The influence of humic substances on the molecular weight distributions of phosphate and iron in epilimnetic lake waters

1. The influence of humic substances on the association of free inorganic iron and phosphate with material of larger molecular weight was investigated in epilimnetic samples from two humus-rich lakes of contrasting ionic strength. After modification of the molecular weight distribution of the humic substances in the samples using dialysis and ultrafiltration, the molecular weight distribution of added radioisotopes (Fe-55(3+) and (PO43-)-P-32) was assessed using gel filtration chromatography. 2. The association of Fe-55(3+) and (PO43-)-P-32 With larger molecular weight fractions (>50000 and 10000-50000 Da) was not in general related to the quantity of humic substances of the same molecular weight in the samples. However, the proportions of Fe-55(3+) and (PO43-)-P-32 observed in higher molecular weight peaks were strongly correlated to the quantity of humic substances of the same molecular weight in (a) the 10000-50000 Da peak in the sample of low ionic strength at pH 5.5 and pH 7.0, and (b) the >50000 Da peak in the sample of higher ionic strength at pH 4.0. 3. It was concluded that humic substances promote the association of Fe-55(3+) and (PO43-)-P-32 With higher molecular weight fractions primarily by acting as peptizing agents for inorganic colloids containing Fe and P. Association of Fe-55(3+) and (PO43-)-P-32 With material of higher molecular weight via tho formation of humic substance-Fe3+-PO43- complexes was identified but only at specific pH and within specific molecular weight ranges for each of the epilimnetic lake water samples studied.