EFFECTS OF DISSOLVED HUMIC SUBSTANCES ON THE SPECIATION OF IRON AND PHOSPHATE AT DIFFERENT PH AND IONIC-STRENGTH

The effects of dissolved humic substances (DHS) on the speciation of iron and phosphate at different pH in waters of contrasting ionic strength and DHS content were studied by Sephadex gel filtration following incubation of filtered (0.2 mum) epilimnetic lake water samples to which both (FeCl3.6H2O)-Fe-55 and (PO43-)-P-32 had been added. The simultaneous movement of Fe-55 and P-32 to fractions of greater apparent molecular size depended on the presence of DHS and diminished in response to decreasing pH. In the absence of significant quantities of DHS, much of the added 55Fe precipitated from the incubation mixture or was retained within the gel column. (PO43-)-P-32 added to clear water of high ionic strength underwent minor transformations to large apparent molecular size, while the (PO43-)-P-32 recovered in eluates from experiments with clear waters of low ionic strength was dependent on pH and did not undergo transformation, the remainder being coprecipitated with 55Fe and retained within the gel column.