Isotopically exchangeable phosphate in freshwater sediments: Effects of u.v.-irradiation,formaldehyde, solid/solution ratio,and pH on its experimental determination

The influence of bioactivities, solid/solution ratio and the pH on the isotopic exchangeability of phosphate in a freshwater sediment was investigated. From the comparison of the results obtained for the same sample in the presence or absence of formaldehyde, it is concluded that microorganisms can effect the analysis for isotopically exchangeable phosphate. Irradiation with u.v.-light caused a sharp rise in isotopic exchangeability. In the pH-range 6.6–8.4 isotopic exchangeability of phosphate increases with decreasing pH-value which is attributed to an easier exchange of H₂PO₄⁻ than of HPO₄²⁻. The influence of the solid/solution ratio on the isotopic exchangeability of phosphate in the solid phase is small or nil. However, because of the relatively large amount of phosphate that goes into solution, the total isotopic exchangeability E_i of phosphate in the solid and liquid phase together is strongly increased at a low solid/solution ratio. From these results it is concluded that in order to make a meaningful comparison of isotopically exchangeable phosphate in different soils or sediments it is essential to work at a nearly constant pH and solid/solution ratio. Such a comparison was made for 26 freshwater sediments from the Rhine/Meuse delta, in the presence and absence of 0.17 mol l⁻¹ formaldehyde as a biological inhibitor and it is concluded that addition of the latter is essential. The lowest total isotopic exchangeabilities of phosphate, (15–25) were measured in the sediments collected from the Haringvliet, whereas higher values (40–80) were found in the sediments from the Brielse Meer and the Grote Rug. This could well be indicative of a similar variance in the biological availability of the phosphate in the investigated sediments.