THE ROLE OF THE IRONHYDROXIDE-PHOSPHATE-SULFIDE SYSTEM IN THE PHOSPHATE EXCHANGE BETWEEN SEDIMENTS AND OVERLYING WATER

The accumulation of inorganic phosphate in lake sediments and a possib le following release is due to the adsorption of phosphate onto Fe(OOH ) and, especially in hard waters, to the precipitation of apatite. Att empts are made to quantify both processes. For the quantification of t he P adsorbed, P(ads), onto Fe(OOH) the Freundlich adsorption isotherm , P(ads) = A(o-P)B, gave good results. The constants A and B could be quantified. Constant A appeared to depend on the pH and the Ca2+ and M g2+ concentrations in the water. Constant B appeared to approach 0.333 . The full equation becomes then: P(ads)=23600.(10(-0.4)pH)(2.77-1.77; e-Ca)3square-root o - P, with the Ca concentration in mmol 1(-1) and the o-P and P(ads) concentrations in mg 1(-1). For the quantification of the solubility of calcium-bound phosphate the solubility product of apatite being 10(-50), as found in the two hard water rivers Rhine an d Rhone, was used. With this solubility product the solubility of o-P can be calculated as function of the Ca2+ concentration and the pH. Th e two equations, for adsorption and precipitation, are put together in a so-called solubility diagramme, which describes the o-P concentrati on as function of the Fe(OOH) concentration in the sediments, and the pH and Ca2+ concentration in the overlying water. The release of phosp hate from the Fe(OOH) almost-equal-to P complex under anoxic condition s after adding H2S in inorganic suspensions was shown to be limited. O nly when a large excess of H2S was added there was some release, but i f less than 75% of the Fe(OOH) was converted into FeS, there was no re lease. The possibility of organic phosphate as the source of phosphate release under anoxic conditions is discussed. For a full understandin g of this possibility, fractionation of sediment bound phosphate must be carried out in such a way, that these organic phosphates are not hy drolysed.