM-Fe³⁺ is higher than that of Fe²⁺, and that of low-molecular-weight DOM with Fe²⁺ and Fe³⁺ is higher. The experiment indicates that high stability of DOM-Fe in peat, and relatively low-molecular-weight DOM has stronger ability to complex iron. Even Fe²⁺ is oxidized to Fe³⁺, it can still complex with DOM stably, which is conducive to the output of terrestrial dissolved iron to aquatic ecosystem. The complexing stability of iron organic complexes from peat will also affect the bioavailability of iron.