SORPTION OF PHOSPHATE BY SEDIMENTS AS A RESULT OF ENHANCED EXTERNAL LOADING

In artificial test ditches, originally poor in nutrients, the effects of enhanced external loading with phosphorus were studied. An important term in the mass balance of phosphorus is retention by sediment. Parameters concerning the uptake of phosphorus by the sandy sediment of a ditch have been measured or were obtained from curve-fitting and were used in a mathematical model to describe diffusion into the sediment and subsequent sorption by soil particles. On a time scale of hours uptake of phosphorus from the overlying water by intact sediment cores could be simulated well with a simple diffusion-adsorption model. Mixing of the overlying water resulted in an enhanced uptake rate caused by an increased effective diffusion coefficient in the top layer of the sediment. Laboratory experiments revealed that after a fast initial adsorption, a slow uptake process followed that continued for a period of at least several months. This slow sorption can immobilize a substantial part of the phosphorus added. It may physically be described as an intraparticular diffusion process, in which the adsorbed phosphate penetrates into metaloxides, probably present as sand grain coating, and thereby reaches sorption sites not immediately accessible otherwise. The total sorption capacity of the soil particles is ca. 3.3 times the maximum instantaneous surficial adsorption capacity.