The distribution of phosphate over iron-bound and calcium-bound phosphate in stratified sediments

Release of phosphate from, and adsorption onto sediments is calculated as a chemical equilibrium between dissolved o-phosphate and two solid phosphates, i.e. iron- and calcium-bound phosphate. Organic phosphates play a minor role, if any at all. Using chemical equilibrium equations, the distribution of the two solid inorganic phosphates is calculated from the accumulated phosphate quantity as function of time and depth in sediment layers of shallow lakes or wetlands. It is shown that this distribution depends on water depth, pH, Ca super(2+) concentration in the water, Fe(OOH) concentration in the sediments and maximal binding capacity of the sediments. By comparing values of dissolved phosphate at different pH values, it is shown that acidification, which usually takes place in hypolimnia, will cause release of phosphate, which is not necessarily dependent on the redox potential. The release does depend on pH, Ca super(2+) concentration in the water, CaCO sub(3) concentration in the sediments and the saturation stage of the two P-pools in the surface layers of these sediments.