The role of the ironhydroxide-phosphate-sulphide system in the phosphate exchange between sediments and overlying water

The accumulation of inorganic phosphate in lake sediments and a possible following release is due to the adsorption of phosphate onto Fe(OOH) and, especially in hard waters, to the precipitation of apatite. Attempts are made to quantify both processes. For the quantification of the solubility of calcium-bound phosphate the solubility product of apatite being 10 super(-50), as found in the two hard waters rivers Rhine and Rhone, was used. With this solubility product the solubility of o-P can be calculated as function of the Ca super(2+) concentration and the pH. The two equations, for adsorption and precipitation, are put together in a so-called solubility diagramme, which describes the o-P concentration as function of the Fe(OOH) concentration in the sediments, and the pH and the Ca super(2+) concentration in the overlying water. The release of phosphate from the Fe(OOH) approximately P complex under anoxic conditions after adding H sub(2)S inorganic suspensions was shown to be limited. Only when a large excess of H sub(2)S was added there was some release, but if less than 75% of the Fe(OOH) was converted into FeS, there was no release. The possibility of organic phosphate as the source of phosphate release under anoxic conditions is discussed. For a full understanding of this possibility, fractionation of sediment bound phosphate must be carried out in such a way, that these organic phosphates are not hydrolysed.