Abiotic transformations of iron and phosphate in humic lake water revealed by double‐isotope labeling and gel filtration

Abstract

Abiotic transformations of Fe and PO 4 3− in humic water from Finnish forest lakes were studied by Sephadex G‐100 gel filtration following incubation of filtered (0.2 µ m) epilimnetic water samples to which both 55 FeCl 3 ·6H 2 O and 32 PO 4 3− had been added. The simultaneous movement of 55 Fe and 32 P to higher molecular‐weight fractions (10,000–20,000 MW) depended on the presence of dissolved humic substances (DHS). In the absence of DHS almost all 55 Fe, probably as inorganic hydrolyzed iron particles, sorbed to the Sephadex. The binding of 55 Fe by DHS was not affected by increasing the natural ionic strength ( I = ∼3 × 10 −4 M) by a factor of 10 2 . In the absence of quasi‐equilibrium between free and DHS‐bound 32 PO 4 3− , the DHS‐Fe‐PO 4 3− complex readily released 32 PO 4 3− ‐P. Compared to the 32 P binding, 55 Fe binding was rapid. More than 20% of the Fe had been bound after only 1 min, whereas for labeled PO 4 3− this percentage was reached only after 24 h.